Voluntary Activation of Human Motor Axons in the Absence of Muscle Afferent Feedback

Abstract

The ability to activate human motoneurons supplying individual intrinsic muscles of the hand was examined during acute deafferentation of the muscles. Tungsten microelectrodes were inserted percutaneously into motor fascicles of the ulnar nerve of 5 subjects, which was then blocked distally with local anaesthetic. In 4 subjects unitary action potentials were recorded from 16 motor axons, which were identified with respect to their target muscles. In the complete absence of muscle afferent feedback, subjects could voluntarily recruit motoneurons, grade their discharge and sustain a constant level of activity. Significant facilitation of motor efforts was provided by cutaneous feedback from the digits via the median nerve. During attempted maximal voluntary efforts the mean discharge frequencies of single motor axons were significantly lower than those of normally-innervated motor units. This finding suggests that peripheral afferents have a net facilitatory influence on motoneurons. However, during prolonged (20-30 s) maximal voluntary efforts the deafferented motoneurons did not display the progressive decline in discharge frequency shown by normally-innervated motor units during contractile fatigue, a finding consistent with two possible explanations: disfacilitation or reflex inhibition of the motoneuron pool by peripheral afferents. The results also indicate that the otherwise intact nervous system can perform some simple motor tasks with no proprioceptive input other than knowledge of the motor commands. Other factors may contribute to the poor motor performance reported for patients with severe sensory deficits.

Fatigue-Sensitive Afferents Inhibit Extensor but Not Flexor Motoneurons in Humans

“…The reduced motoneuronal excitability probably contributes to the decline in the discharge rate of single motor units and the development of central fatigue that occurs with sustained MVCs. The reduction in motoneuron firing rates observed in many studies (Grimby et al, 1981;Bigland-Ritchie et al, 1983;Gandevia et al, 1990) has often been attributed to reflex inhibition from group III and IV muscle afferents (Woods et al, 1987;Garland, 1991). In the current study, when ischemia was maintained after fatiguing contractions of elbow extensors, CMEPs in triceps failed to recover.…”

“…Although observed in several human muscles (Grimby et al, 1981;BiglandRitchie et al, 1983;Gandevia et al, 1990), the cause of the decline is controversial. It is widely believed that activity in group III and IV muscle afferents, which are sensitive to ischemia and metabolites, inhibits spinal motoneurons (for review, see Garland and Kaufman, 1995).…”

Walking Strategy in Diabetic Patients With Peripheral Neuropathy

“…6. It is thus reasonable to infer that a larger contact time could result not only from the loss of proprioception (25,26), which should entail a general increase in contact time of the whole plantar surface with the ground, but also from hypothesized compartmental muscle weakness. Neuropathy affects ankle dorsal flexors more than other muscle compartments (21,26) and consequently alters foot-to-floor interaction.…”

Fusimotor reflexes in relaxed forearm muscles produced by cutaneous afferents from the human hand.

“…Cutaneous input from the hand may exert a more dominant role in the motor control of the hand than the cutaneous input from the leg has in control of the leg. When subjects attempt to contract their acutely paralysed intrinsic hand muscles, some motoneurones cannot be activated without cutaneous feedback from the hand (Gandevia, Macefield, Burke & McKenzie, 1990), but this is not so for motoneurones innervating ankle flexor muscles (Gandevia, Macefield, Bigland-Ritchie, Gorman & Burke, 1993). Accordingly the cutaneous reflex control of the fusimotor system might be expected to differ for the leg and the hand.…”