The effect of neonatal nerve crush on adult rat muscle spindles

Abstract

A methodology for producing a workable number of chainless/bag-only spindles, (otherwise well developed), in the medial gastrocnemius muscle of the adult rat was explored by means of a series of neonatal nerve crushes. The single-fibre and two-fibre spindles thus produced as permanent structures in the adult were termed "model adult" spindles. Nuclear bags were well preserved in all spindles designated as model-adult. A histological study was made of these spindles. Length was the most affected spindle dimension. Most model-adult spindles were shorter than normal, the decrease involving the polar lengths (regions B and C; see Banks et al, 1976) rather than the periaxial lengths (region A). Apart from some myofibrillar disarrangement and a general increase in sarcoplasm, the fine structure and histochemistry of model-adult bag fibres was comparable to that of control bag fibres (certainly, in the two-fibre spindles). Thus, the bag1 fibre (Banks et al,'76) lacked an M line throughout its length whereas the bagi fibre lacked thisfeature only in region A. The presence or absence of an M line in m any of the single-fibre spindles could not be ascertained because of sarcomere 11 blurring ", but in some, a double M line could be recognised throughout the entire length of the bag fibre. The identity of this fibre is discussed in the light of its M line structure, its consistently pale reaction with alkaline ATP'ase and its apparent greater susceptibility to denervation, as evidenced by monitoring the process of degeneration with electron microscopy, primary sensory endings were present in all model- adult spindles, but secondary endings occurred in fewer spindles than normal. As regards the form of the primary ending, fewer annulospirals were in evidence, Ultra structurally, however, the sensory terminals appeared quite normal. There were fewer fusimotor axons (particularly, trail axons) per model-adult spindle, the reduction reflecting the absence of chain fibres. Of the two types of fusimotor plate recognised in the control spindles, the smaller type remained intact in model-adult spindles. However, the larger type appeared to have been radically altered by the nerve crush; they were reduced to simple, button-like swellings. The possible physiological implications of the findings are discussed.