中脳ネコ歩行標本の制御歩行時における外側前庭神経核細胞の機能的役割

Abstract

In the acute mesencephalic cat, stimulation delivered to the mesencephalic locomotor region (MLR) elicits "controlled locomotion" on a surface of moving treadmill. In each locomotion, double support phase is observed during slow walk with a cycle time (interstep interval) longer than 0.8s, and single support phase is observed during fast walk with a cycle time shorter than 0.8s.In this experiment, the possibility that well coordinated locomotion requires control of static and dynamic equilibrium of posture was studied with an assumption that during slow walk static equilibrium of posture dominates, and during fast walk dynamic equilibrium of posture dominates. I analysed discharge characteristics of vestibulospinal neurons (lateral vestibular neurons, VL neurons) during locomotion, because the vestibulospinal tract has been known as one of the control system for equilibrium of upright posture and locomotion. The speed of limb movements during locomotion was systematically changed by increasing the speed of moving treadmill, while keeping the stimulus strength to the MLR constant. With an increase in the speed of moving treadmill, cycle time shortened from 1.7s to 0.5s.VL neurons were recorded extracellularly by means of microelectrodes filled with Woods' alloy and identified antidromically by stimulating the L2 segment of the spinal cord. Fifty-four VL neurons were recorded during locomotion. Mean conduction velocity and mean firing frequency of these neurons were 93.9m/s and 11.5 spikes/s respectively. VL neurons were divided into 3 groups from their discharge characteristics. The first group of neurons (15/54) maintained tonic state of discharges with no relation to the shortening of cycle time. The second group of neurons (26/54) changed their tonic state of discharges during slow walk to rhythmic bursting state of discharges during fast walk. The third group of neurons (13/54) maintained rhythmic bursting state of discharges both during slow walk and fast walk. Each bursting discharge was phase-locked with each stance phase of the unilateral or bilateral hindlimbs.These results demonstrate that, during slow walk, most VL neurons discharge tonically playing roles to regulate the overall excitability of extensor muscle tone of hindlimbs, and to maintain static eqilibrium of posture, and that during fast walk, most VL neurons discharge rhythmically playing roles to control the rhythmicity of hindlimb movements, and to maintain dynamic equilibrium of posture. These results indicate that the functional role played by each one of the VL neuron changes depending on the requirement whether static or dynamic equilibrium of posture should be controlled during locomotion.