CORTICORETICULAR PATHWAYS IN THE CAT - II - DISCHARGE ACTIVITY OF NEURONS IN AREA-4 DURING VOLUNTARY GAIT MODIFICATIONS

We propose that the descending command from area 4 that is responsible , in part, for the change in limb trajectory required to step over an obstacle in one's path also plays a role in triggering the anticipator y postural modifications that accompany this movement. To test this hy pothesis, we recorded the discharge characteristics of identified clas ses of corticofugal neurons in area 4 of the cat. Neurons were identif ied either as: pryamidal tract neurons (PTNs) if their axon projected to the caudal pyramidal tract (PT) but not to the pontomedullary retic ular formation (PMRF); as corticoreticular neurons (CRNs) if their axo n projected to the PMRF but not to the PT; and as PTN/CRNs if their ax on projected to both structures. Altogether, the discharge properties of 212 corticofugal neurons (109 PTNs, 66 PTN/CRNs, and 37 CRNs) withi n area 4 were recorded during voluntary gait modifications. Neurons in all three classes showed increases in their discharge frequency durin g locomotion and included groups that increased their discharge either during the swing phase of the modified step, during the subsequent st ance phase, or in the stance phase of the cycle preceding the step ove r the obstacle. A slightly higher percentage of CRNs (39%) discharged in the stance phase prior to the gait modification than did the PTNs o r PTN/CRNs (20% and 17% respectively). In 37 electrode penetrations, w e were able to record clusters of 3 or more neurons within 500 mu m of each other. In most cases, PTN/CRNs recorded in close proximity to PT Ns had similar receptive fields and discharged in a similar, but not i dentical, manner during the gait modifications. Compared with adjacent PTNs, CRNs normally showed a more variable pattern of activity and fr equently discharged earlier in the step cycle than did the PTNs or PTN /CRNs. We interpret the results as providing support for the original hypothesis. We suggest that the collateral branches to the PMRF from c orticofugal neurons with axons that continue at least as far as the ca udal PT provide a signal that could be used to trigger dynamic postura l responses that are appropriately organized and scaled for the moveme nts that are being undertaken. We suggest that the more variable and e arlier discharge activity observed in CRNs might be used to modify the postural support on which the movements and the dynamic postural adju stments are superimposed.