Classification of muscle spindle receptors.

The conduction velocities of muscle spindle afferent fibers have a bimodal distribution, and classifications of spindle receptors based on afferent fiber diameter have therefore divided these receptors into two groups, the well known primary and secondary endings. However, measures of spindle function that are likely to be important for kinesthetic sensibility such as dynamic response, adaptation and linear directionality (hysteresis) are distributed rather uniformly. Therefore, from this functional perspective it might be argued that muscle spindle receptors should not be subdivided at all. On the other hand, different receptors demonstrate these properties to varying degrees, and there are simple, linear correlations among log (dynamic response), log (adaptation), linear directionality and conduction velocity. Thus, the receptors can be divided into as many as 5-10 different subpopulations that differ significantly in one or more of these properties.