Response Profiles of Human Muscle: Afferents During Active Finger Movements

Abstract

Twenty-five human muscle afferents from the extensor digitorum muscles of the forearm were studied with the microneurographic method. Single unit impulses were recorded while the subjects performed alternating movements of moderate speed at the appropriate metacarpophalangeal joint. For comparison, responses to imposed movements of similar amplitudes and velocities were also studied. Most spindle afferents (n = 17) provided a stretch response with both kinds of movement. However, the impulse rate was slightly higher and the interspike interval variability much larger during active movement. Two units provided deviating response profiles: a flat profile and a converse stretch response. Small and constant torque loads usually failed to modify the response profile but gave rise to a moderate increase of impulse rate in 50% of the spindle afferents. In one single unit, a converse stretch response appeared with opposing loads. Tendon organ afferents (n = 8) were totally unmodulated by imposed stretch in the relaxed muscle. In contrast, their impulse rate was highly modulated during active movements, often following the rectified EMG which resulted in a converse relationship to muscle length and velocity. The findings support the view that, in general, human muscle spindles monitor muscle length and velocity in routine movements of moderate speed as long as opposing loads are small, whereas Golgi tendon organs monitor the amount of muscle recruitment. The significance of the deviating response profiles from spindle afferents remains obscure.

Ensemble firing of muscle afferents recorded during normal locomotion in cats

“…This is far in excess of the highest firing rates recorded in human neurography (e.g. Al-Falahe et al 1990), which raises the question of whether a species difference is involved. Although our study does not resolve this issue, muscle velocity clearly emerged as a dominant variable in determining spindle primary firing rates.…”

Responses of somatosensory area 2 neurons to actively and passively generated limb movements

“…Passiveonly neurons seem to require selective suppression of afferent input to eliminate responses that would otherwise occur during active movement. When the limb is moved volitionally, muscles change length, resulting in spindle activation which is transmitted centrally (Al-Falahe et al 1990). To respond preferentially to passive movements, efference copy must have a suppressive effect on these passive-only cells (Seki and Fetz 2012).…”

Limited independent flexion of the thumb and fingers in human subjects.

“…Matthews & Miles, 1988). It is unlikely that fusimotor outflow is sufficiently selective to focus the relevant motor commands via spindle-mediated facilitation, given the complex mechanical interactions (see below), in-series connections of spindles, and the likelihood of spindle unloading during shortening contractions (Al-Falahe, Nagaoka & Vallbo, 1990;cf. Burke, Hagbarth & Lofstedt, 1978).…”