Reconstruction of the nerve supply to a human muscle spindle

Distributions of one sensory and 13 motor axons to intrafusal fibers in a human spindle from the biceps brachii muscle were reconstructed from serial, 1 micron thick transverse sections. The primary afferent was distributed predominantly to nuclear bag fibers. Motor innervation in the human spindle was characterized by the presence of shared innervation among different types of intrafusal fiber, long unmyelinated preterminal segments of axon, and numerous short motor endings on both bag1 and bag2 fibers. These neuroanatomical features differ grossly from those in the cat tenuissimus spindles and may reflect a major functional difference.