Projection to the cerebral cortex from proximal and distal muscles in the human upper limb

Motor-point stimulation using insulated microelectrodes has been used to assess the cerebral projection from a number of single muscles in the human upper limb. Previous studies with intrafascicular neural stimulation have revealed a short-latency cortical projection from group I muscle afferents of intrinsic muscles of the hand but this technique is unsuitable to study the projection of afferents from proximal muscles. In 6 subjects, stimuli were delivered to the motor point of the following muscles: anterior deltoid, biceps brachii (short head), flexor carpi radialis, extensor digitorum communis, abductor pollicis brevis and first dorsal interosseous. The mean latency of the early cortical negativity was 15.0 ms for deltoid, 16.4 ms for biceps, 18.8 ms for flexor carpi radialis, 18.8 ms for extensor digitorum communis, 21.9 ms for abductor pollicis brevis and 25.0 ms for first dorsal interosseous. The amplitude of cerebral potentials from individual muscles was smaller than that produced by stimulation of the digital nerves of the index or little finger and did not vary systematically between proximal and distal muscles. This study provides direct evidence for a short-latency cortical projection from proximal muscles of the upper limb. The results are consistent with psychophysical studies which have proposed a role for intramuscular receptors in kinaesthesia at both proximal and distal joints.