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15 The Special Senses. Section 1: Olfaction and Gustation. Learning Outcomes 15.1 Describe the sensory organs of smell, trace the olfactory pathways to their destinations in the cerebrum, and explain how olfactory perception occurs. 15.2 Describe the sensory organs of gustation.

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The Special Senses

Section 1: Olfaction and Gustation

Learning Outcomes

15.1 Describe the sensory organs of smell, trace the olfactory pathways to their destinations in the cerebrum, and explain how olfactory perception occurs.

15.2 Describe the sensory organs of gustation.

15.3 Describe gustatory reception, briefly describe the physiologic processes involved in taste, and trace the gustatory pathway.

Section 1: Olfaction and Gustation

Special senses introduction

Special sense organs provide us with information about external environment

Two types of receptors used

Dendrites of specialized neurons

Bind chemicals producing a depolarization of the cell or generator potential

Example: olfactory (smell) receptors

Specialized receptors that synapse with sensory neurons

Stimulated receptor releases chemical transmitters that depolarize sensory neuron (generator potential)

Small delay due to synapse

Examples: vision, hearing, taste, equilibrium

Figure 15 Section 1 1

The function of olfactory receptors








Generator potential


to CNS

Specialized olfactory neuron

Figure 15 Section 1 2

The function of receptors for the senses of

taste, vision, equilibrium, and hearing

Receptor cell














to CNS


Axon of sensory neuron

Receptor cell

Generator potential

Module 15.1: Olfaction


Provided by olfactory organs

Located in nasal cavity, either side of nasal septum


Inferior surface of cribiform plate

Superior portion of perpendicular plate

Superior nasal conchae of ethmoid

Module 15.1: Olfaction

Olfactory pathway

Sensory neurons in olfactory organ stimulated by chemicals

Olfactory epithelium axons collect into 20 or more bundles penetrating cribiform plate of ethmoid bone

Synapse with olfactory bulb

Axons leaving bulb travel along olfactory tract to olfactory cortex, hypothalamus, and portions of limbic system

Explains why smells can produce profound emotional and behavioral responses

Figure 15.1 1

Olfactory Pathway to the Cerebrum

The sensory

neurons within

the olfactory

organ are

stimulated by

chemicals in the


Axons leaving

the olfactory


collect into 20 or

more bundles

that penetrate the

cribriform plate

of the ethmoid.

The first

synapse occurs

in the olfactory

bulb, which is

located just

superior to the

cribriform plate.

Axons leaving the

olfactory bulb travel

along the olfactory

tract to reach the

olfactory cortex, the

hypothalamus, and

portions of the limbic


The distribution of olfactory

information to the limbic

system and hypothalamus

explains the profound

emotional and behavioral

responses, as well as the

memories, that can be

triggered by certain smells.

Cribriform plate

of ethmoid

Olfactory organ

Olfactory epithelium

Superior nasal concha

Module 15.1: Olfaction

Olfactory organ composition

Two layers

Olfactory epithelium

Olfactory receptor cells

Each cell produces knob (base of 20 cilia)

10–20 million receptors in 5 cm2 area

Supporting cells

Basal (stem) cells

Replace worn-out receptors

One of the few examples of neuronal replacement

Lamina propria

Contains olfactory glands that produce mucus

Figure 15.1 2

A portion of an olfactory organ, which consists of the

olfactory epithelium and the lamina propria







Olfactory nerve fibers

Lamina propria

Basal cell:

divides to replace worn-out

olfactory receptor cells

Developing olfactory

receptor cell

Olfactory receptor cell

Olfactory epithelium

Supporting cell

Mucous layer


Olfactory cilia: surfaces

contain receptor proteins

Module 15.1: Olfaction

Steps of olfactory reception

Binding of odorant (dissolved chemical) to receptor protein

Activates adenylyl cyclase (enzyme converting ATP to cAMP)

cAMP opens sodium channels, depolarizing membrane

With sufficient depolarization, an action potential may be generated and relayed to CNS

Module 15.1: Olfaction


Generally small organic molecules

Strongest smells associated with molecules with either high water or lipid solubilities

As few as four odorant molecules can activate receptor

Figure 15.1 3

Step 3: If sufficient

depolarization occurs, an

action potential is triggered in

the axon, and the information

is relayed to the CNS.

Step 1: The binding of an odorant

to its receptor protein leads to the

activation of adenylyl cyclase, the

enzyme that converts ATP to

cyclic-AMP (cAMP).

Step 2: The cAMP then opens

sodium channels in the

plasma membrane, which, as a

result, begins to depolarize.




ions enter














The process of olfactory reception on the surface membranes of the olfactory cilia

Module 15.1 Review

a. Describe olfaction.

b. Which neurons associated with olfaction are capable of regenerating?

c. Trace the olfactory pathway, beginning at the olfactory epithelium.

Module 15.2: Gustation

Gustation or taste provides information about consumed food and liquids

Taste (gustatory) receptors

Found mainly on superior surface of tongue within taste buds

Also some located in pharynx and larynx but decrease in importance and abundance with age

Module 15.2: Gustation

Taste bud structure

Gustatory cells

Each has slender microvilli into surrounding fluids through narrow opening (taste pore) of taste bud

Each only survives ~10 days

Approximately 40–100 receptor cells/bud

Basal cells

Stem cells that divide and mature to produce more gustatory cells

Figure 15.2 3 – 4



Taste hairs


The structure of taste buds

Gustatory cell

Basal cell



Diagrammatic view

of a taste bud



Taste bud

LM x 650

Taste buds

LM x 280

Module 15.2: Gustation

Taste bud location

Recessed along epithelium lining tongue projections (lingual papillae; papilla, nipple-shaped mound)

Papillae types

Circumvallate (circum-, around + vallate, wall) papillae

Large with deep folds containing ~100 taste buds

Located in V-shape on tongue posterior

Fungiform (fungus, mushroom) papillae

Shaped like small buttons with shallow depressions

Each contains ~5 taste buds

Filiform (filum, thread) papillae

Provide friction but contain no taste buds

Figure 15.2 1 – 2

Circumvallate Papillae

Are relatively large and are surrounded by

deep epithelial folds; each contains as many

as 100 taste buds

The lingual papillae on the superior surface of the tongue

Water receptors





Fungiform Papillae

Circumvallate papillae





Contain about five taste buds each

Filiform Papillae

Provide friction that

helps the tongue

move objects around

in the mouth but do

not contain taste


Module 15.2: Gustation

Taste sensations

Four primary sensations: sweet, salty, sour, and bitter

Found in taste buds all over tongue

Two other sensations


Meaty or savory

Receptor binds amino acids

Discovered in Japan

Water receptors

Demonstrated in human pharynx

Information sent to hypothalamus to manage thirst

Module 15.2: Gustation

Taste receptor sensitivity

More sensitive to unpleasant stimuli

100,000× more sensitive to bitter, 1000× more sensitive to sour (acids) compared to sweet and salty

May have survival value

Toxic compounds are often bitter

Acids can create chemical burns

Overall sensitivity declines with age

Number of taste receptors declines

Number of olfactory receptors declines

Module 15.2 Review

a. Define gustation.

b. Describe filiform papillae.

c. Relate the adaptive sensitivity of taste receptors for bitter and sour sensations, to sweet and salty sensations.

Module 15.3: Gustatory receptors and pathways

Mechanism of gustatory reception

Two types

Chemically gated ion channels whose stimulation produces depolarization of the cell and release of neurotransmitters

Salt and sour receptors

Taste receptor activates G-proteins (gustducins) that activate 2nd messenger system to release neurotransmitters

Sweet, bitter, and umami receptors

Figure 15.3 1

The mechanisms involved in gustatory reception

Sweet, Bitter, and Umami Receptors

Salt and Sour Receptors

Salt receptors and sour receptors are

chemically gated ion channels whose

stimulation produces depolarization of the


Receptors responding to stimuli that

produce sweet, bitter, and umami

sensations are linked to G proteins called

gustducins (GUST-doos-inz)—protein

complexes that use second messengers to

produce their effects.



Sweet, bitter,

or umami





Gated ion


Resting plasma



G protein


G protein

Channel opens

Plasma membrane


Plasma membrane



G protein


2nd messenger


2nd messenger

Depolarization of membrane stimulates

release of chemical neurotransmitters.

Activation of second messengers

stimulates release of chemical


Module 15.3: Gustatory receptors and pathways

Gustatory information is relayed to the cerebral cortex along three different cranial nerves dependent on the location of the receptor

Facial nerve (VII) – anterior 2/3 of tongue to line of circumvallate papillae

Glossopharyngeal nerve (IX) – circumvallate papillae and posterior 1/3 of tongue

Vagus nerve (X) – surface of epiglottis

Module 15.3: Gustatory receptors and pathways

Gustatory pathway

Receptors respond to stimulation

Relay information to appropriate cranial nerve

Sensory afferents synapse in solitary nucleus of medulla oblongata

Postsynaptic neuron axons cross over at medial lemniscus with other somatic sensory information and relay to thalamus

After synapse in thalamus, impulse is routed to appropriate area of primary sensory cortex

Figure 15.3 2

The components of the gustatory pathway

After another synapse in the thalamus,

the information is projected to the

appropriate portions of the gustatory

cortex of the insula.

The axons of the postsynaptic neurons

cross over and enter the medial

lemniscus of the medulla oblongata.

Cranial Nerves Carrying

Gustatory Information

The sensory afferents carried by

these three cranial nerves

synapse in the solitary

nucleus of the medulla


The facial nerve (VII)

innervates all the taste buds

located on the anterior

two-thirds of the tongue,

from the tip to the line of

circumvallate papillae.

The glossopharyngeal

nerve (IX) innervates the

circumvallate papillae and

the posterior one-third of

the tongue.

The vagus nerve (X)

innervates taste buds

scattered on the surface of

the epiglottis.


Receptors respond

to stimulation.

Module 15.3: Gustatory receptors and pathways

Central processing of gustatory sensations

Conscious perception of taste occurs at the primary sensory cortex

Taste sensation is analyzed with taste-related sensations

“Peppery” or “burning hot” from afferents in trigeminal nerve (V)

Olfactory stimulation significantly contributes to taste perception

Central adaptation quickly reduces sensitivity to new tastes

Module 15.3 Review

a. What are gustducins?

b. Identify the cranial nerves that carry gustatory information.

c. Trace the gustatory pathway from the taste receptors to the cerebral cortex.

Section 2: Equilibrium and Hearing

Learning Outcomes

15.4 Describe the structures of the external, middle, and inner ear, and explain how they function.

15.5 Describe the structures and functions of the bony labyrinth and membranous labyrinth.

15.6 Describe the functions of hair cells in the semicircular ducts, utricle, and saccule.

Section 2: Equilibrium and Hearing

Learning Outcomes

15.7 Describe the structure and functions of the organ of Corti.

15.8 Explain the anatomical and physiological basis for pitch and volume sensations for hearing.

15.9 Trace the pathways for the sensations of equilibrium and hearing to their respective destinations in the brain.

Section 2: Equilibrium and Hearing

Equilibrium and Hearing

Chemoreceptors compared to mechanoreceptors

Olfactory and gustatory receptors are located in epithelia exposed to the external environment

Olfactory receptors are modified neurons

Gustatory receptors communicate with sensory neurons

Equilibrium and hearing receptors are isolated and protected from external environment

Located in inner ear

Information is integrated and organized locally before forwarding to CNS

Figure 15 Section 2 1

Sensory receptors that are located within epithelia exposed to the

external environment





Section 2: Equilibrium and Hearing

Hair cell receptors of the inner ear

Free surfaces covered with specialized processes

80–100 stereocilia (like long microvilli)

May contain single large kinocilium

Hair cells are mechanoreceptors that are not actively moved

External forces push against processes causing distortion of cell membrane and neurotransmitter release

Provide information about direction and strength of mechanical stimuli

Complex inner ear structure determines what stimuli can reach different hair cells

Figure 15 Section 2 2 - 3

Inner ear

Location of the receptors for

equilibrium and hearing


in this direction

stimulates hair cell


in this direction

inhibits hair cell



Receptors for equilibrium

and hearing, which are

isolated and protected from

the external environment

Hair cell

Dendrite of

sensory neuron

Supporting cell

A hair cell, the receptor located in the inner ear

Module 15.4: Ear regions and structures

Three anatomical regions of the ear

External ear – visible portion that collects and directs sound waves toward middle ear


External acoustic meatus (passageway in temporal bone)

Lined with

Ceruminous glands (secrete waxy cerumen)


Has some protection against entering foreign objects, insects, and bacteria

Figure 15.4 1

The ear’s three anatomical regions: the external ear, the middle ear, and the inner ear

Middle Ear

External Ear

Inner Ear

The visible portion of the ear;

collects and directs sound waves

toward the middle ear

Site of sensory organs for hearing and

equilibrium; receives amplified sound

waves from the middle ear

An air-filled chamber; is connected to the

nasopharynx by the auditory tube

Elastic cartilages

Auditory ossicles

Semicircular canals

Petrous part of

temporal bone


Facial nerve (VII)


nerve (VIII)

Bony labyrinth





Tympanic membrane

(tympanum or eardrum)

Auditory tube

(pharyngotympanic tube

or Eustachian tube)




Module 15.4: Ear regions and structures

Three anatomical regions of the ear (continued)

Middle ear

Tympanic membrane (also tympanum or eardrum)

Border between external and middle ear

Thin, transparent sheet

Auditory ossicles (3)

Bones that connect tympanic membrane to receptor complexes of inner ear (smallest bones and synovial joints of body)

Malleus (attached at three points to tympanum)

Incus (middle ossicle)

Stapes (bound to oval window of cochlea)

Module 15.4: Ear regions and structures

Three anatomical regions of the ear (continued)

Middle ear (continued)

Auditory tube (pharyngotympanic or Eustachian tube)

Connects middle ear to pharynx to equalize pressure on either side of tympanic membrane

Also can lead to bacterial infection (otitis media)

Middle ear muscles

Tensor tympani muscle

Connects to malleus and can dampen tympanic membrane vibrations

Stapedius muscle (smallest muscle in body)

Connects to stapes and reduces its movement at oval window

Figure 15.4 2

The structures of the middle ear

Auditory Ossicles




Temporal bone

(petrous part)





Branch of

facial nerve

VII (cut)

Muscles of the Middle Ear

Tensor tympani muscle




Stapedius muscle

Tympanic cavity

(middle ear)

Auditory tube

Round window

Tympanic membrane

Module 15.4: Ear regions and structures

Sound impulse pathway

Sound waves vibrate tympanic membrane

Tympanic membrane and ossicles amplify and conduct vibrations to oval window of inner ear

Vibrations can be dampened by actions of middle ear muscles

Animation: The Ear: Balance

Animation: The Ear: Ear Anatomy

Module 15.4 Review

a. Name the three tiny bones located in the middle ear.

b. What is the function of the auditory tube?

c. Why are external ear infections relatively uncommon?

Module 15.5: Labyrinths of the inner ear

Bony labyrinth

Shell of dense bone

Surrounds and protects membranous labyrinth

Filled with perilymph (similar to CSF)

Consists of three parts

Semicircular canals



Module 15.5: Labyrinths of the inner ear

Membranous labyrinth

Collection of fluid-filled tubes and chambers

Houses receptors for hearing and equilibrium

Filled with endolymph

Receptors only function when exposed to unique ionic composition

Module 15.5: Labyrinths of the inner ear

Membranous labyrinth (continued)

Consists of three parts

Semicircular ducts (within semicircular canals)

Receptors stimulated by rotation of head

Utricle and saccule (within vestibule)

Provide sensations of gravity and linear acceleration

Cochlear duct (within cochlea)

Sandwiched between pair of perilymph-filled chambers

Resembles snail shell

Receptors stimulated by sound

Figure 15.5 1 – 2

The structures of the inner ear

Bony Labyrinth

Surrounds and protects the membranous labyrinth and

contains a fluid called perilymph

Semicircular canals





Membranous Labyrinth

Houses the receptors for equilibrium

and hearing and contains a fluid

called endolymph

Semicircular duct

Utricle and saccule

Cochlear duct

Bony labyrinth


Membranous labyrinth





Bony labyrinth

A cross section of a semicircular canal

Module 15.5 Review

a. Identify the components of the bony labyrinth.

b. Describe hair cells.

c. Explain the regional differences among the receptor complexes in the membranous labyrinth.

Module 15.6: Receptors for equilibrium

Receptors for equilibrium

Semicircular ducts


Three ducts continuous with utricle and filled with endolymph




Each contains an enlarged region (ampulla) with area (crista) housing receptors

Hair cells with kinocilia and stereocilia embedded within gelatinous matrix

Figure 15.6 1

The location and structure of an ampulla, which

contains receptors that respond to rotation

The location of the ampullae within the inner ear

Semicircular Ducts







Ampulla filled

with endolymph

Hair cells


Supporting cells

Sensory nerve

The structure of an ampulla

Module 15.6: Receptors for equilibrium

Semicircular ducts


Head rotating in plane of one duct causes endolymph movement and cupula bends, causing distortion of hair cells

Movement one way causes stimulation

Opposite movement causes inhibition

Horizontal rotation (“no”) stimulates lateral duct receptors

Nodding (“yes”) stimulates anterior duct receptors

Tilting head stimulates posterior duct receptors

Figure 15.6 2

Direction of

duct rotation

Direction of

duct rotation

Direction of relative

endolymph movement

Semicircular duct


At rest

The response when the head rotates in the plane of a semicircular duct

Figure 15.6 3

Anterior semicircular

duct for “yes”



duct for “no”

Posterior semicircular

duct for tilting head to the side

The analysis of complex movements, which involves the response of each

semicircular duct to rotational movements

Module 15.6: Receptors for equilibrium

Utricle and saccule

Provide equilibrium sensations, whether body is stationary or moving

Connected by slender passageway continuous with endolymphatic duct ending in endolymphatic sac

After being secreted in cochlear duct, endolymph returns to general circulation in endolymphatic sac

Module 15.6: Receptors for equilibrium

Utricle and saccule (continued)

Contain hair cells clustered in maculae

Processes embedded in gelatinous mass with calcium carbonate crystals (statoconia; statos, standing + conia, dust)

Whole complex called otolith (“earstone”)

With head in upright position, statoconia sit on hair cells compressing, but not bending, them

With tilted position or with linear movement, otolith movement bends hair cell processes stimulating macular receptors

Figure 15.6 4 – 5

The anatomy of equilibrium receptors in the utricle and saccule


Endolymphatic sac

The locations of the utricle and

saccule in the inner ear





Gelatinous material

An otolith, which consists of a

gelatinous matrix and statoconia



Nerve fibers

Figure 15.6 6

The effects of gravity and linear acceleration on the hair cell processes in the maculae

When your head is tilted, the pull of gravity on the statoconia shifts

them to the side, thereby distorting the hair cell processes and stimu-

lating the macular receptors. A similar mechanism accounts for your

perception of linear acceleration, as when your car speeds up sud-

denly. The statoconia lag behind, and the effects on the hair cells is

comparable to tilting your head back.

When your head is in the normal, upright

position, the statoconia sit atop the macula.

Their weight presses on the macular

surface, pushing the hair cell processes

down rather than to one side or another.






distorting hair

cell processes




Module 15.6 Review

a. Define statoconia.

b. Cite the function of receptors in the saccule and utricle.

c. Damage to the cupula of the lateral semicircular duct would interfere with what perception?

Module 15.7: Receptors for hearing

Receptors for hearing

Cochlear duct

Lies between pair of perilymphatic chambers

Vestibular duct (separated by vestibular membrane)

Tympanic duct (separated by basilar membrane)

Both ducts connect at tip of cochlear spiral creating one long chamber

Begins at oval window

Ends at round window

Hair cells for hearing located in organ of Corti on basilar membrane

Animation: The Ear: Receptor Complexes

Figure 15.7 1

Round window

The location of the cochlea in the inner ear

Stapes at

oval window

Vestibular duct

Cochlear duct

Tympanic duct






nerve (VIII)




From oval window

to tip of spiral

From tip of spiral

to round window

Figure 15.7 2

A cross section of the cochlea

From oval


Vestibular membrane

Basilar membrane

Vestibular duct

Organ of Corti

Cochlear duct

Tympanic duct

Temporal bone

(petrous part)

Cochlear nerve

Vestibulocochlear nerve (VIII)

To round


Figure 15.7 2

Sectional view of the

cochlear spiral

LM x 200

Module 15.7: Receptors for hearing

Organ of Corti

Hair cells arranged in longitudinal rows

Lack kinocilia

In contact with overlying tectorial membrane

Sound waves cause pressure waves within perilymph, vibrating the basilar membrane and organ of Corti

Hair cells press into tectorial membrane and are distorted/stimulated

Sensory neurons relay the message through the spiral ganglion and cochlear branch of vestibulocochlear nerve (VIII)

Figure 15.7 3

A sectional view showing a single

turn of the cochlea

Bony cochlear wall

Spiral ganglion

Vestibular duct

Vestibular membrane

Cochlear duct

Basilar membrane

Tympanic duct

Cochlear branch of the


nerve (VIII)

Organ of Corti

Figure 15.7 4

The structure of the organ of Corti

Tectorial membrane


hair cell

Inner hair cell

Basilar membrane

Nerve fibers

Figure 15.7 4


Pressure wave

in perilymph

At rest

The distortion of hair cells in response to pressure changes within the

perilymph triggered by sound waves arriving at the tympanic membrane

Module 15.7 Review

a. Where is the organ of Corti located?

b. Name the fluids found within the vestibular duct, tympanic duct, and cochlear duct.

c. Identify the features visible in the LM sectional view of the cochlear spiral.

Module 15.8: Sensations of pitch and volume

Physical characteristics of sound

Consists of waves of pressure conducted through a medium

In air, a pressure wave consists of alternating regions of compressed and separated molecules

Wavelength of sound

Distance between two adjacent wave crests (peaks) or between two adjacent wave troughs

Figure 15.8 1

Sounds, which consist of pressure waves

conducted through a medium such as air

Wavelength of

a sound

Air molecules



Tuning fork

Module 15.8: Sensations of pitch and volume


Sensory response to wave frequency

Frequency = number of waves passing a fixed point in a given time

Often measured in waves or cycles/sec called hertz (Hz)

Pitch and wave frequency are directly related

Example: high-pitched sound might be 15,000 Hz while low-pitched sound might be 100 Hz or less

All sound travels at same speed so as frequency increases, wavelength must become shorter

Figure 15.8 2

A graph showing the relationships among the

characteristics of sound waves.

Wavelength, which is

inversely related to


Amplitude of a sound

1 wavelength

Sound energy arriving at

tympanic membrane

Time (sec)

Module 15.8: Sensations of pitch and volume

Volume or intensity (energy in sound waves)

Energy variation in sound is represented by changes in wave height or amplitude

Wave amplitude = wave energy = perceived loudness

Directly related

Sound energy reported in decibels (dB)

Figure 15.8 3

Module 15.8: Sensations of pitch and volume

Sound wave energy causes movements of flexible structures in the ear

At a particular frequency and amplitude, object will vibrate at same frequency

= Resonance

Examples: tympanic membrane, basilar membrane

Module 15.8: Sensations of pitch and volume

Basilar membrane flexibility changes along length

Different sound frequencies vibrate different areas of basilar membrane

Location = pitch

Number of hair cells stimulated: volume

As stapes pushes on oval window, basilar membrane distorts toward round window

Opposite action when stapes retracts

Figure 15.8 4

The movement of the flexible basilar membrane in response to

sound waves with a frequency of 6000 Hz



at oval


1000 Hz

16,000 Hz

6000 Hz



Basilar membrane




Basilar membrane distorts

toward round window












Basilar membrane distorts

toward oval window

Figure 15.8 5

Events Involved in Hearing

Information about

the region and the

intensity of

stimulation is

relayed to the CNS

over the cochlear

branch of cranial

nerve VIII.


waves arrive

at the



Movement of the

stapes at the oval



pressure waves

in the perilymph

of the vestibular


Movement of

the tympanic



displacement of

the auditory


The pressure

waves distort the


membrane on

their way to the

round window of

the tympanic


Vibration of the



causes vibration

of hair cells

against the



Tympanic duct

Basilar membrane

Cochlear duct

Vestibular membrane


of sound


Vestibular duct





Module 15.8 Review

a. Define decibel.

b. Beginning at the external acoustic meatus, list, in order, the structures involved in hearing.

c. How would sound perception be affected if the round window could not bulge out as a result of increased perilymph pressure?

Module 15.9: Sensory pathways for equilibrium and hearing

Sensory pathway for equilibrium

Hair cells in vestibule and semicircular canals become stimulated

Sensory neurons take information through vestibular branch of vestibulocochlear nerve (VIII)

Vestibular nuclei integrate information from both ears and relay to cerebral cortex, cerebellum, and motor nuclei of brain stem and spinal cord

Module 15.9: Sensory pathways for equilibrium and hearing

Motor responses for equilibrium

Automatic movements of eyes

Directed by superior colliculi

Keep gaze focused on point despite movement

Distribution of motor commands to motor nuclei for cranial nerves controlling eye, head, neck movements (CN III, IV, VI, XI)

Vestibular nuclei relay information to cerebellum

Information relayed down vestibulospinal tracts of spinal cord to adjust peripheral muscle tone and coordinate with head and neck movements

Figure 15.9 1

The path of equilibrium information from receptors to the brain stem to muscular effectors

Sensory neurons located in

adjacent vestibular ganglia

carry information from the

hair cells. These sensory

fibers form the vestibular

branch of the

vestibulocochlear nerve (VIII).

The automatic movements of

the eyes that occur in

response to sensations of

motion are directed by the

superior colliculi. These

movements attempt to keep

your gaze focused on a

specific point in space,

despite changes in body

position and orientation.

The vestibular nuclei in the

medulla oblongata integrate

sensory information from both

ears, and relay that information

to the cerebral cortex,

cerebellum, and motor nuclei in

the brain stem and spinal cord.

Hair cells of

the vestibule



ducts monitor

body position

and motion.

The reflexive motor

commands issued by the

vestibular nuclei are

distributed to the motor

nuclei for the cranial verves

involved with eye, head, and

neck movements (N III, N IV,

N VI, and N XI).



The vestibular nuclei relay

information about position

and balance to the





Instructions descending in

the vestibulospinal tracts of

the spinal cord adjust

peripheral muscle tone and

complement the reflexive

movements of the head or



nerve (VIII)



Figure 15.9 2

Module 15.9: Sensory pathways for equilibrium and hearing

Sensory pathway for hearing

Hair cells in specific area of basilar membrane become stimulated

Sensory neurons relay information through cell bodies in spiral ganglion to cochlear branch of vestibulocochlear nerve (VIII)

Information reaches cochlear nuclei of medulla oblongata and ascends to midbrain

From inferior colliculi of midbrain, auditory sensations synapse at medial geniculate nucleus of thalamus

Projection fibers relay information to different areas of auditory cortex in temporal lobe

Module 15.9: Sensory pathways for equilibrium and hearing

Sensory pathway for hearing

Most auditory information from one cochlea is projected to the auditory cortex on the opposite side

Some information from cochlea on the same side also is received by auditory cortex

These interconnections aid in localizing sounds and reduce functional impact of damage to a cochlea or ascending pathway

Module 15.9: Sensory pathways for equilibrium and hearing

Motor responses for hearing

Inferior colliculi coordinate a number of reflexive responses involving skeletal muscles of head, neck, and trunk

Reflexes automatically change position of head in response to a noise (usually toward source)

Module 15.9 Review

a. Where are the hair cells for equilibrium located?

b. Which cranial nerves are involved with eye, head, and neck movements?

c. What is your reflexive response to hearing a loud noise, such as a firecracker?

Section 3: Vision

Learning Outcomes

15.10 Identify the accessory structures of the eye and explain their functions.

15.11 Describe the layers of the wall of the eye and the anterior and posterior cavities of the eye.

15.12 Explain how light is directed to the fovea of the retina.

15.13 Describe the process by which images are focused on the retina.

Section 3: Vision

Learning Outcomes

15.14 Describe the structure and function of the retina’s layers of cells, and the distribution of rods and cones and their relation to visual acuity.

15.15 Explain photoreception, describe the structure of the photoreceptors, explain how visual pigments are activated, and describe how we are able to distinguish colors.

15.16 Explain how the visual pathways distribute information to their destinations in the brain.

Section 3: Vision

Learning Outcomes

15.17 CLINICAL MODULE Describe various accommodation problems associated with the cornea, lens, or shape of the eye.

15.18 CLINICAL MODULE Describe age-related disorders of olfaction, gustation, vision, equilibrium, and hearing.

Section 3: Vision

Eye development

Optic vesicles form in prosencephalon lateral walls

Contain cavity continuous with neurocoel

Optic cups form as lateral bulges become indented

Remain connected to diencephalon by slender stalks

Overlying epidermis pinches off and becomes lens

Retina develops

Ependymal cells on optic cup outer wall become photo receptors

Ependymal cells on optic cup inner wall become pigmented cells

Neural tissue on optic cup outer wall becomes neurons, ganglion cells, and specialized glial cells

Section 3: Vision

Eye development (continued)

Supporting layers of connective tissue develop from aggregating mesoderm around optic cup

Fluid-filled interior chambers develop

Figure 15 Section 3 1

The formation of optic

vesicles in the lateral

walls of the


Optic vesicle

Figure 15 Section 3 2

Layers of Developing Retina

The ependymal cells on the outer wall of the

optic cup develop into the photoreceptors.

The formation of optic cups that

remain connected

to the diencephalon

by slender stalks

Optic cup

The ependymal cells on the inner wall of the

optic cup develop into pigment cells that

absorb light that has passed through the

photoreceptor layer. The separation gradually

decreases until the photoreceptors and

pigment cell layers are in contact.

Developing lens

The neural tissue of the outer wall of the optic

cup forms layers of neurons, ganglion cells,

and specialized glial cells that are responsible

for preliminary processing and integration of

visual information.

Week 5

Figure 15 Section 3 3

The development of interior

chambers filled with fluid

that is continuously

generated and reabsorbed

Optic nerve, N II (follows path of

original connecting stalk)



Fluid-filled chambers

Connective tissue layers

Week 6

Module 15.10: Eye accessory structures

Eye accessory structures

Eyelids (palpebrae)

Eyelashes (hairs that help prevent foreign particles from reaching the eye)

Medial canthus (medial connection)

Lateral canthus (lateral connection)

Tarsal glands (along inner margin of lids, secretion keeps eyelids from sticking together)

Module 15.10: Eye accessory structures

Eyelids (continued)

Palpebral fissure (space between eyelids)

Eye structures seen within:

Cornea (transparent anterior surface)

Iris (colored part of eye)

Pupil (hole that light passes through in center of iris)

Lacrimal caruncle (produces thick secretion during sleep)

Figure 15.10 1

The accessory structures of the eye


Eyelids and Eyelashes


Lateral canthus

Palpebra (eyelid)

Medial canthus

Pupil within iris

Palpebral fissure

Lacrimal caruncle

Module 15.10: Eye accessory structures


Epithelium lining surface of eye and inner eyelids

Ocular conjunctiva

Continuous with thin corneal conjunctiva

Palpebral conjunctiva

Fornix (pocket where conjunctivae meet)

Mucous membrane covered by stratified squamous epithelium


Swelling associated with conjunctiva due to damage to/irritation of conjunctiva

May be caused by infection, physical, allergic, or chemical irritation

Figure 15.10 2

The conjunctiva, the specialized epithelium

covering the inner surfaces of the eyelids

and the outer surface of the eye

Tarsal glands

(Meibomian glands)


Palpebral conjunctiva

Ocular conjunctiva



Figure 15.10 4

Conjunctivitis, or pinkeye

Module 15.10: Eye accessory structures

Lacrimal apparatus components

Lacrimal gland

Almond-shaped gland that produces tears (~1 mL/day)

Tear ducts (10–12)

Deliver tears from gland to under upper eyelid

Lacrimal puncta

Two small pores that drain lacrimal lake

Module 15.10: Eye accessory structures

Lacrimal apparatus components (continued)

Lacrimal canaliculi

Small canals connecting puncta to lacrimal sac

Lacrimal sac

Chamber in lacrimal sulcus of orbit

Nasolacrimal duct

Delivers tears from lacrimal sac into nasal cavity inferior and lateral to inferior nasal concha

Animation: The Eye: Accessory Structures

Figure 15.10 3

The lacrimal apparatus


rectus muscle

Components of the Lacrimal Apparatus

Lacrimal gland

Tear ducts

Upper eyelid

Lacrimal puncta

Lower eyelid

Lacrimal canaliculi

Lacrimal sac

Orbital fat

Nasolacrimal duct

Inferior rectus muscle

Drainage of the nasolacrimal duct

into the inferior meatus

Inferior oblique muscle

Module 15.10: Eye accessory structures

Function of tears

Reduce friction

Remove debris

Prevent bacterial infection with lysozyme and antibodies

Provide nutrients and oxygen to portions of conjunctiva

Module 15.10 Review

a. List the accessory structures associated with the eye.

b. Explain conjunctivitis.

c. Which layer of the eye would be the first affected by inadequate tear production?

Module 15.11: Eye layers and cavities

Three layers of the eye (tunics)

Fibrous tunic

Outermost layer of eye

Consists of cornea (clear) and sclera (white)

Joined at corneal limbus


Provides mechanical support and some physical protection

Attachment site for extrinsic eye muscles

Contains structures assisting in focusing process

Module 15.11: Eye layers and cavities

Three layers of the eye (continued)

Vascular tunic (uvea)

Iris (colored part of eye that controls size of pupil)

Anterior: incomplete layer of fibroblasts and melanocytes

Posterior: pigmented epithelium of neural tunic

Color determined by:

Genes influencing density and distribution of melanocytes

Density of pigmented epithelium

Blue: less melanin, light reaches pigmented layer

Green, brown, black: increasing melanin

Module 15.11: Eye layers and cavities

Three layers of the eye (continued)

Vascular tunic (continued)

Ciliary body (thickened region connecting to lens)

Ciliary muscle (smooth-muscle ring)

Ciliary processes (epithelial folds covering muscle)

Suspensory ligaments (attach to lens)

Choroid (vascular layer covered by sclera)

Contains extensive capillary network delivering oxygen and nutrients to neural tissue in neural tunic

Animation: The Eye: Uvea Parts

Module 15.11: Eye layers and cavities

Three layers of the eye (continued)

Neural tunic (retina)

Innermost layer of the eye

Two layers

Pigmented layer (outer)

Absorbs light

Ora serrata (jagged anterior edge)

Neural layer (inner)

Contains photoreceptors sensitive to light

Animation: The Eye

Figure 15.11 1

The three tunics of the wall of the eye

Fibrous Tunic

The outermost layer of the eye, consisting of the cornea

and the sclera

Corneal limbus



Vascular Tunic

The middle layer of the eye; also

called the uvea; contains numerous

blood vessels, lymphatic vessels,

and the intrinsic (smooth) muscles

of the eye



Optic nerve

Ciliary body


Neural Tunic

The innermost layer of the eye; also known as the retina;

contains photoreceptors

Figure 15.11 2

A sectional view showing that the

ciliary body and the lens divide the

interior of the eye into a small

anterior cavity and a large

posterior cavity

Anterior Cavity


Anterior chamber


Ciliary body


Posterior chamber

Posterior Cavity

Contains the vitreous body

Optic nerve

Module 15.11: Eye layers and cavities

Eye cavities

Anterior cavity (cornea to lens)

Filled with aqueous humor

Two chambers

Anterior chamber (cornea to iris)

Posterior chamber (iris to ciliary body and lens)

Posterior cavity (main volume of eye)

Filled with gelatinous vitreous body (humor)

Module 15.11: Eye layers and cavities

Aqueous humor

Secreted by epithelia of ciliary processes

Rate of 1–2 µL/min

Similar to CSF

Circulates between anterior and posterior chambers

Distributes nutrients and wastes

Acts as fluid cushion

Helps to retain eye shape

Reabsorbed at canal of Schlemm (at corneal limbus)

Into veins in sclera

Reabsorption rate is approximately the same as production

Tonometry (measures intraocular pressure)

Figure 15.11 3 – 4

A view of the anterior and posterior cavities of the eye

Site of secretion

of aqueous humor



Ciliary muscle



Canal of Schlemm



Ciliary processes

Posterior cavity

(vitreous chamber)

Suspensory ligaments

Ora serrata

Module 15.11 Review

a. Name the three tunics of the eye.

b. What give eyes their characteristic color?

c. Where in the eye is aqueous humor located?

Module 15.12: Visual axis structures

Visual axis

Imaginary line drawn from object in view through structures of the eye to retina



Dense matrix of collagen fibers organized to permit light

Has no blood vessels

Must get oxygen and nutrients from tears

Animation: The Eye: Light Path

Module 15.12: Visual axis structures

Visual axis (continued)

Structures (continued)

Pupil (size controlled by two sets of muscles)

Pupillary dilator muscles

Extend radially from pupil edge

Contraction enlarges pupil

Stimulated by sympathetic nervous system

Pupillary constrictor muscles

Concentrically arranged around pupil

Contraction constricts pupil

Stimulated by parasympathetic nervous system

Animation: The Eye: Interior Parts of Eye

Module 15.12: Visual axis structures

Visual axis (continued)

Structures (continued)


Concentric layers of cells

Cells filled with transparent proteins (crystallins)

Give clarity and focusing power

Dense fibrous capsule

Connects to suspensory ligaments

Animation: The Eye: Lens and Retina

Module 15.12: Visual axis structures

Visual axis (continued)

Structures (continued)


Macula lutea (highest photoreceptor concentration)

Contains fovea (shallow depression) that is the site of sharpest vision

Figure 15.12 1

A sectional view showing aspects of eye anatomy associated with

positioning the eye and allowing light to reach the

photoreceptors of the retina

Cornea: transparent and lacks blood vessels

Lens: consists of concentric layers of cells

filled with transparent proteins called


Suspensory ligaments: resist the

tendency of the lens to assume a

spherical shape


Ciliary body: supports the lens and

controls its shape.

Retina: contains the photoreceptors, pigment

cells, supporting cells, and neurons

Blood vessels of the choroid: directly or

indirectly provide nutrients to all

structures within the eye.

Sclera (“white of the eye”): stabilizes the

shape of the eye during eye movements

and is site of insertion of the six extrinsic

eye muscles

Optic nerve (N II): carries visual

information to the brain

Figure 15.12 2

How the two layers of the pupillary muscles of the iris control the amount of light

entering the eye

Pupillary constrictor


Pupillary dilator


The pupillary constrictor

muscles form a series of

concentric circles around

the pupil. When these

sphincter muscles contract,

the diameter of the pupil


The pupillary dilator

muscles extend radially

away from the edge of the

pupil. Contraction of these

muscles enlarges the pupil.

Decreased light intensity

Increased sympathetic stimulation

Increased light intensity

Increased parasympathetic stimulation

Figure 15.12 1

How light passing along the visual axis of the eye

strikes a specific location that contains the

highest density of photoreceptors


Visual axis of the eye


Neural portion of the retina:

site of the photoreceptors




Fovea in center of macula lutea:

site of sharpest vision

Orbital fat

Module 15.12 Review

a. Which eye structure does not contain blood vessels?

b. List the structures and fluids that light passes through from the cornea to the retina.

c. What happens to the pupils when light intensity decreases?

Module 15.13: Focusing at the retina

Creating a focused image of an object involves bending (refracting) light rays together to create a focal point on the retina

Two refracting structures of eye



Focal distance (distance between center of lens and focal point)

Determined by distance from object to lens and shape of lens

Accommodation (changing lens shape to keep focal distance constant)

Figure 15.13 1

Focal distance

Focal distance, which is determines by the

shape of the lens and the distance between

the lens and the object being viewed






Focal distance






The closer the light source,

the longer the focal distance

Focal distance

The rounder the lens,

the shorter the focal distance

Module 15.13: Focusing at the retina


Lens shape changed by ciliary muscle action

For close vision:

Ciliary muscle contracts, moving toward lens 

suspensory ligaments’ tension relaxes lens 

lens shape becomes more spherical 

lens increases refractive power

Near point of vision (inner limit of clear vision)

Increases with age as lens becomes inflexible

Module 15.13: Focusing at the retina

  • Accommodation (continued)

    • Lens shape changed by ciliary muscle action (continued)

      • For distance vision:

        • Ciliary muscle relaxes 

          suspensory ligament tension increases 

          lens becomes flatter  lens decreases refractive power

Animation: The Eye: Cilliary Muscles

Figure 15.13 2

Accommodation, the process by which the eye focuses images on the retina by changing the shape of the lens to

keep the focal distance constant

Accommodation when objects viewed are far from

the eye

Accommodation when objects viewed are near the eye

For Close Vision: Ciliary Muscle Contracted,

Lens Rounded

For Distant Vision: Ciliary Muscle Relaxed, Lens


Focal point

on fovea

When the ciliary muscle contracts, the ciliary body moves

toward the lens, thereby reducing the tension in the

suspensory ligaments. The elastic capsule of the lens then

pulls it into a more spherical shape that increases the

refractive power of the lens, enabling it to bring light from

nearby objects into focus on the retina.

When the ciliary muscle relaxes, the suspensory ligaments

pull at the circumference of the lens, making the lens flatter

and bringing the image of a distant object into focus on the


Module 15.13: Focusing at the retina

Image formation

Most objects are more than just one point

Full images invert and reverse passing through lens

Brain compensates during processing

Figure 15.13 3

The inversion and reversal of images projected onto the retina

A sagittal section through an eye showing

the inversion of the image of a viewed object

A sagittal section through an eye showing

the reversal of the image of a viewed object

Module 15.13 Review

a. Define focal point.

b. When the ciliary muscles are relaxed, are you viewing something close up or something in the distance?

c. Why does the near point of vision typically increase with age?

Module 15.14: Retinal layers and cells

Retinal layers

Pigmented part

Absorbs light passing through neural part

Keeps light from bouncing around retina

Has important biochemical interactions with neural part

Neural part

Ganglion cells (innermost layer)

Axons converge on optic disc

Also called blind spot since lacking photoreceptors

Axons exit eye through optic nerve

Animation: The Eye: Blind Spot

Figure 15.14 1

A diagrammatic sectional view through the eye

showing the retina near the origin of the optic nerve

Neural Part of the Retina

Pigmented Part of the Retina

Contains photoreceptors, supporting cells,

and neurons that perform preliminary

processing and integration of visual


Absorbs light that passes through the neural part,

preventing light from bouncing back and producing

visual “echoes”

Layer closest to the pigmented part of the

retina; contains the photoreceptors

Ganglion cells

Optic disc (blind spot)

Central retinal vein

Central retinal artery

Optic nerve

Blood vessels entering and leaving

the interior of the eye within the

optic nerve



Figure 15.14 2

A photograph of the retinal surface, taken through the cornea, pupil, and lens of the right eye

Optic disc

(blind spot)


Macula lutea

Central retinal artery and vein

emerging from center of optic disc

Module 15.14: Retinal layers and cells

Retinal layers (continued)

Neural part (continued)

Bipolar cells

Synapse with photoreceptors and ganglion cells

Amacrine cells

Facilitate or inhibit communication between ganglion cells and bipolar cells

Module 15.14: Retinal layers and cells

Retinal layers (continued)

Neural part (continued)

Horizontal cells

Facilitate or inhibit communication between photoreceptors and bipolar cells

Photoreceptors (outermost layer)

Rods (low light, monochromatic vision)

Cones (high light, color vision)

Animation: The Eye: The Retina

Figure 15.14 3

A sectional view showing the retina’s multiple

layers of specialized cells

Pigmented part of retina

Photoreceptors of the Retina

Rods (for vision under dimly

lit conditions)

Horizontal and Amacrine Cells

Facilitate or inhibit communication

between photoreceptors and

ganglion cells, thereby altering the

sensitivity of the retina

Cones (provide color vision

under brightly lit conditions)

Horizontal cells

Amacrine cells

Bipolar cells

Ganglion cells


Module 15.14: Retinal layers and cells

Photoreceptor distribution in retina


Maximum density at fovea (no rods)

Has highest visual acuity (sharp vision)

~6 million in retina overall


Maximum density at retina periphery

~125 million in retina overall

Figure 15.14 4

The retina of each eye contains approximately

6 million cones. The density of cones reaches

its maximum at the fovea of the macula lutea,

where there are no rods.

Low Density of Cones

High Density of Cones

Optic disc


The retina contains approximately 125 million

rods. The density of rods is highest at the

periphery of the retina, where there are very

few cones.

Low Density of Rods

High Density of Rods

Visual acuity

Plot of sharpness of vision (visual acuity) along the

horizontal line; note the direct correlation between

visual acuity and cone density


Lateral border

Nasal border

The relative densities of cones and rods on either

side of a horizontal line passing through the fovea

and optic disc of the right eye

Module 15.14 Review

a. Define rods and cones.

b. If you enter a dimly lit room, will you be able to see clearly? Why or why not?

c. If you had been born without cones in your eyes, explain why you would or would not be able to see.

Module 15.15: Photoreception


Detect photons of light

Light energy also occurs as a wave

Our visible spectrum of light is 400–700 nm

Contain visual pigments that transduce light

Are derivatives of rhodopsin (pigment in rods)

Consist of:

Retinal (pigment synthesized from vitamin A)

Opsin (protein that determines wavelength absorption of pigment)

Figure 15.15 2





The structure of rhodopsin (visual

purple), the visual pigment found

in rods

Module 15.15: Photoreception

Photoreceptor structure

Outer segment

Contains flattened membranous plates or discs

Contain visual pigment

In cones, are plasma membrane infoldings

In rods, each disc is separate entity

In cones, tapered

In rods, elongate cylinder

Inner segment

Contains organelles for maintaining cell

Figure 15.15 1

Structure of Cones

Structure of Rods

Pigment Epithelium

Discs of cones: are infoldings of the

plasma membrane and taper to a

blunt point

The pigment

epithelium absorbs

photons that are not

absorbed by visual


Discs of rods: are independent

entities and form an elongated


Melanin granules

Outer Segment

The outer segment of a

photoreceptor contains

flattened membranous

plates, or discs, that contain

the visual pigments.




Inner Segment


The inner segment contains

the photoreceptor’s major

organelles and is responsible

for all cell functions other

than photoreception.






Each photoreceptor

synapses with a bipolar cell.

Bipolar cell


The major structural features of rods and cones, and the adjacent

pigment epithelium and bipolar cells

Module 15.15: Photoreception

Steps of phototransduction

Light absorption makes retinal molecule more linear

Opsin activity changes Na+ outer segment permeability and changes neurotransmitter release to bipolar cell

Bipolar cell activity changes are relayed to ganglion cells

Steps of photopigment regeneration

Pigment breaks down into retinal and opsin (= bleaching)

Retinal converted to original shape (requires ATP)

Converted retinal can recombine with opsin

Animation: Photoreception

Module 15.15: Photoreception

Color vision

Three types of cones

Blue cones (16% of all cones)

Green cones (10%)

Red cones (74%)

Combined differential stimulation allows brain to discern colors

All stimulated equally = white

Figure 15.15 4

The range of wavelength

sensitivities for the three

types of cones, each of

which contains a different

form of opsin








Light absorption

(percent of maximum)







Module 15.15 Review

a. Identify the three types of cones.

b. Compare rods with cones.

c. How could a diet deficient in vitamin A affect vision?

Module 15.16: Visual pathways

Visual pathway

Receptors  bipolar cells  ganglion cells

~1 million ganglion cells converge at optic disc

Optic nerves converge at optic chiasm to reach thalamus

At thalamus

Half of the fibers to lateral geniculate nucleus on same side

Half of the fibers to lateral geniculate nucleus on opposite side

Fibers radiate to visual cortex of occipital lobe

= Optic radiation

Figure 15.16

How the visual pathways transmit information

from both eyes to the visual cortex of each

cerebral hemisphere

Combined Visual Field

Right side

Left side

Left eye


Right eye


Binocular vision

The Visual Pathways

The visual pathways begin at the

photoreceptors in the retina. Each

photoreceptor monitors a specific receptive

field, and when stimulated, passes the

information through a bipolar cell and to a

ganglion cell.

Axons from the approximately 1 million

ganglion cells converge on the optic disc,

penetrate the wall of the eye, and proceed

toward the diencephalon as the optic nerve (II).


Optic disc

The two optic nerves, one from each eye, reach

the diencphalon at the optic chiasm.

From that point, approximately half the fibers

proceed toward the lateral geniculate nucleus of

the same side of the brain, whereas the other

half cross over to reach the lateral geniculate

nucleus of the opposite side.

Collaterals from fibers

synapsing in the lateral

geniculate nuclei



brain stem

Optic tract

From each lateral geniculate nucleus, visual

information travels to the occipital cortex of the

cerebral hemisphere on that side. The bundle of

projection fibers linking each lateral geniculate

nucleus with the visual cortex is known as the

optic radiation.

Lateral geniculate




Projection fibers

(optic radiation)

The perception of a visual image reflects the

integration of information that arrives at the

visual cortex of the occipital lobes. Each eye

receives a slightly different visual image,

because (1) the foveae are 5–7.5 cm (2–3.0 in.)

apart, and (2) the nose and eye socket block the

view of the opposite side.

Right cerebral


Left cerebral


Module 15.16: Visual pathways

Depth perception

Interpretation of 3-D relationships of objects in view

Obtained by comparing relative positions of objects between images from both eyes

Perception varies due to position of each eye

Each visual cortex receives overlapping visual field information for both eyes

Module 15.16 Review

a. Define optic radiation.

b. Where are visual images perceived?

c. Trace the visual pathway, beginning at the photoreceptors in the retina.

CLINICAL MODULE15.17: Accommodation problems

Accommodation problems

Emmatropia (emmetro-, proper + opia, vision)

Normal vision

Distant image focused on retinal surface

Ciliary muscle relaxed and lens flattened

Myopia (myein, to shut + ops, eye)


Focal distance too short (in front of retina)

Cause may be:

Eyeball too deep

Resting curvature of lens too great

Corrected with diverging (concave) lens in front of eye

CLINICAL MODULE15.17: Accommodation problems



Focal distance too long (in back of retina)

Cause may be:

Eyeball is too shallow

Lens is too flat

Corrected with converging (convex) lens in front of eye

Figure 15.17

The shape of the eye and the site at which light is focused for three conditions

Myopia, or nearsighted vision

Enmetropia, or normal vision





In the normal healthy eye, when

the ciliary muscle is relaxed and

the lens is flattened, the image of

a distant object will be focused on

the retina’s surface. This condition

is called emmetropia (emmetro-,

proper + opia, vision), or normal


If the eyeball is too deep or the resting

curvature of the lens is too great, the image

of a distant object is projected in front of the

retina. Such individuals are said to be

nearsighted because vision at close range is

clear but distant objects are blurry and out

of focus. Their condition is more formally

termed myopia (myein, to shut + ops, eye).

Myopia can be treated by

placing a diverging lens in

front of the eye. Diverging

lenses have at least one

concave surface and spread

the light rays apart as if the

object were closer to the


Hyperopia, or farsighted vision


If the eyeball is too shallow or the lens is too

flat, hyperopia results. The ciliary muscle

must contract to focus even a distant object

on the retina, and at close range the lens

cannot provide enough refraction to focus

an image on the retina. Individuals with this

problem are said to be farsighted, because

they can see distant objects most clearly.

Hyperopia can be corrected by

placing a converging lens in

front of the eye. Converging

lenses have at least one

convex surface and provide

the additional refraction

needed to bring nearby

objects into focus.

CLINICAL MODULE15.17: Accommodation problems

Surgical correction

Photorefractive keratectomy (PRK)

Laser shapes cornea

Removes 10–20 µm (<10%) of cornea

Laser-assisted in-situ keratomileusis (LASIK)

Interior corneal layers reshaped and covered by normal corneal epithelium

~70% of LASIK patients achieve normal vision

~10 million people have had corrective procedure

Immediate and long-term visual problems can occur

Figure 15.17

Myopia, or nearsighted vision




If the eyeball is too deep or the resting

curvature of the lens is too great, the image

of a distant object is projected in front of the

retina. Such individuals are said to be

nearsighted because vision at close range is

clear but distant objects are blurry and out

of focus. Their condition is more formally

termed myopia (myein, to shut + ops, eye).

Myopia can be treated by

placing a diverging lens in

front of the eye. Diverging

lenses have at least one

concave surface and spread

the light rays apart as if the

object were closer to the



a. Define emmetropia.

b. Discuss two surgical procedures for correcting myopia and hyperopia.

c. Which type of lens would correct hyperopia?

CLINICAL MODULE15.18: Disorders of the special senses

Olfaction disorders

May relate to nerve (N I) or receptor damage

Aging issues

Number of receptors decline with age

Remaining receptors are less sensitive

Gustation disorders

Damage to taste buds (mouth infection, inflammation)

Damage to cranial nerves (N VII, IX, X)

Problems with olfactory receptors

Figure 15.18 1

Figure 15.18 2

CLINICAL MODULE15.18: Disorders of the special senses

Vision disorders

Many mentioned previously

Cataracts (opaque lens)

Can result from:



Reaction to drugs

Aging (senile cataracts)

Most common

Damaged lens can be replaced by synthetic lens

Figure 15.18 3

Normal eye

Eye with


CLINICAL MODULE15.18: Disorders of the special senses

Equilibrium disorders

Vertigo (illusion of movement)

Caused by conditions that alter function of:

Inner ear receptor complex

Usually affect endolymph

Vestibular branch of N VII

Sensory nuclei and CNS pathways

Motion sickness (most common cause)

CLINICAL MODULE15.18: Disorders of the special senses

Hearing disorders

Conductive deafness (issue between tympanic membrane and oval window)

Causes include:

Excess wax or trapped water in external ear

Scarring or perforation of tympanic membrane

Immobility of ear ossicles (fluid or tumor)

CLINICAL MODULE15.18: Disorders of the special senses

Hearing disorders (continued)

Nerve deafness (issue with cochlea or along auditory pathway)

Damage to receptors

20–20,000 Hz in young children but hearing loss later

Neural damage to cochlear branch of N VIII

Caused by loud noises or pathogenic infections


a. Which cranial nerves provide taste sensations from the tongue?

b. Indentify two common classes of hearing-related disorders.

c. What causes vertigo?

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