Anatomy and physiology

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Chapter 17

The Special

Senses

Special Senses

Recall that a sensation is the conscious or subconscious awareness of an internal or external stimulus

For this chapter, “external stimulus” means light rays striking the retina of the eye, sound waves impinging on the tympanic membrane of the ear, molecules in the air and food transmitting smells and tastes to the chemical sensors in the nose an on the tongue, and the force of gravity acting on equilibrium receptors in the inner ear which sense changes in inertia

Special Senses

Receptors for the special senses of smell, taste, vision, hearing, and equilibrium are anatomically distinct from one another and are concentrated in specific locations in the head

In addition to the stimuli and the receptors, there are specific afferent pathways and translation sites in the brain for information assembled from these special senses

Special Senses

General Senses

Include somatic sensations (tactile, thermal, pain, and proprioceptive) and visceral sensations

Are scattered throughout the body

Are relatively simple structures

Special Senses

Include smell, taste, vision, hearing and equilibrium

Are concentrated in specific locations in the head

Are anatomically distinct structures

Form complex neural pathways

Comparing the general senses and the special senses

Olfaction and Taste

Olfaction is the process of perceiving smells. Smell and taste are brought about through the interpretation of chemicals present in the environment

Olfactory and gustatory (taste) impulses travel not only to the cerebral cortex, but also to the limbic system

this is why we can have emotional responses and trigger strong memories to certain smells and tastes
gustation and olfaction work together but olfaction

is much stronger/more sensitive (when someone has a cold it is difficult to taste food)

Olfaction

The olfactory epithelium is located in the superior part of the nasal cavity covering the surface of the cribriform plate and extending along the superior nasal concha

Olfaction

The olfactory epithelium consists of 3 kinds of cells:

The olfactory receptor is a bipolar neuron with cilia (called olfactory hairs). There are 10-100 million of these receptors in the nose that respond to odorant molecules

Supporting cells provide

support and nourishment

Basal cells are stem cells

that replace olfactory

receptors

Olfaction

The olfactory apparatus can detect about 10,000 different odors, often in concentrations as low as 1/25 billionth of a milligram per milliliter of air
When an odorant binds to the

receptor of an olfactory hair it

initiates a cascade of intracellular

events through a G-protein and

a 2nd messenger ( production of

cAMP  opening of Na+ channels

 inflow of Na+  generator potentials)

Because olfaction is much more sensitive than taste, a given concentration of a food substance may stimulate the olfactory system thousands of times more strongly than it stimulates the gustatory system. When you have a cold or are suffering from allergies and cannot taste your food, it is actually olfaction that is blocked, not taste.

A smell can be detected in a concentration as low as 1/25 billionth of a milligram per milliliter of air (the concentration of the methyl mercaptan additive to natural gas).

Olfaction

Once generated, nerve impulses travel through the two olfactory nerves  olfactory bulbs  olfactory tract  primary olfactory area in the temporal lobe of the cortex

Olfaction is the only sensory

system that has direct

cortical projections

without first going

through relay stations

in the thalamus

Olfaction

Olfactory sensory pathways (centrally) are rapidly adapting, decreasing activity by 50% in the first second, and completely accommodating in 1–2 minutes
Olfactory supporting cells and glands are innervated by the facial (VII) nerve, a component of which provides parasympathetic motor innervation to lacrimal glands and the mucous membranes in the nasal cavity. This is why certain odors will make our nose run and cause us to produce tears

Olfactory Rc adapt very little. It’s the central pathways that adapt.

Gustation

Gustation, or taste, is much simpler than olfaction in that only five primary tastes can be distinguished: sour, sweet, bitter, salty, and umami (“meaty” or “savory”)

Umami is believed to arise from taste receptors that are stimulated by monosodium glutamate (MSG), a substance naturally present in many foods and added to others as a flavor enhancer

All other flavors, such as chocolate, pepper, and coffee, are combinations of the five primary tastes, plus accompanying olfactory and tactile (touch) sensations

Chemicals that stimulate gustatory receptor cells are known as tastants. Once a tastant is dissolved in saliva, it can make contact with the plasma membrane of the gustatory hairs, which are the sites of taste transduction. The result is a receptor potential that stimulates exocytosis of synaptic vesicles from the gustatory receptor cell. In turn, the liberated neurotransmitter molecules trigger nerve impulses in the first-order sensory neurons that synapse with gustatory receptor cells.

We have nearly 10,000 taste buds located on the tongue, soft palate, pharynx, and larynx (the number  with age)

Each taste bud is composed of about 50 gustatory receptor cells, surrounded by a number of supporting cells

Basal cells located near the CT base multiply and differentiate, first to become

the supporting cells

around the bud, then

the gustatory receptor

cells inside the taste

bud

Gustation

A single, long microvillus, called a gustatory hair, projects from each receptor cell to the surface through the taste pore
Each gustatory

receptor cell

has a lifespan

of about

10 days

Gustation

Taste buds are found in 3 different types of papillae (elevations on the tongue which provide a rough texture

About 12 very large vallate papillae form a row at the back of the tongue (each houses 100–300 taste buds)

Fungiform papillae are mushroom-shaped and are scattered over the entire surface of the tongue (containing about 5 taste buds each)

Foliate papillae are located in small trenches on the lateral margins of the tongue, but most of their taste buds degenerate in early childhood

Vallate papillae are also called circumvallate papillae (or circular vallate papillae).

In addition, the entire surface of the tongue has filiform papillae that contain tactile

receptors but no

taste buds

They increase

friction between the

tongue and food,

making it easier to

move food in the oral cavity

Gustation

Three cranial nerves contain axons of the first-order gustatory neurons that innervate the taste buds

The facial (VII) nerve serves taste buds in the anterior 2/3 of the tongue

The glossopharyngeal (IX)

nerve serves taste buds in the

posterior 1/3 of the tongue

The vagus (X) nerve serves taste

buds in the throat and epiglottis

Nerve impulses propagate along these cranial nerves to the gustatory nucleus in the medulla oblongata. From there, axons carrying taste signals project

to the hypothalamus, limbic

system, and thalamus

Taste is perceived consciously as

signals from the thalamus arrive

at the primary gustatory area at

the base of the somatosensory

cortex in the parietal lobe

Gustation

The threshold for taste varies for each of the primary tastes

We are most sensitive to bitter substances, such as quinine. Because poisonous substances are often bitter, this high sensitivity may have a protective function

The threshold for sour substances is somewhat higher, followed by salty and sweet substances

Complete adaptation to a specific taste can occur in

1–5 minutes of continuous stimulation

Vision

Our visual perception is dependent on the eye, its accessory structures, the optic tracts, and the 1o visual cortex and it’s association areas

Vision is possible because of

photoreceptors that are able

to “catch” photons of EM

radiation in the 400-700 nm

wavelengths – what we

perceive as visual light

Vision

The eyeball is about 2.5 cm in diameter, with only about 16% of it viewable by just looking at a person

The accessory structures

of the eye are the

extraocular muscles,

palpebra, conjunctiva,

and the lacrimal glands

and ducts. The pupil is

an opening for light to

pass into the back of the eye

The upper and lower palpebrae are the eyelids, with the fissure being the space between them
CN III supplies 4 of the 6 extraocular muscles, plus the levator palpebrae superioris

muscles that raise the

upper eyelid

The conjunctiva is a clear

mucous membrane that

covers the white (avascular)

part of the eye

Accessory Eye Structures

Visceral motor to parasympathetic innervation of the constrictor pupillae and ciliary muscles. CN IV to the superior oblique. CN VI to the lateral rectus

Production of tears superiorly and laterally, drain inferomedially. Tears have lysozymes which help to destroy bacteria.

Accessory Eye Structures

The lacrimal glands are each about the size an almond, situated superolateral to the eyeball. Leading from the lacrimal glands are 6 to 12 excretory lacrimal ducts

Tears (lacrimal fluid) run from the lacrimal glands, into the excretory lacrimal ducts,

onto the surface of the

conjunctiva, over the

surface of the eyeball

some lacrimal fluid

also evaporates

Accessory Eye Structures

Tears drain into the lacrimal puncta, which are two openings on the nasal side of the extreme edge of the eyeball. Superior and inferior lacrimal canals empty the tears into the nasolacrimal sac and nasolacrimal duct

The right and left sided nasolacrimal ducts empty into each side of

the nose

Accessory Eye Structures

Watery eyes occur when lacrimal fluid builds up, as when something obstructs the nasolacrimal ducts for instance

Blocked nasolacrimal ducts can be caused by an inflammation of the nasal mucosa, such as a cold

Over production of lacrimal fluid occurs in response to parasympathetic stimulation, caused by an emotional response (crying), and tears spill over the edges of the eyelids and drain into the nasal cavity (causing nasal stuffiness)

The wall of the eyeball consists of three layers or tunics: The fibrous tunic is the outer layer and is composed of the sclera (“white” of the eye) and the cornea (the transparent epithelium the protects the front of the eye)

The vascular tunic or uvea is

the middle layer and is

composed of the choroid,

the ciliary body and the iris

The nervous tunic is the

inner retinal layer

Anatomy of the Eye

Even though you can’t easily see it, the cornea is a very important structure in the outer avascular fibrous tunic

It’s composed of a transparent epithelium that covers the anterior eye and helps focus light onto the retina

LASIK is a common visual corrective procedure that is performed on the cornea of the eye
Because of the amount of collagen fibers in the sclera it forms the tough, white part of the eye

The sclera gives the eye it’s shape and protects the inner anatomical parts

Anatomy of the Eye

Of the 3 parts of the middle tunic the choroid forms the major vascular portion that lines the internal surface of the sclera

The ciliary body consists of two parts:

The ciliary processes that secrete aqueous humor
The ciliary muscle that changes the shape of the lens to adapt to near and far vision

The iris is the colored portion of the eyeball consisting of circular and radial smooth muscle fibers

Ciliary process is folded epithelial tissue

Iris has pigmented epithelium + two groups of smooth muscle cells

The uvea (Lat. uva, grape), also called the uveal layer, uveal coat, uveal tract, or vascular tunic, is the pigmented middle of the three concentric layers that make up an eye. The name is possibly a reference to its almost black colour, wrinkled appearance and grape-like size and shape when stripped intact from a cadaveric eye. Its use as a technical term in anatomy and ophthalmology is relatively modern.

The inner nervous tunic (retina) lines the posterior 2/3 of the eye

The retina consist of a layer of melanin pigmented epithelium that allows light to be absorbed rather than scattered. Without

the melanin, scattered

light in our eye would

cause us to always be

squinting, even in a

moderately lit room

Anatomy of the Eye

We don’t need the retina to cover the entire inside of the eye because the light only strikes the back of the eye.

The exact center of the retina is called the macula lutea, and in its center is a small depression called the central fovea (or fovea centralis)

There are no rods or nerve cells in the fovea, only a high concentration of cones - this gives us the sharp central vision

necessary in any

activity where

detail is of

primary importance

Anatomy of the Eye

The sharp central vision is also called foveal vision.

The retina can be viewed through the pupil using an ophthalmoscope, allowing direct inspection of the retinal vessels for any pathological changes. This is the only place in the body where arterial vessels can be so viewed (without

opening

the body)

Anatomy of the Eye

The optic disc is where the optic nerve and retinal vessels enter and exit the eyeball. Its existence creates a necessary defect on the retina – an area where there are no cones or rods. Bilateral vision, and

saccade (involuntary,

quick) muscle movements

allow our brain to correct

for this “blind spot”, and

most are not even aware

they have one

(try the test on the next page)

The blind spot can be demonstrate using this chart

Instructions:

Situate yourself so that your nose is pointing in-between the cross and the black circle.

Cover your LEFT eye and stare at the cross with your RIGHT eye.

Now SLOWLY move towards the computer screen while still staring at the cross with your RIGHT eye.

At somewhere around 10-14 inches from the computer screen – the black circle will disappear and the area where the black circle was…will now be all white - this is your BLIND SPOT.

If you move closer to the screen or farther away - the circle will re-appear. At just the right distance – the circle will disappear.

Now try the OTHER eye…but this time cover your RIGHT eye and look at the CIRCLE with your LEFT eye…..move closer and you will see that the CROSS now disappears!!

Anatomy of the Eye

The retina consists of two types of photoreceptor cells, rods and cones

Rods are abundant in the periphery of the retina whereas cones are found more frequently in the central areas

Anatomy of the Eye

Each eye contains ≈ 120 million rod-shaped photoreceptors that are adapted for a low light threshold (high sensitivity) - they produce low resolution, black and white images

a loss of rods with age makes it difficult to drive at night

Anatomy of the Eye

Cone-shaped photoreceptors function in bright light to produce high resolution color images

They exists in three varieties,

corresponding to the type of

pigment they contain: red, green or blue

The photopigments are concentrated in

the outer segment of the receptor, while the

inner segment contains the

nucleus and organelles

A complete loss of cones will result in legal blindness. A relative loss, or a deficiency of one type results in color blindness.

Eye Cavities and Chambers

The lens is an avascular refractory structure situated posterior to the pupil and iris. It consists of a capsule with crystallin proteins arranged in layers, and like the cornea, the lens is transparent

It attaches to the ciliary muscle

of the ciliary body

by suspensory

ligaments that fine

tune the focusing of

light on the retina

Eye Cavities and Chambers

The lens divides the eyeball into two cavities: An anterior cavity anterior to the lens, and a posterior cavity (vitreous chamber) behind the lens

The anterior cavity is further

divided at the level of the

iris into anterior and

posterior chambers

(both filled with

aqueous humor)

Eye Cavities and Chambers

The much larger posterior cavity of the eyeball (vitreous chamber) lies between the lens and the retina

Within the vitreous chamber is the vitreous body, a transparent jellylike substance that holds the retina flush against the choroid, giving the retina an even surface for the reception of clear images

occasionally, collections of debris called vitreal floaters cast shadows on the retina and create a spot in our field of vision (they are usually harmless and do not require treatment)

Eye Cavities and Chambers

This cow eye dissection shows an eye bisected into anterior and posterior sections along its coronal axis. The anterior structures of the

iris and pupil are seen in

the bottom half; the

posterior retina,

choroid, and optic

disc are seen in the

top half

Aqueous Humor

The eye requires a constant bath in a nourishing fluid to deliver enough O2 to support the avascular lens and cornea

It also needs fluid to help “inflate” the walls of the eyeball (maintain a constant intraocular pressure – IOP) and support the vitreous body

this need is accomplished through the production of aqueous humor, which flows through the anterior cavity of the eye and is replaced every 90 minutes

Aqueous Humor

Aqueous humor is produced at the ciliary body and flows first through the posterior chamber (of the anterior cavity of the eye)

Traveling along the posterior surface of the iris it passes through the pupil to enter the anterior chamber

It proceeds along the anterior surface of the iris until it is reabsorbed into the scleral venous sinus (canal of Schlemm) and returned to the venous system

Aqueous Humor

Any sort of blockage to aqueous humor flow, or overproduction at the ciliary body may result in an increase of pressure inside the eye –

a condition called glaucoma

If not treated,

glaucoma can lead

to a degeneration

of eye function

Too high or too low of an IOP are both problems. IOP can be measured with a device called an applanation tonometer that sends a puff of air to ricochet off the cornea and back to the machine to be measured.

Retinal Detachment

The vitreous body (humor) also contributes to maintain proper intraocular pressure as it holds the retina against the choroid. The vitreous humor, however, is only formed during embryological development and is not replaced. As we age, shrinkage of the vitreous body may lead to a detachment of the retina from the choroid

A retinal detachment is considered a medical emergency and needs immediate repair before vision loss becomes permanent

The pupil is an opening in the center of the iris. It is composed of a radial muscle that “radiates” away from the center, and a circular muscle that is in the center

Contraction of the inner circular muscle fibers cause the pupil to constrict

while contraction

of the radial fibers

cause it to dilate

The Pupillary Response

Refraction and Image

Normal image formation depends on refraction of light waves, accommodation of the lens, constriction of the pupil, and convergence of the two eyes

Refraction is the process of bending light rays. Both the cornea and the lens refract light rays, and both must be functioning in order to

properly focus light onto

the right spot on the retina

to produce clear vision

Refraction and Image

Since the cornea has a fixed shape, its “focal length” is also fixed; and its ability to refract light is likewise fixed
In order to focus light that has already been bent by the cornea the lens must change shape – the amount depending on the type of light rays we are trying to “see”

Refraction and Image

An increase in the curvature of the lens for near vision is called accommodation

The near point of vision is the minimum distance from the eye that an object

can be clearly

focused - about 4 in

(a distance that

increases with age

due to a loss of

elasticity in the lens)

Images focused on the retina are inverted and reversed, but our brain learns to flip them around.

Refraction and Image

Convergence is the inward movement of the eyes so that both are directed at the object being viewed - becoming a little cross-eyed when viewing things close up

The nearer the object, the greater the degree of convergence needed to maintain binocular vision

the coordinated action of the extrinsic eye muscles brings about convergence.

Convergence helps us maintain our binocular vision and see in three dimensions

Refraction and Image

With nearsightedness (myopia), only close objects can be seen clearly: Light rays coming in from distant objects are naturally focused in front of the retina and appear blurry

Correction involves the use of a concave

(negative) lens

With farsightedness (hyperopia), only distant objects can be seen clearly: Light rays coming in from nearer objects are naturally focused behind the retina

Correction involves the use of a convex (positive) lens

Abnormal refractive capabilities of the eye are the result of a misshapen eyeball (usually too long or too short), or because the lens becomes stiff (usually

with age). Corrections are

accomplished using either

a positive (convex) or

negative (concave) lens

(eyeglasses, contacts, or

lens replacements)

Refraction and Image

Astigmatism is an irregular curvature of the lens or cornea that causes portions of objects to be out of focus.

Visual Transduction

Once light waves have been successfully focused on the retina, the information “stored” in that electromagnetic energy must be changed by photopigments in the photoreceptors into signals our brain can interpret - a process called visual transduction
The single type of photopigment in rods is rhodopsin, whereas there are 3 different cone photopigments

Color vision results from different colors of light selectively activating the different cone photopigments

Visual Transduction

The first step in visual transduction is absorption of light by a photopigment, a colored protein that undergoes structural changes when it

absorbs light in the outer

segment of a photoreceptor

Light absorption initiates

a series of events that

lead to the production

of a receptor potential

(number 4 in the diagram)

Visual Transduction

All photopigments associated with vision contain two parts: a glycoprotein known as opsin and a derivative of vitamin A called retinal

Although there are 4 different opsins, retinal is the light-absorbing part of all visual photopigments

To simplify the process we can say that there is a cyclical bleaching and regeneration of photopigment

Bleaching is a term describing a conformational change in the retinal molecule in response to light

Visual Transduction

In darkness, retinal has a bent shape called cis-retinal

Absorption of a photon of light causes it to straighten into the trans-retinal form in a process called isomerization

Trans-retinal completely separates from the opsin; since the final products look colorless, this part of the cycle is called bleaching of photopigment

An enzyme converts trans-retinal→ cis-retinal

The cis-retinal regenerates the photopigment

In darkness, the neurotransmitter glutamate is released keeping Na+ channels open and inhibiting the bipolar cell: This inflow of Na+is called the “dark current”. Photons cause Na+ channels to close, and the rod hyperpolarizes. It’s strange that when your eyes are closed and you are asleep the rods are the most active.

Bleaching and regeneration of photopigments are summarized here

Horizontal cells transmit inhibitory signals to bipolar cells in the areas lateral to excited rods and cones.

Visual Transduction

In daylight, regeneration of rhodopsin cannot keep up with the bleaching process, so rods contribute little to daylight vision. In contrast, cone photopigments regenerate rapidly enough that some of the cis form is always present, even in very bright light

As a consequence, light adaptation (from dark conditions  light conditions) happens in seconds; dark adaptation (from light  dark) takes minutes to occur (up to 40 minutes to fully adapt)

After complete bleaching, regeneration of half of the rhodopsin takes 5 minutes; half of the cone photopigments regenerate in only 90 seconds. Full regeneration of bleached rhodopsin takes 30 to 40 minutes.

Visual Transduction

Most forms of color blindness, an inherited inability to distinguish between certain colors, result from the absence or deficiency of one of the three types of cones

Most common type is red-green color blindness in which red cones or green cones are missing

Prolonged vitamin A deficiency and the resulting below-normal amount of rhodopsin may cause night blindness or nyctalopia, an inability to see well at low light levels

The graded potentials generated by the photoreceptors undergo considerable processing at synapses among the various types of neurons in the retina (horizontal cells, bipolar cells, and amacrine cells)- certain features of visual

input are enhanced while others

are discarded

Overall, convergence pre-

dominates as 126 million

photo-receptors impinge on only

1 million ganglion cells

The Visual Pathway

Horizontal cells transmit inhibitory signals to bipolar cells in the areas lateral to excited rods and cones. Horizontal cells also assist in the differentiation of various colors. Amacrine cells, which are excited by bipolar cells, synapse with ganglion cells and transmit information to them that signals a change in the level of illumination of the retina. When bipolar or amacrine cells transmit excitatory signals to ganglion cells, the ganglion cells become depolarized and initiate nerve impulses.

The Visual Pathway

The axons of retinal ganglion cells provide output that travels back “towards the light”, exiting the eyeball as the

optic nerve, which emerges from the vitreous surface of the retina

The axons then pass

through a crossover point

called the optic chiasm

Some axons cross to the opposite side, while others remain uncrossed. Once through the

optic chiasm the axons enter the

brain matter as the optic tracts

(most terminate in thalamus)

Here they synapse with

neurons that project to the

1o visual cortex in the

occipital lobes

The Visual Pathway

Visual field of

left eye

Temporal

half

Visual field of

right eye

Temporal

half

Nasal

half

Midbrain

Left eye

Temporal

retina

Optic

radiations

Left eye and its pathways

Primary visual area of cerebral

cortex (area 17) in occipital lobe

Lateral geniculate nucleus

of the thalamus

Optic

radiations

Midbrain

Temporal

retina

Nasal

retina

Right eye

Right eye and its pathways

Nasal

half

Nasal retina

Visual field of

left eye

Temporal

half

Visual field of

right eye

Temporal

half

Nasal

half

Midbrain

Left eye

Temporal

retina

Optic

radiations

Left eye and its pathways

Primary visual area of cerebral

cortex (area 17) in occipital lobe

Lateral geniculate nucleus

of the thalamus

Optic

radiations

Midbrain

Temporal

retina

Nasal

retina

Right eye

Right eye and its pathways

Nasal

half

Nasal retina

Visual field of

left eye

Temporal

half

Visual field of

right eye

Temporal

half

Nasal

half

Midbrain

Left eye

Temporal

retina

Optic

radiations

Left eye and its pathways

Primary visual area of cerebral

cortex (area 17) in occipital lobe

Lateral geniculate nucleus

of the thalamus

Optic

radiations

Midbrain

Temporal

retina

Nasal

retina

Right eye

Right eye and its pathways

Nasal

half

Nasal retina

Visual field of

left eye

Temporal

half

Visual field of

right eye

Temporal

half

Nasal

half

Midbrain

Left eye

Temporal

retina

Optic

radiations

Left eye and its pathways

Optic

tract

Primary visual area of cerebral

cortex (area 17) in occipital lobe

Lateral geniculate nucleus

of the thalamus

Optic

radiations

Midbrain

Temporal

retina

Nasal

retina

Right eye

Right eye and its pathways

Nasal

half

Nasal retina

Visual field of

left eye

Temporal

half

Visual field of

right eye

Temporal

half

Nasal

half

Midbrain

Left eye

Temporal

retina

Optic

radiations

Left eye and its pathways

Optic

tract

Primary visual area of cerebral

cortex (area 17) in occipital lobe

Lateral geniculate nucleus

of the thalamus

Optic

radiations

Midbrain

Temporal

retina

Nasal

retina

Right eye

Right eye and its pathways

Nasal

half

Nasal retina

Visual field of

left eye

Temporal

half

Visual field of

right eye

Temporal

half

Nasal

half

Midbrain

Left eye

Temporal

retina

Optic

radiations

Left eye and its pathways

Optic

tract

Primary visual area of cerebral

cortex (area 17) in occipital lobe

Lateral geniculate nucleus

of the thalamus

Optic

radiations

Midbrain

Temporal

retina

Nasal

retina

Right eye

Right eye and its pathways

Nasal

half

Nasal retina

The Ear

Audition, the process of hearing, is accomplished by the organs of the ear. The ear is an engineering marvel because its sensory receptors can transduce sound vibrations with amplitudes as small as the diameter of an atom of gold into electrical

signals 1000 times faster than

the eye can respond to light

The ear also contains

receptors for equilibrium

The Ear

The ear has 3 principle regions

The external ear, which uses air to collect and channel sound waves

The middle ear, which uses a

bony system to amplify

sound vibrations

The internal ear, which

generates action potentials to transmit

sound and balance information to the brain

The pathways for sound transmission starts in the air, the to the solid bone of the middle ear, then to the endolymph of the inner ear.

The External Ear

The anatomy of the external ear includes

The auricle (pinna), a flap of elastic cartilage covered by skin and containing ceruminous glands

A curved 1” long external auditory canal situated in the temporal bone leading from

the meatus to the tympanic

membrane (TM – or ear

drum) which separates the

outer ear from the cavity

of the middle ear

The middle ear is an air-filled cavity in the temporal bone. It is lined with epithelium and contains 3 auditory ossicles (bones)

The stapes (stirrup)

The incus (anvil)

The handle of

the malleus

(hammer) attaches

to the TM

The Middle Ear

Two small skeletal muscles (the tensor tympani and stapedius) attach to

the ossicle and

dampen vibrations

to prevent damage

from sudden,

loud sounds

The Middle Ear

The Eustachian (auditory) tube connects the middle ear with the nasopharynx (upper portion of the throat)

It consists of bone and hyaline

cartilage and is normally

passively collapsed. It opens to

equalize pressures on each side

of the TM

(allowing it

to vibrate freely)

The chamber of the middle ear is continuous through the eustachian tube with the nasopharynx, but also with the mastoid antrum and mastoid air cells. Infection of the mucosa lining the middle ear will extend to the mastoid air cells in the bone behind the ear.

The Inner Ear

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

Structurally, it consists of two main divisions: an outer bony labyrinth that encloses an inner membranous labyrinth

the bony labyrinth is sculpted out of the petrous part of the temporal bone, and divided into three areas: (1) the semicircular canals, (2) the vestibule,

and (3) the cochlea

The oval window starts the inner ear. As the stapes rocks back and forth the oval window and round window oscillates (like pushing on the end of a waterbed).

The Inner Ear

The vestibule is the middle part of the bony labyrinth

The membranous labyrinth in the vestibule consists of two sacs called the utricle and the saccule (both contain rc for static equilibrium)

The cochlea , located anterior to the

vestibule, contains rc for hearing

The three semicircular canals

are above the vestibule, each

ending in a swollen

enlargement called the ampulla

(for dynamic equilibrium)

The Inner Ear

The snail shaped cochlea contains the hearing apparatus

Two types of fluid (perilymph and endolymph) fill its 3 different internal channels: The scala vestibuli, scala tympani, and cochlear duct

A section

through one

turn of the

cochlea is shown

The Inner Ear

Perilymph transmits the vibrations coming from the stapes in the oval window up and around the scala vestibuli, and then back down and around the scala tympani – causing the endolymph in the cochlear duct to vibrate

Pressure waves in the endolymph cause the basilar membrane of the cochlear duct to vibrate, moving the hair cells of the spiral organ of Corti against an overhanging flexible gelatinous membrane called the tectorial membrane

The Inner Ear

Note how the sound waves between the number 1 and number 2 in this diagram are shown impacting different parts of membranous labyrinth. This is a representation of sounds waves of different frequencies being transduced at the segment of the basilar membrane that

is “tuned” for

a particular

pitch

The Inner Ear

Movements of the hair cells in contact with the tectorial membrane transduce mechanical vibrations into electrical signals which generate nerve impulses along the cochlear branch of CN VIII

The Auditory Pathway

This graphic depicts the events in the stimulation of auditory receptors, from channeling sound waves into the external ear and onto the TM, to the transduction of those vibrations

into local

receptor

potentials

Sound waves enter the external auditory canal and strike the eardrum. The vibrations of the eardrum cause the ossicles to vibrate and the stapes pushes the membrane of the oval window in and out. The movement of the oval window sends fluid pressure waves into the perilymph of the scala vestibuli which then transmit them to the scala tympani and eventually to the round window (causing it to bulge outward into the middle ear). The pressure waves move into the endolymph of the cochlear duct and cause the basilar membrane to vibrate which moves the hair cells of the spiral organ against the tectorial membrane. This leads to bending of the stereocilia and ultimately to the generation of nerve impulses in first-order neurons in cochlear nerve fibers. Sound waves of various frequencies cause certain regions of the basilar membrane to vibrate more intensely than other regions.

Each segment of the basilar membrane is “tuned” for a

Scala

vestibuli

Cochlear duct

(contains endolymph)

Scala

tympani

Perilymph

Basilar

membrane

Cochlea

Sound waves

Helicotrema

Stapes vibrating

in oval window

Malleus

Incus

External auditory

canal

Tympanic

membrane

Secondary tympanic

membrane vibrating

in round window

Auditory tube

Vestibular membrane

Middle ear

Tectorial membrane

Spiral organ

(organ of Corti)

Scala

vestibuli

Cochlear duct

(contains endolymph)

Scala

tympani

Perilymph

Basilar

membrane

Cochlea

Sound waves

Helicotrema

Stapes vibrating

in oval window

Malleus

Incus

External auditory

canal

Tympanic

membrane

Secondary tympanic

membrane vibrating

in round window

Auditory tube

Vestibular membrane

Middle ear

Tectorial membrane

Spiral organ

(organ of Corti)

Scala

vestibuli

Cochlear duct

(contains endolymph)

Scala

tympani

Perilymph

Basilar

membrane

Cochlea

Sound waves

Helicotrema

Stapes vibrating

in oval window

Malleus

Incus

External auditory

canal

Tympanic

membrane

Secondary tympanic

membrane vibrating

in round window

Auditory tube

Vestibular membrane

Middle ear

Tectorial membrane

Spiral organ

(organ of Corti)

Scala

vestibuli

Cochlear duct

(contains endolymph)

Scala

tympani

Perilymph

Basilar

membrane

Cochlea

Sound waves

Helicotrema

Stapes vibrating

in oval window

Malleus

Incus

External auditory

canal

Tympanic

membrane

Secondary tympanic

membrane vibrating

in round window

Auditory tube

Vestibular membrane

Middle ear

Tectorial membrane

Spiral organ

(organ of Corti)

Scala

vestibuli

Cochlear duct

(contains endolymph)

Scala

tympani

Perilymph

Basilar

membrane

Cochlea

Sound waves

Helicotrema

Stapes vibrating

in oval window

Malleus

Incus

External auditory

canal

Tympanic

membrane

Secondary tympanic

membrane vibrating

in round window

Auditory tube

Vestibular membrane

Middle ear

Tectorial membrane

Spiral organ

(organ of Corti)

Scala

vestibuli

Cochlear duct

(contains endolymph)

Scala

tympani

Perilymph

Basilar

membrane

Cochlea

Sound waves

Helicotrema

Stapes vibrating

in oval window

Malleus

Incus

External auditory

canal

Tympanic

membrane

Secondary tympanic

membrane vibrating

in round window

Auditory tube

Vestibular membrane

Middle ear

Tectorial membrane

Spiral organ

(organ of Corti)

Scala

vestibuli

Cochlear duct

(contains endolymph)

Scala

tympani

Perilymph

Basilar

membrane

Cochlea

Sound waves

Helicotrema

Stapes vibrating

in oval window

Malleus

Incus

External auditory

canal

Tympanic

membrane

Secondary tympanic

membrane vibrating

in round window

Auditory tube

Vestibular membrane

Middle ear

Tectorial membrane

Spiral organ

(organ of Corti)

Scala

vestibuli

Cochlear duct

(contains endolymph)

Scala

tympani

Perilymph

Basilar

membrane

Cochlea

Sound waves

Helicotrema

Stapes vibrating

in oval window

Malleus

Incus

External auditory

canal

Tympanic

membrane

Secondary tympanic

membrane vibrating

in round window

Auditory tube

Vestibular membrane

Middle ear

Tectorial membrane

Spiral organ

(organ of Corti)

Scala

vestibuli

Cochlear duct

(contains endolymph)

Scala

tympani

Perilymph

Basilar

membrane

Cochlea

Sound waves

Helicotrema

Stapes vibrating

in oval window

Malleus

Incus

External auditory

canal

Tympanic

membrane

Secondary tympanic

membrane vibrating

in round window

Auditory tube

Vestibular membrane

Middle ear

Tectorial membrane

Spiral organ

(organ of Corti)

The Auditory Pathway

The cell bodies of the sensory neurons are located in the spiral ganglia. Nerve impulses pass along the axons of these neurons, which

form the cochlear

branch of the

vestibulo-

cochlear (VIII)

nerve

The Auditory Pathway

The nerve impulses follow CN VIII en route to the medulla, pons, midbrain, and thalamus, and finally to the primary auditory cortex in the temporal lobe. Slight differences in the timing of nerve

impulses arriving from the

two ears at the superior

olivary nuclei in the

pons allow us to

locate the source

of a sound

Equilibrium

Equilibrium is another function of the inner ear - controlled by the vestibular apparatus (the saccule and utricle of the vestibule, and the 3 semicircular canals)

Static equilibrium refers to a

state of balance relative to

the force of gravity

Dynamic equilibrium

involves the maintenance

of balance during sudden movements

Static Equilibrium

Static equilibrium is controlled by the sensory hairs within the macula of the utricle

and saccule

Static Equilibrium

An otolithic membrane, studded with dense calcium carbonate crystals (otoliths), responds to gravity when head position is changed

This movement opens

transduction channels

in the hair cells,

producing local potentials

which summate to

form nerve AP

Dynamic Equilibrium

Dynamic equilibrium is controlled by the sensory hairs within the ampulla of the

semicircular canals

Within each ampulla

is a small elevation

called the crista

Dynamic Equilibrium

Each crista contains hair cells and supporting cells covered by gelatinous material called the cupula

With movement, the endolymph within the ampulla lags behind the moving cupola, causing a difference in the inertial forces –

the hair bundle

of the cupola

bends and nerve

impulses are generated

Equilibrium Pathway

Once generated, nerve impulse travel up the vestibular branch of CN VIII. Most of these axons synapse in the major integrating centers for equilibrium, in the medulla and pons, which also receive input from the eyes and proprioceptors

Ascending neurons continue  primary auditory area in the parietal lobe to provide us with conscious awareness of the position and movements of the head and limbs

Homeostatic Imbalances

A cataract is an opaque defect in the cornea or lens of the eye – most cataracts are in the lens

Cataracts are causes by injury, medications, and diseases like diabetes. They are common in old age

Conjunctivitis is an inflammation of the conjunctival membrane which covers part of the front of the eye

Conjunctivitis is caused most frequently by viral infections (pink eye) and allergy. It can also result from bacterial infections and many other irritants

Homeostatic Imbalances

Age Related Macular Degeneration results in a loss of vision in the center of the visual field (the macula) because of damage to the retina. It is a major cause of visual impairment in older adults (>50 years)

It can become impossible

to recognize faces, yet

enough peripheral vision

remains to allow other

activities of daily life

Homeostatic Imbalances

Myringitis is an inflammation of the ear drum

Infections of the middle ear cavity (otitis media) are common in children between 6 mo. – 5 yrs. old, and usually presents with a crying child and a TM (viewed through an otoscope), that looks angry, red, and bulging

Otitis externa (commonly called “swimmer’s ear”) is a dermatitis of the epithelium of the outer ear (infectious and noninfectious). The chlorine, water, and ear plugs associated with swimming can result in irritated, inflamed tissues of the outer ear and ear canal

Distended Eardrum Caused by
Otitis Media

Dr. P. Marazzi/Photo Researchers, Inc.

Homeostatic Imbalances

Meniere’s disease is a disorder of the inner ear that can affect hearing and balance, and is thought to be due to increased pressure in the cochlea and semicircular canals (extra endolymph)

Episodes of vertigo (the room spinning) and ringing in the ears (tinnitus) can be a mild annoyance, or a chronic, disabling disability

End of Chapter 17

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