Neural and muscular control of the human extensor digitorum muscle

Abstract

The human hand has incredible dexterity which depends, in large part, on the ability to move the fingers relatively independently. Interestingly, many of the primary finger flexor and extensor muscles possess a single belly that gives rise distally to multiple tendons that insert onto all the fingers and consequently might produce movement in all of the fingers. Therefore, the objective of this dissertation was to examine the neuromuscular organization of a multi-tendoned finger extensor muscle, the human extensor digitorum (ED). Initially, we found that ED spike-triggered average motor unit force was broadly distributed across the digits. Consequently, we hypothesized that linkages between the distal tendons of ED may cause force developed in a single compartment to be transmitted to neighboring tendons. However, force arising from intramuscular stimulation was fairly focused to a single digit suggesting that inter-tendonous connections account for little of the broad distribution of motor unit force. An alternative possibility was that our spike-triggered averages of motor unit force were contaminated by correlated activity among motor units residing in different compartments. Strong motor unit synchrony was found for motor unit pairs within compartments and a modest degree of synchrony for motor unit pairs in neighboring compartments which likely contributed to the appearance of spike-triggered average motor unit force on multiple fingers. These results suggest that last-order synaptic projections appear to supply predominantly sub-sets of motor neurons innervating specific finger compartments of ED but also branch to supply motor neurons innervating other compartments. Finally, single motor axons branch to innervate muscle fibers situated in multiple compartments of ED. Interestingly, force resulting from intraneural micro stimulation of single motor axons innervating ED was highly focused to a single digit. Therefore, it appears that the muscle fibers innervated by a motor axon are primarily confined to one of four distinct compartments of ED. Based on these experiments we believe that each finger is acted upon by ED through a discreet population of motor units. Consequently, extension of an individual finger would require the selective activation of motor neurons innervating a specific compartment of ED.