Initiation of muscle activity in spinalized pigeons during spinal cord cooling and warming

1. The effect of spinal cord temperature changes on muscle activity was investigated in unanaesthetized intact and chronically spinalized pigeons and in acutely spinalized pigeons which were artifically respirated and lightly anaesthetized with ether. 2. Spinal cord cooling regularly produced an increase in muscle activity and visible muscle tremor in intact and spinalized pigeons. This motor cold defence reaction was less intensive in spinalized animals, but was qualitatively identical in all groups with regard to spindle shaped firing patterns and grouped discharges. 3. Intravenous injection of 60-100 mg/kg L-Dopa enhanced the motor response to spinal cord cooling in acutely spinalized pigeons. It is suggested that L-Dopa may act on dopaminergic or noradrenergic neurones in the spinal cord. 4. The results demonstrate that the generation of the motor cold defence response to cooling of the spinal cord in pigeons is basically independent from supra-spinal nervous mechanisms. The decreased intensity of cold induced muscle activity in spinalized animals may be attributed to the loss of excitatory or disinhibitory descending inputs to the spinal cord. 5. Spinal cord warming above normal body temperature (41.5 degrees C) produced an increase in muscle activity and slow muscle movements. The pattern was qualitatively different from that of the cold induced tremor.