Behavior of neurons in the abducens nucleus of the alert cat--II. Internuclear neurons.

The activity of 43 antidromically identified abducens internuclear neurons with conduction velocities ranging from 14 to 54 m/s was analyzed in alert cats during spontaneous and vestibular induced eye movements. The discharge rate of internuclear neurons significantly increased with successive adducting positions of the contralateral eye. Slopes of rate-position (k) relationships ranged from 3.1 to 17.9 spikes/deg (mean 12.01 +/- 3.1). Threshold ranged from -19 degrees to +3 degrees. Frequency saturation was never observed for any internuclear neuron within the oculomotor range. Although straight lines were selected to illustrate the rate-position relationships, exponential curves always provided the best statistical fit demonstrating that an enhancement in frequency potentiation (k) must accompany more eccentric fixations in the on direction. Internuclear neurons showed a low variability in firing rate (less than 3.0%) for fixations less than 1 s. Variability increased with both longer and repeated fixations of the same eye position. Discharge rates were found to depend upon both the direction of the preceding eye movement and the animal's level of alertness. Separate regression lines of rate-position relations following saccades in the on and off directions differed significantly in slope (100%), but not threshold. The observed static hysteresis in an identified non-motoneuron shows this property to be in a central neural circuit prior to the extraocular motoneuron. The slopes (k) of rate-position plots for all internuclear neurons decreased significantly (100%) when level of alertness changed from "alert" (1 +/- 0.2 saccades/s) to "drowsy" (0.5 +/- 0.2 saccades/s). Thresholds, however, were not significantly altered. Discharge rate of abducens internuclear neurons increased abruptly 10.4 +/- 2.5 ms preceding saccades in the on direction, and decreased 20.5 +/- 7.8 ms before saccades in the off direction. Internuclear neuronal activity was not affected by pure vertical saccades. During on direction saccades, firing frequency did not saturate, but increased with velocity in a linear fashion. Exponential functions often fit the data better due to the difference in slopes of rate-velocity plots for on vs off direction saccades. Slopes (rs) of rate-velocity regression lines during spontaneous saccades ranged from 0.99 to 4.10 spikes/s/deg/s (mean 2.16 +/- 0.93). During saccades in the off direction activity always decreased, but it seldom ceased. Rate-velocity regression lines measured during the fast phase of vestibular nystagmus (rsv = 2.09 +/- 0.88) showed no significant differences from rs slopes in 82% of the cases.(ABSTRACT TRUNCATED AT 400 WORDS)