Histological analysis of cat muscle spindles following direct observation of the effects of stimulating dynamic and static motor axons.

Abstract

Cat tenuissimus spindles (11) were analyzed mainly by cutting serial, transverse, 1 .mu.m thick sections following direct observation of the effects of dynamic motor (.gamma. or .beta.) stimulation. Histological results from these spindles were used to interpret the effects of static fusimotor stimulation of other spindles. Dynamic motor stimulation usually produced contractions seen as convergent movements of sarcomeres in single bag fibers, identified as bag1 fibers. In 1 spindle a single dynamic axon produced a translational movement in one pole of a bag1 fiber and a convergent movement in each pole of a bag2 fiber, together with movements in other unidentified (presumably chain) fibers. Subsequent analysis showed that besides innervating both bag fibers the axon also supplied 2 chain fibers. Contrary to expectation, motor endings on the bag1 fibers seldom occurred at the sites of convergent movement. Only 2 cases of coincidence occurred among 16 foci and 21 motor endings. Focus and nearest ending were separated by distances of 0.85-2.5 mm. Most of the convergent movements of sarcomeres observed in bag1 fibers occurred in a region of the pole that is ultrastructurally distinct from the region where most of the motor endings were located. The possible relevance of this to the production of contractions in the bag1 fiber is discussed. Convergent movement foci in bag2 fibers produced by the stimulation of static axons occurred largely within the same regions of the pole as the motor endings were located; although, whereas foci were observed in both intra- and extracapsular regions, most of the endings were intracapsular.