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

In decerebrate cats with the cerebellum intact we recorded the activity of lateral vestibulospinal neurons projecting to lumbosacral segments of the spinal cord (IVS 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. A slight negative correlation was found between resting discharge rate and conduction velocity of the whole population of IVS 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 IVS 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 IVS neurons can only in part be related to cell size. If we consider the IVS neurons responsive to roll tilt of the animal (76 neurons) and neck rotation (75 neurons) at the standard parameters of 0.026 Hz, +/- 10 degrees, no significant correlation was found between gain (impulses X s-1 X 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 IVS neurons of increasing size. No significant differences in the means of resting discharge, conduction velocity, gain and sensitivity were found between all the IVS 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. The IVS 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.(ABSTRACT TRUNCATED AT 400 WORDS)