The sensory and motor innervation of muscle spindles in cat tail dorsolateral muscles.

The sensory and motor innervation of a sample of 202 muscle spindles from the dorsolateral tail muscles of eight cats were studied by silver and gold staining techniques. The range of diameters of Group Ia afferent nerve fibres to primary (P) sensory endings was 2.0-14.0 micron with a peak at 8.5 micron. The range of Group II fibres to secondary (S) sensory endings was 2.0-8.5 micron with a peak at 5.0 micron. In a sample of 234 spindle capsules, 171 (84.2%) were single capsule spindles and 31 (15.8%) were tandem spindles. With respect to the sensory endings of single capsule spindles, 19.7% showed only a P-ending, 32.5% had an additional S-ending and 15.8%, 3.0% and 2.1% had two, three and four S-endings, respectively. In the tandem spindle capsules, 0.8% of the larger proximal capsules showed only a P-ending, 5.1% had an additional S-ending and 4.7%, 2.6% and 0.4% had two, three and four S-endings, respectively. In the 31 tandem spindles, one of which had a triple capsule, there were 14 types of sensory ending combination in the proximal and distal capsules. The most common (44.4%) innervation was by a P-ending only, while 27.0% had an additional S-ending and 17.5%, 9.5% and 1.6% had two, three and four S-endings, respectively. Of the distal capsules, 92.9% showed only a P-ending. There were three types of motor nerve ending, namely the p1-plate, the p2-plate and the trail (tr) ending, in spindle polar regions. Spindle poles without a motor innervation also occurred. There were 16 types of motor ending combination in the two polar regions of muscle spindles. The most common type of combination was the trp2-ending (41.8%), receiving an average of 5.8 fusimotor fibre branches and the next common was the tr-ending only (38.7%), innervated by an average of 3.2 fibres. The least common combination was with the p1-plate ending (9.5%), receiving an average of 6.9 fibres. The mean number of fusimotor fibre branches per spindle pole was 4.2.