Faculty Bibliography

1. In nine alert chronically prepared cats the activity of 177 neurons was recorded in the prepositus nucleus during either spontaneous eye movement or that induced by natural vestibular and optokinetic stimulation. 2. In 116 cells, eye position and/or eye velocity was precisely and unequivocally encoded whatever the origin of the eye movement. These cells were separated into different populations according to the eye movement variable encoded and the directionality of the neuronal response. The firing rates of the remaining 61 cells were loosely related to eye movements because a variety of discharge patterns were observed during identical eye movements. In the latter case, some other unmeasured variable (e.g., neck or visual) was suggested to be encoded in the firing frequency. 3. Discharge rate changed before the eyes began to move and reached a new steady level during fixation following a saccade into a particular direction of the orbit. The ondirection was horizontal for 59% of the neurons, vertical for 17%, and oblique for 24%. 4. Regardless of their preferred direction, the discharge rate in 19% of the neurons was closely proportional to eye position. The range in sensitivity was from 1.1 to 7.5 spikes X s-1/deg. Weak velocity responses were occasionally observed during the slow phase of vestibular and optokinetic nystagmus including during saccades. This class of neurons exhibited a very regular interspike interval for a given position of fixation. Since mainly eye position was encoded, these cells were called position neurons. 5. Other prepositus neurons showed both position and velocity sensitivity during saccades and fixation. Their firing rate encoded eye position over the same range as above and also coded velocity during the slow phase of vestibular and optokinetic nystagmus. Depending on the weighting between the position and velocity proportionality constants, these neurons were classified into position-velocity (48%) or velocity-position (33%) groups. 6. The distribution of the above responses led to the conclusion that the prepositus nucleus plays a role in vertical and horizontal spatial integration. The predominance of horizontal activity suggested that the nucleus may be a significant site underlying genesis of horizontal eye position.

1. Motoneurons innervating the cat retractor bulbi muscle have been identified by retrograde axonal transport of horseradish peroxidase (HRP). Following injections of the four slips of the retractor bulbi muscle, labeled motoneurons were found in the abducens nucleus overlapping the distribution of lateral rectus motoneurons and in the oculomotor nucleus partially overlapping the distribution of medial rectus motoneurons. Retractor bulbi motoneurons also were found in the accessory abducens nucleus situated ventral and lateral to the abducens nucleus. 2. Retractor bulbi motoneurons varied considerably in shape and size, but in all instances contained similar cytoplasmic organelles. Quantitative analyses of mean soma diameter indicated that the average size of retractor bulbi motoneurons was larger than the average size of lateral rectus and medial rectus motoneurons. 3. Retractor bulbi motoneurons in the accessory abducens nucleus were identified electrophysically and stained by intracellular injection of HRP. Neuronal reconstructions demonstrated a dorsomedial axonal trajectory directed toward the abducens nucleus and elongated dendritic fields oriented in a dorsomedial-ventrolateral axis. Another major dendritic extension was directed toward the magnocellular division of the spinal trigeminal nucleus, a major source of excitatory input to these motoneurons. 4. Quantitative analyses of synaptic density indicated that the somata of retractor bulbi motoneurons were contacted by significantly fewer synaptic endings than the somata of motoneurons in the abducens nucleus. Retractor bulbi motoneurons in the abducens nucleus exhibited variations in synaptic density that were similar to the densities on lateral rectus motoneurons. 5. Given the morphological differences in location, size, and somadendritic extent between motoneurons in the accessory abducens, abducens and oculomotor nuclei, it is suggested that such features reflect functional differences between the motoneurons with respect to fiber composition of the muscles they innervate, and subsequently to the role each muscle plays in eye movement. 6. Since the morphological features of retractor bulbi motoneurons in the accessory abducens nucleus are quite different from those in either the abducens or oculomotor nuclei, it appears that each motoneuronal population may perform unique oculomotor functions.

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.