Innervation of Muscle Spindles
Abstract: Mammalian muscle spindles are generally composed of two types of intrafusal muscle fibers: nuclear bag fibers and nuclear chain fibers. Each intrafusal muscle fiber is innervated by sensory and motor nerves. However, the distribution of nerve endings is different between nuclear bag fibers and nuclear chain fibers. The nuclear bag fibers are supplied by sensory (primary) endings in the equatorial region and motor endings from the juxtaequatorial region to the polar region. On the other hand, the nuclear chain fibers are widely innervated by sensory (primary and secondary) endings from the equatorial region to the polar region and motor endings are restricted only in the polar region. Sensory endings generally lie on the surface of the muscle fibers in shallow grooves. However, in the intrinsic laryngeal muscle such as human being, guinea pig and marmoset, sensory endings branch and deeply penetrate into the muscle fibers. Motor endings are classified into at least three types. In addition to sensory endings and motor endings, multiaxonal endings consisting of a bundle of axons occasionally exist in the muscle spindles of the adult Chinese hamster. From a structural organization similar to the autonomic nerves, these multiaxonal endings are considered to be autonomic in nature.
Key words: nuclear bag fiber, nuclear chain fiber, sensory ending, motor ending, electron microscopy