The Evolving Gamma Loop

Abstract

The term gamma loop was introduced by Granit to refer to the activation of alpha motoneurones indirectly through the effect of gamma efferent drive to the muscle spindles. Merton went on to popularise the notion that this type of action was essential to achieve negative feedback control of voluntary movement. However, subsequent development of detailed knowledge of the static and dynamic fusimotor systems and recordings from spindles during natural movements combined to show that the true significance of the gamma loop must be much more subtle. This theme was dealt with in penetrating fashion by Matthews in his 1981 review lecture. He pointed out that though the idea of servo control of movement through gamma action had provided a great stimulus to thought and to experiment, it was based on much too simple a knowledge of spindle structure and function. The decade from 1960 to 70 saw great advances with the recognition of the primary and secondary afferents and the division of fusimotor actions into static and dynamic. The elucidation of detail continued in the next decade with the description of the three types of intrafusal muscle fibres, but there was also increasing interest in the broader issues of spindle function in movement control. This was particularly dependent on the recording of spindle afferent activity during natural movements in man by microneurography (Vallbo & Hagbarth, 1968; Vallbo, 1973) and in animals by chronic electrode implantation (e.g. Taylor & Cody, 1974; Goodwin & Luschei, 1975; Prochazka, 1975; Prochazka, Westerman & Ziccone, 1976). In fact, in 1981 Matthews noted that this approach had “quite properly led to a shift of interest away from the fascinations of the muscle spindle itself towards the wider questions of its functional role in the body”. It is therefore surprising to see the extent to which interest in the details of spindles has persisted since then and how few workers have involved themselves in the study of the ‘wider questions’. A positive reason for this is that knowledge of some details has been seen to be incomplete and it is not very attractive or useful to construct theoretical models of function in these circumstances. The less worthy reasons are that chronic experiments on sophisticated animals require a lot of committment and resources. The present wave of intolerance of animal experiments, whipped up by extremists posing as liberal thinkers, is also impeding progress in this subject as in others.