SPH405
| Neurological Foundations of Speech, Language and Hearing |
THE VESTIBULAR APPARATUS: CONNECTING THE BODY TO THE EARTH
The vestibular apparatus has sensory receptors in the semicircular
canals and in the utricle and saccule, located within the petrous portion
of the temporal bone.
It conveys action potentials though the Vestibular division of
the VIIIth cranial nerve: The Vestibulocochlear Nerve.
These impulses evoke contractions of the extraocular muscles to
keep the eyes focused on objects as the head changes position.
They also evoke movements of the postural muscles for maintenance
of appropriate postures in response to changes in gravity pullover
a wide range of possible body positions relative to the center of
the planet.
There is great interconnectivity between the vestibular apparatus,
brainstem nuclei, spinal cord and the cerebellum.
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This interconnectivity between the vestibular nerve and some
cranial nerve nuclei encourages postural control by vision,
A most important option in man.
Long descending vestibular fibers leave the lateral recess of the fourth ventricle's floor (called the area vestibularis) and form columns of neuronal cells which extend along the medulla to the sensory decussation.
The Lateral vestibular nuclei are the origins of the vestibulospinal tracts, which have input to the lower motor neurons in spinal segments.
Cerebellar connections enhance synergy of postural muscles. There are vestibular nerve fibers which course into the cerebellum without synapse in the brainstem.
Most vestibular fibers bifurcate into ascending and descending divisions
as they course to their terminal nuclei.
These nuclei are located in the lateral recess of the floor of
the fourth ventricle.
Ascending fibers go to three nuclei:
The Medial Vestibular Nucleus
Lateral Vestibular Nucleus (of Deiters)
- As mentioned above, fibers of the vestibulospinal tract originate here.
- Axons of the Lateral Vestibular Nucleus convey vestibular impulses to the lower motor neurons of the spinal cord.
Superior Vestibular Nucleus (of Bechterew)
Descending fibers form the long descending vestibular nucleus
down the medulla as far as the sensory decussation.
Some vestibular nerve fibers inter the cerebellum and terminate
there without interruption.
After they leave the brainstem nuclei, vestibular system fibers go
in three directions.
Some form an association tract:
These convey vestibular impulses to the various cranial nerve
nuclei.
Such impulses may be associated with head/neck responses to
labyrinthine stimulation.
Some enter the cerebellum through the inferior cerebellar peduncle
without synapse.
Those from the lateral vestibular nucleus form the vestibulospinal
tracts (see above).
In general, vestibular functions are not conscious. While we are
aware when we are "off balance" or when we change postures
to maintain balance, such changes happen so fast that we probably wouldn't
be able to keep up if we had to wait the extra milliseconds it takes
to make conscious responses.
Once you have finished you should:
Go on to Assignment 1
or
Go back to The Vestibular System
E-mail Bill Culbertson
at bill.culbertson@nau.edu
Call Bill Culbertson
at (520) 523-7440
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