[UW Anatomy | Contents]

Function:
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.

Pathway:
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.

Deficits:
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 ATROPHY.

Other Note:
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.