ADVANCE SPECIAL SENSES PHYSIOLOGYFOR POSTGRADUATE STUDENTS

Rabiu AbduSSALAM Magaji, Ph.D.

Department of Human Physiology,Faculty of Medicine,

Ahmadu Bello University, Zaria – Nigeria(www.abu.edu.ng)

Mobile: 08023558721E-mails: ramagaji@abu.edu.ng and rabiumagaji@yahoo.co.uk

Rabiu AbduSSALAM Magaji, Ph.D.

Department of Human Physiology,Faculty of Medicine,

Ahmadu Bello University, Zaria – Nigeria(www.abu.edu.ng)

Mobile: 08023558721E-mails: ramagaji@abu.edu.ng and rabiumagaji@yahoo.co.uk

SPECIAL SENSES OF VISIONLearning Objectives

At the end of the Lesson, the students should be able to:

Describe the various parts of the eye and list the functions of each.

Explain how light rays in the environment are brought to a focus onthe retina and the role of accommodation in this process.

Describe the electrical responses produced by rods and cones andexplain how these responses are produced.

Trace the neural pathways that transmit visual information from therods and cones to the visual cortex.

Define the following terms: hyperopia, myopia, astigmatism,presbyopia, and strabismus.

Learning Objectives

At the end of the Lesson, the students should be able to:

Describe the various parts of the eye and list the functions of each.

Explain how light rays in the environment are brought to a focus onthe retina and the role of accommodation in this process.

Describe the electrical responses produced by rods and cones andexplain how these responses are produced.

Trace the neural pathways that transmit visual information from therods and cones to the visual cortex.

Define the following terms: hyperopia, myopia, astigmatism,presbyopia, and strabismus.

One of the senses that make or destroy a Man

The eyes are complex sense organs.

Within its protective casing, each eye has:

a layer of receptors;

a lens system that focuses light on these receptors; and

a system of nerves that conducts impulses from thereceptors to the brain.

The way these components operate to set up conscious visualimages is the subject of this lesson.

The eyes are complex sense organs.

Within its protective casing, each eye has:

a layer of receptors;

a lens system that focuses light on these receptors; and

a system of nerves that conducts impulses from thereceptors to the brain.

The way these components operate to set up conscious visualimages is the subject of this lesson.

Anatomy of the Eye

The outer protective layerof the eyeball, the sclera, ismodified anteriorly to formthe transparent cornea,through which light raysenter the eye.

Inside the sclera is thechoroid, a layer thatcontains many of the bloodvessels that nourish thestructures in the eyeball.

Lining the posterior two thirdsof the choroid is the retina, theneural tissue containing thereceptor cells.

Anatomy of the Eye

The outer protective layerof the eyeball, the sclera, ismodified anteriorly to formthe transparent cornea,through which light raysenter the eye.

Inside the sclera is thechoroid, a layer thatcontains many of the bloodvessels that nourish thestructures in the eyeball.

The internal anatomy of the eye (Adapted from Medical Physiology: a SystemsApproach by Hershel and Michael. McGraw-Hill Company, 2011).

The crystalline lens is atransparent structure held inplace by a circular lenssuspensary ligament (zonule)that is attached to the thickenedanterior part of the choroid, theciliary body.

The ciliary body containscircular and longitudinal musclefibers that attach near thecorneoscleral junction.

In front of the lens is thepigmented and opaque iris, thecolored portion of the eye, whichcontains circular muscle fibersthat constrict and radial fibers thatdilate the pupil.

Variations in the diameter of the pupilcan produce up to a 5-fold change inthe amount of light reaching theretina.

The crystalline lens is atransparent structure held inplace by a circular lenssuspensary ligament (zonule)that is attached to the thickenedanterior part of the choroid, theciliary body.

The ciliary body containscircular and longitudinal musclefibers that attach near thecorneoscleral junction.

In front of the lens is thepigmented and opaque iris, thecolored portion of the eye, whichcontains circular muscle fibersthat constrict and radial fibers thatdilate the pupil.

The internal anatomy of the eye (Adapted from Medical Physiology: a SystemsApproach by Hershel and Michael. McGraw-Hill Company, 2011).The internal anatomy of the eye (Adapted from Medical Physiology: a SystemsApproach by Hershel and Michael. McGraw-Hill Company, 2011).

The space between the lensand the retina is filled primarilywith a clear gelatinous materialcalled the vitreous humor.

Aqueous humor, a clear liquidthat nourishes the cornea andlens, is produced in the ciliarybody by diffusion and activetransport from plasma.

It flows through the pupil andfills the anterior chamber of theeye.

It is normally reabsorbedthrough the canal of Schlemm, avenous channel at the junctionbetween the iris and the cornea(anterior chamber angle).

The space between the lens and theretina is filled primarily with a cleargelatinous material called the vitreoushumor.

The space between the lensand the retina is filled primarilywith a clear gelatinous materialcalled the vitreous humor.

Aqueous humor, a clear liquidthat nourishes the cornea andlens, is produced in the ciliarybody by diffusion and activetransport from plasma.

It flows through the pupil andfills the anterior chamber of theeye.

It is normally reabsorbedthrough the canal of Schlemm, avenous channel at the junctionbetween the iris and the cornea(anterior chamber angle).

The internal anatomy of the eye (Adapted from Medical Physiology: a SystemsApproach by Hershel and Michael. McGraw-Hill Company, 2011).The internal anatomy of the eye (Adapted from Medical Physiology: a SystemsApproach by Hershel and Michael. McGraw-Hill Company, 2011).

Obstruction of this outletleads to increased intraocularpressure.

One cause of increasedpressure is decreasedpermeability through:

the trabecular meshwork, the tissue around the base

of the cornea that drainsthe aqueous humor fromthe eye (open-angleglaucoma); and

forward movement of theiris, obliterating the angle(angle- closureglaucoma).

Obstruction of this outletleads to increased intraocularpressure.

One cause of increasedpressure is decreasedpermeability through:

the trabecular meshwork, the tissue around the base

of the cornea that drainsthe aqueous humor fromthe eye (open-angleglaucoma); and

forward movement of theiris, obliterating the angle(angle- closureglaucoma).

The internal anatomy of the eye (Adapted from Medical Physiology: a SystemsApproach by Hershel and Michael. McGraw-Hill Company, 2011).The internal anatomy of the eye (Adapted from Medical Physiology: a SystemsApproach by Hershel and Michael. McGraw-Hill Company, 2011).

Glaucoma can be treated with β-adrenergic blocking drugsor carbonic anhydrase inhibitors, both of which decrease theproduction of aqueous humor, or with cholinergic agonists,which increase aqueous outflow.

The eye is well protected from injury by the bony walls of theorbit.

The cornea is moistened and kept clear by tears that coursefrom the lacrimal gland in the upper portion of each orbit acrossthe surface of the eye to empty via the lacrimal duct into thenose.

Blinking helps keep the cornea moist.

Glaucoma can be treated with β-adrenergic blocking drugsor carbonic anhydrase inhibitors, both of which decrease theproduction of aqueous humor, or with cholinergic agonists,which increase aqueous outflow.

The eye is well protected from injury by the bony walls of theorbit.

The cornea is moistened and kept clear by tears that coursefrom the lacrimal gland in the upper portion of each orbit acrossthe surface of the eye to empty via the lacrimal duct into thenose.

Blinking helps keep the cornea moist.

RetinaThe retina extends

anteriorly almost to the ciliarybody. It is organized into:

10 layers within which;

Are found rods and cones,which are the visual receptorsand some non-visualPhotoreceptors (e.g.melanopsin); and

four types of neurons:bipolar cells, ganglion cells,horizontal cells, andamacrine cells.

RetinaThe retina extends

anteriorly almost to the ciliarybody. It is organized into:

10 layers within which;

Are found rods and cones,which are the visual receptorsand some non-visualPhotoreceptors (e.g.melanopsin); and

four types of neurons:bipolar cells, ganglion cells,horizontal cells, andamacrine cells. Neural components of the extrafoveal portion of the retina (Adapted

from Medical Physiology: a Systems Approach by Hershel andMichael. McGraw-Hill Company, 2011).

Neural components of the extrafoveal portion of the retina (Adapted from Medical Physiology: a Systems Approachby Hershel and Michael. McGraw-Hill Company, 2011). Key: C, cone; R, rod; MB, RB, and FB, midget, rod, and flat bipolarcells; DG and MG, diffuse and midget ganglion cells; H, horizontal cells; A, amacrine cells.

Rods and cones, which arenext to the choroid, synapse withbipolar cells, and bipolar cellssynapse with ganglion cells.

The axons of ganglion cellsconverge and leave the eye asthe optic nerve.

Horizontal cells connectreceptor cells to the otherreceptor cells in the outerplexiform layer.

Amacrine cells connectganglion cells to one another inthe inner plexiform layer viaprocesses of varying length andpatterns.

Rods and cones, which arenext to the choroid, synapse withbipolar cells, and bipolar cellssynapse with ganglion cells.

The axons of ganglion cellsconverge and leave the eye asthe optic nerve.

Horizontal cells connectreceptor cells to the otherreceptor cells in the outerplexiform layer.

Amacrine cells connectganglion cells to one another inthe inner plexiform layer viaprocesses of varying length andpatterns.

Neural components of the extrafoveal portion of the retina (Adaptedfrom Medical Physiology: a Systems Approach by Hershel andMichael. McGraw-Hill Company, 2011).

Gap junctions also connectretinal neurons to one another.

The receptor layer of theretina rests on the pigmentepithelium next to the choroid,so light rays must pass throughthe ganglion cell and bipolar celllayers to reach the rods andcones.

The pigment epitheliumabsorbs light rays, preventing thereflection of rays back throughthe retina.

Such reflection would produceblurring of the visual images.

Gap junctions also connectretinal neurons to one another.

The receptor layer of theretina rests on the pigmentepithelium next to the choroid,so light rays must pass throughthe ganglion cell and bipolar celllayers to reach the rods andcones.

The pigment epitheliumabsorbs light rays, preventing thereflection of rays back throughthe retina.

Such reflection would produceblurring of the visual images. Neural components of the extrafoveal portion of the retina (Adapted

from Medical Physiology: a Systems Approach by Hershel andMichael. McGraw-Hill Company, 2011).

The optic nerve leaves theeye and the retinal bloodvessels enter it at a point 3 mmmedial to and slightly above theposterior pole of the globe.

This region is visible throughthe ophthalmoscope as theoptic disk.

There are no visual receptorsover the disk, and consequentlyit is a blind spot.

Near the posterior pole of theeye is a yellowish pigmentedspot, the macula lutea.

The optic nerve leaves theeye and the retinal bloodvessels enter it at a point 3 mmmedial to and slightly above theposterior pole of the globe.

This region is visible throughthe ophthalmoscope as theoptic disk.

There are no visual receptorsover the disk, and consequentlyit is a blind spot.

Near the posterior pole of theeye is a yellowish pigmentedspot, the macula lutea.

Neural components of the extrafoveal portion of the retina (Adaptedfrom Medical Physiology: a Systems Approach by Hershel andMichael. McGraw-Hill Company, 2011).

This marks the location of thefovea centralis, a thinned-out,rod-free portion of the retina.

In it, the cones are denselypacked, and each synapses to asingle bipolar cell, which, in turn,synapses on a single ganglioncell, providing a direct pathway tothe brain.

There are very few overlyingcells and no blood vessels; thus,the fovea is the point wherevisual acuity is greatest.

When attention is attracted to orfixed on an object, the eyes arenormally moved so that light rayscoming from the object fall on thefovea.

This marks the location of thefovea centralis, a thinned-out,rod-free portion of the retina.

In it, the cones are denselypacked, and each synapses to asingle bipolar cell, which, in turn,synapses on a single ganglioncell, providing a direct pathway tothe brain.

There are very few overlyingcells and no blood vessels; thus,the fovea is the point wherevisual acuity is greatest.

Neural components of the extrafoveal portion of the retina (Adaptedfrom Medical Physiology: a Systems Approach by Hershel andMichael. McGraw-Hill Company, 2011).

Visual Receptors in the Retina

• Rods are responsible for visionin low light (night vision) andprovide only black and whitevision.

• Cones are responsible forcolor vision.

• Each rod and cone is dividedinto an outer segment, an innersegment that includes a nuclearregion, and a synaptic zone.

Visual Receptors in the Retina

• Rods are responsible for visionin low light (night vision) andprovide only black and whitevision.

• Cones are responsible forcolor vision.

• Each rod and cone is dividedinto an outer segment, an innersegment that includes a nuclearregion, and a synaptic zone.

Schematic diagram of a rod and a cone

The outer segments aremodified cilia and are made upof regular stacks of flattenedsaccules or disks composed ofmembrane.

These saccules and diskscontain the photosensitivecompounds that react to light,initiating action potentials in thevisual pathways.

The inner segments are rich inmitochondria.

The outer segments aremodified cilia and are made upof regular stacks of flattenedsaccules or disks composed ofmembrane.

These saccules and diskscontain the photosensitivecompounds that react to light,initiating action potentials in thevisual pathways.

The inner segments are rich inmitochondria.

Schematic diagram of a rod and a cone

The rods are named for thethin, rod like appearance of theirouter segments.

Cones generally have thickinner segments and conicalouter segments, although theirmorphology varies from place toplace in the retina.

In cones, the saccules areformed in the outer segments byinfoldings of the cell membrane,but in rods the disks areseparated from the cellmembrane.

In the extrafoveal portions ofthe retina, rods predominate,and there is a good deal ofconvergence.

The rods are named for thethin, rod like appearance of theirouter segments.

Cones generally have thickinner segments and conicalouter segments, although theirmorphology varies from place toplace in the retina.

In cones, the saccules areformed in the outer segments byinfoldings of the cell membrane,but in rods the disks areseparated from the cellmembrane. Schematic diagram of a rod and a cone

• Flat bipolar cells make synaptic contact with several cones,and rod bipolar cells make synaptic contact with several rods.

• Because there are approximately 6 million cones and 120million rods in each human eye but only 1.2 million nerve fibersin each optic nerve, the overall convergence of receptors throughbipolar cells on ganglion cells is about 105:1. However, there isdivergence from this point on.

• There are twice as many fibers in the geniculocalcarine tractsas in the optic nerves, and in the visual cortex, the number ofneurons concerned with vision is 1,000 times the number offibers in the optic nerves.

• Flat bipolar cells make synaptic contact with several cones,and rod bipolar cells make synaptic contact with several rods.

• Because there are approximately 6 million cones and 120million rods in each human eye but only 1.2 million nerve fibersin each optic nerve, the overall convergence of receptors throughbipolar cells on ganglion cells is about 105:1. However, there isdivergence from this point on.

• There are twice as many fibers in the geniculocalcarine tractsas in the optic nerves, and in the visual cortex, the number ofneurons concerned with vision is 1,000 times the number offibers in the optic nerves.

The Image-forming Mechanism

The eyes convert energy in the visible spectrum into actionpotentials in the optic nerve.

The images of objects in the environment are focused on theretina.

The light rays striking the retina generate potentials in therods and cones.

Impulses initiated in the retina are conducted to the cerebralcortex, where they produce the sensation of vision.

The Image-forming Mechanism

The eyes convert energy in the visible spectrum into actionpotentials in the optic nerve.

The images of objects in the environment are focused on theretina.

The light rays striking the retina generate potentials in therods and cones.

Impulses initiated in the retina are conducted to the cerebralcortex, where they produce the sensation of vision.

Light rays are bent when they pass from a medium of onedensity into a medium of a different density, except when theystrike perpendicular to the interface.

The bending of light rays is called refraction and is themechanism that allows one to focus an accurate image onto theretina.

Parallel light rays striking a biconvex lens are refracted to apoint behind the lens.

In the eye, light is actually refracted at the anterior surface ofthe cornea and at the anterior and posterior surfaces of the lens.

The process of refraction can be represented diagrammaticallyby drawing the rays of light as if all refraction occurs at theanterior surface of the cornea.

Light rays are bent when they pass from a medium of onedensity into a medium of a different density, except when theystrike perpendicular to the interface.

The bending of light rays is called refraction and is themechanism that allows one to focus an accurate image onto theretina.

Parallel light rays striking a biconvex lens are refracted to apoint behind the lens.

In the eye, light is actually refracted at the anterior surface ofthe cornea and at the anterior and posterior surfaces of the lens.

The process of refraction can be represented diagrammaticallyby drawing the rays of light as if all refraction occurs at theanterior surface of the cornea.

The retinal image is inverted.

The connections of the retinal receptors are such that from birthany inverted image on the retina is viewed right side up andprojected to the visual field on the side opposite to the retinal areastimulated.

This perception is present in infants and is innate.

Common Defects of the Image-forming Mechanism

In some individuals, the eyeball is shorter than normal and theparallel rays of light are brought to a focus behind the retina.

This abnormality is called hyperopia or farsightedness.

The retinal image is inverted.

The connections of the retinal receptors are such that from birthany inverted image on the retina is viewed right side up andprojected to the visual field on the side opposite to the retinal areastimulated.

This perception is present in infants and is innate.

Common Defects of the Image-forming Mechanism

In some individuals, the eyeball is shorter than normal and theparallel rays of light are brought to a focus behind the retina.

This abnormality is called hyperopia or farsightedness.

Sustained accommodation (focusing due to contraction ofthe ciliary muscle), even when viewing distant objects, canpartially compensate for the defect, but the prolonged musculareffort is tiring and may cause headaches and blurring of vision.

The defect can be corrected by using glasses with convexlenses, which aid the refractive power of the eye in shorteningthe focal distance.

In myopia (nearsightedness), the anteroposterior diameter ofthe eyeball is too long.

The shape of the eye appears to be determined in part by therefraction presented to it.

Sustained accommodation (focusing due to contraction ofthe ciliary muscle), even when viewing distant objects, canpartially compensate for the defect, but the prolonged musculareffort is tiring and may cause headaches and blurring of vision.

The defect can be corrected by using glasses with convexlenses, which aid the refractive power of the eye in shorteningthe focal distance.

In myopia (nearsightedness), the anteroposterior diameter ofthe eyeball is too long.

The shape of the eye appears to be determined in part by therefraction presented to it.

In young adult humans, the extensive close work involved inactivities such as studying accelerates the development of myopia.

This defect can be corrected by glasses with biconcave lenses,which make parallel light rays diverge slightly before they strike theeye.

Astigmatism is a common condition in which the curvature of thecornea is not uniform.

When the curvature in one meridian is different from that inothers, light rays in that meridian are refracted to a different focus,so that part of the retinal image is blurred.

Astigmatism can usually be corrected with cylindrical lensesplaced in such a way that they equalize the refraction in allmeridians.

In young adult humans, the extensive close work involved inactivities such as studying accelerates the development of myopia.

This defect can be corrected by glasses with biconcave lenses,which make parallel light rays diverge slightly before they strike theeye.

Astigmatism is a common condition in which the curvature of thecornea is not uniform.

When the curvature in one meridian is different from that inothers, light rays in that meridian are refracted to a different focus,so that part of the retinal image is blurred.

Astigmatism can usually be corrected with cylindrical lensesplaced in such a way that they equalize the refraction in allmeridians.

Strabismus is a misalignment of the eyes usually due toproblems with eye muscles and one of the most common eyeproblems in children, affecting about 4% of children under 6years of age.

It is characterized by one or both eyes turning inward(crossed-eyes), outward (wall eyes), upward, or downward.

Strabismus is also commonly called “wandering eye” or“crossed-eyes”. It occurs when visual images do not fall oncorresponding retinal points.

When visual images chronically fall on non correspondingpoints in the two retinas in young children, one is eventuallysuppressed (suppression scotoma).

Strabismus is a misalignment of the eyes usually due toproblems with eye muscles and one of the most common eyeproblems in children, affecting about 4% of children under 6years of age.

It is characterized by one or both eyes turning inward(crossed-eyes), outward (wall eyes), upward, or downward.

Strabismus is also commonly called “wandering eye” or“crossed-eyes”. It occurs when visual images do not fall oncorresponding retinal points.

When visual images chronically fall on non correspondingpoints in the two retinas in young children, one is eventuallysuppressed (suppression scotoma).

AccommodationWhen the ciliary muscle is relaxed, parallel light rays striking

the optically normal (emmetropic) eye are brought to a focuson the retina.

As long as this relaxation is maintained, rays from objectscloser than 6 m from the observer are brought to a focus behindthe retina, and consequently the objects appear blurred.

The problem of bringing diverging rays from close objects toa focus on the retina can be solved by increasing the curvatureof the lens, a process called accommodation.

At rest, the lens is held under tension by the lens ligamentsand is pulled into a flattened shape.

When the ciliary muscle is relaxed, parallel light rays strikingthe optically normal (emmetropic) eye are brought to a focuson the retina.

As long as this relaxation is maintained, rays from objectscloser than 6 m from the observer are brought to a focus behindthe retina, and consequently the objects appear blurred.

The problem of bringing diverging rays from close objects toa focus on the retina can be solved by increasing the curvatureof the lens, a process called accommodation.

At rest, the lens is held under tension by the lens ligamentsand is pulled into a flattened shape.

The ciliary muscle contracts when the gaze is directed at a nearobject.

This decreases the distance between the edges of the ciliarybody and relaxes the lens ligaments, so that the lens springs intoa more convex shape.

The degree to which the lens curvature can be increased islimited, and light rays from an object very near the individualcannot be brought to a focus on the retina, even with the greatestof effort.

The nearest point to the eye at which an object can be broughtinto clear focus by accommodation is called the near point ofvision.

Due to increasing hardness of the lens, the near point recedesthroughout life, slowly at first and then rapidly with advancing age,from 9 cm at age 10 to 83 cm at age 60.

The ciliary muscle contracts when the gaze is directed at a nearobject.

This decreases the distance between the edges of the ciliarybody and relaxes the lens ligaments, so that the lens springs intoa more convex shape.

The degree to which the lens curvature can be increased islimited, and light rays from an object very near the individualcannot be brought to a focus on the retina, even with the greatestof effort.

The nearest point to the eye at which an object can be broughtinto clear focus by accommodation is called the near point ofvision.

Due to increasing hardness of the lens, the near point recedesthroughout life, slowly at first and then rapidly with advancing age,from 9 cm at age 10 to 83 cm at age 60.

By the time a healthy individual reaches age 40–45, the lossof accommodation is usually sufficient to make reading andclose work difficult.

This condition, which is known as presbyopia, can becorrected by wearing glasses with convex lenses.

The Photoreceptor Mechanism- Ionic Basis of PhotoreceptorPotentials

Na+ channels in the outer segments of the rods and conesare open in the dark, so current flows from the inner to the outersegment.

Current also flows to the synaptic ending of the photoreceptor.

By the time a healthy individual reaches age 40–45, the lossof accommodation is usually sufficient to make reading andclose work difficult.

This condition, which is known as presbyopia, can becorrected by wearing glasses with convex lenses.

The Photoreceptor Mechanism- Ionic Basis of PhotoreceptorPotentials

Na+ channels in the outer segments of the rods and conesare open in the dark, so current flows from the inner to the outersegment.

Current also flows to the synaptic ending of the photoreceptor.

The Na+, K+-ATPase in the inner segment maintains ionicequilibrium.

Release of synaptic transmitter is steady in the dark.

When light strikes the outer segment, the reactions that areinitiated close some of the Na+ channels, and the result is ahyperpolarizing receptor potential.

The hyperpolarization reduces the release of synaptictransmitter, and this generates a signal in the bipolar cells thatultimately leads to action potentials in ganglion cells.

The photosensitive compounds in the rods and cones of theeyes are made up of a protein called an opsin and retinal, thealdehyde of vitamin A.

The Na+, K+-ATPase in the inner segment maintains ionicequilibrium.

Release of synaptic transmitter is steady in the dark.

When light strikes the outer segment, the reactions that areinitiated close some of the Na+ channels, and the result is ahyperpolarizing receptor potential.

The hyperpolarization reduces the release of synaptictransmitter, and this generates a signal in the bipolar cells thatultimately leads to action potentials in ganglion cells.

The photosensitive compounds in the rods and cones of theeyes are made up of a protein called an opsin and retinal, thealdehyde of vitamin A.

The photosensitive pigment in the rodsis called rhodopsin, one of the manyreceptors coupled to G proteins with Itsopsin called scotopsin.

Rhodopsin has a peak sensitivity tolight at a wavelength of 505 nm.

Sequence of Events in Photoreceptors

Light activates rhodopsin that thenactivates the associated heterotrimeric Gprotein, transducin.

The G protein exchanges GDP forGTP, and the α-subunit separates.

The photosensitive pigment in the rodsis called rhodopsin, one of the manyreceptors coupled to G proteins with Itsopsin called scotopsin.

Rhodopsin has a peak sensitivity tolight at a wavelength of 505 nm.

Sequence of Events in Photoreceptors

Light activates rhodopsin that thenactivates the associated heterotrimeric Gprotein, transducin.

The G protein exchanges GDP forGTP, and the α-subunit separates.

This subunit remains active until itsintrinsic GTPase activity hydrolyzes theGTP.

The α-subunit activates cGMPphosphodiesterase, which convertscGMP to 5′-GMP.

cGMP normally acts directly on Na+channels to maintain them in the openposition, so the decline in thecytoplasmic cGMP concentration causessome Na+ channels to close.

This produces the hyperpolarizingpotential.

This subunit remains active until itsintrinsic GTPase activity hydrolyzes theGTP.

The α-subunit activates cGMPphosphodiesterase, which convertscGMP to 5′-GMP.

cGMP normally acts directly on Na+channels to maintain them in the openposition, so the decline in thecytoplasmic cGMP concentration causessome Na+ channels to close.

This produces the hyperpolarizingpotential.

This cascade of reactions occurs very rapidly and amplifiesthe light signal.

The amplification helps explain the remarkable sensitivity ofrod photoreceptors; these receptors are capable of producing adetectable response to as little as one photon of light.

Cone receptors subserve color vision and respond maximallyto light at wavelengths of 440, 535, and 565 nm.

The cone opsin resembles rhodopsin.

The cell membrane of cones is invaginated to form thesaccules, but the cones have no separate intracellular disks likethose in rods.

The details of theresponses of cones to light are similar tothose in rods.

This cascade of reactions occurs very rapidly and amplifiesthe light signal.

The amplification helps explain the remarkable sensitivity ofrod photoreceptors; these receptors are capable of producing adetectable response to as little as one photon of light.

Cone receptors subserve color vision and respond maximallyto light at wavelengths of 440, 535, and 565 nm.

The cone opsin resembles rhodopsin.

The cell membrane of cones is invaginated to form thesaccules, but the cones have no separate intracellular disks likethose in rods.

The details of theresponses of cones to light are similar tothose in rods.

Visual PathwaysThe axons of the retinal

ganglion cells pass caudally in theoptic nerve and optic tract to endin the lateral geniculate body inthe thalamus.

The fibers from each nasalhemiretina decussate in the opticchiasm.

In the geniculate body, thefibers from the nasal half of oneretina and the temporal half of theother synapse on the cells whoseaxons form the geniculocalcarinetract.

The axons of the retinalganglion cells pass caudally in theoptic nerve and optic tract to endin the lateral geniculate body inthe thalamus.

The fibers from each nasalhemiretina decussate in the opticchiasm.

In the geniculate body, thefibers from the nasal half of oneretina and the temporal half of theother synapse on the cells whoseaxons form the geniculocalcarinetract.

This tract passes to theoccipital lobe of the cerebralcortex.

The primary visual receivingarea (primary visual cortex,Brodmann’s area 17; alsoknown as V1) is locatedprincipally on the sides of thecalcarine fissure.

This tract passes to theoccipital lobe of the cerebralcortex.

The primary visual receivingarea (primary visual cortex,Brodmann’s area 17; alsoknown as V1) is locatedprincipally on the sides of thecalcarine fissure.

Effect of Lesions in the Optic Pathways

Lesions along the neuralpathways from the eyes to thebrain can be localized with ahigh degree of accuracy by theeffects they produce in the visualfields.

The fibers from the nasal halfof each retina decussate in theoptic chiasm, so that the fibers inthe optic tracts are those fromthe temporal half of one retinaand the nasal half of the other.

Lesions along the neuralpathways from the eyes to thebrain can be localized with ahigh degree of accuracy by theeffects they produce in the visualfields.

The fibers from the nasal halfof each retina decussate in theoptic chiasm, so that the fibers inthe optic tracts are those fromthe temporal half of one retinaand the nasal half of the other.

Since each optic tractsubserves half of the field ofvision, a lesion of one opticnerve causes blindness in thateye, but a lesion in one optictract causes blindness in half ofthe visual field.

This defect is classified as ahomonymous (same side ofboth visual fields) hemianopia(half-blindness).

Since each optic tractsubserves half of the field ofvision, a lesion of one opticnerve causes blindness in thateye, but a lesion in one optictract causes blindness in half ofthe visual field.

This defect is classified as ahomonymous (same side ofboth visual fields) hemianopia(half-blindness).

Lesions affecting the opticchiasm (e.g., pituitary tumors)cause disruption of the fibersfrom both nasal hemiretinas andproduce a heteronymous(opposite sides of the visualfields) hemianopia.

Because the fibers from themaculas are located posteriorlyin the optic chiasm, hemianopicscotomas develop before visionin the two hemiretinas iscompletely lost.

Lesions affecting the opticchiasm (e.g., pituitary tumors)cause disruption of the fibersfrom both nasal hemiretinas andproduce a heteronymous(opposite sides of the visualfields) hemianopia.

Because the fibers from themaculas are located posteriorlyin the optic chiasm, hemianopicscotomas develop before visionin the two hemiretinas iscompletely lost.

Selective visual fielddefects are further classifiedas bitemporal, binasal, andright or left.

The optic nerve fibers fromthe upper retinal quadrantssubserving vision in the lowerhalf of the visual fieldterminate in the medial half ofthe lateral geniculate body,whereas the fibers from thelower retinal quadrantsterminate in the lateral half.

Selective visual fielddefects are further classifiedas bitemporal, binasal, andright or left.

The optic nerve fibers fromthe upper retinal quadrantssubserving vision in the lowerhalf of the visual fieldterminate in the medial half ofthe lateral geniculate body,whereas the fibers from thelower retinal quadrantsterminate in the lateral half.

Color VisionColors have three attributes: hue, intensity, and saturation

(degree of freedom from dilution with white).

For any color there is a complementary color that, whenproperly mixed with it, produces a sensation of white.

Black is the sensation produced by the absence of light, but itis probably a positive sensation because the blind eye does not“see black;” rather, it “sees nothing.”

The sensation of white, any spectral color, and even theextraspectral color, purple, can be produced by mixing variousproportions of red light (wavelength 723 - 647 nm), green light(575 - 492 nm), and blue light (492 - 450 nm).

Red, green, and blue are therefore called the primary colors.

Colors have three attributes: hue, intensity, and saturation(degree of freedom from dilution with white).

For any color there is a complementary color that, whenproperly mixed with it, produces a sensation of white.

Black is the sensation produced by the absence of light, but itis probably a positive sensation because the blind eye does not“see black;” rather, it “sees nothing.”

The sensation of white, any spectral color, and even theextraspectral color, purple, can be produced by mixing variousproportions of red light (wavelength 723 - 647 nm), green light(575 - 492 nm), and blue light (492 - 450 nm).

Red, green, and blue are therefore called the primary colors.

Also, the color perceived depends in part on the color of otherobjects in the visual field.

Thus, for example, a red object is seen as red if the fieldis illuminated with green or blue light, but as pale pink or white ifthe field is illuminated with red light.

Color is mediated by ganglion cells that subtract or add inputfrom one type of cone to input from another type.

Processing inthe ganglion cells and the lateral geniculatenucleus produces impulses that pass along three types of neuralpathways that project to V1:

a red-green pathway that signals differences between L- andM-cone responses;

Also, the color perceived depends in part on the color of otherobjects in the visual field.

Thus, for example, a red object is seen as red if the fieldis illuminated with green or blue light, but as pale pink or white ifthe field is illuminated with red light.

Color is mediated by ganglion cells that subtract or add inputfrom one type of cone to input from another type.

Processing inthe ganglion cells and the lateral geniculatenucleus produces impulses that pass along three types of neuralpathways that project to V1:

a red-green pathway that signals differences between L- andM-cone responses;

a blue-yellow pathway that signals differences between S-coneand the sum of L- and M-cone responses; and

a luminance pathway that signals the sum of L- and M-coneresponses.

• Blue-yellow color vision deficits are less common and show nogender selectivity.

• Color blindness is usually due to an inherited absence of conesfor specific colors.

• It can also occur in individuals with lesions of area V8 of thevisual cortex.

a blue-yellow pathway that signals differences between S-coneand the sum of L- and M-cone responses; and

a luminance pathway that signals the sum of L- and M-coneresponses.

• Blue-yellow color vision deficits are less common and show nogender selectivity.

• Color blindness is usually due to an inherited absence of conesfor specific colors.

• It can also occur in individuals with lesions of area V8 of thevisual cortex.

These pathways project to the blobs and the deep portion oflayer 4 of V1.

From the blobs and layer 4, color information is projected toV8.

However, it is not known how V8 converts color input into thesensation of color.

Color blindness is most often an inherited condition inwhich individuals are unable to distinguish certain colors.

The most common type is a red-green color vision deficit, agenetically sex-linked condition that occurs in about 8% ofmales and 0.4% of females.

These pathways project to the blobs and the deep portion oflayer 4 of V1.

From the blobs and layer 4, color information is projected toV8.

However, it is not known how V8 converts color input into thesensation of color.

Color blindness is most often an inherited condition inwhich individuals are unable to distinguish certain colors.

The most common type is a red-green color vision deficit, agenetically sex-linked condition that occurs in about 8% ofmales and 0.4% of females.

Pupillary Light ReflexWhen light is directed into one eye, the pupil constricts

(pupillary light reflex).

The optic nerve fibers that carry the impulses initiating thesepupillary responses leave the optic nerves near the lateralgeniculate bodies.

On each side, they enter the midbrain via the brachium of thesuperior colliculus and terminate in the pretectal nucleus.

From this nucleus, the second-order neurons project to theipsilateral and contralateral Edinger–Westphal nucleus.

The third-order neurons pass from this nucleus to the ciliaryganglion in the oculomotor nerve, and the fourth-order neuronspass from this ganglion to the ciliary body.

When light is directed into one eye, the pupil constricts(pupillary light reflex).

The optic nerve fibers that carry the impulses initiating thesepupillary responses leave the optic nerves near the lateralgeniculate bodies.

On each side, they enter the midbrain via the brachium of thesuperior colliculus and terminate in the pretectal nucleus.

From this nucleus, the second-order neurons project to theipsilateral and contralateral Edinger–Westphal nucleus.

The third-order neurons pass from this nucleus to the ciliaryganglion in the oculomotor nerve, and the fourth-order neuronspass from this ganglion to the ciliary body.

Eye MovementsThe eye is moved within the orbit by six ocular muscles.

These are innervated by the oculomotor, trochlear, andabducens (cranial) nerves.

Because the oblique muscles pull medially, their actions varywith the position of the eye.

When the eye is turned nasally, the inferior oblique elevates itand the superior oblique depresses it.

When it is turned laterally, the superior rectus elevates it andthe inferior rectus depresses it.

Because much of the visual field is binocular, a very high orderof coordination of the movements of the two eyes is necessary ifvisual images are to fall at all times on corresponding points in thetwo retinas and to avoid diplopia (double vision).

The eye is moved within the orbit by six ocular muscles.

These are innervated by the oculomotor, trochlear, andabducens (cranial) nerves.

Because the oblique muscles pull medially, their actions varywith the position of the eye.

When the eye is turned nasally, the inferior oblique elevates itand the superior oblique depresses it.

When it is turned laterally, the superior rectus elevates it andthe inferior rectus depresses it.

Because much of the visual field is binocular, a very high orderof coordination of the movements of the two eyes is necessary ifvisual images are to fall at all times on corresponding points in thetwo retinas and to avoid diplopia (double vision).

There are four types of eye movements, each controlled by adifferent neural system but sharing the same final common path,the motor neurons that supply the external ocular muscles.

Saccades, sudden jerky movements, occur as the gaze shiftsfrom one object to another.

They bring new objects of interest onto the fovea and reduceadaptation in the visual pathway that would occur if gaze werefixed on a single object for long periods.

Smooth pursuit movements are tracking movements of theeyes as they follow moving objects.

Vestibular movements, adjustments that occur in responseto stimuli initiated in the semicircular canals, maintain visualfixation as the head moves.

There are four types of eye movements, each controlled by adifferent neural system but sharing the same final common path,the motor neurons that supply the external ocular muscles.

Saccades, sudden jerky movements, occur as the gaze shiftsfrom one object to another.

They bring new objects of interest onto the fovea and reduceadaptation in the visual pathway that would occur if gaze werefixed on a single object for long periods.

Smooth pursuit movements are tracking movements of theeyes as they follow moving objects.

Vestibular movements, adjustments that occur in responseto stimuli initiated in the semicircular canals, maintain visualfixation as the head moves.

Convergence movements bring the visual axes toward eachother as attention is focused on objects near the observer.

Saccadic movements seek out visual targets, pursuitmovements follow them as they move about, and vestibularmovements stabilize the tracking device as the platform on whichthe device is mounted (i.e., the head) moves about.

Saccades are programmed in the frontal cortex and thesuperior colliculi and pursuit movements in the cerebellum.

Convergence movements bring the visual axes toward eachother as attention is focused on objects near the observer.

Saccadic movements seek out visual targets, pursuitmovements follow them as they move about, and vestibularmovements stabilize the tracking device as the platform on whichthe device is mounted (i.e., the head) moves about.

Saccades are programmed in the frontal cortex and thesuperior colliculi and pursuit movements in the cerebellum.

Visual Neuroscience Research Group at the University of Alicante, Alicante, Spain, July, 2011

PHYSIOLOGY OF HEARING

HEARING AND EQUILIBRIUMLearning Objectives

At the end of this lesson, it is expected that the student can:

Describe the components and functions of the external, middle, and inner ear.

Describe the way that movements of molecules in the air are converted intoimpulses generated in hair cells in the cochlea.

Trace the path of auditory impulses in the neural pathways from the cochlearhair

cells to the auditory cortex, and discuss the function of the auditory cortex.

Explain how pitch and loudness are coded in the auditory pathways.

Describe the various forms of deafness and tests for their diagnosis.

Explain how the receptors in the semicircular canals detect rotationalacceleration

and how the receptors in the saccule and utricle detect linear acceleration.

List the major sensory inputs that provide the information synthesized inthe brain into the sense of position in space.

Learning ObjectivesAt the end of this lesson, it is expected that the student can:

Describe the components and functions of the external, middle, and inner ear.

Describe the way that movements of molecules in the air are converted intoimpulses generated in hair cells in the cochlea.

Trace the path of auditory impulses in the neural pathways from the cochlearhair

cells to the auditory cortex, and discuss the function of the auditory cortex.

Explain how pitch and loudness are coded in the auditory pathways.

Describe the various forms of deafness and tests for their diagnosis.

Explain how the receptors in the semicircular canals detect rotationalacceleration

and how the receptors in the saccule and utricle detect linear acceleration.

List the major sensory inputs that provide the information synthesized inthe brain into the sense of position in space.

Receptors for hearing and equilibrium are housed in the ear.

The external ear, middle ear, and cochlea of the inner ear areconcerned with hearing.

The semicircular canals, utricle, and saccule of the innerear are concerned with equilibrium.

Receptors in:

the semicircular canals (hair cells) detect rotationalacceleration;

receptors in the utricle detect linear acceleration in thehorizontal direction; and

receptors in the saccule detect linear acceleration in thevertical direction.

Receptors for hearing and equilibrium are housed in the ear.

The external ear, middle ear, and cochlea of the inner ear areconcerned with hearing.

The semicircular canals, utricle, and saccule of the innerear are concerned with equilibrium.

Receptors in:

the semicircular canals (hair cells) detect rotationalacceleration;

receptors in the utricle detect linear acceleration in thehorizontal direction; and

receptors in the saccule detect linear acceleration in thevertical direction.

Anatomy of the External and Middle EarThe external ear funnels

sound waves to the externalauditory meatus.

Sound waves pass inward tothe tympanic membrane(eardrum).

The middle ear is an air-filledcavity in the temporal bone thatopens via the auditory(Eustachian) tube into thenasopharynx and through thenasopharynx to the exterior.

The external ear funnelssound waves to the externalauditory meatus.

Sound waves pass inward tothe tympanic membrane(eardrum).

The middle ear is an air-filledcavity in the temporal bone thatopens via the auditory(Eustachian) tube into thenasopharynx and through thenasopharynx to the exterior.

The tube is usually closed, but during swallowing, chewing, andyawning it opens, equalizing air pressure on the two sides of theeardrum.

The three auditory ossicles(malleus, incus, and stapes)are in the middle ear.

The manubrium (handle ofthe malleus) is attached to theback of the tympanicmembrane.

Its head is attached to thewall of the middle ear, and itsshort process is attached to theincus, which articulates with thehead of the stapes.

The three auditory ossicles(malleus, incus, and stapes)are in the middle ear.

The manubrium (handle ofthe malleus) is attached to theback of the tympanicmembrane.

Its head is attached to thewall of the middle ear, and itsshort process is attached to theincus, which articulates with thehead of the stapes.

The foot plate of the stapes is attached by an annular ligamentto the walls of the oval window.

Two small skeletal muscles(tensor tympani andstapedius) are located in themiddle ear.

Contraction of the tensortympani pulls the manubrium ofthe malleus medially anddecreases the vibrations of thetympanic membrane;

Contraction of the stapediuspulls the foot plate of the stapesout of the oval window.

Two small skeletal muscles(tensor tympani andstapedius) are located in themiddle ear.

Contraction of the tensortympani pulls the manubrium ofthe malleus medially anddecreases the vibrations of thetympanic membrane;

Contraction of the stapediuspulls the foot plate of the stapesout of the oval window.

Anatomy of the Inner Ear and CochleaThe inner ear (labyrinth) is

made up of two parts, onewithin the other.

The bony labyrinth is aseries of channels in thetemporal bone.

Inside these channels,surrounded by a fluid(perilymph) is themembranous labyrinth that isfilled with a K+-rich fluid(endolymph).

The inner ear (labyrinth) ismade up of two parts, onewithin the other.

The bony labyrinth is aseries of channels in thetemporal bone.

Inside these channels,surrounded by a fluid(perilymph) is themembranous labyrinth that isfilled with a K+-rich fluid(endolymph).

There is no communicationbetween the spaces filled withendolymph and those filled withperilymph.

The cochlear portion of thelabyrinth is a coiled tube that,in humans, is 35-mm long andmakes approximately 2.75turns.

The basilar membrane andReissner’s membrane divideit into three chambers orscalae.

The cochlear portion of thelabyrinth is a coiled tube that,in humans, is 35-mm long andmakes approximately 2.75turns.

The basilar membrane andReissner’s membrane divideit into three chambers orscalae.

The upper scala vestibuli and the lower scala tympanicontain perilymph and communicate with each other at the apexof the cochlea via a small opening (helicotrema).

At the base of the cochlea, the scala vestibuli ends at the ovalwindow, which is closed by the footplate of the stapes.

The scala tympani end at theround window, a foramen on themedial wall of the middle ear that isclosed by the flexible secondarytympanic membrane.

The scala media is continuouswith the membranous labyrinth anddoes not communicate with theother two scalae.

The organ of Corti contains theauditory receptors (hair cells)whose processes pierce thereticular lamina that is supportedby the pillar cells or rods of Corti

The scala tympani end at theround window, a foramen on themedial wall of the middle ear that isclosed by the flexible secondarytympanic membrane.

The scala media is continuouswith the membranous labyrinth anddoes not communicate with theother two scalae.

The organ of Corti contains theauditory receptors (hair cells)whose processes pierce thereticular lamina that is supportedby the pillar cells or rods of Corti

The hair cells are arranged infour rows:

three rows of outer hair cellslateral to the tunnel formed by therods of Corti; and

one row of inner hair cells medialto the tunnel.

Covering the rows of hair cells isthe tectorial membrane in whichthe tips of the hairs of the outercells are embedded.

The cell bodies of the sensoryneurons are located in the spiralganglion within the modiolus:

The hair cells are arranged infour rows:

three rows of outer hair cellslateral to the tunnel formed by therods of Corti; and

one row of inner hair cells medialto the tunnel.

Covering the rows of hair cells isthe tectorial membrane in whichthe tips of the hairs of the outercells are embedded.

The cell bodies of the sensoryneurons are located in the spiralganglion within the modiolus:

~95% of these sensoryneurons innervate inner haircells; and

~5% innervate outer haircells, and each sensoryneuron innervates severalouter hair cells.

By contrast, most efferent fibersin the auditory nerve terminate onthe outer hair cells.

The axons of afferent neuronsthat innervate hair cells form theauditory (cochlear) division of theeighth cranial nerve.

The semicircular canals areoriented in the three planes.

Inside the bony canals, themembranous canals aresuspended in perilymph.

By contrast, most efferent fibersin the auditory nerve terminate onthe outer hair cells.

The axons of afferent neuronsthat innervate hair cells form theauditory (cochlear) division of theeighth cranial nerve.

The semicircular canals areoriented in the three planes.

Inside the bony canals, themembranous canals aresuspended in perilymph.

A receptor structure (cristaampullaris) is located in theexpanded end (ampulla) of eachof the membranous canals.

Each crista consists of haircells and supporting(sustentacular) cellssurmounted by a gelatinouspartition (cupula) that closes offthe ampulla.

The processes of the hair cellsare embedded in the cupula, andthe bases of the hair cells contactthe afferent fibers of the vestibulardivision of the eighth cranialnerve.

Within each membranouslabyrinth is an otolithic organ(macula).

Each crista consists of haircells and supporting(sustentacular) cellssurmounted by a gelatinouspartition (cupula) that closes offthe ampulla.

The processes of the hair cellsare embedded in the cupula, andthe bases of the hair cells contactthe afferent fibers of the vestibulardivision of the eighth cranialnerve.

Within each membranouslabyrinth is an otolithic organ(macula).

A receptor structure (cristaampullaris) is located in theexpanded end (ampulla) ofeach of the membranouscanals.

Another macula is located onthe wall of the saccule in asemivertical position.

The maculae containsupporting cells and hair cells,surmounted by an otolithicmembrane in which areembedded crystals of calciumcarbonate, the otoliths, which arealso called otoconia or ear dust.

The processes of the hair cellsare embedded in the membrane.

Another macula is located onthe wall of the saccule in asemivertical position.

The maculae containsupporting cells and hair cells,surmounted by an otolithicmembrane in which areembedded crystals of calciumcarbonate, the otoliths, which arealso called otoconia or ear dust.

The processes of the hair cellsare embedded in the membrane.

The nerve fibers from thehair cells join those from thecristae in the vestibulardivision of the eighth cranialnerve.

Auditory Receptors: Hair CellsThe hair cells:

in the organ of Corti signal hearing;

in the utricle signal horizontal acceleration;

in the saccule signal vertical acceleration; and

a patch in each of the three semicircular canals signalsrotational acceleration.

These hair cells have a common structure.

Each is embedded in an epithelium made up of supportingcells, with the basal end in close contact with afferent neurons.

Projecting from the apical end are 30–150 rod-shapedprocesses or hairs.

The hair cells:

in the organ of Corti signal hearing;

in the utricle signal horizontal acceleration;

in the saccule signal vertical acceleration; and

a patch in each of the three semicircular canals signalsrotational acceleration.

These hair cells have a common structure.

Each is embedded in an epithelium made up of supportingcells, with the basal end in close contact with afferent neurons.

Projecting from the apical end are 30–150 rod-shapedprocesses or hairs.

Except in the cochlea, one of these, the kinocilium, is a truebut nonmotile cilium with nine pairs of microtubules around itscircumference and a central pair of microtubules.

It is one of the largest processes and has a clubbed end.

The kinocilium is lost from the hair cells of the cochlea inadults; however, the other processes (stereocilia) are found inall hair cells.

They have cores composed of parallel filaments of actin that iscoated with isoforms of myosin.

Within the clump of processes on each cell there is an orderlystructure.

Along an axis toward the kinocilium, the stereocilia increaseprogressively in height; along the perpendicular axis, allstereocilia are the same height.

Except in the cochlea, one of these, the kinocilium, is a truebut nonmotile cilium with nine pairs of microtubules around itscircumference and a central pair of microtubules.

It is one of the largest processes and has a clubbed end.

The kinocilium is lost from the hair cells of the cochlea inadults; however, the other processes (stereocilia) are found inall hair cells.

They have cores composed of parallel filaments of actin that iscoated with isoforms of myosin.

Within the clump of processes on each cell there is an orderlystructure.

Along an axis toward the kinocilium, the stereocilia increaseprogressively in height; along the perpendicular axis, allstereocilia are the same height.

Electrical ResponsesThe resting membrane potential of the hair cells is about –60 mV.

When the stereocilia are pushed toward the kinocilium, themembrane potential is decreased to about –50 mV.

The hair processes provide a mechanism to generate changes inmembrane potential proportional to the direction and distance the hairmoves.

When the bundle of processes is pushed in the opposite direction,the cell is hyperpolarized.

Displacing the processes in a direction perpendicular to this axisprovides no change in membrane potential.

On the other hand, displacing the processes in directions that areintermediate between these two directions produces depolarization orhyperpolarization that is proportionate to the degree to which thedirection is toward or away from the kinocilium.

Electrical ResponsesThe resting membrane potential of the hair cells is about –60 mV.

When the stereocilia are pushed toward the kinocilium, themembrane potential is decreased to about –50 mV.

The hair processes provide a mechanism to generate changes inmembrane potential proportional to the direction and distance the hairmoves.

When the bundle of processes is pushed in the opposite direction,the cell is hyperpolarized.

Displacing the processes in a direction perpendicular to this axisprovides no change in membrane potential.

On the other hand, displacing the processes in directions that areintermediate between these two directions produces depolarization orhyperpolarization that is proportionate to the degree to which thedirection is toward or away from the kinocilium.

Very fine processescalled tip links tie the tip ofeach stereocilium to theside of its higher neighbor,and at the junction aremechanosensitive cationchannels.

If shorter stereocilia arepushed toward higher ones,the open time of thechannels increases.

K+ and Ca2+ enter viathe channel and producedepolarization.

Genesis of Action Potentials in Afferent Nerve FibersVery fine processes

called tip links tie the tip ofeach stereocilium to theside of its higher neighbor,and at the junction aremechanosensitive cationchannels.

If shorter stereocilia arepushed toward higher ones,the open time of thechannels increases.

K+ and Ca2+ enter viathe channel and producedepolarization.

A molecular motor in thehigher neighbor then may movethe channel toward the base,releasing tension in the tip link.

This causes the channel toclose and permits restoration ofthe resting state.

Depolarization of hair cellscauses them to release aneurotransmitter that initiatesdepolarization of neighboringafferent neurons.

The K+ that enters hair cells viathe mechanosensitive cationchannels is recycled.

It enters supporting cells andthen passes on to other supportingcells via tight junctions.

In the cochlea, it eventuallyreaches the stria vascularis and issecreted back into the endolymph,completing the cycle.

Depolarization of hair cellscauses them to release aneurotransmitter that initiatesdepolarization of neighboringafferent neurons.

The K+ that enters hair cells viathe mechanosensitive cationchannels is recycled.

It enters supporting cells andthen passes on to other supportingcells via tight junctions.

In the cochlea, it eventuallyreaches the stria vascularis and issecreted back into the endolymph,completing the cycle.

The processes of the haircells project into theendolymph and the basesare bathed in perilymph.

The perilymph is formed mainlyfrom plasma; endolymph is formedin the scala media by the striavascularis and has a highconcentration of K+ and a lowconcentration of Na+.

Cells in the stria vascularis havea high concentration of Na+,K+-ATPase.

The perilymph is formed mainlyfrom plasma; endolymph is formedin the scala media by the striavascularis and has a highconcentration of K+ and a lowconcentration of Na+.

Cells in the stria vascularis havea high concentration of Na+,K+-ATPase.

HearingSound Waves

Sound is the sensation produced when vibrations of moleculesin the external environment strike the tympanic membrane.

The loudness of a sound is typically correlated with theamplitude of a sound wave and its pitch with its frequency(number of waves per unit of time).

The amplitude of a sound wave is expressed on a relativescale, called a decibel scale.

The intensity of a sound in bels is the logarithm of the ratio ofthe intensity of that sound to a standard sound.

A value of 0 dB does not mean the absence of sound; rather, itis a sound level whose intensity is equal to that of a standard.

Sound Waves

Sound is the sensation produced when vibrations of moleculesin the external environment strike the tympanic membrane.

The loudness of a sound is typically correlated with theamplitude of a sound wave and its pitch with its frequency(number of waves per unit of time).

The amplitude of a sound wave is expressed on a relativescale, called a decibel scale.

The intensity of a sound in bels is the logarithm of the ratio ofthe intensity of that sound to a standard sound.

A value of 0 dB does not mean the absence of sound; rather, itis a sound level whose intensity is equal to that of a standard.

The 0–160-dB range from threshold pressure to a pressurethat is potentially damaging to the organ of Corti actuallyrepresents a 107-fold variation in sound pressure.

A range of 120–160 dB (e.g., firearms, jackhammer, jet planeon takeoff) is painful;

90–110 dB (e.g., subway, bass drum, chain saw, lawn mower)is extremely high;

60–80 dB (e.g., alarm clock, busy traffic, dishwasher,conversation) is very loud;

40–50 dB (e.g., moderate rainfall, normal room noise) ismoderate; and

30 dB (e.g., whisper, library) is faint.

The 0–160-dB range from threshold pressure to a pressurethat is potentially damaging to the organ of Corti actuallyrepresents a 107-fold variation in sound pressure.

A range of 120–160 dB (e.g., firearms, jackhammer, jet planeon takeoff) is painful;

90–110 dB (e.g., subway, bass drum, chain saw, lawn mower)is extremely high;

60–80 dB (e.g., alarm clock, busy traffic, dishwasher,conversation) is very loud;

40–50 dB (e.g., moderate rainfall, normal room noise) ismoderate; and

30 dB (e.g., whisper, library) is faint.

The sound frequencies audible to humans range from about20 to 20,000 cycles per second (cps, Hz).

The range decreases with age, especially difficulty detectinghigher frequency sounds.

The threshold of the human ear varies with the pitch of thesound; the greatest sensitivity is in the 1,000–4,000-Hz range.

The pitch of the average male and female voice inconversation is 120 and 250 Hz, respectively.

The number of pitches that can be distinguished by anaverage individual is about 2,000, but trained musicians canimprove on this figure considerably.

The sound frequencies audible to humans range from about20 to 20,000 cycles per second (cps, Hz).

The range decreases with age, especially difficulty detectinghigher frequency sounds.

The threshold of the human ear varies with the pitch of thesound; the greatest sensitivity is in the 1,000–4,000-Hz range.

The pitch of the average male and female voice inconversation is 120 and 250 Hz, respectively.

The number of pitches that can be distinguished by anaverage individual is about 2,000, but trained musicians canimprove on this figure considerably.

Sound TransmissionThe ear converts sound

waves in the environment intoaction potentials in the auditorynerves.

The waves are transformedby the eardrum and auditoryossicles into movements of thefoot plate of the stapes.

These movements set upwaves in the fluid of the innerear.

The ear converts soundwaves in the environment intoaction potentials in the auditorynerves.

The waves are transformedby the eardrum and auditoryossicles into movements of thefoot plate of the stapes.

These movements set upwaves in the fluid of the innerear.

The action of the waves onthe organ of Corti generatesaction potentials in the nerve.

The tympanic membrane movesin and out in response to thepressure changes produced bysound waves on its externalsurface.

Thus, the membrane functionsas a resonator that reproducesthe vibrations of the sound source.

It stops vibrating almostimmediately when the sound wavestops.

The motions of the tympanicmembrane are imparted to themanubrium.

The tympanic membrane movesin and out in response to thepressure changes produced bysound waves on its externalsurface.

Thus, the membrane functionsas a resonator that reproducesthe vibrations of the sound source.

It stops vibrating almostimmediately when the sound wavestops.

The motions of the tympanicmembrane are imparted to themanubrium.

The malleus rocks on an axisthrough the junction of its longand short processes, so that theshort process transmits thevibrations of the manubrium tothe incus.

The incus moves in such a waythat the vibrations are transmittedto the head of the stapes.

Movements of the head of thestapes swing its foot plate to andfro like a door hinged at theposterior edge of the oval window.

The auditory ossicles function asa lever system that converts theresonant vibrations of the tympanicmembrane into movements of thestapes against the perilymph filledscala vestibuli of the cochlea.

The incus moves in such a waythat the vibrations are transmittedto the head of the stapes.

Movements of the head of thestapes swing its foot plate to andfro like a door hinged at theposterior edge of the oval window.

The auditory ossicles function asa lever system that converts theresonant vibrations of the tympanicmembrane into movements of thestapes against the perilymph filledscala vestibuli of the cochlea.

This system increases thesound pressure that arrives atthe oval window, because:

the lever action of the malleusand incus multiplies the force 1.3times; and

the area of the tympanicmembrane is much greater thanthe area of the foot plate of thestapes.

When the middle ear muscles(tensor tympani and stapedius)contract, the manubrium of themalleus pulls inward and the footplate of the stapes pushes outward,decreasing sound transmission.

the lever action of the malleusand incus multiplies the force 1.3times; and

the area of the tympanicmembrane is much greater thanthe area of the foot plate of thestapes.

When the middle ear muscles(tensor tympani and stapedius)contract, the manubrium of themalleus pulls inward and the footplate of the stapes pushes outward,decreasing sound transmission.

Loud sounds initiate thetympanic reflex, whichcontracts the middle earmuscles to prevent strongsound waves from causingexcessive stimulation of theauditory receptors.

Bone and Air ConductionOssicular conduction is the normal conduction of sound

waves to the fluid of the inner ear via the tympanic membraneand the auditory ossicles.

Sound waves also initiate vibrations of the secondarytympanic membrane that closes the round window; this process,unimportant in normal hearing, is called air conduction.

Bone conduction is the transmission of vibrations of thebones of the skull to the fluid of the inner ear; this plays a role intransmission of extremely loud sounds.

Considerable bone conduction also occurs when a vibratingtuning fork is applied directly to the skull.

Bone and Air ConductionOssicular conduction is the normal conduction of sound

waves to the fluid of the inner ear via the tympanic membraneand the auditory ossicles.

Sound waves also initiate vibrations of the secondarytympanic membrane that closes the round window; this process,unimportant in normal hearing, is called air conduction.

Bone conduction is the transmission of vibrations of thebones of the skull to the fluid of the inner ear; this plays a role intransmission of extremely loud sounds.

Considerable bone conduction also occurs when a vibratingtuning fork is applied directly to the skull.

Traveling Waves• The movements of the foot plate of the stapes set up a seriesof traveling waves in the perilymph of the scala vestibuli.

• The bony walls of the scala vestibuli are rigid, but Reissner’smembrane is flexible.

• The basilar membrane is not under tension, and it also isreadily depressed into the scala tympani by the peaks of wavesin the scala vestibuli.

• Displacements of the fluid in the scala tympani are dissipatedinto air at the round window.

• Sound distorts the basilar membrane, and the site at whichthis distortion is maximal is determined by the frequency of thesound wave.

Traveling Waves• The movements of the foot plate of the stapes set up a seriesof traveling waves in the perilymph of the scala vestibuli.

• The bony walls of the scala vestibuli are rigid, but Reissner’smembrane is flexible.

• The basilar membrane is not under tension, and it also isreadily depressed into the scala tympani by the peaks of wavesin the scala vestibuli.

• Displacements of the fluid in the scala tympani are dissipatedinto air at the round window.

• Sound distorts the basilar membrane, and the site at whichthis distortion is maximal is determined by the frequency of thesound wave.

The tops of the hair cells in the organ of Corti are held rigid bythe reticular lamina, and the processes of the outer hair cells areembedded in the tectorial membrane.

When the stapes moves, both membranes move in the samedirection, but they are hinged on different axes, so a shearingmotion bends the hairs.

The processes of the inner hair cells are not attached to thetectorial membrane, but they are bent by fluid moving betweenthe membrane and the underlying hair cells.

The tops of the hair cells in the organ of Corti are held rigid bythe reticular lamina, and the processes of the outer hair cells areembedded in the tectorial membrane.

When the stapes moves, both membranes move in the samedirection, but they are hinged on different axes, so a shearingmotion bends the hairs.

The processes of the inner hair cells are not attached to thetectorial membrane, but they are bent by fluid moving betweenthe membrane and the underlying hair cells.

Inner hair cells are the primary sensory cells that generateaction potentials in auditory nerves and are stimulated by thefluid movements noted above.

Outer hair cells respond to sound, but depolarization makesthem short and hyperpolarization makes them lengthy.

They do this over a very flexible part of the basal membrane,and this action increases the amplitude and clarity of sounds.

The frequency of the action potentials in auditory nerve fibersis proportional to the loudness of the sound stimuli.

The major determinant of the pitch perceived when a soundwave strikes the ear is the place in the organ of Corti that ismaximally stimulated.

Action Potentials in Auditory Nerve FibersInner hair cells are the primary sensory cells that generate

action potentials in auditory nerves and are stimulated by thefluid movements noted above.

Outer hair cells respond to sound, but depolarization makesthem short and hyperpolarization makes them lengthy.

They do this over a very flexible part of the basal membrane,and this action increases the amplitude and clarity of sounds.

The frequency of the action potentials in auditory nerve fibersis proportional to the loudness of the sound stimuli.

The major determinant of the pitch perceived when a soundwave strikes the ear is the place in the organ of Corti that ismaximally stimulated.

The traveling wave set up by a tone produces peakdepression of the basilar membrane, and consequently maximalreceptor stimulation, at one point.

The distance between this point and the stapes is inverselyrelated to the pitch of the sound, with low tones producingmaximal stimulation at the apex of the cochlea and high tonesproducing maximal stimulation at the base.

The traveling wave set up by a tone produces peakdepression of the basilar membrane, and consequently maximalreceptor stimulation, at one point.

The distance between this point and the stapes is inverselyrelated to the pitch of the sound, with low tones producingmaximal stimulation at the apex of the cochlea and high tonesproducing maximal stimulation at the base.

The afferent fibers in theauditory division of the eighthcranial nerve end in dorsaland ventral cochlear nuclei.

From there, auditoryimpulses pass by variousroutes to the auditory cortexvia:

the inferior colliculi; the centers for auditory

reflexes; and the medial geniculate

body in the thalamus.

Central Pathway

Tocerebellum

The afferent fibers in theauditory division of the eighthcranial nerve end in dorsaland ventral cochlear nuclei.

From there, auditoryimpulses pass by variousroutes to the auditory cortexvia:

the inferior colliculi; the centers for auditory

reflexes; and the medial geniculate

body in the thalamus. Auditory Pathway

Other impulses enter thereticular formation.

Information from both earsconverges on each superiorolive, and beyond this, most ofthe neurons respond to inputsfrom both sides.

The primary auditory cortexis Brodmann’s area 41.

Low tones are representedanterolaterally and high tonesposteromedially in the auditorycortex.

Central Pathway

Tocerebellum

Other impulses enter thereticular formation.

Information from both earsconverges on each superiorolive, and beyond this, most ofthe neurons respond to inputsfrom both sides.

The primary auditory cortexis Brodmann’s area 41.

Low tones are representedanterolaterally and high tonesposteromedially in the auditorycortex. Auditory Pathway

In the primary auditory cortex, most neurons respond to inputsfrom both ears, but strips of cells are stimulated by input from thecontralateral ear and inhibited by input from the ipsilateral ear.

There are several additional auditory receiving areas, just asthere are several receiving areas for cutaneous sensation.

The auditory association areas adjacent to the primaryauditory receiving areas are widespread.

The olivocochlear bundle is a prominent bundle of efferentfibers in each auditory nerve that arises from both ipsilateral andcontralateral superior olivary complexes and ends primarilyaround the bases of the outer hair cells of the organ of Corti.

In the primary auditory cortex, most neurons respond to inputsfrom both ears, but strips of cells are stimulated by input from thecontralateral ear and inhibited by input from the ipsilateral ear.

There are several additional auditory receiving areas, just asthere are several receiving areas for cutaneous sensation.

The auditory association areas adjacent to the primaryauditory receiving areas are widespread.

The olivocochlear bundle is a prominent bundle of efferentfibers in each auditory nerve that arises from both ipsilateral andcontralateral superior olivary complexes and ends primarilyaround the bases of the outer hair cells of the organ of Corti.

DeafnessHearing loss is the most common sensory defect in humans.

Presbycusis, the gradual hearing loss associated with aging,affects more than one third of those over 75 and is probably dueto gradual cumulative loss of hair cells and neurons.

In most cases, hearing loss is a multifactorial disorder causedby both genetic and environmental factors.

Conductive deafness refers to impaired sound transmissionin the external or middle ear and impacts all sound frequencies.

Causes of conduction deafness include:

plugging of the external auditory canals with wax or foreignbodies;

Hearing loss is the most common sensory defect in humans.

Presbycusis, the gradual hearing loss associated with aging,affects more than one third of those over 75 and is probably dueto gradual cumulative loss of hair cells and neurons.

In most cases, hearing loss is a multifactorial disorder causedby both genetic and environmental factors.

Conductive deafness refers to impaired sound transmissionin the external or middle ear and impacts all sound frequencies.

Causes of conduction deafness include:

plugging of the external auditory canals with wax or foreignbodies;

fluid accumulation due to otitis externa (inflammation of theouter ear, “swimmer’s ear”);

otitis media (inflammation of the middle ear);

perforation of the eardrum; and

Osteosclerosis in which bone is resorbed and replaced withsclerotic bone that grows over the oval window.

Sensorineural deafness is usually due to the loss ofcochlear hair cells but can also be due to problems with theeighth cranial nerve or within central auditory pathways.

It can impair the ability to hear certain pitches while othersare unaffected.

fluid accumulation due to otitis externa (inflammation of theouter ear, “swimmer’s ear”);

otitis media (inflammation of the middle ear);

perforation of the eardrum; and

Osteosclerosis in which bone is resorbed and replaced withsclerotic bone that grows over the oval window.

Sensorineural deafness is usually due to the loss ofcochlear hair cells but can also be due to problems with theeighth cranial nerve or within central auditory pathways.

It can impair the ability to hear certain pitches while othersare unaffected.

Aminoglycoside antibiotics such as streptomycin andgentamicin obstruct the mechanosensitive channels in thestereocilia of hair cells and can cause the cells to degenerate,producing sensorineural hearing loss and abnormal vestibularfunction.

Damage to the outer hair cells by prolonged exposure to noiseis associated with hearing loss.

Other causes include tumors of the eighth cranial nerve andcerebellopontine angle and vascular damage in the medulla.

Conduction and sensorineural deafness can be differentiatedby simple tests with a tuning fork.

Aminoglycoside antibiotics such as streptomycin andgentamicin obstruct the mechanosensitive channels in thestereocilia of hair cells and can cause the cells to degenerate,producing sensorineural hearing loss and abnormal vestibularfunction.

Damage to the outer hair cells by prolonged exposure to noiseis associated with hearing loss.

Other causes include tumors of the eighth cranial nerve andcerebellopontine angle and vascular damage in the medulla.

Conduction and sensorineural deafness can be differentiatedby simple tests with a tuning fork.

Common tests with a tuning fork to distinguish between sensorineural and conduction deafness.

Three of these tests, named for the individuals who developedthem, are outlined below.

The Weber and Schwabach tests demonstrate the importantmasking effect of environmental noise on the auditory threshold.

VESTIBULAR SYSTEMThe vestibular system is divided into the vestibular

apparatus and central vestibular nuclei.

The vestibular apparatus within the inner ear detects headmotion and position and transduces this information into aneural signal.

The vestibular nuclei are concerned with maintaining theposition of the head in space; the tracts that descend fromthese nuclei mediate head-on-neck and head-on-bodyadjustments.

The vestibular ganglia contain the cell bodies of the neuronssupplying the cristae and maculae.

The vestibular system is divided into the vestibularapparatus and central vestibular nuclei.

The vestibular apparatus within the inner ear detects headmotion and position and transduces this information into aneural signal.

The vestibular nuclei are concerned with maintaining theposition of the head in space; the tracts that descend fromthese nuclei mediate head-on-neck and head-on-bodyadjustments.

The vestibular ganglia contain the cell bodies of the neuronssupplying the cristae and maculae.

Each vestibular nerveterminates in the ipsilateralvestibular nucleus and in theflocculonodular lobe of thecerebellum.

Fibers from the semicircularcanals end in the superior andmedial divisions of thevestibular nucleus and projectmainly to nuclei controlling eyemovement.

Fibers from the utricle andsaccule end in Deiters’nucleus, which projects to thespinal cord.

Each vestibular nerveterminates in the ipsilateralvestibular nucleus and in theflocculonodular lobe of thecerebellum.

Fibers from the semicircularcanals end in the superior andmedial divisions of thevestibular nucleus and projectmainly to nuclei controlling eyemovement.

Fibers from the utricle andsaccule end in Deiters’nucleus, which projects to thespinal cord.

The vestibular nuclei also project to the thalamus and fromthere to the primary somatosensory cortex.

The ascending connections to cranial nerve nuclei areconcerned with eye movements.

At the end of this lesson, it is expected that the student can: Describe the basic features of the olfactory epithelium and

olfactory bulb.

Explain signal transduction in odorant receptors.

Outline the pathway by which impulses generated in theolfactory epithelium reach the olfactory cortex.

Describe the location and cellular composition of taste buds.

Name the five major taste receptors and their signaltransduction mechanisms.

Outline the pathways by which impulses generated in tastereceptors reach the insular cortex.

Smell and TasteAt the end of this lesson, it is expected that the student can:

Describe the basic features of the olfactory epithelium andolfactory bulb.

Explain signal transduction in odorant receptors.

Outline the pathway by which impulses generated in theolfactory epithelium reach the olfactory cortex.

Describe the location and cellular composition of taste buds.

Name the five major taste receptors and their signaltransduction mechanisms.

Outline the pathways by which impulses generated in tastereceptors reach the insular cortex.

Smell and taste are classified as visceral senses because oftheir close association with gastrointestinal function.

Physiologically, they are related to each other; the flavors ofvarious foods are in large part a combination of their taste andsmell.

This explains why food may taste “different” if one has a coldthat depresses the sense of smell.

Both smell and taste receptors are chemoreceptors that arestimulated by molecules in solution in mucus in the nose andsaliva in the mouth respectively.

IntroductionSmell and taste are classified as visceral senses because of

their close association with gastrointestinal function.

Physiologically, they are related to each other; the flavors ofvarious foods are in large part a combination of their taste andsmell.

This explains why food may taste “different” if one has a coldthat depresses the sense of smell.

Both smell and taste receptors are chemoreceptors that arestimulated by molecules in solution in mucus in the nose andsaliva in the mouth respectively.

A specialized portion of thenasal mucosa which is yellowishand pigmented is known asolfactory epithelium.

It contains 10–20 millionbipolar olfactory sensoryneurons interspersed with glia-like supporting (sustentacular)cells and basal stem cells.

The olfactory epithelium is theplace in the body where thenervous system is closest to theexternal world.

Physiology of SmellOlfactory Epithelium and Olfactory Bulbs

A specialized portion of thenasal mucosa which is yellowishand pigmented is known asolfactory epithelium.

It contains 10–20 millionbipolar olfactory sensoryneurons interspersed with glia-like supporting (sustentacular)cells and basal stem cells.

The olfactory epithelium is theplace in the body where thenervous system is closest to theexternal world.

Each neuron has a short,thick dendrite that projects intothe nasal cavity where itterminates in a knob containing10–20 cilia.

The cilia are unmyelinatedprocesses that contain odorantreceptors.

The axons of the olfactorysensory neurons pass through thecribriform plate of the ethmoid boneand enter the olfactory bulbs.

New olfactory sensory neuronsare generated by basal stem cells asneeded to replace those damagedby exposure to the environment.

In the olfactory bulbs, the axons ofthe olfactory sensory neurons (firstcranial nerve) contact the primarydendrites of the mitral cells andtufted cells

The cilia are unmyelinatedprocesses that contain odorantreceptors.

The axons of the olfactorysensory neurons pass through thecribriform plate of the ethmoid boneand enter the olfactory bulbs.

New olfactory sensory neuronsare generated by basal stem cells asneeded to replace those damagedby exposure to the environment.

In the olfactory bulbs, the axons ofthe olfactory sensory neurons (firstcranial nerve) contact the primarydendrites of the mitral cells andtufted cells

This combination forms ananatomically discrete synapticunits called the olfactoryglomeruli.

Both types of neurons send axons into the olfactory cortex.

The olfactory bulbs also contain periglomerular cells, whichare inhibitory neurons connecting one glomerulus to another, andgranule cells, which have no axons and make reciprocalsynapses with the lateral dendrites of the mitral and tufted cells.

At these synapses, the mitral or tufted cell excites the granulecell by releasing glutamate, and the granule cell in turn inhibitsthe mitral or tufted cell by releasing γ-Aminobutyric acid(GABA).

Both types of neurons send axons into the olfactory cortex.

The olfactory bulbs also contain periglomerular cells, whichare inhibitory neurons connecting one glomerulus to another, andgranule cells, which have no axons and make reciprocalsynapses with the lateral dendrites of the mitral and tufted cells.

At these synapses, the mitral or tufted cell excites the granulecell by releasing glutamate, and the granule cell in turn inhibitsthe mitral or tufted cell by releasing γ-Aminobutyric acid(GABA).

Olfactory Cortex

The axons of the mitral and tufted cells pass posteriorly throughthe lateral olfactory stria to terminate on apical dendrites ofpyramidal cells in five regions of the olfactory cortex: anterior olfactory nucleus, olfactory tubercle, Piriform cortex, Amygdala; and entorhinal cortex

From these regions, information travels directly to the frontalcortex or via the thalamus to the orbitofrontal cortex.

Conscious discrimination of odors relies on the pathway tothe orbitofrontal cortex.

The orbitofrontal activation is generally greater on the rightside than the left; thus, cortical representation of olfaction isasymmetric.

The pathway to the amygdala is involved with the emotionalresponses to olfactory stimuli, and the pathway to the entorhinalcortex is concerned with olfactory memories.

From these regions, information travels directly to the frontalcortex or via the thalamus to the orbitofrontal cortex.

Conscious discrimination of odors relies on the pathway tothe orbitofrontal cortex.

The orbitofrontal activation is generally greater on the rightside than the left; thus, cortical representation of olfaction isasymmetric.

The pathway to the amygdala is involved with the emotionalresponses to olfactory stimuli, and the pathway to the entorhinalcortex is concerned with olfactory memories.

Taste BudsThe specialized sense organ for taste (gustation) consists of

approximately 10,000 taste buds.

There are four morphologically distinct types of cells within eachtaste bud: basal cells, dark cells, light cells, and intermediatecells.

Physiology of Taste

The latter three cell types are referred to as Type I, II, and IIItaste cells.

They are the sensory neurons that respond to taste stimuli.

The apical ends of taste cells have microvilli that project intothe taste pore, a small opening on the dorsal surface of thetongue where taste cells are exposed to the oral contents.

Each taste bud is innervated by about 50 nerve fibers, andconversely, each nerve fiber receives input from an average offive taste buds.

The basal cells arise from the epithelial cells surrounding thetaste bud.

The latter three cell types are referred to as Type I, II, and IIItaste cells.

They are the sensory neurons that respond to taste stimuli.

The apical ends of taste cells have microvilli that project intothe taste pore, a small opening on the dorsal surface of thetongue where taste cells are exposed to the oral contents.

Each taste bud is innervated by about 50 nerve fibers, andconversely, each nerve fiber receives input from an average offive taste buds.

The basal cells arise from the epithelial cells surrounding thetaste bud.

They differentiate into new taste cells, and the old cells arereplaced with a half-time of about 10 days.

If the sensory nerve is cut, the taste buds it innervatesdegenerate and eventually disappear.

The taste buds are located in the mucosa of the epiglottis, palate,and pharynx and in the walls of papillae of the tongue:

the fungiform papillae are rounded structures most numerousnear the tip of the tongue and consists of up to 5 taste buds,mostly located at the top of the papilla;

the circumvallate papillae are prominent structures arranged ina V on the back of the tongue with up to 100 taste buds, mostlylocated along the sides of the papillae; and

the foliate papillae are on the posterior edge of the tongue.

They differentiate into new taste cells, and the old cells arereplaced with a half-time of about 10 days.

If the sensory nerve is cut, the taste buds it innervatesdegenerate and eventually disappear.

The taste buds are located in the mucosa of the epiglottis, palate,and pharynx and in the walls of papillae of the tongue:

the fungiform papillae are rounded structures most numerousnear the tip of the tongue and consists of up to 5 taste buds,mostly located at the top of the papilla;

the circumvallate papillae are prominent structures arranged ina V on the back of the tongue with up to 100 taste buds, mostlylocated along the sides of the papillae; and

the foliate papillae are on the posterior edge of the tongue.

The sensory nerve fibers fromthe taste buds on the anterior twothirds of the tongue.

They travel in the chordatympani branch of the facialnerve, and those from theposterior third of the tonguereach the brain stem via theglossopharyngeal nerve.

The fibers from areas otherthan the tongue (e.g., pharynx)reach the brain stem via thevagus nerve.

Taste PathwaysThe sensory nerve fibers from

the taste buds on the anterior twothirds of the tongue.

They travel in the chordatympani branch of the facialnerve, and those from theposterior third of the tonguereach the brain stem via theglossopharyngeal nerve.

The fibers from areas otherthan the tongue (e.g., pharynx)reach the brain stem via thevagus nerve.

On each side, the myelinated butrelatively slowly conducting tastefibers in these three nerves unite inthe gustatory portion of the nucleusof the tractus solitarius (NTS) inthe medulla oblongata.

From there, axons of second-order neurons ascend in theipsilateral medial lemniscus to passdirectly to the ventralposteromedial nucleus of thethalamus

Then fibers project to theanterior insula and frontaloperculum in the ipsilateralcerebral cortex.

On each side, the myelinated butrelatively slowly conducting tastefibers in these three nerves unite inthe gustatory portion of the nucleusof the tractus solitarius (NTS) inthe medulla oblongata.

From there, axons of second-order neurons ascend in theipsilateral medial lemniscus to passdirectly to the ventralposteromedial nucleus of thethalamus

Then fibers project to theanterior insula and frontaloperculum in the ipsilateralcerebral cortex.

This region is rostral to the facearea of the postcentral gyrus,which may be the area thatmediates conscious perception oftaste and taste discrimination.