Central compensation of vestibular deficits. II. Influences of roll tilt on different-size lateral vestibular neurons after ipsilateral labyrinth deafferentation.

The activity of 168 Deiters' neurons projecting to lumbosacral segments of the spinal cord has been recorded in precollicular decerebrate cats after ipsilateral acute (aVN) or chronic vestibular neurectomy (cVN), and their response characteristics to sinusoidal stimulation of contralateral labyrinth receptors at the standard parameters (roll tilt at 0.026 Hz, 10 degrees peak displacement) have been related to cell size inferred from the conduction velocity of the corresponding axons. These findings were compared with those elicited in decerebrate cats with both vestibular nerves intact. In all experimental conditions, the higher the coefficient of variation (CV) of the vestibulospinal neurons, reflecting a more irregular unit discharge, the lower was the mean discharge rate at rest. However, the proportion of regularly discharging units (with the lowest CV) decreased after aVN but increased after cVN. The relation found in control experiments, i.e., the faster the conduction velocity of vestibulospinal axon the lower was the unit discharge at rest, was lost after aVN due to a decrease in resting discharge rate of the slow neurons. The mean discharge rate of these units, however, recovered after cVN, so that the negative correlation between resting discharge rate and axonal conduction velocity was reestablished. After aVN, the decrease in resting discharge rate of the slow vestibulospinal neurons was not associated with significant changes in gain (impulses per second per degree) of the unit responses to standard parameters of tilt, so that the sensitivity of these units (percentage change of the mean discharge rate per degree) increased; on the other hand, the resting discharge rate of the fast neurons, which remained almost unchanged after aVN, was associated with a significant increase in gain, thus leading to an average increase in response sensitivity of these units.(ABSTRACT TRUNCATED AT 400 WORDS)