Central regulation of motor cortex neuronal responses to forelimb nerve inputs during precision walking in the cat

1. The responses of neurones in forelimb motor cortex to impulse volleys ev oked by single pulse electrical stimulation (at 1 5 or 2 times the threshol d for most excitable nerve fibres) of the superficial radial (SR) and ulnar (UL) nerves of the contralateral forelimb were studied in awake cats both resting quietly and walking on a horizontal ladder. Nerve volley amplitude was monitored by recording the compound action potential elicited by the st imulus. 2. In the resting animal 34/82 (41%) cells yielded statistically significan t responses to SR stimulation, and 20/72 (28%) responded to UL stimulation. Some responses were confined to or began with an increase;se in firing pro bability ('excitatory' responses) and others with a decrease in firing ('in hibitory' responses), typically including a brief interruption of the spike train (zero rate). Cells responding to both nerves usually yielded respons es similar in type. Most (78%) response onset latencies were less than 30 m s. Responses involved the addition or subtraction of from 3.4 to 0.1 impuls es stimulus(-1) (most ( 1 impulse stimulus(-1). The distribution of respons e sizes was continuous down to the smallest values, i.e. there was no 'gap' which would represent a clear separation into 'responsive' and 'unresponsi ve' categories. Responses were commonest in the lateral part of the pericru ciate cortex, and commoner among pyramidal tract neurones (PTNs) than non-P TNs. 3. During ladder walking most cells generated a rhythmic step-related disch arge; in assessing the size of responses to nerve stimulation (20 studied, from 13 cells) this activity was first subtracted. Response onset latencies (90% < 30 ms) and durations showed little or no change. Although most cell s were overall more active than during rest both 'excitatory' and 'inhibito ry' responses in both PTNs and non-PTNs were often markedly reduced in larg e parts of the step cycle; over some (usually brief) parts responses approa ched or exceeded their size during rest, i.e. response size was step phase dependent. Such variations occurred without parallel change in the nerve co mpound action potential, nor were they correlated with the level of backgro und firing at the time that the response was evoked. when responses to both nerves were studied in the same neurone they differed in their patterns of phase dependence. 4. The findings are interpreted as evidence for central mechanisms that, du ring 'skilled', cortically controlled walking, powerfully regulate the exci tability of the somatic afferent paths from forelimb mechanoreceptors (incl uding low threshold cutaneous receptors) to motor cortex. Retention (or enh ancement) of responsiveness often occurred (especially for ulnar nerve) aro und footfall, perhaps reflecting a behavioural requirement for sensory inpu t signalling the quality of the contact established with the restricted sur face available for support.