EXTENSOR REFLEXES OF THE CHRONIC SPINAL CAT

¹ Department of Physiology, Oxford

1. The fraction of the maximum tension capacity of m. soleus which can be enlisted in reflex contraction from a large contralateral nerve is 75-85 per cent., in chronic spinal animals.

2. The rate and type of recruitment in the crossed extensor reflex of spinal animals are largely determined by the proportion of excitatory and inhibitory fibres excited by a given intensity of stimulus.

3. A single shock is a more effective stimulus to contralateral extension than is a tetanic series of such shocks. This discrepancy increases with strong stimuli. An interpretation is offered in terms of different time relations in the development of central excitation and inhibition from the crossed nerve.

4. The "spinality" of crossed extensor reflexes is more apparent with tetanic than with single-shock stimulation.

5. Spinal "stepping" in the isolated soleus evoked by contralateral nerve-trunk stimulation of varied intensity is described, and evidence is given for interaction of excitation and inhibition as a factor in the genesis of stepping.

6. The myotatic reflex to slowly applied and to maintained stretch in chronic spinal cats usually remains scanty, despite recovery of proprioceptive arcs enabling the hind-limbs to stand. An instance of well-developed stretch-reflex in soleus muscle of a chronic spinal cat is recorded.

Expenses for materials were in part met by the Rockefeller Foundation. We wish to thank Professor Sherrington for his advice, and for receiving us into his laboratory.