Ultrastructure of dynamic and static skeletofusimotor endings in a cat muscle spindle

Endings of four beta skeletofusimotor axons in a spindle of the cat tenuissimus muscle were examined in semithin (1-micron thick) and ultrathin transverse serial sections. Two (dynamic) beta axons terminated on the nuclear bag1 intrafusal muscle fiber and on extrafusal fibers of the dark type. Two (static) beta axons terminated on the nuclear chain intrafusal fibers and extrafusal fibers of the intermediate type. The degree of indentation of axon terminals into the muscle surface, thickness of the sole plate and extent of folding of subjunctional membranes differed among intrafusal and extrafusal terminations of the same axon. Endings of beta axons on the bag1 and chain fibers were also morphologically dissimilar. Motor axons may not determine ending morphology. Rather the form and structure of a beta bag1 or chain ending may be determined by the type of intrafusal fiber on which the ending lies and the ending's distance from the primary sensory axon.