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

(1) The activity of 136 Deiters' neurons projecting to lumbosacral segments of the spinal cord has been recorded in decerebrate, partially cerebellectomized cats, and their response characteristics to sinusoidal stimulation of labyrinth and neck receptors have been related to cell size inferred from the conduction velocity of the corresponding axons. (2) Vestibulospinal neurons with faster conduction velocity and, by inference, having thicker axons and larger cell bodies differed from those neurons having lower axonal conduction velocity by displaying (i) a relatively irregular interspike interval distribution; (ii) a lower resting discharge rate; (iii) a periodically modulated response to the labyrinth input elicited by sinusoidal tilt around the animal's longitudinal axis (0.026 Hz, 10 degrees) characterized by an increase in firing rate during side-down roll tilt; (iv) an increase in gain (impulses per sec per degree) and phase lag relative to the displacement of the labyrinth response to increasing angular acceleration; (v) a greater gain in labyrinth than neck input, the latter elicited by sinusoidal neck rotation (0.026 Hz, 5 or 10 degrees); and (vi) due to the imbalance of the gains of the separate labyrinth and neck responses and their predictable vectorial summation, a response to both inputs elicited by head rotation resembling that obtained by labyrinth stimulation alone. (3) These findings are discussed in terms of the reciprocal distribution of synaptic contacts of vestibular and neck afferents on vestibulospinal neurons as a function of cell size. 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 vestibulospinal neurons.