Distribution of fusimotor axons to intrafusal muscle fibres in cat tenuissimus spindles as determined by the glycogen‐depletion method.

Abstract

The distribution of fusimotor axons to bag1, bag2 and chain muscle fibers in cat tenuissimus spindles was studied using a modification of the glycogen-depletion technique of Edstroem and Kugelberg. Single fusimotor axons were stimulated intermittently at 40-100/s for long periods (30-90 s) during blood occlusion. Portions of muscle containing the activated spindles were quick-frozen, fixed in absolute ethanol during freeze-substitution and then embedded in paraffin wax. Serial transverse sections were stained for glycogen using the periodic acid-Schiff method, and examined for depletion. Dynamic .gamma. axons (i.e., those that increase the dynamic index of primary-ending responses to ramp stretches of large amplitude) depleted bag1 fibers almost exclusively. Static .gamma. axons (i.e., those that reduce or abolish the dynamic index) depleted both bag and chain fibers. Bag1 and bag2 fibers were depleted about equally. A single static .gamma. axon may activate both bag and chain fibers in 1 spindle (the most common pattern), chain fibers only in another, and bag fibers only in a 3rd spindle. Static .gamma. axons with conduction velocities < 25 m/s also had a nonselective distribution, but no depletion was observed in bag2 fibers. The zones of depletion produced by dynamic .gamma. axons were distributed more or less equally in the intra- and extracapsular parts of spindle poles, whereas those produced by static .gamma. axons were mainly intracapsular. The results are compared with the glycogen-depletion studies of Brown and Butler and a study of the distribution of static .gamma. axons to spindles in which all other motor axons had degenerated. The implications of the finding that both static .gamma. and dynamic .gamma. axons activate bag1 fibers are discussed.