Chapter 15 - Coastal Bend College

Chapter 15 

The Special Senses 

AP2 Chapter 15: The Special Senses

Sections of the 

Chapter 

I. Olfaction (Smell) 

II. Gustation (Taste) 

III. Visual System 

IV. Hearing and Balance 

V. FX of Aging on the 

Special Senses 

AP2 Chapter 15: The Special Senses 

What are the 

special senses? 

• Smell 

• Taste 

• Sight 

• Hearing 

• Balance

I. Olfaction 

The sense of smell 


Olfaction 

Superior region of the nasal 

cavity houses the Olfactory 

region wh/is covered in 

Olfactory Epithelium (epi) 

Olfactory (O) epi 

a) Supporting Cells- nonsensory 

b) Basal Cells- non-sensory 

c) Olfactory Neurons 

(ON’s) - sensory 

~10,000,000 

Their axons project 

through the Olfactory 

foramina in cribriform 

plate to the O bulbs.

Olfactory Neurons (ON) 

Dendrites extend into the epi 

surface of nasal cavity 

Ends Olfactory vesicles 

(OV) 

OV possess cilia (aka O hairs) 

These lie in the mucus film of 

the epi surface 

“Odorants” (air borne molecules) 

dissolve in mucus and bind to 

Transmembrane odorant receptors 

(TOR) on ON’s cells surface.

Olfaction 

• ~ 1000 different TOR’s can be prod’d each reacts to it’s own 

odorants 

• Thus: the intracellular pathways are GPCR’s that each have their 

own unique rxns prod’ing their own unique smells ~ 4000/ person 

• Only 7 1 o classes of odors have been proposed 

– 1. Camphoraceous (moth balls) 

– 2. Musky 

– 3. Floral 

– 4. Pepperminty 

– 5.Ethereal (Fresh pears) 

– 6.Pungent 

– 7.Putrid 

• Threshold for odorants VERY LOW 

• Low specificity receptors 

• The entire epi surface is replaced ~ 2 months 

• Basal Cells replace ON”s

Process: Odorant Binding to O-hair’s membrane

Neuronal Pathways for Olfaction 


II. Gustation 

The sense of taste 


d) 

Gustation 

b) 

a) 

c) 

• Sensory Structure 

– Taste Buds 

• 4 types of Papillae 

a) Vallate 

• Largest in size 

• Least in # 

• 8-12 form an arch dividing the 

front & back of tongue 

b) Fungiform 

• Scatted irregularly all over 

tongue surface 

• Higher # than filiform 

c) Foliate 

• Distributed on folds on tongue 

sides 

• House most sensitive taste 

buds 

d) Filiform 

• Most # in children decrease 

with age 

• Located posterior in adults 

• Provide rough surface for food 

manipulation

Histology -Taste Bud 

• Oval Structures embedded in 

tongue surface 

• 10,000/tongue 

• 3 cells types in each TB 

a) Supporting Cells: non-sensory 

b) Basal Cells: non-sensory 

c) Gustatory/Taste Cells: sensory 

~ 50/TB each w/10 day lifespan 

Continuously replaced 

Plasma Membrane Gustatory 

Hairs 

There in a tiny space called a Taste 

Pore that allows tastants to reach 

the G-hairs

Fxn of Taste 

• Tastants: chemicals 

dissolved in saliva that 

enter taste pores and by 

various mechanisms cause 

the taste cell’s 

depolarization 

• TC’s do not have classic 

axons, but have short 

connections with 2 o sensory 

neurons 

– This serves the same 

purpose as a synapse and is 

where NT’s are released to 

begin the chain rxn of neuron 

stimulation

5 primary tastes and threshold 

1. Salty 

Na + Channel opening causes depolarization 

Low sensitivity…very high Threshold 

Metal Ions 

2. Sour 

H + causes depolarization 

3 ways: 

a) H + enters cell thru H + channels 

b) H + binds to ligand-gated K+ channels and blocks K+ exit 

c) H + opens ligand-gated Ch’s for + ions allowing them to enter cell 

Acids 

3. Sweet 

Tastants binds to GPCR receptors causing depolarization 

Low sensitivity…very high Threshold 

Sugars 

4. Bitter 

Tastants binds to GPCR receptors causing depolarization 

High Sensitivity…very low Threshold 

Bases 

5. Umami 

AA’s bind to GPCR’s causing depolarization 

Proteins and AA’s

Salty and Sour 


Sweet, Bitter, and Umami (GPCR) 


Δ’s in Taste 

• Temperature can Δ taste fxn 

– Hot disrupts TB fxn 

– Cold take time to warm…results in 

enhanced taste 

• Texture can also Δ taste perception 

• Taste is a rapid Adaptor 

• There is also a strong correlation with 

smell 

• Threshold varies with specific taste 


Taste receptors 

 

Cranial nerves 7, 9, 10 

 

Brain Stem synapse with nucleus of tractus solitarius 

 

Axons synapse w/thalamus 

 

Axons terminate in taste area of the cortex 

Neural 

Pathways: 

Taste 

Cranial nerve VII 

Facial Nerve 

Anterior 2/3 of the 

tongue 

Cranial nerve IX 

Glossphyrengeal 

Nerve 

Posterior 1/8 of the 

tongue 

Cranial nerve X 

Vagus Nerve 

Epiglotis

III. The Visual System 

The sense of sight 


Visual System 

• Visual input is important to learning 

• Light/Dark; Color/Hue; Distance 

• Eye 

– Eyeball 

– Lens 

– Responds to light and begins afferent action 

potentials 

• Eye Optic Nerve Optic Tract Brain (Visual Cortex) 

• Accessory Structure 

– Protection from direct sunlight and damaging particles 

• Optic 

– Nerves 

– Tracts 

– Pathways 


Accessory Structures 

Protect, Lubricate, Move, & Aid in eye fxn 

A. Eyebrows 

B. Eyelids & Eyelashes 

C. Conjunctiva 

D. Lacrimal Apparatus 

E. Extrinsic Eye 

Muscles 

A. Eyebrows 

– Protects eyes from 

dripping perspiration 

– Helps shade from 

direct sunlight 



B. Eyelids w/associated 

lashes 

– A.k.a. Palpebral 

– Protect the eye from foreign 

objects 

– Palpebral fissure- Space 

between the 2 lids 

– Canthi (pl)- corners where 

eyelids meet. 

• One lateral one medial 

– Caruncle- pinkish mound in 

medial canthus that houses 

modified sebaceous & 

sweat glands


B. Eyelids w/associated lashes 

– 5 layers (out in) 

1. Thin layer integument 

2. Thin Areolar CT 

3. Skeletal Muscle 

4. Tarsal Plate (keep eyelid shape) 

5. Palpebral Conjunctiva 

– Blink reflex 

• Protection, lubrication of eye, and 

light entry regulation. 

– Eyelashes double/triple rows of 

hairs 

• Ciliary Glands sweat glands at base 

of EL’s to keep hair lubricated 

– Tarsal Glands sebaceous glands 

prod sebum wh/lubricates lids & 

limits tear leakage

Accessory Struc’s 

C. Conjunctiva 

Thin transparent mucus membrane 

Inner surface of eyelid Palpebral 

conjunctiva 

White of the eye Bulbar 

conjunctiva 

E. Lacrimal Apparatus: 

– Lacrimal gland: produces tears that 

exit through lacrimal ducts. 

– Tears pass over the surface of the 

eye 

– They enter the lacrimal cananiculi 

– They are carried through the 

lacrimal duct 

– They enter the nasal cavity via the 

nasolacrimal duct


E. Extrinsic Eye muscles 

Fxn eyeball mvmt 

Rectus Muscles: arranged 

front to back 

Inferior (Oculomotor Nerve) 

Superior (Oculomotor Nerve) 

Medial (Oculomotor Nerve) 

Lateral (Abducent nerve) 

Oblique Muscles: wrap 

around eye 

Superior (Trochlear nerve) 

Inferior (Oculomotor Nerve) 

H-test- move eye in “H” 

pattern if not indicative of eye 

muscle damage

1. Fibrous Layer 

Outer most 

a) Sclera 

b) Cornea 

2. Vascular Layer 

Middle 

a) Choroid 

b) Ciliary Body 

c) Iris 

3. Retina 

Inner 

4. Chambers of the eye 

5. Lens 

Anatomy of the eye 



1. Fibrous Layer 

a) Sclera 

Anatomy: 

Back 5/6 of the eye 

Dense Collagenous CT & elastic fibers 

White of eye 

Physiology 

Helps maintain eye shape 

Protect inner eye structures 

Attachment point for muscles to move 

eye 

b) Cornea 

Anatomy 

Continuous with sclera 

Anterior 1/6 of eye 

CT-matrix: collagen, Elastic Fibers, & 

proteoglycans (very little H 2O to prevent 

light scatter 

Outer surface Strat squ epi 

Inner surface simp squ epi 

Physiology 

Transparent to allow light to enter eye.



• It contains most of the blood vessels 

• Large # of melanocytes (looks black) 

a) Choroid (Posterior) 

Thin portion associated 

with sclera 

b) Anterior 

Ciliary Body 

Middle btwn iris and 

choroid 

2 major parts: 

Ciliary ring (outer) 

Ciliary process 

(inner) 

Prod’s 

aqueous 

humor 

Both are 

connected to the 

lens via 

suspensatory 

ligaments 

Contains smooth 

muscle to move and Δ 

shape of lens 

Iris



a) Choroid (Posterior) 

b) Anterior 

Ciliary Body 

Iris 

Pigmented portion of the 

eye 

Smooth muscle ring 

surrounding the hole 

(Pupil) 

Regulates the amount of 

light entering the eye by 

Δing hole size 

2 smooth muscle groups 

a) Sphincter Pupillae 

• Circular, 

contraction 

reduces pupil 

size 

b) Dilator Pupillae 

• Radial, 

contraction 

increases pupil 

size


3. Retina 

– Inner layer of eyeball 

– Pigmented layer layer closest 

to the choroid 

– Neuronal Layer faces inner 

chamber of the eye and responds 

to light 

• Relay neurons 

• Photoreceptors: 

– Rods 120 million 

– Cones 6-7 million 

– Landmarks of the retina: 

• Macula- region where light is 

focused 

• Fovea Centralis- high # of 

photoreceptors; greatest visual 

acuity 

• Optic Disc/Blind Spot- No 

photoreceptors entrance for arteries 

and veins of eye


4. The 3 Chambers of the eye 

a. Anterior Chamber 

Lies btwn the cornea and the iris 

b. Posterior Chamber 

Smaller than anterior lies btwn the iris and the 

lens 

c. Vitreous Chamber 

Largest chamber of the eye behind the lens



4. Chambers of the Eye 

Anterior & Posterior Chamber 

• Both filled w/Aqueous Humor 

– Fxn of AqH. helps maintain 

intraocular pressure thus 

maintaining eye shape 

– Refracts light (bends) 

– Provides nutrition for structures 

of the anterior eye 

– Prod’d by cilliary processes as 

bld filtrate



4. Chambers of the Eye 

Vitreous Chamber 

• Filled with vitreous humor 

– Transparent jelly-like substance 

– Its turnover is very slow 

compared to AqH 

– Fxn of VitH helps maintain 

intraocular pressure and thus 

the shape of the eyeball 

– Holds lens and retina in place 

with that pressure 

– Fxns in the refraction of light

5. Lens 

– Transparent biconvex 

with greatest curve 

posterior 

– Anterior surface 

simp cube epi 

• Give rise to cells that b/ 

c lens fibers 

– Posterior surface 

simp col. epi 

• “Lens fibers” 

– Cells that loose their 

nuclei, organelles, 

and accumulate a 

protein 

crystallines 

– Covered by highly 

elastic transparent 

capsule 



Fxns of the complete eye 

Light entry 

• Light entry into the eye 

is regulated by the iris. 

• It must pass thru the 

Cornea, Lens and 

various humors to be 

focused on the retina 

– The retina is responsible 

for converting the light 

into an action potential 

that can be carried to the 

brain for interpretation 

– What does the retina 

respond to??? 

• Electromagnetic 

spectrum range of 

wavelengths 

positioned by 

frequency 

• Visible light 

– 380-750 nM This short 

range is the only range 

our eyes can detect

Fxns of the complete eye 

• Refraction: 

– Light can be bent when it goes 

from air into a different 

medium 

• Reflection 

– When light rays strike an 

object that isn’t transparent, 

they bounce off of its surface 

and come back to the eyeball 

in a specific pattern 

• “Focal Point” 

– Point at which refracted light 

rays converge and focus by 

changing lens shape 

• Distant Vision 

– Ciliary bodies and muscles 

relax lens flattens 

• Near Vision 

– Ciliary bodies and muscles 

contract lens thickens

3 events to bring object into focus 

on retina (Closer than 20 ft) 

1. Accommodation 

– Ciliary muscles and ligaments contract, less 

tension to the lens, lens becomes thicker and 

there is a greater refraction of light 

2. Pupil Constriction 

– The smaller the pupil the more centered light is 

on lens the more focused 

3. Convergence 

– To keep an object in focus both eyes must focus 

simultaneously. This means the closer the 

object the eyes rotate medially to keep in focus 

for both eyes…it is a reflex

Visual System 

Physiology of the Retina 


Structure and fxn of retina 

2 Layers: 

1. Neural Layer 

• 2 networks of communication 

1. Outer Plexiform 

Communication 

2. Inner Plexiform Layer 

• 3 main cell types 

1. Photoreceptors (next to 

pigmented layer) 

1. Respond to light 

2. Bipolar Cells 

1. React to photoreceptors 

response to light and pass 

action potential to ganglionic 

cells 

3. Ganglionic Cells (next to 

Vitreous Humor) 

1. Carry signal to Optic Nerve 

2. Pigmented layer 

• Outer layers connected to 

the choroid 

• Simple Cub. Epi, filled with 

melanin to enhance visual 

acuity by isolating individual 

photoreceptors and reducing 

light scatter

Retina orientation & Photoreceptors 

• Photoreceptor Cell Types: 

1. Rods 

• Fxn in non-color vision 

• Vision in low light conditions 

(night vision) 

• Responds to entire visual 

spectrum 

• Found over most of the retina, 

missing from fovea 

2. Cones 

• Function in color vision 

• Visual acuity (fine vision) 

• Require relatively bright light 

to fxn

Rods 

• Outer “rod” contains ~ 700 

double layered membranous 

discs that contain a pigment 

called rhodopsin 

– Combo of Opsin (a protein) 

covalently bound to retinal (yellow 

photosynthetic pigment derived 

from Vitamin A) 

– Rhodopsin is associated with a Gprotein 

– When activated the G-protein 

activates cGMP phophodiesterase 

– cGMP phophodiesterase breaks 

down cGMP into GMP 

– cGMP can keep Na+ channels 

open 

– Without cGMP Na+ channels close

The Players in retina function

Dark Confirmation 

• When there is no light the G-protein remains 

inactive thus the enzyme (cGMP 

phosphodiesterase) also remains inactive 

• Because the sodium channel has cGMP bound 

to it, the channel remains open allowing Na+ to 

flood (influx) into the rod. 

• As a result of Na+ influx the rod releases the 

inhibitory NT- glutamate. 

• Glutamate effectively “shuts down” the bipolar 

cell.

Light confirmation 

Step #1: 

• Light hits the rhodopsin 

changing retinal’s 

confirmation


Step #2: 

• By changing the 

confirmation of retinal it 

activates the G-protein 

• (Alpha)

Step #3: 

• Alpha sub-unit activate 

the cGMP 

phosphodiesterase 

turning it “on.” 


Activated cGMP 

phosphodiesterase


Step #4: 

• Active cGMP phosphodiesterase removes cGMP 

from the Na+ channel 

• The cGMP is converted into GMP 

• Because cGMP is no longer bound to the Na+ 

channel, the Na+ channel closes


Step #4 ½ : 

• Because Na+ is no longer flooding into the rod, 

the glutamate release stops, thus the bipolar cell 

can release its NT activating the ganglionic cells

Cones 

• Found in highest # in fovea (35,000) 

• Outer cone contains double layered 

discs 

• These discs contain the pigment 

Iodopsin 

– A combo of Retinol and 1 of 3 

phodisplaystopigments (each cone only 

contains1 of the 3) 

1. Blue 

2. Red 

Genetically variable 

a) Ser 180 

b) Ala 180 

3. Green 

– Fxn in the same manner as rhodopsin 

but each only responds to a narrow 

region of the visual spectrum 

– Various combinations of cones 

produce gradations of light and hue

Bipolar Cells (BP) 

 

Ganglionic Cells (GC) 

 

GC’s axons extend along retina 

surface (except 4 fovea) 

 

Converge at Optic Disc 

 

Exit at Optic Nerve 

Inner Layers: Retina 

• Rods 

– BP receives input from >1 Rod 

– GC receives input from >1 BP 

Spatial summation results in signal 

enhancement 

THUS: low light still causes 

stimulation 

• Cones 

– BP receives input from 1 Cone 

Lower light sensitivity increases 

visual acuity 

• Interneurons 

– Modify signal from Photoreceptor 

b4 signal leaves retina 

– Can be excitatory or inhibitory 

– 3 types 

• Horizontal Cells 

• Amacrine Cells 

• Interplexiform Cells

Neuronal Pathways for Vision 

• ½ of the eye remain on the same 

side of the brain (lateral) 

• ½ of the eye crosses over to the 

other side of the brain (Medial) 

• Crossover Optic Chiasm 

• Superior Colliculi center of 

visual reflexes 

• Visual Cortex where neurons 

integrate information into single 

message; translate it into a 

mental image; transfer image to 

other parts of the brain for 

evaluation (action/ignore) 

• Binocular Vision visual fields of 

2 eyes overlap (2 separate 

images must be merged (depth 

perception) 


IV. Hearing and Balance 

Ears do more than hear they also 

provide you with body position 

recognition 


3 Major regions of the ear 


1. External Ear 

– Fxn in Hearing 

– Parts 

a) Auricle 

b) External Auditory Meatus 

c) Tympanic Membrane 

2. Middle Ear 

– Fxn in Hearing 

– Parts 

a) Auditory ossicles 

3. Inner Ear 

– Fxn in Hearing and 

Balance 

– Parts 

a) Sensory organs

Auditory Structures & their fxns: 

1. External Ear 

a) Auricle/Pinna 

Anatomy 

• Fleshy portion of external ear 

• Elastic Cartilage covered with Skin 

Physiology 

• Collects sounds waves and directs them toward 

the External Auditory Meatus 

b) External Auditory Meatus 

Anatomy 

• Tube in temporal bone lined with hairs and 

ceriminous glands (ear wax) 

Physiology 

• Prevent foreign objects from reaching Tympanic 

Membrane 

• Guide sound waves to tympanic membrane 

c) Tympanic Membrane/ Ear Drum 

Anatomy 

• Thin, semitransparent, 3 layered oval membrane 

Physiology 

• Sounds waves hitting it make it vibrate


2. Middle Ear 

• Air filled cavity with 3 major 

structures of importance. 

1. Round window and Oval 

Window 

Each of which is essential for 

communication between 

middle and inner ear 

2. Auditory Ossicles 

3 bones used to transfer 

vibrations from the tympanic 

membrane to the inner ear 

a) Malleus/Hammer 

• Attached to the TM 

b) Incus/Anvil 

• Attached to hammer and 

stirrup 

c) Stapes/Stirrup 

• Attached to oval window


2. Middle Ear 

• Air filled cavity with 3 major structures of 

importance. 

3. 2 Muscles of the middle ear 

Tensor Tympani 

Attached to Malleus helps with TM tension 

Stapedius Muscle 

Attached to stapes helps control stapes vibration to sound 

waves


3. Inner Ear 

3 major regions 

1. Cochlea 

– Fxns in hearing 

2. Vestibule 

– Fxns in static (1 o ) balance 

3. Semicircular canals 

– Fxns in dynamic balance 

**Notice the placement of the 

round and oval windows** 



3. Inner Ear 

• Bony Labyrinth series 

of tunnels embedded 

w/in the temporal bone 

– Btwn BL and ML is 

Perilymph 

• W/in the bony labyrinth 

is a membranous 

labyrinth of the same 

shape 

– ML is filled with 

endolymph


3. Inner Ear (Cochlea)


3. Inner Ear (Cochlea) 

• Scala Vestibuli (Sv) 

• Scala Tympani (St) 

• Vestibular Membrane (Vm) 

• Basilar Membrane (Bam) 

• Cochlear Duct (Cd) 

• Perilymph: Sv & St 

• Endolymph : Cd 

• Cd: Spiral Organ. 

• Sv- extends from the oval window 

under the stapes to the 

helicotrema (apex of the cochlea) 

– Part of the bony labyrinth 

• St extends from the helicotrema 

back to the round window. 

– Part of the bony labyrinth 

• Vm- thin membrane btwn Sv and 

Cd. So thin that there are no fx on 

sound wave transmission. 

• Bam: much more complex and of 

greater physiological interest in the 

mechanics of hearing. 

– Width increases with distance



• Bam: 

– 2 major regions 

1. Acellular Portion 

• Collagen fibers 

• Ground substance 

• Sparse Elastic Fibers 

2. Cellular Portion 

• Thin layer of vascular CT 

overlaid with simp squ Epi 

– Collagen fibers near the 

oval window: 

• Short and stiff 

• Activate w/high frequency 

vibrations 

– Collagen fibers near 

helicotrema: 

• Wide and limber 

• Activation w/low frequency 

vibrations



Organ of Corti/Spiral organ 

• Made up of: 

– Supporting epithelial cells 

– Hair Cells- stereocilia (part of the plasma membrane) 

• Arranged in 4 long rows 

1. Inner Row of 1 cell 

– Responsible for hearing 

2. Outer Row of 3 cells 

– Responsible for setting tension of Bam



• Single Inner Row 

– Responsible for hearing 

– “Hairs” form a conical 

structure aka Conical Bundle 

• They increase in length from 1 

to the next 

• “Tip Link” Connects 

neighboring hairs to each 

other that are actually 

mechanically gated K + 

channels. As the hairs bend 

the K+ channels open 

– This isn’t an axon but the K + 

level Δ does cause the 

synaptic terminals at its base 

to activate the sensory neuron 

and carry the signal to the 

cochlear ganglion



• Outer hair cells (3) 

– Responsible for tension of 

the Bam 

– Separated from the outer 

cell by a gap 

– The hairs on these cells are 

arranged in a curved 

pattern 

• The longest is embedded in 

the acellular gelatinous shelf 

of the tectorial membrane

Auditory Fxn 

Terminology: 

• Vibration: 

– Ripples propagated along matter 

• Volume: 

– Fxn of wave amplitude (height) 

measured in decibels 

• Pitch: 

– Fxn of wave frequency (# of waves/ 

second) measured in hertz 

– High Frequency = High pitch 

– Low Frequency = Low Pitch 

• Timbre: 

– resonance quality or overtones of 

sound (Distinguishing between a 

piano hitting a note or someone 

singing the note…the sound is 

different.) 

Normal Range: 

– 20-20,000 Hz 

– 0 or more dbs 

• BUT higher than 125 

db is painful to the 

ear

Auditory 

Fxn 

Steps 

involved in 

the 

mechanical 

portion of 

hearing

Auditory 

Fxn 

Steps 

involved in 

the 

mechanical 

portion of 

hearing 

EE: 

Auricle directs the sound waves into the EAM 

and the TM 

Sound waves are slow (332 m/s) thus they 

reach the ears at different times and allow us 

to distinguish directionality. 

ME: 

TM vibrates through the ossicles (Malleus 

vibrates with the TM and in turn hits the Incus 

which vibrates the stapes on the oval 

window.) 

**Run into an issue going from the ME to the 

IE. The vibration must go from air into the 

liquid of the perilymph. Thus the vibration 

must be amplified significantly! 

How? 

TM is 20X as large as the stapes’ foot and the oval 

window. This alone amplifies the sound waves 

because of differences in size and mechanical 

vibration

Auditory 

Fxn 

Steps 

involved in 

the 

mechanical 

portion of 

hearing 

IE: 

Vibration of the foot plate of the stapes 

causes the perilymph of the Sv to 

vibrate wh/in turn vibrates the Vm 

which transmits vibration to the 

endolymph. The Endolymph causes 

the Bam to move, which have the hair 

cells on top of it. 

Short Waves High Pitch 

Displacement of hairs near the oval window 

Long Waves Low Pitch 

Displacement of hairs near the helicotrema 

Hair cells are bent because tectorial 

membrane doesn’t move

Unstimulated 

• ~ 15% channels are open 

• RMP= -60mV 

• If the hairs are bent toward 

the “short” hairs (stereocilia) 

– It is a negative stimulus . 

– The “springs” go slack, which 

results in closing of the open 

ch’s 

– Cells become hyperpolarized 

(more -) 

Stimulated 

• Hairs bent toward the “long” 

hairs (stereocilia) 

– Positive stimulus 

– Pulls more K+ Ch’s open 

– Cells depolarize with K+ 

causing Voltage-gated Ca2+ 

channels to open 

– Ca2+ influx signals the release 

of NT (glutamate/aa) 

– Induces action potential in 

cochlear neurons that synapse 

with hair cells. (Afferent signal)

Neuronal Pathways for Hearing 

• Some go to the superior colliculus where reflexes will turn 

the head in response to sound

Hearing and Balance 

Divided structurally & fxnally into 2 parts 

1. Static Labyrinth 

• Located w/in the vestibule 

• Internally there are 2 

fxnal units: 

a) Utricle 

b) Saccule 

• Fxns: 

a) Primary position of head 

relative to gravity 

b) Responds to linear 

acceleration and 

deceleration 


2. Kinetic Labyrinth 

• Located w/in the 

semicircular canals 

• Fxn: 

– Evaluating movements of 

the head


Static Labyrinth 

• Utricle & Saccule 

– Simple cuboidal epi 

– Contains a specialized 

patch of epi called the 

macula 

a) Macula Utricle: 

• Oriented parallel to 

the base of the skull 

b) Macula Saccule: 

• Oriented 

perpendicular to the 

base of the skull


Static Labyrinth (Macula) 

• Resembles the spiral 

organ 

– Colum. supporting cells + hair 

cells 

• Numerous hair cells (hc) 

that contain stereocilia & 

1kinocilium 

– These “hairs” are 

embedded in a gelatinous 

matrix (Gm), (weighted 

by otoliths) 

– Gm moves in response to 

gravity because of 

embedded otoliths & 

bends the hc initiating 

action potentials. 

• Bend hair cells: 

• Toward kinocilium= depolarization 

• Away kinocilium= hyperpolarization 

• hc’s are synapsed with 

Vestibulocochlear nerve & use 

glutamate as a NT 

• The “pattern of stimulation” gives 

specific information about head 

position and acceleration/ 

deceleration 

• In response the body Δ’s muscle 

tone of back & neck


Static Labyrinth (Macula urticle)

Macula Saccule 

Effect of gravity on 

the macula Saccule 

that is perpendicular 

to the skull’s base 



Kinetic Labyrinth 

• The 3 semicircular canals 

(sc) occupy the X, Y, and Z 

plane 

– Thus movement in all 

directions can be detected 

• At the base of each sc is 

the ampullae 

– w/in the ampullae the epi is 

specialized to form the crista 

ampullaris. 



Kinetic Labyrinth (crista ampullaris) 

• CA: ridge/crest of epi w/curved gm 

(Cupula) over the crest 

– Crista hc’s are embedded in the 

cupula w/o otoliths there for there isn’t 

a response to gravity 

– This cupula is displaced when the 

endolymph shifts in response to mvmt 

of the head. 

• Cupula mvmt, bend hc’s and initiates an 

action potential 

• Response to mvmt is largely subconscious

Neuronal Pathways for Balance 


Fxn of Aging on the Special Senses 

• Decline in all fxns 

• Loss of appetite, visual impairment, 

disorientation, risk of falling

Chapter 15 - Coastal Bend College

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