Chapter

12

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Somatic and Special Senses

Chapter 12Somatic and Special Senses

Sensory Receptors• specialized cells or multicellular structures that collect information• stimulate neurons to send impulses along sensory fibers to the brain

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Receptor Types

Chemoreceptors• respond to changes in chemical concentrations

Nociceptors (Pain receptors)• respond to tissue damage

Thermoreceptors• respond to changes in temperature

Mechanoreceptors• respond to mechanical forces

Photoreceptors• respond to light 12-3

Sensation and Adaptation

Sensation feeling that occurs when brain interprets sensory impulse

Sensory Adaptation•adjustment of sensory receptors from continuous stimulation• stronger stimulus required to activate receptors• smell and temperature receptors undergo sensory adaptation

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Somatic Senses

• senses associated with skin, muscles, joints, and viscera

• three groups• exteroceptive senses – senses associated with body surface; touch, pressure, temperature, pain• proprioceptive senses – senses associated with changes in muscles and tendons• visceroceptive senses – senses associated with changes in viscera

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Touch and Pressure Senses

Free nerve endings• common in epithelial tissues• detect touch and pressure

Meissner’s corpuscles• abundant in hairless portions of skin• detect light touch• detect motion on skin• detect texture

Pacinian corpuscles• common in deeper subcutaneous tissues, tendons, and ligaments• detect heavy pressure

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Touch and Pressure Senses

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Temperature Senses

Warm receptors• sensitive to temperatures between 25oC (77o F) and 45oC (113oF)

Cold receptors• sensitive to temperature between 10oC (50oF) and 20oC (68oF)

•In between – brain interprets impulses from both•Pain receptors

• respond to temperatures below 10oC (50 F)• respond to temperatures above 45oC (113 F)

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Sense of Pain

• free nerve endings • widely distributed • nervous tissue of brain lacks pain receptors• stimulated by tissue damage, chemical, mechanical forces, or extremes in temperature• do not adapt

Visceral Pain• may exhibit referred pain• not well localized

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Referred Pain

• may occur due to sensory impulses from two regions following a common nerve pathway to brain

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Stretch Receptors

• proprioceptors• send information to CNS concerning lengths and tensions of muscles• 2 main kinds of stretch receptors

• muscle spindles – in skeletal muscles• Golgi tendon organs – in tendons

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Stretch Receptors

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Special Senses

• sensory receptors are within large, complex sensory organs in the head• smell in olfactory organs• taste in taste buds• hearing and equilibrium in ears (hair cells)• sight in eyes (rods and cones)

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Smell

Olfactory Receptors• chemoreceptors• respond to chemicals dissolved in liquids

Olfactory Organs• contain olfactory receptors and supporting epithelial cells• cover parts of nasal cavity, superior nasal conchae, and a portion of the nasal septum • small patch of tissue (12 million cells) on the roof of nasal cavity• smells start as a gas, but must be dissolved in watery fluid that surrounds the cilia of the receptors (each receptor has 10-12) to be detected 12-17

Olfactory Receptors

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Olfactory Nerve Pathways

Once olfactory receptors are stimulated, nerve impulses travel through

• olfactory nerves to • olfactory bulbs to • olfactory tracts to • limbic system (for emotions) and olfactory cortex (for interpretation)

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Taste

Taste Buds• organs of taste• located on papillae of tongue, roof of mouth, linings of cheeks and walls of pharynx

Taste Receptors• chemoreceptors• taste cells – modified epithelial cells that function as receptors• taste hairs –microvilli that protrude from taste cells; sensitive parts of taste cells

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Taste Receptors

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Taste SensationsFour Primary Taste Sensations (or are there five?)

• sweet – stimulated by carbohydrates• most plentiful near tip

• sour – stimulated by acids• most plentiful at margins

• salty – stimulated by salts• most plentiful at tip and upper front

• bitter – stimulated by many organic compound• most plentiful at back• may be protective – spit out

• umami (?)– stimulated by amino acids (meat, cheese)

Spicy foods activate pain receptors12-22

What does food REALLY Taste like?

Taste is affected by:

• Smell

• Temperature

• Texture

• Psychological impacts such as color

Taste Nerve Pathways

Sensory impulses from taste receptors travel along• cranial nerves to • medulla oblongata to• thalamus to • gustatory cortex (for interpretation)

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Hearing

Ear – organ of hearing

3 Sections• External• Middle• Inner

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External Ear

• auricle• collects sounds waves

• external auditory meatus• lined with ceruminous glands• carries sound to tympanic membrane• terminates with tympanic membrane

• tympanic membrane • vibrates in response to sound waves

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Middle Ear

• tympanic cavity• air-filled space in temporal bone• auditory ossicles (bones)

• vibrate in response to tympanic membrane• malleus, incus, and stapes

• oval window • opening in wall of tympanic cavity (cochlea)• stapes vibrates against it to move fluids in inner ear

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Auditory Tube

• eustachian tube • connects middle ear to throat• helps maintain equal pressure on both sides of tympanic membrane• usually closed by valve-like flaps in throat

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Inner Ear

• complex system of labyrinths• osseous (bony) labyrinth

• bony canal in temporal bone• filled with perilymph

• membranous labyrinth• tube within osseous labyrinth• filled with endolymph

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Inner Ear

3 Parts of Labyrinths• cochlea

• functions in hearing• semicircular canals

• functions in equilibrium

• vestibule• functions in equilibrium

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Cochlea

Scala vestibuli• upper compartment

Scala tympani• lower compartment

Cochlear duct **• portion of membranous labyrinth in cochlea

• contains Organ of Corti

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Organ of Corti

• group of hearing receptor cells (hair cells)• on upper surface of basilar membrane• different frequencies of vibration move different parts of basilar membrane• particular sound frequencies cause hairs of receptor cells to bend• nerve impulse generated

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Organ of Corti

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Auditory Nerve Pathways

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Summary of the Generation of Sensory Impulses from the Ear

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Equilibrium

Static Equilibrium• vestibule• sense position of head when body is not moving

Dynamic Equilibrium• semicircular canals• sense rotation and movement of head and body

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Vestibule

• Macula **(sensory organ)• hair cells of utricle and saccule sense static balance

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Macula

• responds to changes in head position• bending of hairs results in generation of nerve impulse

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Semicircular Canals

• three canals at right angles• ampulla

• swelling of membranous labyrinth

• crista ampullaris (in ampulla)• sensory organ of dynamic balance• hair cells and supporting cells• rapid turns of head or body stimulate hair cells

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Crista Ampullaris

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Sight

Visual Accessory Organs• eyelids• lacrimal apparatus• extrinsic eye muscles

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Eyelid

•palpebra (eyelid)•composed of four layers

• skin• muscle • connective tissue• conjunctiva

• orbicularis oculi - closes• levator palpebrae superioris – opens• tarsal glands – secrete oil onto eyelashes• conjunctiva – mucous membrane; lines eyelid and covers portion of eyeball

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Lacrimal Apparatus

• lacrimal gland• lateral to eye• secretes tears

• canaliculi (superior, inferior)• collect tears

• lacrimal sac• collects from canaliculi

• nasolacrimal duct• collects from lacrimal sac• empties tears into nasal cavity

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Extrinsic Eye Muscles

Superior rectus• rotates eye up and medially

Inferior rectus• rotates eye down and medially

Medial rectus• rotates eye medially

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Extrinsic Eye Muscles

Lateral rectus• rotates eye laterally

Superior oblique• rotates eye up and laterally

Inferior oblique• rotates eye down and laterally

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Structure of the Eye

• hollow• spherical• wall has 3 layers

• (outer) fibrous tunic• (middle) vascular tunic• (inner) nervous tunic

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Outer Tunic

Cornea• anterior portion• transparent• light transmission• light refraction

Sclera• posterior portion• opaque• protection

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Middle Tunic

Iris • anterior portion• pigmented• controls light intensity

Ciliary body (muscle)• anterior portion• pigmented• holds lens• moves lens for focusing

Choroid coat• provides blood supply• pigments absorb extra light

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Anterior Portion of Eye

• filled with aqueous humor

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Lens

• transparent• biconvex• lies behind iris• largely composed of lens fibers• elastic• held in place by suspensory ligaments of ciliary body

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Accommodation• changing of lens shape to view objects

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Iris

• composed of connective tissue and smooth muscle• pupil is hole in iris• dim light stimulates radial muscles and pupil dilates• bright light stimulates circular muscles and pupil constricts

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Inner Tunic

• retina• contains visual receptors• continuous with optic nerve• ends just behind margin of the ciliary body• composed of several layers• macula lutea – yellowish spot in retina , center, 1 mm• fovea centralis – center of macula lutea; produces sharpest vision, where lens tries to focus light**• optic disc – blind spot; contains no visual receptors• vitreous humor – thick gel that holds retina flat against choroid coat

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Focusing On Retina• as light enters eye, it is refracted by

• convex surface of cornea• convex surface of lens

• image focused on retina is upside down and reversed from left to right

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Visual Receptors

Rods• long, thin projections• contain light sensitive pigment called rhodopsin• hundred times more sensitive to light than cones• provide vision in dim light• produce colorless vision• produce outlines of objects

Cones• short, blunt projections• contain light sensitive pigments called erythrolabe, chlorolabe, and cyanolabe• provide vision in bright light• produce sharp images• produce color vision

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Rods and Cones

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Visual Pigments

Rhodopsin• light-sensitive pigment in rods• decomposes in presence of light• triggers a complex series of reactions that initiate nerve impulses• impulses travel along optic nerve

Pigments on Cones• each set contains different light-sensitive pigment• each set is sensitive to different wavelengths• color perceived depends on which sets of cones are stimulated• erythrolabe – responds to red• chlorolabe – responds to green• cyanolabe – responds to blue

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Stereoscopic Vision• provides perception of distance and depth• results from formation of two slightly different retinal images

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Visual Pathway

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Life-Span Changes

Age related hearing loss due to • damage of hair cells in organ of Corti• degeneration of nerve pathways to the brain• tinnitus

Age-related visual problems include• dry eyes• floaters (crystals in vitreous humor)• loss of elasticity of lens• glaucoma• cataracts• macular degeneration 12-65

Clinical Application

Refraction Disorders• concave lens corrects nearsightedness (myopia) when eye is too long or lens focuses light in front of retina

• convex lens corrects farsightedness (presbyopia or hyperopia) when eye is too short or lens focuses light behind retina

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