Morphological identification and intrafusal distribution of the endings of static fusimotor axons in the cat

1. Tenuissimus muscles of the cat were prepared in which the motor innervation was reduced to a single γ axon by cutting all the other motor axons and allowing them to degenerate during a period of 7–12 days. The function of the surviving γ axon was then determined, and the distribution of its endings ascertained in teased, silver preparations. 2. In the ten muscles successfully prepared the function of the surviving γ axon was static and the motor innervation distributed to the spindles consisted of trail endings. The conduction velocities of the axons ranged from 33 to 48 m/sec. 3. A detailed histological analysis was made of thirty spindles innervated by six of the surviving static axons. 4. The six static axons distributed trail endings to both bag and chain muscle fibres in the poles of thirty spindles with about twice the frequency of supplying them to poles in which the distribution was restricted exclusively to one type of muscle fibre or the other. 5. The density of trail innervation supplied to the bag fibres, in terms of the mean number of terminals per fibre, was typically from one and a half to twice that supplied to the chain fibres. On the other hand, whereas the number of bag fibres supplied with trail endings in a spindle pole was seldom more than one, the number of chain fibres innervated was usually two in a range of one to four. 6. The possible effects that partial denervation might have had on the spindles are discussed, but it is concluded that they are unlikely to have affected the results.