Electromyographic responses evoked in muscles of the forelimb by intracortical stimulation in the cat

Chronically implanted microwires were used to deliver brief trains of electrical stimuli (11 cathodal pulses at 330 Hz and intensity 5-35 microA) to sixty-two locations in the grey matter of the pericruciate cortex in cats. Electromyographic (e.m.g.) responses in the contralateral forelimb were recorded from a total of ten muscles (four to eight in each animal) acting about the shoulder, elbow and wrist and on the digits. The animals were relaxed with little background e.m.g. in the muscles and as a result only excitatory effects could be described. Five muscles which are flexors in the locomotor context were excited from more electrodes, distributed more widely across the motor cortex, than another five muscles which are extensors during locomotion; this difference in 'accessibility' was present both at 35 microA stimulus intensity and at 15 microA. At a stimulus intensity of 15 microA, effective cortical electrodes tended to cluster either in the most lateral part of the anterior sigmoid gyrus (rostromedial focus) or in the coronal gyrus just caudal to a line prolonged beyond the lateral end of the cruciate sulcus (caudolateral focus). This is consistent with the existence of a double motor representation within the forelimb motor cortex (Pappas & Strick, 1981). The two foci were similar in that both gave rise to more flexor than extensor responses and to fewer responses in digit or wrist muscles than in muscles acting about more proximal joints (elbow and shoulder). At stimulus intensity 35 microA the latency of the earliest e.m.g. responses ranged from 11 to 14 ms in different muscles. For some muscles and electrodes the amplitude of the e.m.g. responses was substantially altered by a quite small postural change. After pyramidectomy the cortical thresholds and the e.m.g. latencies were both greatly increased.