Physiological and developmental implications of motor unit anatomy

Abstract

There is increasing evidence that the architectural design and arrangement of the fibers within a motor unit have important physiological and developmental ramifications. Limited data, however, are available to directly address this issue. In the present study the physiological properties of one motor unit in each of seven cat tibialis anterior (TA) muscles were determined. Each of these units then was repetitively stimulated to deplete the glycogen in all muscle fibers within the unit. Subsequently, the length, type of ending, and spatial distribution of fibers sampled from these physiologically and histochemically typed motor units were determined. Four fast fatigable (FF), one fast, fatigue resistant (FR), and two slow (S) motor units (MU) were studied. The samples consisted of all those glycogen-depleted fibers (9-27) contained within a single fascicle or a circumscribed area of each of the motor unit territories. The mean fiber lengths for the two slow motor units were 35.9 and 45.5 mm. The mean fiber lengths for the fast motor unit samples ranged from 8.8 to 48.5 mm. Some fibers of both the fast and slow units reached lengths of 58 mm. Most of the fibers in the slow units extended the entire distance between the proximal and distal musculotendinous planes, had relatively constant cross-sectional areas, and terminated at the tendon as blunt endings. In contrast, the majority of the fibers in the fast units terminated intrafascicularly at one end, and the cross-sectional area decreased progressively along their lengths, that is, showed a tapering pattern for a significant proportion of their lengths. Therefore, the force generated by units that end midfascicularly would appear to be transmitted to connective tissue elements and/or adjacent fibers. All fibers of a fast unit within a fascicle were located at approximately the same proximo-distal location. Thus, developmentally the selection of muscle fibers by a motoneuron would seem to be influenced by their spatial distribution. The architectural complexities of motor units also have clear implications for the mechanical interactions of active and inactive motor units. For example, the tension capabilities of a motor unit may be influenced not only by the spatial arrangement of its own fibers, but also by the level of activation of neighboring motor units.

Nonlinear summation of force in cat soleus muscle results primarily from stretch of the common-elastic elements

“…Although there are specialized structures where they insert to the tendon plate, because the fibers always taper, the connective tissue matrix is likely to transmit some of the force. Furthermore, recent evidence shows that many muscles have fibers that do not run the length of the muscle (18,20,24). These fibers by necessity transmit their force to this endomysial connective tissue matrix.…”

“…The links between neighboring fibers may prevent sarcomere length heterogeneity observed in isolated fibers, thus preventing damage and altering muscle force (8,21). In some muscles, fibers do not run the length of the muscle but rather rely on this extracellular matrix and other muscle fibers to transmit force (18,24). Thus neighboring fibers are not mechanically independent.…”

Effects of muscle action type on corticospinal excitability and triceps surae muscle-tendon mechanics

“…The fascicle length is defined as the distance between the insertions of the fascicle into superficial and deep aponeurosis (Kubo et al 2000; figure 19), and it is sometimes regarded as equivalent to muscle fiber length, based on observations of tendon-to-tendon fiber arrangement within a fascicle . Although the orientation of fascicles is the same as muscle fibers, the fascicle length may not correspond to the muscle fiber length, because muscle fibers may terminate mid-fascicularly (Ounjian et al 1991). In spite of the fact that fascicle length measurements do not give accurate representation of fiber or sarcomere function due to inhomogeneities in sarcomere lengths in series and compliance of connective tissue, the ultrasonographic method can give more detailed information about muscular function in vivo as compared to the muscle-tendon unit length estimations alone (Finni et al 2001).…”

Extraocular Motor Unit and Whole-Muscle Responses in the Lateral Rectus Muscle of the Squirrel Monkey

“…Other investigators have speculated that such an arrangement could lead to a loss of expected force when the muscle contracts (Katz, 1939;Demieville and Partridge, 1980;Goldberg et al, 1997b). Other skeletal muscles have been shown to have serially arranged and interdigitated fibers as well, and such arrangements could significantly alter the contractile properties of a motor unit as it acts in concert with other units (Loeb et al, 1987;Ounjian et al, 1991;Roy et al, 1995;Trotter et al, 1995).…”