Page 258 - Anatomy and Physiology of Farm Animals, 8th Edition

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right angles to one another and are desig­ would continually slip across the retina
during head movement, and focusing on
nated anterior, posterior, and lateral to
VetBooks.ir describe their orientation. As an extension the visual field would be difficult or
impossible while the head was moving.
of the membranous labyrinth, each duct is
filled with endolymph and surrounded by The vestibular nuclei use information about
perilymph. acceleration to coordinate extraocular
One end of each duct is dilated to form muscle movements with movements of the
an ampulla, within which are housed the head and thereby fix the visual image in
receptor organs of the semicircular ducts one place on the retina. When the excursion
(Fig. 12‐10). One wall of the ampulla features of the moving head carries the fixed image
a transverse ridge of connective tissue, the out of visual range, the eyes dart ahead in
crista ampullaris, which supports a neu­ the direction of movement to fix upon a
roepithelium of hair cells. Attached to the new image. This new image is held on the
crista is a gelatinous cupula; this extends retina while the head continues turning
across the ampulla, forming a flexible until a compensatory jump ahead is again
barrier to the flow of endolymph. The cilia needed. This mechanism results in a cycle
of the hair cells are embedded in the cupula of slow movement opposite the direction
and are therefore bent by movements of it. of turn (eyes fixed on target) followed by a
The semicircular ducts detect angular rapid readjustment in the same direction
acceleration (rotation), and their planes as the turn. This oscillatory reflex eye
of orientation roughly correspond to the movement is called nystagmus.
X‐, Y‐, and Z‐axes of three‐dimensional Nystagmus is a normal reflex, generated
space. When the head rotates, the semicir­ in response to movement of the head.
cular duct lying in that rotational plane Nystagmus is considered abnormal when it
moves with it. The endolymph inside the occurs in absence of head movement; this
duct, however, must overcome its inertia resting (spontaneous) nystagmus is a sign
at the start of rotation and, as a conse­ of vestibular disease.
quence, briefly lags behind the movement Axons of some neurons in the vestibular
of the head. The flexible cupula, acting nuclei project caudad in a motor tract that
as a dam across the ampulla, bulges in influences activity in cervical and upper
response to the endolymph’s push, and in thoracic spinal cord segments. These motor
doing so, bends the cilia of the embedded connections activate neck musculature
hair cells. With three pairs (right and left) and forelimb extensors, producing the
of semicircular ducts detecting movement vestibulocollic reflex, which generates
in the three planes of space, complex rota­ neck movements and forelimb extension to
tional movements of the head are encoded help keep the head level with respect to
in the firing patterns of the six cristae gravity and movement.
ampullares. Some fibers from the vestibular nuclei
Primary afferent neurons synapse with form an ipsilateral descending motor tract
the hair cells of the vestibular apparatus. that extends the length of the spinal cord.
Their cell bodies are in the vestibular gan­ This lateral vestibulospinal tract is part
glion, and their axons constitute the ves­ of the ventromedial motor system (see
tibular nerve, which joins the cochlear Chapter 10). Its activity has the primary
nerve to become the eighth cranial nerve. effect of increasing tone in antigravity
Most axons in the vestibular nerve synapse muscles (proximal limb extensors and axial
in the large vestibular nuclei of the pons muscles). This vestibulospinal reflex uses
and rostral medulla. vestibular information to produce limb
and trunk movements that counteract the
Vestibular Reflexes. If there were no displacement of the head that elicits it. This
mechanism to keep the eyes steady on a mechanism is designed to prevent tilting or
target when the head moved, visual images falling with shifts in head position.