Anatomy & Physiology: Special Senses
I.
The Eye & Vision
A. Accessory Structures of the Eye
1. Orbital Cavity
a. Frontal, maxilla, ethmoid, ethmoid, sphenoid, lacrimal, & palatine bones.
b. Optic foramen – opening in posterior wall through which the optic nerve
and ophthalmic nerve pass.
c. Superior orbital foramen – opening through which other nerves and
arteries pass.
d. Fat pad – cushions & absorbs mechanical blows to the eye.
2. Eyebrow – ridge of frontal bone that is covered by coarse hair. Provides
shade and prevents perspiration from dripping into the eye.
3. Eyelids – thin, skin covered folds that protect the eye from foreign objects
and helps keep eyes moist. The blinking reflex occurs every 3 – 7 seconds.
4. Conjunctiva – transparent mucous membrane that coves the inside of the
eyelid and the white of the eyeball. Functions to keep the eye moist and
prevent objects from going behind the eyeball.
5. lacrimal Apparatus
a. Lacrimal gland – located on the superior, lateral area of the eye.
b. Lacrimal secretion (tears) – dilute saline, mucus, & enzyme solution that
keeps eye moist, pH balanced, and free of germs.
c. Blinking washes tears across the eyes.
d. Nasolacrimal duct – drains excessive tears from the eye into the nasal
cavity.
6. Extrinsic Eye Muscles – 6 muscles that control the movement of the eyes
a. superior rectus
b. inferior rectus
c. lateral rectus
d. medial rectus
e. inferior oblique
f. superior oblique
B. Structure of the Eyeball (three layers: Vascular Tunic, Fibrous Tunic, Nervous
Tunic)
1. Fibrous Tunic – outermost layer of the eye
a. Sclera (White of the eye) – white collagenous fibers that compose 5/6 of
the eye. Very tough & functions in protection & holding the eye together.
b. Cornea – anterior, transparent 1/5 of the eyeball. Contains no blood
vessels and is continuous with the sclera. Functions as a window to allow
light to enter the eye and to bend most of the light entering the eye.
2. Vascular Tunic – middle layer of the eye that has many blood vessels
a. Choroid - Black to brown tissue that lines the back 5/6 of the eye.
Functions to supply nutrients and blood to the inner tissues of the eye and t
absorb excess light.
b. Ciliary body – thickened ring of tissue that encircles the lens.
1) Ciliary Muscles – muscles that control the shape of the lens.
2) Suspensory ligament – connects the ciliary muscles to the lens.
c. Lens – biconvex, transparent, flexible structure that is able to change its
shape to allow for precise focusing.
d. Iris – colored part of the eye. Composed of circular and radial arranged
smooth muscles that function to regulate the amount of light that enters the
eye from its center opening (pupil).
1) Iris dilates to let in more light and constricts to decrease the amount of
light.
2) Colors are brown, blue, green, or gray depending upon the amount of
brown pigment.
e. Aqueous Humor – clear fluid between the cornea and the ciliary body. It
maintains a constant pressure of fluid to maintain the shape of the cornea
and eyeball.
3. Nervous (Sensory) Tunic – innermost layer of the eye
a. Retina – layer that contains visual receptor cells.
1) rods – responsible for black & white vision. Stimulated in dim light.
2) Cones – responsible for color vision. Stimulated in bright light.
a) concentrated near the center of the retina
b) three colors: red, blue, & green.
b. Macula lutea (yellow spot) – central region of the retina. Small depression
in the middle called the Fovea Centralis, which is where most of the light
is focused and most of the cones are concentrated.
c. Optic disc (blind spot) – located just medial of the macula lutea, this area
contains NO photoreceptors and is where the optic nerve exits the eye.
d. Vitreous Humor – clear, jelly-like fluid that functions to support the shape
of the eyeball and to hold the retina against the surface of the eye ball.
C. Vision Physiology
1. Refraction & lenses
a. Refraction is the bending of light as it passes through a transparent object.
b. A lens is a transparent object that is curved on one or both sides such that
light is bent as it passes through.
1) convex lens – cause light waves to converge
2) concave lens – cause light waves to diverge
c. Focal point – the point at which convergent light rays meet.
d. The thicker the lens, the more the light rays will be bent.
e. Real image – the image formed by a convex lens will be upside down and
backwards.
2. Visual focusing
a. As light reaches the eyeball the cornea refracts it towards the lens. Most
of the refraction of the light occurs at the cornea.
b. Light passes through the lens and is focused on the retina.
1) Distant objects require less focusing, thus the lens is stretched by the
ciliary muscles into a thinner shape.
2) Near objects need more focusing thus the ciliary body relaxes and
allows the lens to recoil back to its shorter, fatter shape that refracts the
light more. (accommodation)
c. Emmetropic vision is normal vision and requires NO accommodation or
change in lens shape (usually 20 feet.)
3. Refraction problems
a. Myopia (nearsightedness) – results from an eyeball that is too long.
Objects close up can be seen, but far away objects are focused in front of
the retina. Correction involves concave lenses.
b. Hyperopia (farsightedness) – results from too short of a eyeball. Objects
can be seen far away, but near objects focus beyond the retina. Correction
involves convex lenses.
c. Astigmatism – unequal curvature of the lens that causes unequal focusing
of objects back on the retina. Correction involves special cylindrically
ground lenses.
d. Presbyopia (farsightedness due to age) – elasticity of the lens decreases
with age (usually starting around age 45) and accommodation decreases.
Correction involves bifocal glasses that have increased convex lenses at
the lower half for reading and close up work.
D. Disorders of the Eyes
1. Cataracts – thickening and hardening of the lens that causes it to cloud over
and eventually blindness. Due to age (normally), UV light, smoking,
diabetes. Lens replacement surgery is only cure.
2. Glaucoma – a buildup of aqueous humor in the anterior chamber of the eye
that eventually can cause blindness. Due to decreased drainage of the
aqueous humor past the age of 40. If detected early on, it can be corrected
using eye drops that increase drainage.
3. Colorblindness – sex-linked hereditary condition that results in the absence of
one or more types of cones.
a. Monochromates – black & white only
b. Dichromates – blue-yellow or red-green
II.
The Ear, Hearing and Equilibrium
A. Structure of the Ear (divided into three parts: outer ear, middle ear, & inner ear)
1. Outer Ear
a. Auricle / Pinna – funnel shaped elastic cartilage that directs sound into the
ear canal.
b. External Auditory Meatus (canal) – short (2.5 cm) curved tube that
connects the auricle and the eardrum.
1) Ceruminous glands line the tube and secrete a yellow, sticky wax
(cerumen) that traps dirt & debris
2) Lined with hairs that point out to capture dirt.
c. Tympanic membrane (eardrum) – membrane that converts air vibrations
into mechanical vibrations.
2. Middle Ear
a. Air filled space (tympanic chamber) that connects the outer & inner ears.
b. Pharyngotympanic tube (Eustachian tube) connects the middle ear with the
pharynx (back of the throat). Yawning or swallowing opens this tube and
equalizes the air pressure between the outside air and the middle ear.
(popping your ears)
c. Auditory ossicles – three small bones that connect the tympanic membrane
to the oval window of the inner ear.
1) Smallest bones of the body.
2) Malleus, Incus, Stapes.
3) Transmit and amplify the vibrations from the eardrum to the inner ear.
d. Tensor tympani & Stapedius – small muscles in the middle ear that are
connected to the auditory ossicles. Contractions of these muscles lessen
the vibrations of the auditory ossicles in times of loud noise (tympanic
reflex).
3. Inner Ear
a. Labyrinth – maze of canals and chambers in the temporal bone (behind the
eye). Two major divisions: Bony Labyrinth & Membranous Labyrinth.
1) Bony Labyrinth – channels in the bone
2) Membranous Labyrinth – membranous sacs and ducts in the bony
labyrinth.
3) Perilymph – fluid found between the membranous & bony Labyrinth.
Functions to cushion the organs.
4) Endolymph – fluid found inside the various inner ear organs.
5) The three structures of the inner ear are the
a) Cochlea – snail shell, hearing
b) Vestibule - sacs, static equilibrium
c) Semicircular Canals – three half-circle canals, dynamic equilibrium
B. The Cochlea and Hearing
1. Structure of the Cochlea (from Latin for snail)
a. The cochlea is divided into three ducts
1) Scala Vestibuli - upper duct. Contains perilymph.
2) Scala Tympani – lower duct. Contains perilymph.
3) Scala Media (Cochlear duct) – middle duct. Contains endolymph and
has the Organ of Corti.
b. Oval window – thin, oval membrane at the start of the s. vestibuli that is in
contact with the stapes bone.
c. Tectoral membrane – immovable membrane between the s. vestibuli and s.
media.
d. Basilar membrane – moveable membrane that separates the s. tympani and
s. media.
e. Organ of Corti – receptor complex that lies on top of the basilar
membrane. It contains hair cells that contact the tectoral membrane.
2. Physiology of Hearing
a. Mechanical vibrations of the stapes put pressure on the oval window of the
s. vestibuli. This pressure is transferred to the perilymph, which is
continuous throughout the s. vestibuli and s. tympani.
b. The frequency of the vibration causes a localized distortion of the basilar
membrane.
c. Movement of the basilar membrane presses the hair cells of the organ of
Corti up against the tectoral membrane.
d. The hair cells are neural receptors that transfer the stimuli into an impulse
that travels to the brain via the Auditory nerve.
e. The location of the basilar membrane distortion depends upon the
frequency of the sound.
1) High pitch sound will cause a distortion closer to the oval window.
2) Low pitch sounds will cause a distortion closer to the center of the
shell.
f. The volume of the sound will cause a greater or lesser distortion in the
basilar membrane. The louder the sound, the more distortion and the
greater the number of hair cells will be stimulated.
C. Sound – two physical properties: frequency & amplitude
1. Frequency – the tone or pitch of the sound
a. measured in Hertz (Hz)
b. humans can hear 20 –20,000 Hz
c. Children have the greatest range of hearing, but stiffening membranes,
ossicle damage, or damage to the organ of Corti with age tends to decrease
the adult range.
2. Amplitude – the intensity or loudness of the sound
a. measured in decibels (dB)
b. 0 dB is the lowest audible sound, 30 dB is whispering, 50 dB is normal
conversation, 70 dB is a noisy restaurant, and 130 dB is the threshold of
pain.
D. Deafness
1. Conductive deafness – interference in the transmission of vibrations to the
inner ear.
a. wax plug
b. damage to the eardrum (hardening, tearing, etc.)
c. ossicles are damaged, destroyed by disease, etc.
d. Otosclerosis – age-related disorder due to deposition of new bone on the
ossicles causing them to lose their mobility.
2. Sensorineural deafness – damage to the organ of Corti or auditory nerve that
prevents transmission of the nerve impulse to the brain.
a. can be temporary or permanent
b. caused by tumors in the CNS, brain damage, loud noises etc.
c. Presbycusis – high pitched hearing loss due to age.
E. Vestibule, Semicircular canals, & Equilibrium
1. Vestibule – two sacs of the inner ear that sense static equilibrium.
a. The two sacs are the Utricle and Saccule.
b. The sacs contain masses of sensory hairs called Maculae and calcium
carbonate crystals called Otoliths.
c. Sudden movement in any direction causes the otoliths to lag behind which
causes tension on the hair cells and triggers a impulse.
2. Semicircular canals – three canals that sense dynamic equilibrium.
a. The three canals are at right angle to each other to respond to all possible
rotational movements.
b. Each canal has a swollen region called an Ampulla that contains sensory
hair cells.
c. Endolymph-like fluid flows inside each canal.
d. Movement of the fluid in response to rotational movement of the body
causes the hair cells to be stimulated.