Vestibulospinal and reticulospinal neuronal activity during locomotion in the intact cat. I. Walking on a level surface.

To examine the function of descending brain stem pathways in the control of locomotion, we have characterized the discharge patterns of identified vestibulo- and reticulospinal neurons (VSNs and RSNs, respectively) recorded from the lateral vestibular nucleus (LVN) and the medullary reticular formation (MRF), during treadmill walking. Data during locomotion were obtained for 44 VSNs and for 63 RSNs. The discharge frequency of most VSNs (42/44) was phasically modulated in phase with the locomotor rhythm and the averaged peak discharge frequency ranged from 41 to 165 Hz (mean = 92.8 Hz). We identified three classes of VSNs based on their discharge pattern. Type A, or double peak, VSNs (20/44 neurons, 46%) showed two peaks and two troughs of activity in each step cycle. One of the peaks was time-locked to the activity of extensor muscles in the ipsilateral hindlimb while the other occurred anti-phase to this period of activity. Type B, or single pause, neurons (13/44 neurons, 30%) were characterized by a tonic or irregular discharge that was interrupted by a single pronounced and brief period of decreased activity that occurred just before the onset of swing in the ipsilateral hindlimb; some type B VSNs also exhibited a brief pulse of activity just preceding this decrease. Type C, or single peak, neurons (9/44 neurons, 23%) exhibited a single period of increased activity that, in most cells, was time-locked to the burst of activity of either extensor or flexor muscles of a single limb. The population of RSNs that we recorded included neurons that showed phasic activity related to the activity of flexor or extensor muscles [electromyographically (EMG) related, 26/63, 41%], those that were phasically active but whose activity was not time-locked to the activity of any of the recorded muscles (13/63, 21%) and those that were completely unrelated to locomotion (24/63, 38%). Most of the EMG-related RSNs showed one (15/26) or two (11/26) clear phasic bursts of activity that were temporally related to either flexor or extensor muscles. The discharge pattern of double-burst RSNs covaried with ipsilateral and contralateral flexor muscles. Peak averaged discharge activity in these EMG-related RSNs ranged from 4 to 98 Hz (mean = 35.2 Hz). We discuss the possibility that most VSNs regulate the overall activity of extensor muscles in the four limbs while RSNs provide a more specific signal that has the flexibility to modulate the activity of groups of flexor and extensor muscles, in either a single or in multiple limbs.