A histological study of the motor innervation of the cat's muscle spindle.

The motor innervation of four muscle spindles from the tenuissimus muscle of the cat was demonstrated using reconstructions of 1 micrometer thick, serial transverse sections. Analysis of the results clearly indicates that the bag intrafusal muscle fibre usually does not receive a static fusimotor input via trail innervation. In contrast to the highly selective innervation of bag fibres, almost half the axons supplying bag or chain fibres branched to terminate on both types of fibre. The significance of these results is discussed in relation to previous studies on fusimotor innervation and to their functional implications. The presence of autonomic innervation is a further complication that appears to have led to erroneous conclusions concerning the nature of the trail innervation of chain fibres in a recent study of the distribution of cholinesterase activity in the spindle.