Correspondence

To the Editor:

RE: ‘Functional characteristics of the rat jaw muscles: daily muscle activity and fiber type composition’ by Kawai et al. Journal of Anatomy, Vol. 215, pp. 656–662.

Functional adaptation in jaw muscles

Kawai et al. present an interesting study of the relationship between electromyographic activity in jaw muscles and their fibre type composition (Kawai et al. 2009). The concept of functional adaptation has been widely accepted since I introduced it in the 1970s (Salmons & Sréter, 1976; Salmons, 1980; Salmons & Henriksson, 1981). The evidence comes entirely from muscles of the limbs and trunk, but it is reasonable to expect the same regulatory phenomena to be present in skeletal muscles of cranial origin even though they contain, and often co-express, a greater diversity of myosin heavy chain isoforms. The authors of this paper appear, however, to have misinterpreted the adaptive hypothesis. This states that sustained high levels of activity induce slow, fatigue-resistant properties; low or intermittent levels of activity allow the fibers to retain, or revert to, a native fast, fatigue-susceptible state. Accordingly, where there is a high proportion of type IIX fibres it would be associated not with the generation of large forces, as the authors propose, but with a low aggregate amount of activity. The high-frequency patterns needed to produce large forces do not differ from low-frequency patterns in their effect on myosin isoform composition (Hudlicka et al. 1982; Kernell et al. 1987; Sutherland et al. 2003).

It is certainly true that the muscles of smaller animals have a lower proportion of slow fibres. This is associated with a higher threshold for the transition from fast to slow myosin isoforms (Salmons & Henriksson, 1981; Sutherland et al. 1998, 2006). The transition, although present in the rat, is correspondingly more difficult to demonstrate than in larger species (Jarvis et al. 1996). On this basis a better correlation between the proportion of type I fibres and muscle activity should be seen in a larger experimental animal, and has in fact been found in rabbit jaw muscles by van Wessel et al. (2005). This citation is curiously absent from the present paper.

There is a good deal of published evidence to show that myosin heavy chain composition responds to the aggregate number of impulses reaching the muscle (see Salmons, 2009 for a recent review). Unfortunately this is not measured by the overall duration and amplitude of electromyographic activity over the same period: high-frequency bursts of short duration could deliver the same number of impulses as low-frequency trains of longer duration. If the authors were able to take this into account, the apparent inconsistencies in their data for the anterior temporalis muscle and the seeming departures from Henneman's size principle would, I suggest, disappear.