copyright 2010, john wiley & sons, inc. chapter 12 somatic senses and special senses

Copyright 2010, John Wiley & Sons, Inc.

Chapter 12

Somatic Senses and Special Senses


Special Senses Smell (olfaction) Taste (gustation) Vision Balance Hearing


General Senses: Somatic and Visceral Somatic

Tactile: touch, pressure, vibration Thermal (warm, cold) Pain Proprioception (joint, muscle position sense;

movements of limbs, head) Visceral: internal organ conditions


Definition of Sensation Conscious or subconscious awareness of

change in external or internal environment Requires

1. Stimulus2. Sensory receptor3. Neural pathway4. Brain region for integration


Characteristics Perception: conscious awareness

Occurs in cerebral cortex Adaptation: decreased receptor response

during prolonged stimulation Decreased perception Adaptation speed varies with receptor

Rapid adaptation: pressure, touch, smell Slow adaptation: pain, body position, chemical levels in

blood


Sensory Receptors: Structural Types Free nerve endings

Pain, thermal, tickle, itch, some touch receptors Encapsulated nerve endings

Touch pressure, and vibration Separate, specialized cells

Hair cells in inner ear Photoreceptors in retina of eye


Sensory Receptors: Functional Types Mechanoreceptors

Cell deformation: stretching or bending Touch, pressure, vibration

Thermoreceptors: temperature Nociceptors: pain Photoreceptors: light Chemoreceptors: taste, smell Osmoreceptors

Osmotic pressure of body fluid


Somatic Senses Somatic receptors in skin, mucous

membranes, muscles, tendons, and joints Distributed unevenly: dense concentration of

receptors in very sensitive areas Fingertips, lips, tip of tongue

Include tactile, thermal, pain, proprioceptive


Tactile Sensations Touch, pressure, vibration

Encapsulated mechanoreceptors Itch and tickle

Free nerve endings


Touch Rapidly adapting receptors for touch

Meissner corpuscles Hair root plexuses: detect hair movement

Slowly adapting receptors for touch Type I mechanoreceptors: Merkel discs or tactile

discs Surface receptors: in epidermis

Type I mechanoreceptors: Ruffini corpuscles Deep in dermis and tendons


Pressure and Vibration Pressure

Pacinian (lamellated) corpuscles: layers like onion Rapid adapting Widely distributed: in dermis, subcutaneous,

around joints, tendons, muscles, periosteum Vibration

Response to rapidly repetitive stimuli Receptors: Meissner and pacinian


Structure and Location of Sensory Receptors


Itch and Tickle Itch: chemical stimulation of free nerve

endings Bradykinin from inflammation response

Tickle: from free nerve endings and pacinian corpuscles Tickle requires stimulus from outside of self Effects of attempts to tickle oneself are blocked by

signals to/from cerebellum


Thermal Sensations Two kinds of thermoreceptors

Cold receptors: 10˚–40˚ C (50–105˚ F) Located in epidermis

Warm receptors: 32˚–48˚ C (90–118˚ F) Located in dermis

Both adapt rapidly but continue slow signals during prolonged stimulus

Outside these ranges: nociceptors detect pain


Pain Sensations Nociceptors

Free nerve endings in every tissue except brain Can respond to any excessive stimulus Minimal adaptation

Types of pain Fast pain: acute, sharp pain

Well localized Slow pain: chronic, burning, aching, throbbing

More diffuse (not localized) Referred pain is visceral pain displaced to

surface


Distribution of Referred Pain


Proprioception (Kinesthesia) Awareness of

Body position, movements, weight of objects Sites of receptors

Muscles (muscle spindles) Tendons (tendon organs) Joint kinesthetic receptors (synovial joints) Inner ear (hair cells): head position

Tracts to Somatosensory area of cerebral cortex and Cerebellum

Slight adaptation


Functional Areas of the Cerebrum


Special Senses Smell (olfaction) Taste (gustation) Vision Balance Hearing


Smell: Olfaction Site of olfactory receptors

In mucosa of superior region of nose Three types of olfactory cells

Olfactory receptors Consist of olfactory hairs with chemoreceptors These are first order neurons of olfactory pathway

Supporting cells Epithelial cells: support, protect

Basal cells: stem cells that produce new neurons (receptors) throughout life. Rare!


Smell: Olfaction


Stimulation of Receptors Genetic evidence: 100’s of primary odors

exist Binding of chemical odorants stimulates

receptor Recognition of 10,000 odors from

combination of primary receptor input Rapid adaptation by 50% in 1 second


Olfactory Pathway First-order neurons

Olfactory receptors are neurons in nasal mucosa Axons form olfactory nerves (cranial nerve I)

Extend through cribriform plate into cranium to olfactory bulb

Second-order neurons Neuron cell bodies in olfactory bulb Olfactory tract: axons extend from olfactory bulb

to cerebral cortex (temporal lobe) Limbic system: emotional response to odors


Olfactory Receptors


Taste: Gustation Five primary tastes: salt, sweet, sour,

bitter, and umami Perception of what is called “taste”

includes olfactory input Receptors in 10,000 taste buds

Located on tongue, pharynx, epiglottis In structures called papillae

Vallate (posterior) Fungiform (all over) Filiform: touch receptors only


Taste: Gustation


Structure of Taste Bud Contains 3 types of epithelial cells

Supporting cells that surround Gustatory receptor cells

Gustatory hair projects from receptor through taste pore Basal cells

Stem cells that produce supporting cells that develop into receptor cells (10-day life span)


Taste: Gustation


Stimulation of Taste Receptors Sequence of events

Tastant dissolves in saliva Enters taste pore contacts gustatory hair Electrical signal produced Causes gustatory cell to release neurotransmitter That activates dendrites of first-order neurons

Adaptation occurs within minutes Different tastes arise from activation of

different groups of taste neurons


Gustatory Pathway Cranial nerves transmit impulses

Facial (CN VII) from anterior of tongue Glossopharyngeal (CN IX) from posterior Vagus (CN X) from pharynx, epiglottis

To medulla oblongata Thalamus primary gustatory area of cerebral

cortex Limbic system or hypothalamus


Vision: Eyes Accessory structures

Eyebrows, eyelashes: protection Eyelids: protection and lubrication (blinking) Extrinsic muscles: move eyeball

Superior rectus, inferior rectus, lateral rectus, medial rectus, superior oblique, inferior oblique

Lacrimal apparatus: produces tears Lacrimal glands lacrimal ducts surface of upper

eyelid surface of eye Lacrimal canals lacrimal sac nasolacrimal duct

nasal cavity


Vision: Eyes


Layers of Eyeball First layer: Fibrous tunic

Anteriorly: cornea (clear, colorless) Posteriorly: sclera (“white of eye”)

Second layer: Vascular tunic consists of Choroid: lines most of internal surface of eye

Contains blood vessels that nourish the eye Ciliary body consists of

Ciliary processes: secrete aqueous humor Ciliary muscles: changes lens shape for focusing

Iris: pigmented part of eye (blue, brown, green) Smooth muscle that dilates or constricts pupil Pupil: hole for passage of light


Layers of Eyeball Third layer: Retina—composed of two layers

Neural layer: outgrowth of brain Photoreceptor layer: rods and cones Bipolar cell layer Ganglion cell layer: axons of neurons here form optic

nerve (CN II) that exits eye at optic disc (“blind spot” since no rods/cones here)

Pigmented layer: helps absorb stray light Between choroid and neural layer


Pupil Response to Light


Layers of Eyeball


Photoreceptors: Rods and Cones Rods: black-and-white vision; 120 million Cones: color sensitive; 6 million cones

Three types: sensitive to blue, green or red light Color vision results from combined input Cones mostly in central fovea in center of macula

lutea Point of highest visual acuity (sharpness)

Visual pathway Photoreceptor cells (rods or cones) Bipolar layer Ganglion cells; their axons form optic nerve


Photoreceptors: Rods and Cones


Interior of Eyeball Two cavities separated by the lens

Anterior cavity filled with aqueous humor Clear, colorless fluid secreted from capillaries in

ciliary body Completely replaced every 90 min Establishes intraocular pressure, maintains eye

shape; nourishes lens and cornea Drains into blood in scleral venous sinus (canal of

Schlemm) Vitreous chamber: filled with gel-like vitreous

body (not replaced) Holds retina back against choroid


Physiology of Vision: Three StepsA. Formation of image on retinaB. Stimulation of photoreceptors (rods and

cones)C. Visual pathway: nerve impulses pass to

cerebral cortex


A. Formation of Image on Retina: Four Steps1. Refraction (bending) of light rays to

focus them on retina2. Accommodation: change of lens shape

to focus for near (or far) vision3. Constriction (narrowing) of pupil to

control amount of light entering the eye4. Convergence of eyeballs: for binocular

vision


Step 1: Refraction of Light Definition: bending of light rays as they pass

from medium of one density to another of different density

75% occurs at cornea; lens also helps focus light on retina

Image is inverted but brain adjusts and interprets distance and size


Step 1: Refraction of Light


Step 2: Accommodation Lens adjusts shape for distance to allow

image to focus on retina For distant objects, ciliary muscle relaxes flat

lens For closeup vision, ciliary muscle contracts fat

lens (rounder = more convex) Visual disorders

Myopia (nearsightedness): can see near but not far objects

Eyeball is too long so lens cannot accommodate enough to focus images of distant objects onto retina


Step 2: Accommodation Visual disorders

Hyperopia (farsightedness): can see far but not near

Eyeball is too short so lens cannot accommodate enough to focus images of near objects onto retina

Astigmatism: irregular curvature of cornea or lens Presbyopia: aging change loss of elasticity of

lens farsightedness reading glasses These disorders can be corrected with lenses or

LASIK (laser-assisted in situ keratomileusis)


Step 2: Accommodation


Steps 3 and 4: Constriction and Convergence■ Constriction of pupil

Autonomic (parasympathetic) reflex to prevent excessive light rays from entering eye

By contraction of circular muscles of iris

■ Convergence Eyes rotate inward for binocular vision By contraction of extrinsic eye muscles


B. Stimulation of Photoreceptors Photoreceptors: light neural signal

In rods light is absorbed by a photopigment (rhodopsin) which splits into opsin + retinal and leads receptor potential Vitamin A deficiency decreases rhodopsin production

and leads to night blindness. In cones light is absorbed by 3 opsins

receptor potential for color vision In colorblindness, red or green cones are missing.


C. Visual Pathway Rods or cones bipolar cells ganglion

cells (their axons form optic nerve = CN II) About 50% of these axons cross over to opposite

side of brain in optic chiasm Axons continue on into optic tract

Terminate/synapse in thalamus Occipital lobes of cerebral cortex

Right brain sees left side of object Left brain sees right side of object


Physiology of Vision: Three Steps


Hearing and Equilibrium: Ear Structure Outer ear: auricle, external auditory canal,

and tympanic membrane (ear drum) Canal contains hairs and ceruminous glands

Middle ear: auditory tube (eustachian tube) and ossicles (bones) Ossicles (malleus, incus, stapes: attached to oval

window) Inner ear: bony labyrinth + membranous

labyrinth filled with endolymph Cochlea: sense organ of hearing , Vestibule and semicircular canals: organs of

balance


Hearing and Equilibrium: Ear Structure


Inner Ear Structure: Three Regions Vestibule includes

Two sacs: utricle and saccule Semicircular canals: at right angles

Contain membranous semicircular ducts Each ends in a swelling known as ampulla

Cochlea: 3 levels Cochlear duct: membranous, has endolymph

Contains spiral organ (sensory organ for hearing) Above: scala vestibuli: ends at oval window Below: scala tympani: ends at round window


Spiral Organ Sits on basilar membrane

Floor of cochlear duct Contains supporting cells + hair cells Hair cells

Covered with jellylike tectorial membrane Are receptors for auditory sensations Synapse with sensory neurons in cochlear branch

of vestibulocochlear nerve cranial nerve VIII)


Inner Ear Structure


Physiology of Hearing Sound waves in air auditory canal Tympanic membrane ossicle movement

stapes strikes oval window Pressure waves in perilymph

Conveyed from scala vestibuli scala tympani Pressure waves in endolymph cause

Hair cells bend against tectorial membrane Neurotransmitter released to sensory neurons

Pitch (wavelength): location in cochlea Volume (loudness): intensity of waves


Scalavestibuli

Cochlear duct(contains endolymph)

Scalatympani

Perilymph

Basilarmembrane

Cochlea

Sound waves

HelicotremaStapes vibratingin oval window

Malleus Incus

External auditorycanal

Tympanicmembrane

Secondary tympanicmembrane vibratingin round window Auditory tube

Vestibular membrane

Middle ear

Tectorial membrane

Spiral organ(organ of Corti)

1

Scalavestibuli


Scalatympani

Perilymph

Basilarmembrane

Cochlea

Sound waves


Malleus Incus


Tympanicmembrane


Vestibular membrane

Middle ear

Tectorial membrane


1 2

Scalavestibuli


Scalatympani

Perilymph

Basilarmembrane

Cochlea

Sound waves


Malleus Incus


Tympanicmembrane


Vestibular membrane

Middle ear

Tectorial membrane


1 2

3

Scalavestibuli


Scalatympani

Perilymph

Basilarmembrane

Cochlea

Sound waves


Malleus Incus


Tympanicmembrane


Vestibular membrane

Middle ear

Tectorial membrane


1 2

34

Scalavestibuli


Scalatympani

Perilymph

Basilarmembrane

Cochlea

Sound waves


Malleus Incus


Tympanicmembrane


Vestibular membrane

Middle ear

Tectorial membrane


1 2

34

5Scalavestibuli


Scalatympani

Perilymph

Basilarmembrane

Cochlea

Sound waves


Malleus Incus


Tympanicmembrane


Vestibular membrane

Middle ear

Tectorial membrane


1 2

34

5

6

Scalavestibuli


Scalatympani

Perilymph

Basilarmembrane

Cochlea

Sound waves


Malleus Incus


Tympanicmembrane


Vestibular membrane

Middle ear

Tectorial membrane


1 2

34

5

6

7

Scalavestibuli


Scalatympani

Perilymph

Basilarmembrane

Cochlea

Sound waves


Malleus Incus


Tympanicmembrane


Vestibular membrane

Middle ear

Tectorial membrane


1 2

34

5

6

78

8

Scalavestibuli


Scalatympani

Perilymph

Basilarmembrane

Cochlea

Sound waves


Malleus Incus


Tympanicmembrane


Vestibular membrane

Middle ear

Tectorial membrane


1 2

34

5

6

78

8

9


Auditory Pathway Cochlear neurons (in cranial nerve VIII) end

in medulla On same side: R ear R side medulla

Midbrain thalamus Auditory cortex in temporal lobe

Each side of brain receives input from both ears


Physiology of Equilibrium Static equilibrium: senses position relative to

gravity As when head is tilted or a car is speeding up or

slowing down Dynamic equilibrium: senses position in

response to head movement As in spinning movements


Static Equilibrium Sensed in maculae of utricle and saccule Mechanism

Gravity pulls on otoliths in otolithic membrane Bends hair cells in otolithic membrane Triggers nerve impulses in vestibular branch of

vestibulochochlear nerve


Static Equilibrium


Dynamic Equilibrium Semicircular canals (3)

At right angles to each other Cristae in each ampulla contain

Hair cells embedded in jellylike cupula Supporting cells

Mechanism When head turns, hair cells move Endolymph lags and bends hair cells Nerve impulses in vestibular branch


Dynamic Equilibrium


Equilibrium Pathways Axons from vestibular branch medulla or cerebellum Medulla motor control: eye, head, neck Spinal cord tracts for adjusting muscle

tone and postural muscles


End of Chapter 12

Copyright 2010 John Wiley & Sons, Inc.All rights reserved. Reproduction or translation of this work beyond that permitted in section 117 of the 1976 United States Copyright Act without express permission of the copyright owner is unlawful. Request for further information should be addressed to the Permission Department, John Wiley & Sons, Inc. The purchaser may make back-up copies for his/her own use only and not for distribution or resale. The Publishers assumes no responsibility for errors, omissions, or damages caused by the use of theses programs or from the use of the information herein.

copyright 2010, john wiley & sons, inc. chapter 12 somatic senses and special senses

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