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Results are presented of examining the postcrush sensory reinnervation of cat peroneus brevis muscle spindles previously investigated physiologically by Hyde and Scott. It is shown that primary and secondary endings were successfully restored in their final form in the early stages of recovery. The primary endings were shorter than normal and had fewer transverse bands; 12% were judged to be hyperinnervated. Some secondary endings showed signs of growth through the primary region apparently designed to establish secondary terminals in the opposite pole. This is compared with the collateral regeneration of intact motor axons in partially denervated muscle. It is concluded that the defects observed in the regenerated sensory endings had no effect on their functional recovery.

Direct identification of the endings of skeletofusimotor (beta) axons has been made in muscle spindles deprived for their gamma innervation by degeneration. Hindlimb muscles were prepared in which 1--5 fast-conducting motor axons were left intact while the rest of the motor supply was cut and allowed to degenerate for a period of 7 days. In 3 experiments a single beta axons survived supplying tenuissimus, and in 2 experiments beta axons were among 4 or 5 surviving axons that supplied superficial lumbrical and abductor digiti quinti medius muscles. Motor endings identified as p1 plates were found in teased, silver preparations of all experimental muscles, a total of 35 such plates being located in 15 spindles. The plates were all supplied to bag1 fibres. The experiments show that if a spindle innervated by a beta axon is deprived of its gamma supply by degeneration the motor endings that remain intact are p1 plates.