312 Chapter 10
- When a person is upright, the stereocilia of the utricle
are oriented vertically; those of the saccule are oriented
horizontally. - Linear acceleration produces a shearing force between
the hairs of the otolithic membrane, thus bending the
stereocilia and electrically stimulating the sensory endings.
C. The three semicircular canals are oriented at nearly right
angles to each other, like the faces of a cube. - The hair cells are embedded within a gelatinous
membrane called the cupula, which projects into the
endolymph. - Movement along one of the planes of a semicircular
canal causes the endolymph to bend the cupula and
stimulate the hair cells. - Stimulation of the hair cells in the vestibular apparatus
activates the sensory neurons of the vestibulocochlear
nerve (VIII), which projects to the cerebellum and to the
vestibular nuclei of the medulla oblongata.
a. The vestibular nuclei in turn send fibers to the
oculomotor center, which controls eye movements.
b. Spinning and then stopping abruptly can thus cause
oscillatory movements of the eyes (nystagmus).
10.5 The Ears and Hearing 282
A. The outer ear funnels sound waves of a given frequency
(measured in hertz) and intensity (measured in decibels) to
the tympanic membrane, causing it to vibrate.
B. Vibrations of the tympanic membrane cause movement
of the middle-ear ossicles—malleus, incus, and stapes—
which in turn produces vibrations of the oval window of the
cochlea.
C. Vibrations of the oval window set up a traveling wave of
perilymph in the scala vestibuli.
- This wave can pass around the helicotrema to the scala
tympani, or it can reach the scala tympani by passing
through the scala media (cochlear duct). - The scala media is filled with endolymph.
a. The membrane of the cochlear duct that faces the
scala vestibuli is called the vestibular membrane.
b. The membrane that faces the scala tympani is called
the basilar membrane.
D. The sensory structure of the cochlea is called the spiral organ
or organ of Corti. - The organ of Corti rests on the basilar membrane and
contains sensory hair cells.
a. The stereocilia of the hair cells project upward into
an overhanging tectorial membrane.
b. The hair cells are innervated by the vestibulocochlear
nerve (VIII). - Sounds of high frequency cause maximum displacement
of the basilar membrane closer to its base, near the
stapes; sounds of lower frequency produce maximum
displacement of the basilar membrane closer to its apex,
near the helicotrema.
a. Displacement of the basilar membrane causes the
hairs to bend against the tectorial membrane and
stimulate the production of nerve impulses.
b. Pitch discrimination is thus dependent on the region of the
basilar membrane that vibrates maximally to sounds of dif-
ferent frequencies.
c. Pitch discrimination is enhanced by lateral inhibition.10.6 The Eyes and Vision 290
A. Light enters the cornea of the eye, passes through the pupil
(the opening of the iris) and then through the lens, from
which point it is projected to the retina in the back of the eye.
1. Light rays are bent, or refracted, by the cornea and lens.
2. Because of refraction, the image on the retina is upside
down and right to left.
3. The right half of the visual field is projected to the left
half of the retina in each eye, and vice versa.
B. Accommodation is the ability to maintain a focus on the retina
as the distance between the object and the eyes is changed.
1. Accommodation is achieved by changes in the shape
and refractive power of the lens.
2. When the muscles of the ciliary body are relaxed, the
suspensory ligament is tight, and the lens is pulled to its
least convex form.
a. This gives the lens a low refractive power for
distance vision.
b. As an object is brought closer than 20 feet from
the eyes, the ciliary body contracts, the suspensory
ligament becomes less tight, and the lens becomes
more convex and more powerful.
C. Visual acuity refers to the sharpness of the image. It depends
in part on the ability of the lens to bring the image to a focus
on the retina.
1. People with myopia have an eyeball that is too long, so
that the image is brought to a focus in front of the retina;
this is corrected by a concave lens.
2. People with hyperopia have an eyeball that is too short,
so that the image is brought to a focus behind the retina;
this is corrected by a convex lens.
3. Astigmatism is the condition in which asymmetry of
the cornea and/or lens causes uneven refraction of light
around 360 degrees of a circle, resulting in an image
that is not sharply focused on the retina.10.7 Retina 297
A. The retina contains rods and cones—photoreceptor neurons
that synapse with bipolar cells.
1. When light strikes the rods, it causes the
photodissociation of rhodopsin into retinene and opsin.
a. This bleaching reaction occurs maximally with a
light wavelength of 500 nm.
b. Photodissociation is caused by the conversion of
the 11- cis form of retinene to the all- trans form that
cannot bind to opsin.
2. In the dark, more rhodopsin can be produced, and
increased rhodopsin in the rods makes the eyes more
sensitive to light. The increased concentration of
rhodopsin in the rods is partly responsible for dark
adaptation.