Fig. 2

Typical responses of a muscle spindle to stretch. The responses of an individual muscle spindle from the mouse extensor digitorum longus muscle to ramp and hold stretches applied to the tendon were recorded with an extracellular electrode. Single unit action potentials are shown in (a and d). The stretch was 4-s long, and the length change corresponded to 260 (panel b) and 780 (panel e) μm. The ramp speed in (e) was 3-fold higher compared to that in (b). Panels c and f represent the instantaneous frequencies (action potentials/s). In panel f, three different parameters that are usually analyzed to describe muscle spindle function are illustrated: resting discharge (RD), dynamic peak (DP), and static response (SR). For more information on these parameters, see [32, 33, 44]. Note that the dynamic peak and the static response is higher in (f), compared in (c) due to the higher ramp speed and the longer length change. Since the fusimotor innervation was cut during the dissection of the muscle, no action potentials can be observed directly after the end of the ramp and hold stretch (spindle pause)