Nervous System III
(2) The lens is a transparent, elastic structure. The
ciliary muscles control its shape.
(3) The iris is a muscular diaphragm that controls
the amount of light entering the eye; the pupil
is an opening in the iris.
(4) Spaces in the eye are F
lled with fl uids
(aqueous and vitreous humors) that help
maintain its shape.
c. Light refraction
(1) Light waves are primarily refracted by the
cornea and lens to focus an image on the retina.
(2) The lens must thicken to focus on close
d. Visual receptors
(1) The visual receptors are rods and cones.
(2) Rods are responsible for colorless vision in dim
light, and cones provide color vision.
e. Visual pigments
(1) A light-sensitive pigment in rods (rhodopsin)
decomposes in the presence of light and
triggers a complex series of reactions that
initiate nerve impulses on the optic nerve.
(2) Three sets of cones provide color vision. Each
set has a different light-sensitive pigment, and
each set is sensitive to a different wavelength
of light; the color perceived depends on which
set or sets of cones are stimulated.
f. Stereoscopic vision
(1) Stereoscopic vision provides perception of
distance and depth.
(2) Stereoscopic vision occurs because of the
formation of two slightly different retinal
images that the brain superimposes and
interprets as one image in three dimensions.
(3) A one-eyed person uses relative sizes and
positions of familiar objects to judge distance
and depth.
g. Visual nerve pathways
(1) Nerve F
bers from the retina form the optic
(2) Some F
bers cross over in the optic chiasma.
(3) Most of the F
bers enter the thalamus and
synapse with others that continue to the visual
cortex of the occipital lobes.
(4) Other impulses pass into the brainstem and
function in various visual refl exes.
Diminished senses are often one of the F
rst noticeable
signs of aging.
1. Age-related hearing loss may refl
ect damage to
hair cells of the spiral organ, degeneration of nerve
pathways to the brain, or tinnitus.
2. Age-related visual problems include dry eyes, fl
and light fl
ashes, presbyopia, glaucoma, cataracts,
macular degeneration, and retinal detachment.
3. Sense of hearing
a. The outer ear includes the auricle, the external
acoustic meatus, and the tympanic membrane. It
collects sound waves created by vibrating objects.
b. Middle ear
(1) Auditory ossicles of the middle ear conduct
sound waves from the tympanic membrane
to the oval window of the inner ear. They also
increase the force of these waves.
(2) Skeletal muscles attached to the auditory
ossicles provide the tympanic refl
ex, which
protects the inner ear from the effects of loud
c. Auditory tubes connect the middle ears to the
throat and help maintain equal air pressure on
both sides of the tympanic membranes.
d. Inner ear
(1) The inner ear consists of a complex system of
connected tubes and chambers—the osseous
and membranous labyrinths. It includes the
cochlea, which houses the spiral organ.
(2) The spiral organ includes the hearing receptors
that vibrations in the fl
uids of the inner ear
(3) Different frequencies of vibrations stimulate
different sets of receptor cells; the human ear
can detect sound frequencies from about 20 to
20,000 vibrations per second.
e. Auditory nerve pathways
(1) The nerve F
bers from hearing receptors travel
in the cochlear branch of the vestibulocochlear
(2) Auditory impulses travel into the medulla
oblongata, midbrain, and thalamus and
are interpreted in the temporal lobes of the
4. Sense of equilibrium
a. Static equilibrium maintains the stability of the
head and body when they are motionless. The
organs of static equilibrium are in the vestibule.
b. Dynamic equilibrium balances the head and body
when they are suddenly moved or rotated. The
organs of this sense are in the ampullae of the
semicircular canals.
c. Other structures that help maintain equilibrium
include the eyes and the proprioceptors associated
with certain joints.
5. Sense of sight
a. Visual accessory organs include the eyelids and
lacrimal apparatus that protect the eye and the
extrinsic muscles that move the eye.
b. Structure of the eye
(1) The wall of the eye has an outer, a middle, and
an inner tunic that function as follows:
(a) The outer layer (sclera) is protective, and
its transparent anterior portion (cornea)
refracts light entering the eye.
(b) The middle layer (choroid coat) is vascular
and has pigments that help keep the inside
of the eye dark.
(c) The inner layer (retina) includes visual
receptor cells.
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