The muscle spindle 0f the rat: Peculiarities of motor innervation and ultrastructure and effect of increased activity.

Abstract

The ultrastructures of intrafusal muscle fibers and motor endings in the rat’s Quadrieeps femoris (Q.FEMOR) have some peculiarities. They are expressed by a deficit of static component in motor innervation, as one static nuclear chain intrafusal fiber is devoid of motor innervation, and polyneuronal innervation of the dynamic bag] intrafusal fiber with gamma and beta motor axons of appropriate motoneurons. Reduced motor static component and polyneuronal innervation strengthen the influences of the dynamic intrafusal fiber in muscle spindle of Q. FEMOR muscle. Ability of the bagl plasma membrane to form finger-like projections in the vicinity of motor axonal terminals establishes more close synaptic contact and quick transmission of information concerning changes of muscle length of Q.FEMOR muscle to the CNS. The bag1 intrafusal fiber is coinnervated by the beta axon together with oxidative type of extrafusal muscle fiber.
Comparison of the effect of prolonged muscular activity on the ultrastructure of the muscle spindles in Q.FEMOR and SOL shows that in SOL intrafusal muscle fibres indicate some signs of destruction of myofibrils and other cell organelles.
Intrafusal muscle fibers being located in the SOL muscle adapt themselves to increased activity with a mechanism of response reaction similar to that of extrafusal fibers, although the changes in intrafusal muscle fibers are considerably smaller
than those in extrafusal fibers.