Axon reflexes in human motor nerve fibres

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Abstract
Muscle action potentials were evoked from the small hand muscles in 25 patients with lower motor neuron disease by stimulation of the median or ulnar nerve at the wrist. Atrophy was evident in these due to chronic partial denervation. When compared with control responses, impulses in some of these patients appeared to ascend the arm for a certain distance and then return to the hand. The muscle action potentials occurred after a delay which could be shortened by moving the stimulating cathode in a proximal direction from the wrist but a delay that was too short to allow conduction to the spinal cord and return. The responses studied showed a constant latency and waveform in successive sweeps which followed rapid rates of stimulation without fatigue. Responses have been elicited by paired shocks at intervals of 1 or 2 msec. These results suggest the presence of axon branching. Branching had occurred in motor fibers in the arm which allowed impulses to ascend to the point of branching and then return via the other branch to the muscle. Responses of this nature were found in 7 patients with local peripheral nerve lesions and in 2 cases of generalized neuropathy. The observed axon reflexes might be due to branching at the tip of a regenerating nerve fiber distal to the site of injury or due to collateral sprouting of normal surviving nerve fibers.

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