Each muscle spindle has three essential elements: (1) A group of specialized intrafusal muscle fibers with contractile polar ends and a noncontractile center. (2) Large diameter myelinated afferent nerves (types Ia and II) originating in the central portion of the intrafusal fibers. (3) Small diameter myelinated efferent nerves supplying the polar contractile regions of the intrafusal fibers.
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