Location and completeness of reinnervation by two types of neurons at a single target: the feline muscle spindle.
Muscle spindles from the tenuissimus muscle of the cat were examined microscopically to assess the precision and completeness of reinnervation of intrafusal muscle fibers by efferent and afferent neurons. Positions of motor and sensory nerve terminals were charted relative to the cross-sectional area enclosed by the outer capsule of the spindle. Profiles of nerve endings were measured for normally innervated and reinnervated spindles. The tenuissimus was deprived of innervation by freezing its nerve, sometimes in conjunction with either spinal ganglion removal or ventral rhizotomy. Sensory and motor terminals occupied separate locales along the length of normal muscle spindles. Nerve terminals of efferent and afferent neurons were located in appropriate positions along the length of spindles when axons of both types of neurons regrew together and when either category of axon regenerated alone. Precise reinnervation of muscle spindles occurred in spite of a diminished diameter of intrafusal fibers. Repopulation of the spindle with motor endings was less complete than that by sensory endings, based on the proportion and size of the regenerated terminals. We conclude that under optimal conditions for axonal regrowth, efferent and afferent neurons reinnervate their respective regions along intrafusal muscle fibers but motor lags sensory reinnervation within the spindle. The mechanism by which positional specificity happens during reinnervation of intrafusal fibers requires neither an interaction between terminals of the two types of neurons nor target cells of normal bulk.
DeSantis
M
M
Department of Biological Sciences, University of Idaho, Moscow 83844-3051.
Norman
W P
WP
D000818:Animals; D002415:Cats / Q000033:anatomy & histology*; D005260:Female; D008297:Male; D008854:Microscopy, Electron; D009046:Motor Neurons / Q000648:ultrastructure*; D009470:Muscle Spindles / Q000648:ultrastructure*; D009411:Nerve Endings / Q000648:ultrastructure; D009412:Nerve Fibers / Q000502:physiology; D009416:Nerve Regeneration / Q000502:physiology; D009475:Neurons, Afferent / Q000648:ultrastructure*