S165. Henneman’s size principle for the motor neurons recruitment of human spinal reflex: Evidences from a study of H reflexes during acute compression/ischemia

Abstract

Introduction In H reflex studies, H waves are recruited from the reflex-elicited depolarization of the Alpha motor neurons, which follows the Henneman’s size principle that the small motor neurons are responsible for the early portion of the recruitment of the H-reflex. However, there has not been a successful experiment in human with EMG techniques to demonstrate this principle. The goal of this study is to demonstrate that small motor fibers are responsible for the elicitation of the H-reflex in the soleus and flexor carpi radialis (FCR) muscles by blocking the conduction of large motor nerve fibers of the tibial and median nerve using partial ischemia via blood pressure cuff. Methods This prospective study recruited 8 health resident physicians (4 m, 4 F). Electrical stimulation to the tibial nerve at the popliteal fossa, with recording from the soleus and to median nerve at above the elbow with recording from FCR was titrated to elicit both M and H waves, with ratios of amplitudes at 1:1. Compression by a blood pressure cuff was placed at the proximal tibia during tibial nerve stimulations, and at the forearm just distal to the elbow for median nerve stimulation. Stimulations with pulse duration of 1 ms at intervals of 2 s were delivered to the two nerves before, during, and post compression/ischemia. Results In all the subjects tested, during blood pressure cuff inflation, the M-waves of the two muscles consistently demonstrated significant loss of amplitude (or disappeared) with mild increase of latency, while the H-reflex showed no, or very mild amplitude reductions without changes in latency. After blood pressure cuff deflation, the M-waves immediately returned to their previous amplitude and latency patterns. There was a significant dissociation between the changes of M and H waves during acute partial compression/ischemia. Conclusion Acute compression/ischemia instantly causes nerve conduction block of the tibial and median nerves motor fibers and reduces the amplitude of the M waves, but not the H-reflexes. M waves elicited by electrical stimulation recruit large motor fibers first, and are more susceptible to the ischemic blockade applied. By contrast, the H reflex first recruits small motor neurons, with their smaller motor nerve fibers less susceptible to partial acute compression/ischemia. This experiment demonstrates the Henneman’s size principle that the smallest motor units are recruited first during spinal reflexes and volitional muscle contraction. It is also of important significance in the application of motor rehabilitation with therapeutic and functional electrical stimulation of nerves and muscles.