Distribution of muscle spindles in the human first dorsal interosseus

The problem of accurately locating muscle spindles within the bulk of skeletal muscle has been the major difficulty in histological evaluation of human muscle spindles and in attempts to record their physiological activity. Entire first Dorsal Interossei were taken from full‐term infants, stained in toto by deCastro's silver technique, serially sectioned or squashed, and individual spindles microdissected for study. A model of the muscle with its extrafusal innervation was constructed and the average number of spindles in five muscles was found to be 47 (range 4C–54). The first Dorsal Interosseus muscle is a bi‐pinnate structure divided by a central tendon which inserts on the proximal first phalanx. The spindles within this muscle are nearly equally distributed between the halves. Most of the spindles are concentrated in two well‐defined regions, a lateral, more distal spindle‐rich region and a medial, more proximal, grouping. Each of these two concentrations consists of approximately one‐third of the spindles present, the remaining one‐third being rather uniformly distributed throughout the muscle mass. An idea of the degree of concentration usually seen is illustrated by finding as many as eight spindles present in a 100X field in cross‐section. This density of concentration and apparent consistency of location within the muscle is expected to facilitate procurement of more adequate numbers of spindles at muscle biopsy and to aid the intramuscular recording from human muscle spindles.