A study of the recovery of cat muscle spindles after nerve-crush injury
After nerve-crush injury to a muscle nerve the afferent and efferent axons regenerate and reinnervate their peripheral targets. This process has been examined in the muscle spindles of the peroneus brevis muscle of the cat hindlimb. Following reinnervation there was a rapid restoration of the ending structure which clearly demonstrated the presence of a powerful guidance system enabling a high degree of specificity in the reinnervation process. Although the sensory endings never achieved the full complexity and extent of innervation that characterises the endings of normal spindles, the process of reinnervatlon was highly successful and the specicifity of reinnervation is explained in terms of physical guidance by the basal laminae of the endoneurial tubes and the muscle fibres. During the early stages of recovery the spindles responded abnormally to ramp=and hold stretch, often only firing during the rising phase of the ramp. As recovery progressed, so the responses became more normal and this pattern is explained in terms of a gradual reduction in the threshold of the pacemaker site. Variation of the denervation period showed that on the whole the muscle spindle is resistant to denervation atrophy for at least seven weeks. After all the denervation periods the regenerated endings were fully functional and there was only a slight trend towards slower recovery with increasing denervation period. The morphological appearance of the endings showed a reduction in the regional specificity of reinnervatlon which is attributed to shrinkage of the intrafusal fibres allowing axons to grow between the basal lamina and the muscle fibre.