Comparison of Static Fusimotor Innervation in Cat Peroneus Tertius and Longus Muscles

Emonet-Dénand, Françoise, Yves Laporte, and Julien Petit. Comparison of static fusimotor innervation in cat peroneus tertius and longus muscles. J. Neurophysiol. 80: 249–254, 1998. Static fusimotor innervation was compared in cat peroneus longus and tertius muscles because the γ to spindle ratio is considerably higher in the longus (∼60 γ axons for 17 spindles) than in the tertius (∼24 γ axons for 14 spindles). Single γ axons were identified as static (γs) by their typical effects on the response of primary ending to ramp stretch. The intrafusal muscle fibers that single γs axons activated in the spindles they supplied were identified by the features of cross-correlograms between Ia impulses and stimuli, at 100 Hz, and by those of primary ending responses during stimulation at 30 Hz. In each experiment, a large proportion of the γ population was tested on about nine spindles. A statistical analysis was used to estimate the number of spindles supplied by single γs axons and the proportion of γs axons that supply only one spindle among those the stimulation of which had activated either bag2 or chain fibers alone in a single spindle. In peroneus longus, nearly all γs axons supply one or two spindles, whereas in peroneus tertius, the majority of γs axons supply from three to six spindles. The proportion of nonspecifically distributed γs axons, i.e., of axons that supply both bag2 fibers and chain fibers either in the same or in different spindles, is much lower (56%) in the longus than in the tertius (83%) as previously observed on a population of γs axons that supplied from three to six spindles. Correspondingly, the proportion of specific axons is much higher in the longus (44%) than in the tertius (17%). In none of the two muscles was a strict relationship observed between the conduction velocity of γs axons and their intrafusal distribution (specific bag2, specific chain fibers, nonspecific). However, γs supplying bag2 fibers either specifically or in combination with chain fibers tended to have faster conduction velocities, which suggests that, in various motor acts, the proportion of activated bag2 and chain fibers may be related to the proportions of activated fast and slow γs axons.

Prenatal Development of Muscle Spindles in the Mouse

A closely spaced, serial, light and electron microscopic technique has permitted determination of the patterns of distribution, three dimensional cytoarchitecture and ultrastructural characteristics of developing muscle spindles in the fetal mouse extensor digitorum longus muscle (Figs. 1-6).Spindle development begins at 15 days in utero, when unmyelinated afferent nerves contact large, primary myotubes, located at the muscle's neurovascular hilum. By 17 days in utero the adult number of spindles containing one or two myotubes, can be identified in serial ultrathin sections through the myotube's sensory innervated region. While the primitive capsule of the spindle surrounds the intrafusal myotubes, it is initially confined to the sensory area and does not extend the length of the myotube. At this stage the peripheral regions of the intrafusal myotubes are morphologically indistinguishable from neighboring extrafusal myotubes.It is in this peripheral region that fusimotor innervation will develop.By following the spindles in serial, spaced (i.e., 15μm intervals), ultrathin transverse sections along their length, it is possible to establish the continuity of the sensory innervated region with the peripheral regions of the intrafusal myotube and to determine that fusimotor innervation is not present at 17 days in utero.