Pyramidal tract neurons receptive to different forelimb joints act differently 1 during locomotion

23 During locomotion, motor cortical neurons projecting to the pyramidal tract 24 (PTNs) discharge in close relation to strides. How their discharges vary based on the part 25 of the body they influence is not well understood. We addressed this question with regard 26 to joints of the forelimb in the cat. During simple and ladder locomotion, we compared 27 the activity of four groups of PTNs with somatosensory receptive fields involving 28 different forelimb joints: (1) 45 PTNs receptive to movements of shoulder, (2) 30 PTNs 29 receptive to movements of elbow, (3) 40 PTNs receptive to movements of wrist, and (4) 30 30 non-responsive PTNs. In the motor cortex, a relationship exists between the location 31 of the source of afferent input and the target for motor output. Based on this relationship, 32 we inferred the forelimb joint that a PTN influences from its somatosensory receptive 33 field. We found that different PTNs tended to discharge differently during locomotion. 34 During simple locomotion, shoulder-related PTNs were most active during late 35 stance/early swing, and upon transition from simple to ladder locomotion, often increased 36 activity and stride-related modulation while reducing discharge duration. Elbow-related 37 PTNs were most active during late swing/early stance, and typically did not change 38 activity, modulation, or discharge duration on the ladder. Wrist-related PTNs were most 39 active during swing, and upon transition to the ladder often decreased activity and 40 increased modulation while reducing discharge duration. These data suggest that during 41 locomotion the motor cortex uses distinct mechanisms to control the shoulder, elbow, and 42 wrist. 43 44