Muscle spindle, a. Schematic diagram of a muscle spindle. The diameter of the spindle is expanded to illustrate structural details. Each spindle contains approximately two to four nuclear bag fibers and six to eight nuclear chain fibers. In the nuclear bag fibers, the muscle fiber nuclei are clumped in the expanded central portion of the fiber, hence the name bag. In contrast, the nuclei concentrated in the central portion of the nuclear chain fibers are arranged in a chain. Both afferent (sensory) and efferent (motor) nerve fibers supply muscle spindle cells. The afferent nerve fibers respond to excessive stretching of the muscle, which in turn inhibits the somatic motor stimulation of the muscle. The efferent nerve fibers regulate the sensitivity of the afferent endings in the muscle spindle, b. Photomicrograph of a cross section of a muscle spindle, showing two bundles of spindle cells in
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