The first observations
Following the demonstration by Leksell (1945) of the fusimotor effects of γ-efferent fibres and their detailed study by Kuffler, Hunt & Quilliam (1951), their chief action was thought to be to bias spindle afferent firing. Hunt & Kuffler (1951) emphasized the usefulness of this in preventing silencing of spindles during active muscle shortening, while Merton (1953) visualized it as a way of operating a ‘length follow-up servo’ system. By the end of the 1950s much ground work had been done on the fusimotor system, principally in hindlimb muscles, and Granit's view had become established that the γ loop was generally co-activated with direct drive to α-motoneurones (Granit, 1955; Granit, Kellerth & Szumski, 1966).
In the early 1960s, the respiratory system was recognized as a convenient model for the organization of motor control. Tom Sears was involved in a series of studies laying the foundations for this approach. Intracellular recording demonstrated the monosynaptic connections of muscle afferents to intercostal motoneurones (Eccles, Sears & Shealy, 1962) and the inhibitory as well as excitatory effects of descending respiratory drive (Sears, 1964c). He also provided important background information on the afferent and efferent fibre diameter spectra (Sears, 1964a) and on motor unit types in intercostal muscles (Andersen & Sears, 1964) and introduced recording from natural intramuscular nerve filaments of intercostal muscles, in which the action potentials of a and α motor fibres could be distinguished by their different amplitudes (Sears, 1962, 1963).