Form and distribution of sensory terminals in cat hindlimb muscle spindles

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 ab₁fibre. These were all portions of tandem spindles in which theb₂fibre was continuous from one capsule, where it was accompanied byb₁andcfibres, to another, in which it was accompanied bycfibres only. These have been designated ‘b₂cspindle units’ as distinct from ‘b₁b₂cspindle units’.