In the last decade, a number of laboratories have accumulated data on the firing of single afferent fibres from muscle and skin during movement in awake cats, monkeys and human subjects. While there is general agreement on the firing behaviour of skin afferents and tendon organ (Ib) afferents during movement, there remains a significant divergence of opinion regarding the way in which the response of muscle spindle afferents (Ia and II) to length changes is modified by fusimotor action (e.g., alpha–gamma linkage versus "fusimotor set"). The controversies surrounding the fusimotor system have tended to overshadow the emergence of several important characteristics of propioceptive behaviour, corroborated in separate laboratories, (i) Mean la firing rates during active movements are nearly always higher than at rest. Thus, activation of the fusimotor system is reserved for the control of, or preparation for, movement. In animals, there is now strong evidence that there is usually a tonic component of fusimotor action during rhythmical movements. (ii) During fast, unloaded movements (peak muscle speeds, 0.2 resting lengths/s or more), the firing of both la and II afferents usually increases during lengthening and decreases during shortening. Ib afferents fire during even the most rapid active shortening of their parent muscles, (iii) During powerful shortening contractions performed against significant loads, la firing is often appreciable, suggesting that there is at least some underlying alpha–gamma coactivation. (iv) During fast imposed muscle stretches, la afferents respond with segmented bursts of firing (threshold speed for segmentation, 0.5–1.0 resting length/s). Ib afferents show far less segmentation of discharge under similar circumstances, (v) There are substantial numbers of tendon organ receptors which fire during tasks involving low levels of force.
Role of the human fusimotor system in a motor adaptation task.
