Chapter 17

The Special Senses

•Smell,
taste, vision, hearing and equilibrium

•Housed
in complex sensory organs

•Ophthalmology
is science of the eye

•Otolaryngology
is science of the ear

Chemical Senses

•Interaction
of molecules with receptor cells

•Olfaction
(smell) and gustation (taste)

•Both
project to cerebral cortex & limbic system

–evokes
strong emotional reactions

Olfactory Epithelium

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

•Covers
superior nasal cavity and cribriform plate

•3
types of receptor cells

Olfaction: Sense of Smell

•Odorants
bind to receptors

•Na+
channels open

•Depolarization
occurs

•Nerve
impulse is triggered

Adaptation & Odor Thresholds

•Adaptation
= decreasing sensitivity

•Olfactory
adaptation is rapid

–50%
in 1 second

–complete
in 1 minute

•Low
threshold

–only
a few molecules need to be present

–methyl
mercaptan added to natural gas as warning

Gustatory Sensation: Taste

•Taste
requires dissolving of substances

•Four
classes of stimuli--sour, bitter, sweet, and salty

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

Anatomy of Taste Buds

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

•A
single gustatory hair projects upward through the taste pore

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

Physiology of Taste

•Complete
adaptation in 1 to 5 minutes

•Thresholds
for tastes vary among the 4 primary tastes

–most
sensitive to bitter (poisons)

–least
sensitive to salty and sweet

Accessory Structures of Eye

•Eyelids
or palpebrae

–protect & lubricate

–epidermis, dermis, CT, orbicularis oculi m., tarsal
plate, tarsal glands & conjunctiva

•Tarsal
glands

–oily secretions keep
lids from sticking together

•Conjunctiva

–palpebral
& bulbar

–stops at corneal edge

–dilated BV--bloodshot

Eyelashes & Eyebrows

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

•Sebaceous
glands are found at base of eyelashes (sty)

•Palpebral fissure is gap between the eyelids

Lacrimal Apparatus

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

Extraocular Muscles

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

•.

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

•2
oblique muscles -- inferior and superior

Tunics (Layers) of Eyeball

•Fibrous
Tunic

(outer layer)

•Vascular
Tunic

(middle layer)

•Nervous
Tunic

(inner layer)

Fibrous Tunic -- Description of Cornea

•Transparent

•Helps
focus light(refraction)

–astigmatism

•Transplants

–common & successful

–no blood vessels so no
antibodies to cause rejection

Fibrous Tunic -- Description of Sclera

•“White”
of the eye

•Dense
irregular connective tissue layer -- collagen & fibroblasts

•Provides
shape & support

Vascular Tunic -- Choroid & Ciliary Body

•Choroid

–pigmented epithilial cells (melanocytes) & blood vessels

–provides nutrients to
retina

–black pigment in
melanocytes absorb scattered light

•Ciliary
body

–ciliary processes

•folds on ciliary body

•secrete aqueous humor

–ciliary muscle

•smooth muscle that
alters shape of lens

Vascular Tunic -- Iris & Pupil

•Colored
portion of eye

•Shape
of flat donut suspended between cornea & lens

•Hole
in center is pupil

•Function
is to regulate amount of light entering eye

Vascular Tunic -- Description of lens

•Avascular

•Crystallin proteins arranged like layers in onion

•Clear
capsule & perfectly transparent

Nervous Tunic -- Retina

•Posterior
3/4 of eyeball

•Optic
disc

–optic nerve exiting back
of eyeball

•Central
retina BV

–fan out to supply
nourishment to retina

–visible for inspection

•hypertension &
diabetes

•Detached
retina

–trauma (boxing)

•fluid between layers

•distortion
or blindness

Rods & Cones--Photoreceptors

•Rods----rod
shaped

–shades of gray in dim
light

–120 million rod cells

–discriminates shapes
& movements

–distributed along
periphery

•Cones----cone
shaped

–sharp, color vision

–6 million

–fovea of macula lutea

•densely packed region

•at exact visual axis of
eye

•2nd cells do not cover
cones

•sharpest resolution or
acuity

Pathway of Nerve Signal in Retina

•Light
penetrates retina

•Rods
& cones transduce light into action potentials

•Rods
& cones excite bipolar cells

•Bipolars excite ganglion cells

•Axons
of ganglion cells form optic nerve leaving the eyeball (blind spot)

•To
thalamus & then the primary visual cortex

Aqueous Humor

•Continuously
produced

by ciliary body

•Flows
from posterior chamber

into anterior through the pupil

•Glaucoma

–increased intraocular
pressure that could produce blindness

–problem with drainage of
aqueous humor

Major Processes of Image Formation

•Refraction
of light

–by
cornea & lens

–light
rays must fall upon the retina

•Accommodation
of the lens

–changing
shape of lens so that light is focused

•Constriction
of the pupil

–less
light enters the eye

Definition of Refraction

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

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

Refraction by the Cornea & Lens

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

•Brain
learns to work with that information

•75%
of Refraction is done by

cornea -- rest is done by the lens

Near Point of Vision and Presbyopia

•Near
point is the closest distance from the eye an object can be & still be in
clear focus

–4
inches in a young adult

–8
inches in a 40 year old

•lens
has become less elastic

–31
inches in a 60 to 80 year old

•Reading
glasses may be needed by age 40

–presbyopia

–glasses
replace refraction previously provided by increased curvature of the relaxed,
youthful lens

Correction for Refraction Problems

•Emmetropic eye (normal)

–can refract light from
20 ft away

•Myopia
(nearsighted)

–eyeball is too long from
front to back

–glasses concave

•Hypermetropic (farsighted)

–eyeball is too short

–glasses convex
(coke-bottle)

•Astigmatism

–corneal surface wavy

–parts of image out of
focus

Constriction of the Pupil

•Constrictor
pupillae muscle contracts

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

•Sharpens
vision by preventing blurry edges

•Protects
retina very excessively bright light

Convergence of
the Eyes

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

•As
you look at an object close to your face, both eyeballs must turn inward.

–convergence

Photoreceptors

•Photopigment is integral membrane protein of outer segment
membrane

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

•Photopigments = opsin (protein) +
retinal (derivative of vitamin A)

–rods contain rhodopsin

–cone photopigments contain 3 different opsin
proteins permitting the absorption of 3 different wavelengths (colors) of light

Color Blindness & Night Blindness

•Color
blindness

–inability
to distinguish between certain colors

–absence
of certain cone photopigments

–red-green
color blind person can not tell red from green

•Night
blindness (nyctalopia)

–difficulty
seeing in low light

–inability
to make normal amount of rhodopsin

–possibly
due to deficiency of vitamin A

Light and Dark Adaptation

•Light
adaptation

–adjustments
when emerge from the dark into the light

•Dark
adaptation

–adjustments
when enter the dark from a bright situation

–light
sensitivity increases as photopigments regenerate

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

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

Brain Pathways of Vision

Processing of Image Data in the Brain

•Visual
information in optic nerve travels to

–occipital
lobe for vision

–midbrain
for controlling pupil size & coordination of head and eye movements

–hypothalamus
to establish sleep patterns based upon circadian rhythms of light and darkness

Visual fields

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

•Left
occipital lobe sees  right 1/2 of the
world

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

Anatomy of the Ear Region

External Ear

•Function
= collect sounds

•Structures

–auricle
or pinna

•elastic
cartilage covered with skin

–external
auditory canal

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

•ceruminous
glands produce cerumen = ear wax

–tympanic
membrane or eardrum

•epidermis, collagen & elastic fibers, simple cuboidal epith.

•Perforated
eardrum (hole is present)

–at
time of injury (pain, ringing, hearing loss, dizziness)

–caused
by explosion, scuba diving, or ear infection

Middle Ear Cavity

Middle Ear Cavity

•Air
filled cavity in the temporal bone

•Separated
from external ear by

eardrum and from internal ear by

oval & round window

•3
ear ossicles connected by synovial joints

–malleus attached to eardrum, incus
& stapes attached by foot plate to membrane of oval window

–stapedius and tensor tympani muscles attach to ossicles

•Auditory
tube leads to nasopharynx

–helps
to equalize pressure on both sides of eardrum

•Connection
to mastoid bone =mastoiditis

Inner Ear---Bony Labyrinth

•Bony
labyrinth = set of tubelike cavities in temporal bone

–semicircular canals,
vestibule & cochlea lined with periosteum & filled with perilymph

–surrounds & protects
Membranous Labyrinth

Inner Ear---Membranous Labyrinth

•Membranous
labyrinth = set of membranous tubes containing sensory receptors for hearing
& balance and filled with endolymph

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

Anatomy of the Organ of Corti

•16,000
hair cells have 30-100 stereocilia(microvilli )

•Microvilli make contact with tectorial
membrane (gelatinous membrane that overlaps the spiral organ of Corti)

•Basal
sides of inner hair cells synapse with 1st order sensory neurons whose cell
body is in spiral ganglion

Sound Waves

•Vibrating
object causes compression of air around it = sound waves

–audible
range is 20 to 20,000 Hz

–hear
best within 500 to 5000 cycles/sec or Hz

–speech
is 100 to 3000 Hz

•Frequency
of a sound vibration is pitch

–higher
frequency is higher pitch

•Greater
intensity (size) of vibration, the louder the sound measured in decibels (dB)

–Conversation
is 60 dB;  pain above 140dB

–OSA
requires ear protection above 90dB

Deafness

•Nerve
deafness

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

•the
louder the sound the quicker the hearing loss

–fail
to notice until difficulty with speech

•Conduction
deafness

–perforated
eardrum

–otosclerosis

Physiology of Hearing

•Auricle
collects sound waves

•Eardrum
vibrates

–slow
vibration in response to low-pitched sounds

–rapid
vibration in response to high-pitched sounds

•Ossicles
vibrate since malleus attached to eardrum

•Stapes
pushes on oval window producing fluid pressure waves in scala
vestibuli & tympani

–oval
window vibration 20X more vigorous than eardrum

•Pressure
fluctuations inside cochlear duct move the hair cells against the tectorial membrane

•Microvilli are bent producing receptor potentials

Overview of Physiology of Hearing

Cochlear Implants

•If
deafness is due to destruction of hair cells

•Microphone,
microprocessor & electrodes translate sounds into electric stimulation of
the vestibulocochlear nerve

–artificially
induced nerve signals follow normal pathways to brain

•Provides
only a crude representation of sounds

Physiology of Equilibrium (Balance)

•Static
equilibrium

–maintain
the position of the body (head) relative to the force of gravity

•Dynamic
equilibrium

–maintain
body position (head) during sudden movement of any type--rotation, deceleration
or acceleration

Detection of Position of Head

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

Detection of Rotational Movement

•When
head moves, the attached semicircular ducts and hair cells move with it

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