Form and distribution of sensory terminals in cat hindlimb muscle spindles

Abstract

Abstract The sensory innervation of cat hindlimb muscle spindles was studied by reconstruction, electron microscopy, and examination of teased, silver preparations to ascertain the form of the terminals and their distribution to bag1 (b1), bag2 (b2), and chain (c) muscle fibres. Reconstructions were made of two primary endings, one secondary ending, and the branching of four primary and six secondary axons. For the silver analysis spindles were teased from 14 different hindlimb muscles, the largest samples being from tenuissimus, peroneus brevis, p. longus, p. tertius, superficial lumbrical, extensor digitorum longus, and soleus. Among 310 spindles examined, 40 lacked a b1 fibre. These were all portions of tandem spindles in which the b2 fibre was continuous from one capsule, where it was accompanied by b1 and c fibres, to another, in which it was accompanied by c fibres only. These have been designated ‘b2c spindle units’ as distinct from ‘b1b2c spindle units’.