J.A.M. Korfage

Architecture of the human jaw‐closing and jaw‐opening muscles

The human jaw‐closing and jaw‐opening muscles produce forces leading to the development of three‐dimensional bite and chewing forces and to three‐dimensional movements of the jaw. The length of the sarcomeres is a major determinant for both force and velocity, and the maximal work, force, and shortening range each muscle is capable of producing are proportional to the architectural parameter volume, physiological cross‐sectional area, and fiber length, respectively. In addition, the mechanical role the muscles play is strongly related to their three‐dimensional position and orientation in the muscle–bone–joint system. The objective of this study was to compare relevant architectural characteristics for the jaw‐closing and jaw‐opening muscles and to provide a set of data that can be used in biomechanical modeling of the masticatory system.

Fiber-type composition of the human jaw muscles (Part 1) Origin and functional significance of fiber-type diversity

This is the first of two articles on the fiber-type composition of the human jaw muscles. The present article discusses the origin of fiber-type composition and its consequences. This discussion is presented in the context of the requirements for functional performance and adaptation that are imposed upon the jaw muscles. The human masticatory system must perform a much larger variety of motor tasks than the average limb or trunk motor system. An important advantage of fiber-type diversity, as observed in the jaw muscles, is that it optimizes the required function while minimizing energy use. The capacity for adaptation is reflected by the large variability in fiber-type composition among muscle groups, individual muscles, and muscle regions. Adaptive changes are related, for example, to the amount of daily activation and/or stretch of fibers. Generally, the number of slow, fatigue-resistant fibers is relatively large in muscles and muscle regions that are subjected to considerable activity and/or stretch.

Fiber-type Composition of the Human Jaw Muscles—(Part 2) Role of Hybrid Fibers and Factors Responsible for Inter-individual Variation

This is the second of two articles about fiber-type composition of the human jaw muscles. It reviews the functional relationship of hybrid fibers and the adaptive properties of jaw-muscle fibers. In addition, to explain inter-individual variation in fiber-type composition, we discuss these adaptive properties in relation to environmental stimuli or perturbations. The fiber-type composition of the human jaw muscles is very different from that of limb and trunk muscles. Apart from the presence of the usual type I, IIA, and IIX myosin heavy-chains (MyHC), human jaw-muscle fibers contain MyHCs that are typical for developing or cardiac muscle. In addition, much more frequently than in limb and trunk muscles, jaw-muscle fibers are hybrid, i.e., they contain more than one type of MyHC isoform. Since these fibers have contractile properties that differ from those of pure fibers, this relatively large quantity of hybrid fibers provides a mechanism that produces a very fine gradation of force and movement. The presence of hybrid fibers might also reflect the adaptive capacity of jaw-muscle fibers. The capacity for adaptation also explains the observed large inter-individual variability in fiber-type composition. Besides local influences, like the amount of muscle activation and/or stretch, more general influences, like aging and gender, also play a role in the composition of fiber types.

The adaptive response of jaw muscles to varying functional demands

Jaw muscles are versatile entities that are able to adapt their anatomical characteristics, such as size, cross-sectional area, and fibre properties, to altered functional demands. The dynamic nature of muscle fibres allows them to change their phenotype to optimize the required contractile function while minimizing energy use. Changes in these anatomical parameters are associated with changes in neuromuscular activity as the pattern of muscle activation by the central nervous system plays an important role in the modulation of muscle properties. This review summarizes the adaptive response of jaw muscles to various stimuli or perturbations in the orofacial system and addresses general changes in muscles as they adapt, specific adaptive changes in jaw muscles under various physiologic and pathologic conditions, and their adaptive response to non-surgical and surgical therapeutic interventions. Although the jaw muscles are used concertedly in the masticatory system, their adaptive changes are not always uniform and vary with the nature, intensity, and duration of the stimulus. In general, stretch, increases neuromuscular activity, and resistance training result in hypertrophy, elicits increases in mitochondrial content and cross-sectional area of the fibres, and may change the fibre-type composition of the muscle towards a larger percentage of slow-type fibres. In contrast, changes in the opposite direction occur when neuromuscular activity is reduced, the muscle is immobilized in a shortened position, or paralysed. The broad range of stimuli that affect the properties of jaw muscles might help explain the large variability in the anatomical and physiological characteristics found among individuals, muscles, and muscle portions.

Fibre-type composition of rabbit jaw muscles is related to their daily activity.

Skeletal muscles contain a mixture of fibres with different contractile properties, such as maximum force, contraction velocity and fatigability. Muscles adapt to altered functional demands, for example, by changing their fibre‐type composition. This fibre‐type composition can be changed by the frequency, duration and presumably the intensity of activation. The aim of this study was to analyse the relationship between the spontaneous daily muscle activation and fibre‐type composition in rabbit jaw muscles. Using radio‐telemetry combined with electromyography, the daily activity of five jaw muscles was characterized in terms of the total duration of muscle activity (duty time) and the number of activity bursts. Fibre‐type composition of the muscles was classified by analysing the myosin heavy chain content of the fibres. The amount of slow‐type fibres was positively correlated to the duty time and the number of bursts only for activations exceeding 20–30% of the maximum activity per day. Furthermore, cross‐sectional areas of the slow‐type fibres were positively correlated to the duty time for activations exceeding 30% of the maximum activity. The present data indicate that the amount of activation above a threshold (> 30% peak activity) is important for determining the fibre‐type composition and cross‐sectional area of slow‐type fibres of a muscle. Activation above this threshold occurred only around 2% of the time in the jaw muscles, suggesting that contractile properties of muscle fibres are maintained by a relatively small number of powerful contractions per day.

Heterogeneity of fiber characteristics in the rat masseter and digastric muscles

The functional requirements in muscle use are related to the fiber type composition of the muscles and the cross‐sectional area of the individual fibers. We investigated the heterogeneity in the fiber type composition and fiber cross‐sectional area in two muscles with an opposing function, namely the digastric and masseter muscles (n = 5 for each muscle) of adult male rats, by means of immunohistochemical staining according to their myosin heavy chain (MyHC) content. The digastric and masseter muscles were taken from Wistar strain male rats 10 weeks old. In the masseter six predefined sample locations were examined; in the digastric four. Most regions showed dominant proportions of type IIA and IIX fibers. However, both muscles also revealed a regional heterogeneity in their fiber type distribution. In the digastric, type I fibers were detected only at the central and deep areas of the anterior and posterior belly, respectively. Meanwhile, the peripheral area of the anterior belly contained a higher proportion of type IIB fibers. In the masseter, the type I fibers were absent. In the superficial masseter the distribution of IIA and IIB fibers was significantly different between the superior and inferior regions. In the deep masseter, regional differences were observed among all four examined areas, of which the posterolateral region contained the highest proportion of type IIB fibers. The cross‐sectional areas of type IIB fibers were always the largest, followed by the type IIX and IIA fibers. Only a few differences in cross‐sectional area of corresponding fiber types were detected between the various sites. In conclusion, the masseter and digastric muscles showed an obvious heterogeneity of fiber type composition and fiber cross‐sectional area. Their heterogeneity reflects the complex role of the both muscles during function. This detailed description of the fiber type composition can serve as a reference for future studies examining the muscular adaptations after the onset of various diseases in the masticatory system.

Adaptation of rat jaw muscle fibers in postnatal development with a different food consistency: an immunohistochemical and electromyographic study

The development of the craniofacial system occurs, among other reasons, as a response to functional needs. In particular, the deficiency of the proper masticatory stimulus affects the growth. The purpose of this study was to relate alterations of muscle activity during postnatal development to adaptational changes in the muscle fibers. Fourteen 21‐day‐old Wistar strain male rats were randomly divided into two groups and fed on either a solid (hard‐diet group) or a powder (soft‐diet group) diet for 63 days. A radio‐telemetric device was implanted to record muscle activity continuously from the superficial masseter, anterior belly of digastric and anterior temporalis muscles. The degree of daily muscle use was quantified by the total duration of muscle activity per day (duty time), the total burst number and their average length exceeding specified levels of the peak activity (5, 20 and 50%). The fiber type composition of the muscles was examined by the myosin heavy chain content of fibers by means of immunohistochemical staining and their cross‐sectional area was measured. All muscle fibers were identified as slow type I and fast type IIA, IIX or IIB (respectively, with increasing twitch contraction speed and fatigability). At lower activity levels (exceeding 5% of the peak activity), the duty time of the anterior belly of the digastric muscle was significantly higher in the soft‐diet group than in the hard‐diet group (P < 0.05). At higher activity levels (exceeding 20 and 50% of the peak activity), the duty time of the superficial masseter muscle in the soft‐diet group was significantly lower than that in the hard‐diet group (P < 0.05). There was no difference in the duty time of the anterior temporalis muscle at any muscle activity level. The percentage of type IIA fibers of the superficial masseter muscle in the soft‐diet group was significantly lower than that in the hard‐diet group (P < 0.01) and the opposite was true with regard to type IIB fibers (P < 0.05). The cross‐sectional area of type IIX and type IIB fibers of the superficial masseter muscle was significantly smaller in the soft‐diet group than in the hard‐diet group (P < 0.05). There was no difference in the muscle fiber composition and the cross‐sectional area of the anterior belly of the digastric and anterior temporalis muscles. In conclusion, for the jaw muscles of male rats reared on a soft diet, the slow‐to‐fast transition of muscle fiber was shown in only the superficial masseter muscle. Therefore, the reduction in the amount of powerful muscle contractions could be important for the slow‐to‐fast transition of the myosin heavy chain isoform in muscle fibers.

Postnatal transitions in myosin heavy chain isoforms of the rabbit superficial masseter and digastric muscle

We investigated the early (< 8 weeks) and late (> 8 weeks) postnatal development of the fibre type composition and fibre cross‐sectional area in the superficial masseter and digastric muscle of male rabbits. It was hypothesized, first, that due to the transition between suckling and chewing, during early postnatal development the increase in the proportion of slow fibre types and in fibre cross‐sectional areas would be larger in the masseter than in the digastric; and second, that due to the supposed influence of testosterone during late postnatal development, the proportion of slow fibre types in both muscles would decrease. Fibre types were classified by immunostaining according to their myosin heavy chain (MyHC) content. The proportion of slow fibre types significantly increased in the masseter, from 7% at week 1 to 47% at week 8, and then decreased to 21% at week 20, while in the digastric it increased from 5% in week 1 to 19% at week 8 and remained the same thereafter. The changes in the proportion of fast fibre types were the opposite. The remarkable increase and decrease in the proportion of slow fibre types in the masseter was attributed predominantly to MyHC‐cardiac α fibres. During early development, the cross‐sectional area of all fibres in both muscles increased. However, only the fast fibre types in the masseter continued to grow further after week 8. Before weaning, the fast fibre types in the digastric were larger than those in the masseter, but after week 8, they became larger in the masseter than in the digastric. In adult animals, masseter and digastric had the same percentage of fast fibre types, but these fibres were almost twice as large in masseter as in digastric.

Functional characteristics of the rat jaw muscles: daily muscle activity and fiber type composition

Skeletal muscles have a heterogeneous fiber type composition, which reflects their functional demand. The daily muscle use and the percentage of slow‐type fibers have been shown to be positively correlated in skeletal muscles of larger animals but for smaller animals there is no information. The examination of this relationship in adult rats was the purpose of this study. We hypothesized a positive relationship between the percentage of fatigue‐resistant fibers in each muscle and its total duration of use per day. Fourteen Wistar strain male rats (410–450 g) were used. A radio‐telemetric device was implanted to record muscle activity continuously from the superficial masseter, deep masseter, anterior belly of digastric and anterior temporalis muscles. The degree of daily muscle use was quantified by the total duration of muscle activity per day (duty time) exceeding specified levels of the peak activity (2, 5, 20 and 50%). The fiber type composition of the muscles was examined by the myosin heavy chain content of the fibers by means of immunohistochemical staining. At lower activity levels (exceeding 2 and 5% of the peak activity), the duty time of the anterior belly of digastric muscle was significantly (P < 0.01) longer than those of the other muscles. The anterior belly of digastric muscle also contained the highest percentage of slow‐type fibers (type I fiber and hybrid fiber co‐expressing myosin heavy chain I + IIA) (ca. 11%; P < 0.05). By regression analysis for all four muscles, an inter‐muscular comparison showed a positive relationship between the duty time (exceeding 50% of the peak activity) and the percentage of type IIX fibers (P < 0.05), which demonstrate intermediate physiological properties relative to type IIA and IIB fibers. For the jaw muscles of adult male rats, the variations of fiber type composition and muscle use suggest that the muscle containing the largest amounts of slow‐type fibers (the anterior belly of digastric muscle) is mainly involved in low‐amplitude activities and that the amount of type IIX fibers is positively related to the generation of large muscle forces, validating our hypothesis.

Is fiber-type composition related to daily jaw muscle activity during postnatal development?

Aim: Muscles containing large numbers of slow-contracting fibers are generally more active than muscles largely composed of fast fibers. This relationship between muscle activity and phenotype suggests that (1) changes in fiber-type composition during postnatal development are accompanied by changes in daily activity and (2) individual variations in fiber-type composition are related to similar variations in daily muscle activity. Methods: The masseter and digastric muscles of 23 New Zealand White rabbits (young, juvenile and adult) were examined for their phenotype (myosin heavy chain content) and their daily activity (total daily number of activity bursts). Results: During development, the masseter showed a strong increase in the number of fast-type fibers compared to the number of slow-type fibers. During development, also the number of powerful bursts in the masseter increased. The digastric showed no significant changes in fiber types or burst numbers. Within each muscle, across individual animals, no significant correlations (R < 0.70) were found between any of the fiber types and daily burst numbers in any of the age groups. Conclusions: The results suggest that activity-related influences are of relatively minor importance during development and that other factors are dominant in determining fiber-type composition.