Analysis of primary and secondary afferent responses to stretch during activation of the dynamic bag1 fibre or the static bag2 fibre in cat muscle spindles

The response of the primary sensory ending in an inactive spindle to a ramp and hold stretch exhibits five distinct components: an initial transient peak, thought to be due to stiction in intrafusal fibres, a ‘fast rise’ followed by a ‘slow rise’ reaching a peak at the end of the stretch, then a ‘fast fall’ and a ‘slow fall’ of adaptation when the spindle is held at the new length (Fig.1a). The ‘dynamic index’, or fall in Ia afferent frequency in 0.5sec immediately following the peak, and which is used to distinguish ‘dynamic’ from ‘static’ fusimotor axons (Matthews, 1972) comprises the fast fall and part of the slow fall. The dynamic bag1 intrafusal fibre (Db1), and the static bag2 fibre (Sb2) are responsible, respectively, for the dynamic and static fusimotor actions on the Ia afferent discharge (Boyd, 1981). This paper describes the action of axons selectively innervating the Db1 fibre or the Sb2 fibre, and the effect of changing the velocity of stretch, on the various phases of the group Ia and group II afferent responses.