Axon terminal indentation and postsynaptic folding as criteria for classifying fusimotor nerve endings

Abstract

Arbuthnott et al. (1982) classified cat fusimotor endings into four types in terms of ultrastructure. The types are now redefined as follows. m _a plates: axon terminals superficial to, or partially indented into, the muscle fibre surface, and none substantially indented; postsynaptic folding absent or rudimentary. m _b plates: terminals substantially or partially indented, none lying superficially; postsynaptic folding absent or rudimentary: m _c plates: terminals embedded between finger-like processes protruding from the muscle fibre surface; postsynaptic membrane has wide, shallow folds. m _d plates: terminals fully indented; numerous deep, narrow postsynaptic folds. The earlier, intermediate, class of m _ab plates is eliminated if the above criteria are used. The validity of indentation or postsynaptic folding as criteria for classification has been questioned (Banks, 1981, 1983), on the grounds that indentation may be related to distance of fusimotor endings from the spindle equator, and that substantial variations in form of endings occurs between spindles. A quantitative assessment of these two criteria is given below showing that both are, in fact, of considerable value for the identification of types and locations of fusimotor terminals.