A characterisation of spindle afferents from the intertransverse tail muscles in the rat.

The conduction velocities of myelinated afferent axons from the intersegmental muscles at the tail-base in rat range from 5.0--43.0 m-sec-1, and have a bimodal distribution with a boundary between the groups at 25.0 m-sec-1. Calculations of the dynamic indices of spindle endings, observed when stretching the muscle, show that axons from primary endings conduct at velocities above 28.0 m-sec-1 and those from secondary endings at below 24.0 m-sec-1. These characteristics resemble those of spindles in the rat hind-limb muscles, and do not compare with those in the more distally situated segmental tail muscles.