Motor-unit territories in the masseter muscle of infant pigs

In adult miniature pigs (Sus scrofa) fibres of the masseter contract differentially, helping to produce the successive movements of the mandible during the chewing cycle. In infant pigs, however, most fibres of the masseter contract simultaneously. One hypothesis to explain the ontogenetic change in contraction pattern is that the infant masseter is neurologically immature, with large overlapping motor units incapable of producing differential contractions. This hypothesis was tested by mapping the territories of motor units in the masseters of piglets. Filaments from the masseteric nerve were stimulated repetitively; muscle fibres belonging to the stimulated motor units were identified by their failure to react for glycogen. Just as in older pigs, motor-unit territories were found to be very restricted, occupying only a small portion of total muscle volume. Thus, neural organization does not appear to be immature in piglets. An alternative hypothesis, that the ontogenetic change in activity pattern results from growth changes in the anatomy of the masseter, may be a more likely explanation.