Low-threshold, short-latency cutaneous reflexes during fictive locomotion in the "semi-chronic" spinal cat

Low-threshold, short-latency cutaneous reflexes evoked in ipsilateral hindlimb motor nerves were examined during fictive locomotion. Locomotion in 11 anaemically decerebrated spinal animals (1-3 weeks after transection at T13-L1) was induced by administration of clonidine, L-dopa and nialamide; by administration of the latter two drugs only; or by exteroceptive stimulation in the absence of any drugs. The caudal and lateral cutaneous sural, caudal cutaneous femoral, saphenous and superficial peroneal nerves were stimulated at low threshold (1.5-3 T). Pooled results from all combinations of cutaneous nerves stimulated and muscle nerves recorded show that the initial response was excitatory in 40 of 50 triceps surae and 17 of 20 semitendinosus (St) electroneurograms (ENGs). These excitatory responses occurred at latencies that ranged from 5 to 15 ms and tended to be maximal during the motor nerve's active period in the step cycle (i.e. they were modulated in a phase-dependent manner). Only three inhibitory responses (9-12 ms earliest latency) were encountered in total: in two St ENGs of one animal and in one lateral gastrocnemius-soleus ENG of a different animal. In two animals a "second" excitatory response (15-25 ms latency) was sometimes recorded in triceps surae and St nerves and, interestingly, could be modulated out of phase with the early response. Weak short-latency excitatory reflexes were also found in contralateral St ENGs when examined. Finally, among medial gastrocnemius, lateral gastrocnemius and soleus nerves, excitatory responses due to stimulation of any particular cutaneous nerve tended to be modulated similarly but were of consistently different amplitude among the three. This finding, together with the general observation that excitatory reflexes produced by stimulation of a particular cutaneous nerve were modulated similarly in extensors (or flexors) of different animals, suggests that spinal circuits generating locomotion may indeed exert a stereotypic control over interneurons in specific cutaneous reflex pathways to motoneurons. The results are primarily discussed in terms of the existing evidence for short-latency excitatory cutaneous reflexes in extensors in a variety of locomotive and non-locomotive preparations.