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VESTIBULAR NUCLEI AND ABDUCENS NUCLEUS

VESTIBULAR NUCLEI AND ABDUCENS NUCLEUS - VESTIBULAR NUCLEI...

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VESTIBULAR NUCLEI AND ABDUCENS NUCLEUS Both the vestibular nuclei and abducens nucleus are in close functional association in the control of eye movements. There are four vestibular nuclei within the brain stem (superior, lateral, medial, and inferior). All four can not be seen in the same cross section, since they are present for a considerable rostrocaudal distance from the rostral medulla to the middle of the pons. You only have to be able to identify the MEDIAL and INFERIOR vestibular nuclei, both of which are present at level #4 (shown below on the left). The vestibular nuclei receive their primary input from the vestibular portion of C.N. VIII (vestibular-auditory). The axons in the vestibular nerve are the central processes of neurons that lie in t he vestibular or Scarpa’s ganglion which lies in the internal auditory meatus. These central processes terminate in the vestibular nuclei and the cerebellum. The peripheral processes of these cells receive information from the receptors of the vestibular labyrinth, i.e. hair cells located in the semicircular canals and the saccule and the utricle (otolith organs).
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SEMICIRCULAR CANALS The three (on each side) membranous semicircular canals lie within the bony labyrinth and contain endolymph. As shown in two of the drawings below, the canals, one horizontal and two vertical, lie in three planes that are perpendicular to each other. The HORIZONTAL or lateral canals on the two sides lie in the same plane, while the plane of each anterior canal is parallel to that of the posterior canal of the opposite side. The horizontal semicircular canals communicate at both ends with the utricle, which is a large dilation of the membranous labyrinth. The vertical canals (anterior and posterior) communicate with the utricle at one end, and join together at the other end (the common canal communicates with the utricle). At one end of each semicircular canal is a dilation called the ampulla (L., little jar, is labeled “amp” in the upper right drawing below). The ampulla of a horizontal semicircular canal has been enlarged in the drawing below (upper left). Each ampulla contains a crista (crista ampullaris; ridge), which is a transversely oriented ridge of tissue. The upper surface of the crista contains ciliated sensory hair cells that are embedded in a gelatinous material called the cupula (L., little tube). These ciliated sensory hair cells contain vesicles that possess neurotransmitter. When the neurotransmitter is released from the hair cell, the peripheral process of a cell in the vestibular ganglion is turned on. Interestingly, the hair cells release transmitter even when they
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are not stimulated, so the axons in the vestibular nerve are always firing at a baseline rate. Each hair cell of the crista possesses several shorter stereocilia and a single tall kinocilium at one margin of the cell as shown in the lower figure. Deflection of the stereocilia TOWARD the kinocilium results in an INCREASE in the firing rate of the vestibular fiber associated with the hair cell, while deflection AWAY from the kinocilium results in a DECREASE in the firing rate of the vestibular fiber.
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