Effects of roll tilt of the animal and neck rotation on different size vestibulospinal neurons in decerebrate cats with the cerebellum intact

Abstract

1. In decerebrate cats with the cerebellum intact we recorded the activity of lateral vestibulospinal neurons projecting to lumbosacral segments of the spinal cord (lVS neurons) and related the resting discharge, as well as the response characteristics of these neurons to roll tilt of the animal and neck rotation, with the cell size inferred from the conduction velocity of the corresponding axons. 2. A slight negative correlation was found between resting discharge rate and conduction velocity of the whole population of lVS neurons responsive and unresponsive to animal tilt and neck rotation, so that the faster the conduction velocity, the lower was the unit discharge at rest. This correlation, however, was found only for the dorsocaudal LVN neurons, which contributed to the majority of lVS units, but not for the rostroventral LVN neurons. Moreover, it affected the units unresponsive but not those responsive to vestibular stimulation; the opposite, however, occurred for the units tested to neck stimulation. These findings indicate that the static properties of the lVS neurons can only in part be related to cell size. 3. If we consider the lVS neurons responsive to roll tilt of the animal (76 neurons) and neck rotation (75 neurons) at the standard parameters of 0.026 Hz, ±10°, no significant correlation was found between gain (impulses·s⁻¹·deg⁻¹) of the labyrinth or neck responses and conduction velocity of the axons. Thus, due to the presence of slight negative relation between resting discharge and conduction velocity of the axons, larger neurons exhibited a greater percentage modulation (sensitivity) to the labyrinth and neck inputs than smaller neurons; this correlation involved particularly the dcLVN neurons. These findings suggest that the afferent pathways driven during dynamic stimulation of labyrinth and neck receptors produce an higher number or density of synaptic contacts on lVS neurons of increasing size. 4. No significant differences in the means of resting discharge, conduction velocity, gain and sensitivity were found between all the lVS units responding to labyrinth and neck inputs. These findings indicate that the effectiveness of the two inputs was almost comparable and did not vary in different units as a function of cell size. 5. The lVS neurons were mainly excited during side-down animal tilt and side-up neck rotation. Although these neurons showed the same spectrum of conduction velocity as those displaying the opposite response patterns, the response gains of the predominant populations of units were on the average higher than those obtained from the remaining populations of units. The peak of the unit responses occurred with an average phase lead of +21.0±27.2°, SD with respect to the extreme animal displacements, and an average phase lead of +52.0±18.3, SD with respect to the extreme neck displacements; yet no difference was found in the average phase angle of responses of the two populations of slow and fast units to animal tilt or neck rotation. These findings indicate that the quantitative and qualitative organization of the synaptic inputs represent the critical factor controlling response characteristics of comparable size lVS neurons to vestibular and neck stimulations. 6. A comparison between the present results and those obtained in previous experiments performed after bilateral ablation of the paleocerebellum permitted to evaluate the role that the inhibitory area of the cerebellum exerts on the resting discharge, as well as on the dynamic characteristics of response of different size lVS neurons to animal tilt and neck rotation.