THE ANATOMICAL RECORD 200:127-137 (1981) Adaptation of the Masseter and Temporalis Muscles Following Alteration in Length, With or Without Surgical Detachment

Histochemical properties, muscle fiber cross-sectional area, muscle fiber length, and the oxidative capacity of masticatory muscles of female rhesus monkeys were assessed following alteration in functional length by an intraoral appliance or by detachment of the muscle. Experimental groups received the appliance only (A); the appliance and subsequent detachment of the masseter (AD); the appliance and detached masseter, but with surgical reattachment of the masseter to the pterygomasseteric sling (ADR); no appliance, but detachment and reattachment of masseter (DR); or an appliance which was removed after 24 weeks to study posttreatment responses (F’T). Animals were sacrificed and the muscles were studied at intervals from 4 to 48 weeks after initiation of the experimental period. The results of these studies led to the following conclusions: (1) Stretching the masseter and temporalis muscles within physiological limits did not significantly alter the proportion of fiber types, although oxidative capacity of the fibers was reduced. (2) Fibers with “intermediate” myofibrillar ATPase activity were no more prevalent in experimental than control muscles. (3) The cross-sectional area of Type I fibers of masseter muscles decreased following some experimental procedures, indicating that recruitment of these fibers is the most sensitive to altered jaw function. (4) Minimal alteration of muscle capillarity was induced by any of the experimental procedures. (5) The lengths of masseter muscle fibers in Group PT and of temporalis muscle fibers in groups AD and ADR were greater than in control animals. Skeletal muscle fibers may undergo both morphological and physiological adaptation during growth and in response to altered function due to changes in length, position, and behavior. Morphologically, alteration in functional demand could lead to changes in muscle fiber geometry, cross-sectional area, the amount and distribution of connective tissue within the muscle and its terminal ends, and the length of the individual muscle fibers due to either an addition or deletion of sarcomeres. Physiologically, fibers of limb skeletal muscles have been shown to undergo adaptive change with respect to oxidative capacity, and there is evidence to suggest that fibers may undergo conversion between Type I and Type 11 myosins during growth or following alteration of function (Maxwell et al., 1979, 1980a). Nearly all previous studies of muscle adaptation following alteration of function have been undertaken in mammalian limb muscles. There is relatively little known concerning the effects of alteration of function on the muscles of mastication. This is especially unfortunate since clinicians routinely alter the length, position, and habitual function of the perioral muscles and the muscles of mastication during the course of orthodontic treatment and surgical intervention in both growing and adult individuals. The capability of the muscles of Dr. Maxwell’s current address is University of Texas Health Science Center at San Antonio, Department of Physiology, 7703 Floyd Curl Drive, San Antonio, Texas 78284. Received December 10, 1979; accepted November 7, 1980 0003-276X/81/2002-0127$03.50