Page 256 - Anatomy and Physiology of Farm Animals, 8th Edition

Sense Organs / 241

The axons of the cochlear nerve enter
modiolus, and the axons of the afferent the brainstem with the vestibular nerve at
VetBooks.ir neurons within it gather to form the coch­ the junction of the medulla and pons
lear nerve. Transduction of the mechani­
and terminate in cochlear nuclei on the
cal energy of air pressure waves into the
electrical impulses of neurons in the audi­ lateral side of the medulla. From this first
tory system takes place in the following synapse, multiple pathways produce audi­
manner. tory‐mediated reflexes and conscious
Air pressure waves captured by the perception of sound. Auditory informa­
pinna are funneled down to the tym­ tion destined for the cortex ascends bilat­
panic membrane, and they put that erally in the medulla, pons, and midbrain.
membrane into vibratory motion. This The pathways for conscious perception of
motion is carried across the air‐filled sound continue on to the thalamic relay
cavity of the middle ear by movements nucleus for audition, and from there fibers
of the auditory ossicles. These ossicles project to the primary auditory cortex
transfer the vibrations to the vestibular on the lateral aspect of the cerebrum. The
window via the foot of the stapes. The result of bilaterality of the pathways is
perilymph in the osseous labyrinth that auditory information from both
receives the pressure waves transmitted cochleae reaches both left and right audi­
to it by the vibrations of the vestibular tory cortices.
window (Fig. 12‐8). These are carried The bilaterality of auditory represen-
into the cochlea by the scala vestibuli. tations in the brain means that for a
The vestibular membrane between scala brain lesion to produce complete deaf-
vestibuli and cochlear duct vibrates in ness, it must affect both sides of the
response to the pressure waves and pathway. Such widespread brain injuries
transfers those vibrations across the are often incompatible with life; there-
cochlear duct to the basilar membrane fore, most deafness encountered clini-
and underlying perilymph of the scala cally is peripheral in origin (associated
tympani. Movement of the basilar mem­ with the cochlear nerve or internal ear
brane results in movement of the hair rather than the brain itself). In animals,
cells in the spiral organ that rests upon incomplete loss of hearing is difficult to
it, and this produces bending of the hair detect.
cells’ cilia against the more rigid tectorial Disease processes that affect the abil-
membrane. It is the bending of the cilia ity of the tympanic membrane or audi-
that results in a depolarization within tory ossicles to transmit vibrations to
the hair cells. When this depolarization the vestibular window produce conduc­
reaches sufficient intensity, it initiates tion deafness. Those that affect the spiral
an action potential in the primary affer­ organ or more proximal components of
ents of the spiral ganglion. the auditory system (including the coch-
The ability to discriminate one pitch lear nerves, brainstem, and auditory
from another has its basis in a number of cortices) produce sensorineural deaf­
anatomic and electrical features of the ness. Most inherited deafness is sensori-
spiral organ. Most simply, the basilar neural, brought about by degeneration
membrane varies in width from the base of of cochlear hair cells. Congenital deaf-
the cochlear duct, where it is most narrow, ness has been associated with white hair
to the apex, where it is widest. This differ­ coats or the merle or piebald color genes
ence in width means that each portion of in a variety of species, including dogs
the membrane has a different frequency at (most notably the Dalmatian), cats, and
which it preferentially vibrates (“resonates”), horses. In these individuals, lack of
with the widest part of the membrane pigment‐containing cells inside the coch-
vibrating at low frequency and the narrow leae is linked to degeneration of cochlear
part at high. hair cells within a few weeks of birth.