Point: 13a. Vestibular Nuclei-Reflex Control of
Reflex control of the head and body via pathways that convey
information from receptors in the three pairs of
semicircular canals and the saccule and utricle to the
Peripheral processes of cells in vest. ganglion end on hair
cells in the receptors. Central processes comprise the
vestibular nerve which terminates in the four vest. nuc.
(lie in rost. medulla and pons; important nuclei to identify
in sections=inferior and medial [level #4]). Neurons in lat.
vest. nuc. project to IPSI. spinal cord via LAT.
vestibulospinal tract (L.V.S.T.; vent. funic.). Cells in
med. vest. nuc. project to BOTH sides (contra. is bigger) of
spinal cord via MED. vestibulospinal tract (M.V.S.T.; vent.
Lesion of vestibular nuc. or nerve = IPSI. stumbling and
falling. That is, you stumble TOWARD the lesion, since the
normal side is OK and kind of PUSHING you toward the weak
THE ASSOCIATION OF THE VEST. NUC. WITH THE ABDUCENS (EYE
MOVEMENTS) IS COVERED IN 13b.
Point: 13b. Vestibular nuclei-Reflex Control of Eye Mvmnt
To convey vestibular information to neurons that innervate
eye muscles in order to stabilize images on the retina. For
instance, a fast movement of the head to the RIGHT results
in a compensatory reflex movement of BOTH eyes to the LEFT.
We are concerned only with horizontal eye movements. For
example, both eyes move to the LEFT following head movement
to the the RIGHT. When the head rotates to the RIGHT the
hair cells in the RIGHT horizontal S.C.C. increase their
firing. This information is then conveyed to the RIGHT
vestibular nuclei via the RIGHT vest. nerve. Neurons in the
RIGHT vestibular nuclei project to the LEFT (contra.)
paramedian pontine reticular formation (P.P.R.F.; level #5).
Cells in the P.P.R.F. project to the IPSI. abducens nucleus.
Some cells in abducens nucleus innervate the IPSI. lateral
rectus muscle (are lower motor neurons; L.M.N.s), while
others possess axons that cross the midline and ascend in
the medial longitudinal fasciculus (are not L.M.N.s; M.L.F.)
to reach the oculomotor nucleus (contra. to the cells in
abducens nucleus), and in particular to cells innervating
the medial rectus muscle. These are the circuits that
underlie horizontal movement of BOTH eyes in the direction
opposite to the rotating head movement.
Lesion of the vestibular nerve or nucleus results in
nystagmus to the CONTRA. side. The normal side (vestibular
nuclei and nerve) is unopposed and slowly pushes both eyes
toward the lesion side. Then there is a fast return toward
the normal side. Nystagmus (to and fro) is named by the FAST
COMPONENT. Lesion in the P.P.R.F. results in the inability
to turn both eyes IPSI. past the midline upon attempted
horizontal gaze towards IPSI. side (NO ATROPHY). Lesion in
the abducens nucleus results in the inability to turn both
eyes IPSI. past the midline upon attempted horizontal gaze
towards IPSI. side (ATROPHY of IPSI. lateral rectus). Lesion
of the abducens nerve results in deviation of IPSI. eye
medially and diplopia (to ameliorate, rotate head TOWARDS
the side of the weak muscle). Lesion of the M.L.F. results
in the inability to move the IPSI. (to M.L.F.) eye medially
upon attempted horizontal gaze to the contra. side. NO
Frontal eye field of cerebral cortex project to CONTRA.
P.P.R.F. Therefore, a lesion of the F.E.F. will result in
inability to turn eyes past the midline upon attempted
horizontal gaze to the CONTRA. (to the F.E.F.) side. Eyes
deviate TOWARDS side of CORTICAL lesion.