Mammalian muscle spindle: peripheral mechanisms.

The responses of sensory endings of the muscle spindle to stretch are produced by transduction in the sensory terminals and by impulse initiation in the sensory axon, both of which appear to be largely linear and non-time-dependent processes. The marked nonlinearity of spindle responses to length, the processes of gain compression, and the aftereffects of fusimotor activity and of stretch appear to reside mainly in the mechanical properties of the intrafusal fibers. Although the basis of the dynamic sensitivity of the primary ending in the passive spindle is still not well understood, dynamic fusimotor effects have been shown to depend on activation of the bag 1 fiber. Static fusimotor actions result from contraction in the bag 2 and/or chain fibers. Certainly, a great deal is known about the muscle spindle at the level of changes in sensory discharge to variations in muscle length and to fusimotor stimulation, although new insights continue to arise from experiments of this type. However, there is a need for further quantitative information that will lead to greater understanding of transduction mechanisms, impulse initiation, and intrafusal fiber contractile activation.