Principles of Human Anatomy and Physiology, 11e1 Chapter 17 The Special Senses Lecture Outline.

Principles of Human Anatomy and Physiology, 11e 1

Chapter 17

The Special Senses

Lecture Outline


Chapter 17The Special Senses

• Smell, taste, vision, hearing and equilibrium• Housed in complex sensory organs • Ophthalmology is science of the eye• Otolaryngology is science of the ear


Chemical Senses

• Interaction of molecules with receptor cells• Olfaction (smell) and gustation (taste)• Both project to cerebral cortex & limbic system

– evokes strong emotional reactions


Anatomy of olfactory receptors

• The receptors for olfaction, which are bipolar neurons, are in the nasal epithelium in the superior portion of the nasal cavity (Figure 17.1).

• They are first-order neurons of the olfactory pathway.• Supporting cells are epithelial cells of the mucous

membrane lining the nose.• Basal stem cells produce new olfactory receptors.


Olfactory Epithelium

• 1 square inch of membrane holding 10-100 million receptors

• Covers superior nasal cavity and cribriform plate

• 3 types of receptor cells


Cells of the Olfactory Membrane• Olfactory receptors

– bipolar neurons with cilia or olfactory hairs

• Supporting cells – columnar epithelium

• Basal cells = stem cells– replace receptors monthly

• Olfactory glands– produce mucus

• Both epithelium & glands innervated cranial nerve VII.


Physiology of Olfaction - Overview

• Genetic evidence suggests there are hundreds of primary scents.

• In olfactory reception, a generator potential develops and triggers one or more nerve impulses.

• Adaptation to odors occurs quickly, and the threshold of smell is low: only a few molecules of certain substances need be present in air to be smelled.

• Olfactory receptors convey nerve impulses to olfactory nerves, olfactory bulbs, olfactory tracts, and the cerebral cortex and limbic system.

• Hyposmia, a reduced ability to smell, affects half of those over age 65 and 75% of those over 80. It can be caused by neurological changes, drugs, or the effects of smoking .


Olfaction: Sense of Smell

• Odorants bind to receptors• Na+ channels open• Depolarization occurs• Nerve impulse is triggered


Adaptation & Odor Thresholds

• Adaptation = decreasing sensitivity• Olfactory adaptation is rapid

– 50% in 1 second– complete in 1 minute

• Low threshold– only a few molecules need to be present– methyl mercaptan added to natural gas as warning


Olfactory Pathway

• Axons from olfactory receptors form the olfactory nerves (Cranial nerve I) that synapse in the olfactory bulb– pass through 40 foramina in cribriform plate

• Second-order neurons within the olfactory bulb form the olfactory tract that synapses on primary olfactory area of temporal lobe – conscious awareness of smell begins

• Other pathways lead to the frontal lobe (Brodmann area 11) where identification of the odor occurs


GUSTATORY: SENSE OF SMELL

• Taste is a chemical sense.– To be detected, molecules must be dissolved.– Taste stimuli classes include sour, sweet, bitter, and

salty.


Gustatory Sensation: Taste

• Taste requires dissolving of substances

• Four classes of stimuli--sour, bitter, sweet, and salty– Other “tastes” are a combination

of the four taste sensations plus olfaction.

• 10,000 taste buds found on tongue, soft palate & larynx

• Found on sides of circumvallate & fungiform papillae

• 3 cell types: supporting, receptor & basal cells


Anatomy of Taste Buds

• An oval body consisting of 50 receptor cells surrounded by supporting cells

• A single gustatory hair projects upward through the taste pore

• Basal cells develop into new receptor cells every 10 days.


Physiology of Taste

• Receptor potentials developed in gustatory hairs cause the release of neurotransmitter that gives rise to nerve impulses.

• Complete adaptation in 1 to 5 minutes• Thresholds for tastes vary among the 4 primary tastes

– most sensitive to bitter (poisons)– least sensitive to salty and sweet

• Mechanism– dissolved substance contacts gustatory hairs– receptor potential results in neurotransmitter release– nerve impulse formed in 1st-order neuron


Gustatory Pathway

• First-order gustatory fibers found in cranial nerves– V– VII (facial) serves anterior 2/3 of tongue– IX (glossopharyngeal) serves posterior 1/3 of tongue– X (vagus) serves palate & epiglottis

• Signals travel to thalamus or limbic system & hypothalamus• Taste fibers extend from the thalamus to the primary gustatory

area on parietal lobe of the cerebral cortex– provides conscious perception of taste


VISION

• More than half the sensory receptors in the human body are located in the eyes.

• A large part of the cerebral cortex is devoted to processing visual information.


Accessory Structures of Eye - Overview

• Eyelids or palpebrae– protect & lubricate– epidermis, dermis, CT,

orbicularis oculi m., tarsal plate, tarsal glands & conjunctiva

• Tarsal glands– oily secretions

• Conjunctiva– palpebral & bulbar– stops at corneal edge


Eyelids

• The eyelids shade the eyes during sleep, protect the eyes From superficial to deep, each eyelid consists of epidermis, dermis, subcutaneous tissue, fibers of the orbicularis oculi muscle, a tarsal plate, tarsal glands, and conjunctiva (Figure 17.4a).– The tarsal plate gives form and support to the eyelids.– The tarsal glands secrete a fluid to keep the eye lids from

adhering to each other.– The conjunctiva is a thin mucous membrane that lines the

inner aspect of the eyelids and is reflected onto the anterior surface of the eyeball.

– Eyelashes and eyebrows help protect the eyeballs from foreign objects, perspiration, and the direct rays of the sun.


Eyelashes & Eyebrows

• Eyelashes & eyebrows help protect from foreign objects, perspiration & sunlight

• Sebaceous glands are found at base of eyelashes (sty) • Palpebral fissure is gap between the eyelids

Eyeball = 1 inch diameter

5/6 of Eyeball inside orbit & protected


Lacrimal Apparatus

• About 1 ml of tears produced per day. Spread over eye by blinking. Contains bactericidal enzyme called lysozyme.


Extraocular Muscles

• Six muscles that insert on the exterior surface of the eyeball

• Innervated by CN III, IV or VI.

• 4 rectus muscles -- superior, inferior, lateral and medial

• 2 oblique muscles -- inferior and superior


Tunics (Layers) of Eyeball

• The eye is constructed of three layers (Figure 17.5).– Fibrous Tunic

(outer layer)– Vascular

Tunic (middle layer)

– Nervous Tunic(inner layer)


• Transparent • Helps focus light(refraction)

– astigmatism• 3 layers

– nonkeratinized stratified squamous– collagen fibers & fibroblasts– simple squamous epithelium

• Transplants– common & successful– no blood vessels so no antibodies to cause rejection

• Nourished by tears & aqueous humor

Fibrous Tunic -- Description of Cornea


Fibrous Tunic -- Description of Sclera

• “White” of the eye• Dense irregular connective tissue

layer -- collagen & fibroblasts• Provides shape & support• At the junction of the sclera and

cornea is an opening (scleral venous sinus)

• Posteriorly pierced by Optic Nerve (CNII)


Vascular Tunic -- Choroid & Ciliary Body

• Choroid– pigmented epithilial cells

(melanocytes) & blood vessels– provides nutrients to retina– black pigment in melanocytes

absorb scattered light • Ciliary body

– ciliary processes • folds on ciliary body• secrete aqueous humor

– ciliary muscle• smooth muscle that alters

shape of lens


Vascular Tunic -- Iris & Pupil

• Colored portion of eye• Shape of flat donut suspended

between cornea & lens• Hole in center is pupil• Function is to regulate amount of

light entering eye• Autonomic reflexes

– circular muscle fibers contract in bright light to shrink pupil

– radial muscle fibers contract in dim light to enlarge pupil


Vascular Tunic -- Muscles of the Iris

• Constrictor pupillae (circular) are innervated by parasympathetic fibers while Dilator pupillae (radial) are innervated by sympathetic fibers.

• Response varies with different levels of light


Vascular Tunic -- Description of lens

• Avascular• Crystallin proteins arranged

like layers in onion• Clear capsule & perfectly

transparent• Lens held in place by

suspensory ligaments• Focuses light on fovea


Vascular Tunic -- Suspensory ligament

• Suspensory ligaments attach lens to ciliary process• Ciliary muscle controls tension on ligaments & lens


Nervous Tunic -- Retina

• Posterior 3/4 of eyeball• Optic disc

– optic nerve exiting back of eyeball

• Central retina BV– fan out to supply

nourishment to retina– visible for inspection

• hypertension & diabetes

• Detached retina– trauma (boxing)

• fluid between layers• distortion or blindness

View with Ophthalmoscope


Photoreceptors

• shapes of their outer segments differ• Rods

– specialized for black-and-white vision in dim light– allow us to discriminate between different shades of dark

and light – permit us to see shapes and movement.

• Cones – specialized for color vision and sharpness of vision (high

visual acuity) in bright light– most densely concentrated in the central fovea, a small

depression in the center of the macula lutea.


Photoreceptors

• The macula lutea is in the exact center of the posterior portion of the retina, corresponding to the visual axis of the eye.– The fovea is the area of sharpest vision because of the

high concentration of cones.– Rods are absent from the fovea and macula and increase

in density toward the periphery of the retina.


Layers of Retina

• Pigmented epithelium– nonvisual portion– absorbs stray light &

helps keep image clear• 3 layers of neurons

(outgrowth of brain)– photoreceptor layer– bipolar neuron layer– ganglion neuron layer

• 2 other cell types (modify the signal)– horizontal cells– amacrine cells


Rods & Cones--Photoreceptors• Rods----rod shaped

– shades of gray in dim light– 120 million rod cells– shapes & movements– distributed along periphery

• Cones----cone shaped– sharp, color vision– 6 million– fovea of macula lutea

• densely packed region• at exact visual axis of eye• 2nd cells do not cover

cones• sharpest resolution (acuity)


Pathway of Nerve Signal in Retina

• Light penetrates retina• Rods & cones transduce light

into action potentials• Rods & cones excite bipolar cells• Bipolars excite ganglion cells• Axons of ganglion cells form

optic nerve leaving the eyeball (blind spot)

• To thalamus & then the primary visual cortex


Lens

• The eyeball contains the nonvascular lens, just behind the pupil and iris.

• The lens fine tunes the focusing of light rays for clear vision.– With aging the lens loses elasticity and its ability to

accommodate resulting in a condition known as presbyopia.


Cavities of the Interior of Eyeball

• Anterior cavity (anterior to lens)– filled with aqueous humor

• produced by ciliary body• continually drained• replaced every 90 minutes

– 2 chambers• anterior chamber between cornea and iris• posterior chamber between iris and lens

• Posterior cavity (posterior to lens)– filled with vitreous body (jellylike)– formed once during embryonic life– floaters are debris in vitreous of older individuals


Eye Anatomy

• The pressure in the eye, called intraocular pressure, is produced mainly by the aqueous humor.

• The intraocular pressure, along with the vitreous body, maintains the shape of the eyeball and keeps the retina smoothly applied to the choroid so the retina will form clear images.

• Glaucoma– increased intraocular pressure – problem with drainage of aqueous humor– may produce degeneration of the retina and blindness


Aqueous Humor

• Continuously produced by ciliary body

• Flows from posterior chamberinto anterior through the pupil

• Scleral venous sinus– canal of Schlemm

– opening in white of eyeat junction of cornea & sclera

– drainage of aqueous humor from eye to bloodstream


Major Processes of Image Formation

• Refraction of light– by cornea & lens – light rays must fall upon the retina

• Accommodation of the lens– changing shape of lens so that light is focused

• Constriction of the pupil– less light enters the eye


Definition of Refraction

• Bending of light as it passes from one substance (air) into a 2nd substance with a different density(cornea)

• In the eye, light is refracted by the anterior & posterior surfaces of the cornea and the lens


Refraction by the Cornea & Lens

• Image focused on retina is inverted & reversed from left to right

• Brain learns to work with that information

• 75% of Refraction is done by cornea -- rest is done by the lens

• Light rays from > 20’ are nearly parallel and only need to be bent enough to focus on retina

• Light rays from < 6’ are more divergent & need more refraction– extra process needed to get

additional bending of light is called accommodation


Accommodation & the Lens

• Accommodation is an increase in the curvature of the lens, initiated by ciliary muscle contraction, which allows the lens to focus on near objects (figure 17.10c).

• Convex lens refract light rays towards each other– Lens of eye is convex on both surfaces

• Viewing a distant object– lens is nearly flat by pulling of suspensory ligaments

• View a close object– ciliary muscle is contracted & decreases the pull of the suspensory

ligaments on the lens– elastic lens thickens as the tension is removed from it– increase in curvature of lens is called accommodation– The near point of vision is the minimum distance from the eye that

an object can be clearly focused with maximum effort.


Near Point of Vision and Presbyopia

• Near point is the closest distance from the eye an object can be & still be in clear focus– 4 inches in a young adult– 8 inches in a 40 year old

• lens has become less elastic– 31 inches in a 60 to 80 year old

• Reading glasses may be needed by age 40– presbyopia– glasses replace refraction previously provided by

increased curvature of the relaxed, youthful lens


Refraction Abnormalities

• Myopia is nearsightedness (Figure 17.11).• Hyperopia is farsightedness (Figure 17.11).• Astigmatism is a refraction abnormality due to an irregular

curvature of either the cornea or lens.


Correction for Refraction Problems

• Emmetropic eye (normal)– can refract light from 20 ft away

• Myopia (nearsighted)– eyeball is too long from front to back– glasses concave

• Hypermetropic (farsighted)– eyeball is too short– glasses convex (coke-bottle)

• Astigmatism– corneal surface wavy– parts of image out of focus


Constriction of the Pupil

• Constrictor pupillae muscle contracts• Narrows beam of light that enters the eye

– Prevents light rays from entering the eye through the edge of the lens

– Sharpens vision by preventing blurry edges– Protects retina very excessively bright light


Convergence of the Eyes

• Binocular vision in humans has both eyes looking at the same object

• As you look at an object close to your face, both eyeballs must turn inward.– In convergence, the eyeballs move medially so they

are both directed toward an object being viewed.– required so that light rays from the object will strike

both retinas at the same relative point– extrinsic eye muscles must coordinate this action


Physiology of Vision

• The first step in vision transduction is the absorption of light by photopigments (visual pigments) in rods and cones (photoreceptors) (Figure 17.12).


Photoreceptors

• Named for shape of outer segment

• Receptors transduce light energy into a receptor potential in outer segment

• Photopigment is integral membrane protein of outer segment membrane – photopigment membrane is

folded into “discs” & replaced at a very rapid rate

• Photopigments – opsin (protein) + retinal

(derivative of vitamin A)


Physiology of Vision• Photopigments are undergo structural changes upon light

absorption.• Retinal is the light absorbing part of all visual

photopigments.• All photopigments involved in vision contain a glycoprotein

called opsin and a derivative of vitamin A called retinal.– There are four different opsins

• A cone contains one of three different kinds of photopigments so there are three types of cones.

– permit the absorption of 3 different wavelengths (colors) of light

• Rods contain a single type of photopigment (rhodopsin)


Physiology of Vision

• Figure 16.14 shows how photopigments are activated and restored.

• Bleaching and regeneration of the photopigments accounts for much but not all of the sensitivity change during light and dark adaptation.


Photopigments

• Isomerization– light cause cis-retinal to

straighten & become trans-retinal shape

• Bleaching– enzymes separate the trans-

retinal from the opsin– colorless final products

• Regeneration – in darkness, an enzyme

converts trans-retinal back to cis-retinal (resynthesis of a photopigment)


Application: Color Blindness & Night Blindness

• Most forms of colorblindness (inability to distinguish certain colors) result from an inherited absence of or deficiency in one of the three cone photopigments and are more common in males. A deficiency in rhodopsin may cause night blindness (nyctalopia)

• Color blindness– inability to distinguish between certain colors– absence of certain cone photopigments– red-green color blind person can not tell red from green

• Night blindness (nyctalopia)– difficulty seeing in low light– inability to make normal amount of rhodopsin– possibly due to deficiency of vitamin A


Regeneration of Bleached Photopigments

• Pigment epithelium near the photoreceptors contains large amounts of vitamin A and helps the regeneration process.– After complete bleaching, it takes 5 minutes to

regenerate 1/2 of the rhodopsin • Full regeneration of bleached rhodopsin takes 30

to 40 minutes• Rods contribute little to daylight vision, since they

are bleached as fast as they regenerate.– Only 90 seconds are required to regenerate the cone

photopigments


Light and Dark Adaptation

• Light adaptation– adjustments when emerge from the dark into the light

• Dark adaptation– adjustments when enter the dark from a bright situation– light sensitivity increases as photopigments regenerate

• during first 8 minutes of dark adaptation, only cone pigments are regenerated, so threshold burst of light is seen as color

• after sufficient time, sensitivity will increase so that a flash of a single photon of light will be seen as gray-white


Details: Formation of Receptor Potentials

• In darkness– Na+ channels are held open and photoreceptor is always

partially depolarized (-30mV)– continuous release of inhibitory neurotransmitter onto

bipolar cells suppresses their activity• In light

– enzymes cause the closing of Na+ channels producing a hyperpolarized receptor potential (-70mV)

– release of inhibitory neurotransmitter is stopped– bipolar cells become excited and a nerve impulse will travel

towards the brain


Release of Neurotransmitters


Visual Pathway

• Horizontal cells transmit inhibitory signals to bipolar cells• bipolar or amacrine cells transmit excitatory signals to

ganglion cells• ganglion cells which depolarize and initiate nerve impulses

(Figure 17.8).– Impulses are conveyed through the retina to the optic

nerve, the optic chiasma, the optic tract, the thalamus, and the occipital lobes of the cortex (Figure 17.15).


Retinal Processing of Visual Information

• Convergence– one cone cell synapses onto one bipolar

cell produces best visual acuity – 600 rod cells synapse on single bipolar

cell increasing light sensitivity although slightly blurry image results

– 126 million photoreceptors converge on 1 million ganglion cells

• Horizontal and amacrine cells– horizontal cells enhance contrasts in

visual scene because laterally inhibit bipolar cells in the area

– amacrine cells excite bipolar cells if levels of illumination change


Brain Pathways of

Vision


Processing of Image Data in the Brain

• Visual information in optic nerve travels to– hypothalamus to establish sleep patterns based upon

circadian rhythms of light and darkness– midbrain for controlling pupil size & coordination of head

and eye movements– occipital lobe for vision


Visual fields

• Fibers from nasal 1/2 of each retina cross in optic chiasm

• Left occipital lobe receives visual images from right side of an object through impulses from nasal 1/2 of the right eye and temporal 1/2 of the left eye

• Left occipital lobe sees right 1/2 of the world and Right occipital lobe sees left 1/2 of the world.


Anatomy of the Ear Region


HEARING AND EQUILIBRIUM - Overview

• The external (outer) ear collects sound waves.• The middle ear (tympanic cavity) is a small, air-filled cavity

in the temporal bone that contains auditory ossicles (middle ear bones, the malleus, incus, and stapes), the oval window, and the round window (Figure 17.17).

• The internal (inner) ear is also called the labyrinth because of its complicated series of canals (Figure 17.18).


Anatomy of the Ear Region


External Ear• The external (outer) ear collects

sound waves and passes them inwards (Figure 17.16)

• Structures– auricle or pinna

• elastic cartilage covered with skin– external auditory canal

• curved 1” tube of cartilage & bone leading into temporal bone

• ceruminous glands produce cerumen = ear wax– tympanic membrane or eardrum

• epidermis, collagen & elastic fibers, simple cuboidal epith.• Perforated eardrum (hole is present)

– at time of injury (pain, ringing, hearing loss, dizziness)– caused by explosion, scuba diving, or ear infection


Middle Ear Cavity


Middle Ear Cavity

• Air filled cavity in the temporal bone • Separated from external ear by

eardrum and from internal ear by oval & round window

• 3 ear ossicles connected by synovial joints– malleus attached to eardrum, incus & stapes attached by

foot plate to membrane of oval window– stapedius and tensor tympani muscles attach to ossicles

• Auditory tube leads to nasopharynx– helps to equalize pressure on both sides of eardrum

• Connection to mastoid bone =mastoiditis


Muscles of the Ear

• Stapedius m. inserts onto stapes– prevents very large vibrations of stapes from loud noises

• Tensor tympani attaches to malleus– limits movements of malleus & stiffens eardrum to prevent damage


Bony Labyrinth

• The bony labyrinth is a series of cavities in the petrous portion of the temporal bone.

• It can be divided into three areas named on the basis of shape: the semicircular canals and vestibule, both of which contain receptors for equilibrium, and the cochlea, which contains receptors for hearing.

• The bony labyrinth is lined with periosteum and contains a fluid called perilymph. This fluid, chemically similar to cerebrospinal fluid, surrounds the membranous labyrinth.


Inner Ear---Bony Labyrinth

• Bony labyrinth = set of tubelike cavities in temporal bone– semicircular canals, vestibule & cochlea lined with periosteum & filled

with perilymph– surrounds & protects Membranous Labyrinth


Inner Ear---Membranous Labyrinth

• Membranous labyrinth = set of membranous tubes containing sensory receptors for hearing & balance

– utricle, saccule, ampulla, 3 semicircular ducts & cochlea


Membranous Labyrinth

• The membranous labyrinth is a series of sacs and tubes lying inside and having the same general form as the bony labyrinth.– lined with epithelium.– contains a fluid called endolymph, chemically similar to

intracellular fluid.– The vestibule constitutes the oval central portion of the

bony labyrinth. The membranous labyrinth in the vestibule consists of two sacs called the utricle and saccule.

• Anterior to the vestibule is the cochlea, which consists of a bony spiral canal that makes almost three turns around a central bony core called the modiolus (Figure 17.19a).


Semicircular Canals

• Projecting upward and posteriorly from the vestibule are the three bony semicircular canals. – arranged at approximately right angles (X-Y-Z axis)– The anterior and posterior semicircular canals are

oriented vertically; the lateral semicircular canal is oriented horizontally.

– Two parts• One end of each canal enlarges into a swelling called

the ampulla.• The portions of the membranous labyrinth that lie

inside the semicircular canals are called the semicircular ducts (membranous semicircular canals).


Cranial nerves of the Ear Region


Nerve

• Vestibulocochlear nerve = CN VIII– The vestibular branch of the vestibulocochlear nerve

consists of 3 parts• ampullary, utricular, and saccular nerves

– cochlear branch has spiral ganglion in bony modiolus


Overview of Physiology of Hearing


Physiology of Hearing - Overview

• Auricle collects sound waves• Eardrum vibrates

– slow vibration in response to low-pitched sounds– rapid vibration in response to high-pitched sounds

• Ossicles vibrate since malleus is attached to the eardrum• Stapes pushes on oval window producing fluid pressure

waves in scala vestibuli & tympani– oval window vibration is 20X more vigorous than

eardrum (but the frequency of vibration is unchanged)• Pressure fluctuations inside cochlear duct move the hair

cells against the tectorial membrane• Microvilli are bent producing receptor potentials


Tubular Structures of the Cochlea

• Stapes pushes on fluid of scala vestibuli at oval window• At helicotrema, vibration moves into scala tympani• Fluid vibration dissipated at round window which bulges• The central structure is vibrated (cochlear duct)


Cochlea

• Cross sections through the cochlea show that it is divided into three channels by partitions that together have the shape of the letter Y (Figure 17.19 a-c).– The channel above the bony partition is the scala vestibuli, which

ends at the oval window.– The channel below is the scala tympani, which ends at the round

window. – The scala vestibuli and scala tympani both contain perilymph and

are completely separated except at an opening at the apex of the cochlea called the helicotrema.

– The third channel (between the wings of the Y) is the cochlear duct (scala media).

– The vestibular membrane separates the cochlear duct from the scala vestibuli, and the basilar membrane separates the cochlear duct from the scala tympani.


Cochlear Anatomy –

Zoom In

Section thru one turn of Cochlea • Partitions that separate the channels are Y shaped

– bony shelf of central modiolus– vestibular membrane above & basilar membrane below form

the central fluid filled chamber (cochlear duct)• Fluid vibrations affect hair cells in cochlear duct


Cochlear Anatomy – Zoom Out

• 3 fluid filled channels found within the cochlea– scala vestibuli, scala tympani and cochlear duct

• Vibration of the stapes upon the oval window sends vibrations into the fluid of the scala vestibuli


Anatomy

• Resting on the basilar membrane is the spiral organ (organ of Corti), the organ of hearing (Figure 17.19, c,d).

• Projecting over and in contact with the hair cells of the spiral organ is the tectorial membrane, a delicate and flexible gelatinous membrane.


Anatomy of the Organ of Corti

• 16,000 hair cells have 30-100 stereocilia(microvilli )• Microvilli make contact with tectorial membrane (gelatinous

membrane that overlaps the spiral organ of Corti)• Basal sides of inner hair cells synapse with 1st order sensory

neurons whose cell body is in spiral ganglion


Sound Waves• Sound waves result from the alternate compression and

decompression of air molecules.– The sounds heard best by human ears are at

frequencies between 1000 and 4000 Hertz (Hz; cycles per minute), but many people perceive a range of 20 to 20,000 Hz

– speech is 100 to 3000 Hz• Frequency of a sound vibration is percieved as pitch

– higher frequency is higher pitch• The volume of a sound is its intensity (the greater the size

of the vibration, the louder the sound, measured in decibels, dB).– Conversation is 60 dB; pain above 140dB– OSA requires ear protection above 90dB


Deafness

• Nerve deafness– possibly nerve damage (CN VIII), but usually

damage to hair cells from antibiotics, high pitched sounds, anticancer drugs, etc.

• the louder the sound the quicker the loss of hearing

– person may fail to notice loss until they have difficulty hearing frequencies of speech

• Conduction deafness– perforated eardrum– otosclerosis– vibrations are not “conducted” to hair cells


Physiology of Hearing

• The events involved in hearing are seen in Figure 17.20.• The auricle directs sound waves into the external auditory

canal.• Sound waves strike the tympanic membrane, causing it to

vibrate back and forth.• The vibration conducts from the tympanic membrane

through the ossicles (through the malleus to the incus and then to the stapes).

• The stapes moves back and forth, pushing the membrane of the oval window in and out.


Physiology of Hearing - Review

• The movement of the oval window sets up fluid pressure waves in the perilymph of the cochlea (scala vestibuli).

• Pressure waves in the scala vestibuli are transmitted to the scala tympani and eventually to the round window, causing it to bulge outward into the middle ear.

• As the pressure waves deform the walls of the scala vestibuli and scala tympani, they push the vestibular membrane back and forth and increase and decrease the pressure of the endolymph inside the cochlear duct.


Physiology of Hearing - Review

• The pressure fluctuations of the endolymph move the basilar membrane slightly, moving the hair cells of the spiral organ against the tectorial membrane; the bending of the hairs produces receptor potentials that lead to the generation of nerve impulses in cochlear nerve fibers.

• Pressure changes in the scala tympani cause the round window to bulge outward into the middle ear.


Hair Cell Physiology - Review

• Hair cells convert mechanical deformation into electrical signals

• As microvilli are bent, mechanically-gated channels in the membrane let in K+ ions

• This depolarization spreads & causes voltage-gated Ca+2 channels at the base of the cell to open

• Triggering the release of neurotransmitter onto the first order neuron – more neurotransmitter means more nerve impulses


More on Pitch and Volume

• Differences in pitch are related to differences in the width and stiffness of the basilar membrane and sound waves of various frequencies that cause a “standing wave.” – High-frequency (high-pitch) tone causes the basilar membrane to

vibrate near the base of the cochlea (where it is stiff and narrow.)– Low-frequency (low-pitch) tone causes the basilar membrane to

vibrate near the apex of the cochlea (where it is flexible and wide.)– Hair cells beneath the vibrating region of the basilar membrane

convert the mechanical force (stimulus) into an electrical signal (receptor potential)

• Sounds of the same pitch vibrate the same region of the membrane, and thus stimulate the same cells, but a louder sound causes a greater vibration amplitude -- which our brain interprets as “louder.”


Auditory Pathway

• Cochlear branch of CN VIII sends signals to cochlear and superior olivary nuclei (of both sides) within medulla oblongata– differences in the arrival of impulses from the

ears, allows us to locate the source of a sound along the horizon (right vs. left)

• Fibers ascend to the– medulla, most impulses then cross to the opposite

side and then travel to the – midbrain (inferior colliculus)– to the thalamus– to the auditory area of the temporal lobe

• primary auditory cortex (areas 41 & 42)


Otoacoustic Emissions

• The cochlea can produce sounds called otoacoustic emissions. – caused by vibrations of the outer hair cells that occur in

response to sound waves and to signals from motor neurons.

– vibration travels backwards toward the eardrum – can be recorded by sensitive microphone next to the

eardrum• Purpose

– as outer hair cells shorten, they stiffen the tectorial membrane

– amplifies the responses of the inner hair cells– increasing our auditory sensitivity


Cochlear Implants

• If deafness is due to destruction of hair cells• Microphone, microprocessor & electrodes translate

sounds into electric stimulation of the vestibulocochlear nerve– artificially induced nerve signals follow normal

pathways to brain• Provides only a crude representation of sounds


Applications

• Otosclerosis – a condition is which there is an overgrowth of spongy

bone over the oval window that immobilizes the stapes. – prevents the transmission of sound waves to the inner

ear and leads to conductive hearing loss


Vestibular Apparatus

• Notice: semicircular ducts with ampulla, utricle & saccule


Physiology of Equilibrium (Balance)

• Static equilibrium– maintain the position of the body (head) relative to the

force of gravity– macula receptors within saccule & utricle

• Dynamic equilibrium– maintain body position (head) during sudden movement

of any type--rotation, deceleration or acceleration– crista receptors within ampulla of semicircular ducts


Otolithic Organs: Saccule and Utricle

• The maculae of the utricle and saccule are the sense organs of static equilibrium.

• They also contribute to some aspects of dynamic equilibrium (Figure 17.21).


Otolithic Organs: Saccule & Utricle

• Cell types in the macula region– hair cells with stereocilia (microvilli) & one cilia (kinocilium)– supporting cells that secrete gelatinous layer

• Gelatinous otolithic membrane contains calcium carbonate crystals called otoliths that move when you tip your head


Detection of Position of Head

• Movement of stereocilia or kinocilium results in the release of neurotransmitter onto the vestibular branches of the vestibulocochler nerve


Membranous Semicircular Ducts

• The three semicircular ducts, along with the saccule and utricle maintain dynamic equilibrium (Figure 17.22).– anterior, posterior & horizontal ducts detect different

movements (combined 3-D sensitivity)• The cristae in the semicircular ducts are the primary sense

organs of dynamic equilibrium.


Crista: Ampulla of Semicircular Ducts

• Small elevation within each of three semicircular ducts• Hair cells are covered with cupula (gelatinous material)

• When you move, fluid in canal tends to stay in place, thus bending the cupula and bending the hair cells - and altering the release of neurotransmitter


Detection of Rotational Movement

• Nerve signals to the brain are generated indicating which direction the head has been rotated


Equilibrium Pathways in the CNS

Most vestibular branch fibers of the vestibulocochlear nerve (CN VIII) enter the brain stem and terminate in the medulla; the remaining fibers enter the cerebellum.

Fibers from these areas connect to:• cranial nerves that control eye and head and neck

movements (III,IV,VI & XI)• vestibulospinal tract that adjusts postural skeletal muscle

contractions in response to head movementsThe cerebellum receives constant updated sensory

information which it sends to the motor areas of the cerebral cortex

• motor cortex can then adjust its signals to maintain balance


DEVELOPMENT OF THE EYES AND EARS


Eyes

• Eyes begin to develop when the ectoderm of the lateral walls of the prosencephalon bulges to form a pair of optic grooves (Figure 17.23a)

• As the neural tube closes the optic grooves enlarge and move toward the surface of the ectoderm and are known as optic vesicles (Figure 17.23b)

• When the optic vesicles reach the surface, the surface ectoderm thickens to form the lens placodes and the distal portions of the optic vesicles invaginate to form the optic cups (Figure 17.23c).

• The optic cups remain attached to the prosencephalon by the optic stalks (Figure 17.23d).


Ears

• Inner ear develops from a thickening of surface ectoderm called the otic placode (Figure 17.24a).

• Otic placodes invaginate to form otic pits (Figure 17.24 a and b)

• Optic pits pinch off from the surface ectoderm to form otic vesicles (Figure 17.24d)

• Otic vesicles will form structures associated with the membranous labyrinth of the inner ear.

• Middle ear develops from the first pharyngeal (branchial) pouch.

• The external ear develops from the first pharyngeal cleft (Figure 17.24).


AGING AND THE SPECIAL SENSES

• Age related changes in the eyes– Presbyopia– Cataracts– Weakening of the muscles that regulate the size of the

pupil– Diseases such as age related macular disease, detached

retina, and glaucoma– Decrease in tear production– Sharpness of vision as well as depth and color

perception are reduced.


AGING AND THE SPECIAL SENSES

• After age 50 some individuals experience loss of olfactory and gustatory receptors.

• Age related changes in the ears– Presbycusis – hearing loss due to damaged or loss of

hair cells in the organ of Corti– Tinnitus (ringing in the ears) becomes more common


DISORDERS: HOMEOSTATIC IMBALANCES

• A cataract is a loss of transparency of the lens that can lead to blindness.

• Glaucoma is abnormally high intraocular pressure, due to a buildup of aqueous humor inside the eyeball, which destroys neurons of the retina. It is the second most common cause of blindness (after cataracts), especially in the elderly.


DISORDERS: HOMEOSTATIC IMBALANCES

• Deafness is significant or total hearing loss. It is classified as sensorineural (caused by impairment of the cochlear or cochlear branch of the vestibulocochlear nerve) or conduction (caused by impairment of the external and middle ear mechanisms for transmitting sounds to the cochlea).

• Meniere’s syndrome is a malfunction of the inner ear that may cause deafness and loss of equilibrium.

• Otitis media is an acute infection of the middle ear, primarily by bacteria. It is characterized by pain, malaise, fever, and reddening and outward bulging of the eardrum, which may rupture unless prompt treatment is given. Children are more susceptible than adults.


end

Principles of Human Anatomy and Physiology, 11e1 Chapter 17 The Special Senses Lecture Outline.

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