Certain sensory neurons innervate muscles and provide the CNS with information about muscle length and force. These and other sensory neurons have cell bodies in a dorsal root ganglion, with one axon projecting to sensory receptors in the periphery and another terminating in the CNS.
The diameter of muscle-afferent axons determines whether they belong to group I or group II. Group I, subdivided into groups Ia and Ib, has the larger fiber diameter and therefore faster transmission rates. Group Ia and II fibers innervate muscle spindles and are therefore called muscle spindle afferents. The term spindle refers to fine intrafusal muscle fibers that taper at the end and contain a fluid-filled capsule at the center. Muscle-afferent fibers wrap around muscular elements within the capsule. Group Ia fibers are termed primary muscle spindle afferents; group II fibers are called secondary muscle spindle afferents. Group Ib fibers innervate golgi tendon organs (GTOs), which are located in the transitional region between extra-fusal muscle fibers and tendons. The role of muscle afferents in reflex responses is discussed in Section II.B.
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