Eye movement related activity and morphology of second order vestibular neurons terminating in the cat abducens nucleus.

Intracellular records were obtained from axons of second order vestibular neurons in, and around, the left abducens nucleus in alert cats implanted with stimulating electrodes on both vestibular nerves and the left VIth nerve. Twelve secondary vestibular neurons were identified by their increase in firing rate with horizontal head rotation to the left and/or increasing eye position to the right. Following HRP injection, somatic location, axonal trajectory and termination sites were determined. Each of the above cells collateralized extensively in the abducens nucleus in a fashion consistent with their being either inhibitory (n = 7; left) or excitatory (n = 6; right) vestibular neurons in the disynaptic horizontal vestibulo-ocular reflex pathway. These vestibular neurons also arborized extensively in other posterior brainstem eye-movement related areas as well as sending an axon to the spinal cord.