Relation between cell size and response characteristics of medullary reticulospinal neurons to labyrinth and neck inputs.

The activity of presumably inhibitory reticulospinal neurons with cell bodies located in the medial aspects of the medullary reticular formation and axons projecting to lumbosacral cord has been recorded in decerebrate cats and their response characteristics to sinusoidal stimulation of labyrinth receptors (134 neurons) and neck receptors (110 neurons) have been related to cell size inferred from the conduction velocity of the corresponding axons. No significant correlation was found between resting discharge and conduction velocity of the axons. Among the recorded reticulospinal neurons, 64/134 (i.e. 47.8%) units responded to roll tilt, while 66/110 (i.e. 60.0%) units responded to neck rotation (0.026 Hz, +/- 10 degrees). A positive correlation was found between gain (imp./s/deg) of the labyrinth and neck responses and conduction velocity of the axons. Thus, due to absence of correlation between resting discharge and conduction velocity of the axons, larger neurons exhibited a greater percentage modulation (sensitivity) to the labyrinth and the neck input than smaller neurons. These findings are attributed to an overall increase in density or efficacy of the synaptic contacts made by the vestibular and neck afferent pathways on reticulospinal neurons of increasing size. Units receiving neck-macular vestibular convergence showed on the average an higher gain of the neck (GN) response with respect to the labyrinth (GL) response (GN/GL: 1.95 +/- 1.49, S.D.; n = 43); however, due to a parallel increase in gain of the reticulospinal neurons to both neck and labyrinth inputs, the relative effectiveness of the two inputs did not vary in different units as a function of cell size. The reticulospinal neurons were mainly excited by the direction of animal orientation and/or neck displacement. In particular, most of these positional sensitive units were excited by side-up animal tilt (37/58, i.e. 63.8%) and by side-down neck rotation (47/60, i.e. 78.3%). These predominant response patterns were particularly found between large size neurons, whereas small size neurons tended to show also other response patterns. The evidence indicates that in addition to intrinsic neuronal properties related to cell size, the quantitative and qualitative organization of synaptic inputs represents the critical factor controlling the responsiveness of reticulospinal neurons to vestibular and neck stimulation.