Motor reflex responses elicited by cutaneous stimulation in the regenerating nerve of man: Axon reflex or ephaptic response?

Abstract

In 57 of 60 nerves (29 median and 31 ulnar) sutured at the wrist, forearm and arm, we recorded motor responses in thenar or hypothenar muscles by electrical stimulation of the corresponding fingers. Recordings were made at different times during the process of regeneration, ranging from 3 months up to 11 years. The responses showed a constant shape and latency to every stimulation (simple or repetitive). The latency was shorter the more distal the level of injury and the greater the elapsed time from the reinnervation. The point of "reflexion" of the responses is at or very near the line of nerve suture. The electrophysiological behavior of the responses fits well with either the criterion of axon reflex or ephaptic response. We discuss both possibilities and conclude that it is not possible, with the electrophysiological technique that we used, to distinguish between an axon reflex and an ephaptic response.

Reflex excitability of facial motoneurons at onset of muscle reinnervation after facial nerve palsy

“…The responses generated in the OOris by supraorbital nerve stimuli are usually considered the result of axonal branching or ephaptic transmission. 19 However, our results suggest that they can also be due to reflex, transynaptic, recruitment of hyperexcitable motor units at the level of the facial nucleus. Two different findings in this study give support to such an hypothesis.…”

Obstetric lesions of the brachial plexus

“…75 Similar sensory-motor communications were, almost without exception, also found in adults with previously sutured peripheral nerves. 64 In "simple misdirection" the axon does not split, so the motor unit forms in its entirety in the wrong muscle. 21 Both types of misdirection can result in cocontraction of various muscles, and can explain "inactive MUPs"; instead these belong in another muscle.…”

Movement disorders in patients with peripheral facial palsy

“…Similar mechanisms of abnormal regeneration have been demonstrated in mixed nerves, where sensory fibers may enter motor funiculi and vice versa. When this occurs, sensory stimuli can induce motor reflex responses at latencies compatible with axon reflexes 41…”

“…Regenerating errors will later complicate the picture by adding peripheral nerve misdirectionality of impulses. This will ultimately result in the so‐called postparalytic facial syndrome (PFS), consisting in synkinetic activity between hemifacial muscles, movement triggered muscle tightness or spasms, myokymic discharges, and activation of facial muscles together with voluntary or automatic activation of nonfacial muscles or to inputs from peripheral nerve afferents 29, 41, 52, 53. The severity of PFS may range from a subclinical stage, only evident in electrophysiological examinations, to a complex disturbance of voluntary and automatic activation of facial muscles, which may occasionally lead to severe emotional complications 54…”