Physiological Reviews, Vol 70, 643-663, Copyright © 1990 by American Physiological Society
Mammalian muscle spindle: peripheral mechanisms
C. C. Hunt
Department of Neurology and Neurological Surgery, Washington University School of Medicine, St. Louis, Missouri.
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.
