Innervation of normal human muscle spindles

The innervation of the human intercostal muscle spindle is described and compared to that of the cat, which it closely resembles. Differences were found principally in the primary ending, which does not have an annulospiral configuration, and the secondary ending, which has only rare spiral forms and which innervates nuclear bag as well as nuclear chain intrafusal muscle fibers. A multibranched axon system is described which innervates the intrafusal fibers in the area equivalent to the P2 zone of the cat by depositing pairs of end plates in series as the branches parallel individual muscle fibers. Endings similar to the PI endings of cats are described at the poles of the human muscle spindle. Preliminary observations indicate that the innervation described for the intercostal spindle is essentially unchanged for spindles of the muscles of the extremities. A study of the innervation of these structures in human disease is expected to yield additional information as to the function of the various components, as well as providing additional criteria in differential diagnosis of some of the neuromuscular disorders.