Myosin Heavy Chain Expression Can Vary over the Length of Jaw and Leg Muscles.

Muscle fiber type classification can be determined by its myosin heavy chain (MyHC) composition based on a few consecutive sections. It is generally assumed that the MyHC expression of a muscle fiber is the same over its length since neural stimulation and systemic influences are supposed to be the same over its length. We analyzed this in detail in three muscle types: the temporalis (closer) and digastricus (opener; both first brachial arch), and the medial gastrocnemius (somite). Sections of the muscles were incubated with monoclonal antibodies against various MyHC isoforms, and the distribution of these isoforms within individual fibers was followed over a distance of approximately 1 mm. The staining intensity of a fiber was measured and compared with the other fibers in the section. In the temporalis, digastricus, and gastrocnemius, 46, 11, and 15%, respectively, of their MyHC-I fibers showed a variation in the staining intensity over the length of their fibers, as well as 47, 87, and 22%, respectively, of their MyHC-IIA fibers. Most variable fibers were found amongst those with an overall relative intermediate staining intensity, which are presumably hybrid fibers. We conclude that different parts of a muscle fiber can have different fiber type compositions and, thus, contractile properties. Some muscle parts might reach their maximum contraction peak sooner or later than a muscle part a few microns further away. Next to stimulation by the nerve and systemic influences, local influences might also have an impact on the MyHC expression of the fiber.

Changes in activity and structure of jaw muscles in Parkinson's disease model rats.

Parkinson's disease (PD), a major neurological disease, is characterised by a marked loss of dopaminergic neurons in the substantia nigra. Patients with PD frequently show chewing and swallowing dysfunctions, but little is known about the characteristics of their stomatognathic functions. The purpose of this study was to evaluate the influence of PD on jaw muscle fibre and functions. PD model rats were made by means of the injection of 6-hydroxydopamine (6-OHDA) into the striatum of 8-week-old Sprague-Dawley male rats. Five weeks after the injection, a radio-telemetric device was implanted to record muscle activity continuously from the superficial masseter and anterior belly of digastric muscles. Muscle activity was recorded for 3 days and was evaluated by the total duration of muscle activity per day (duty time). After recording the muscle activities, jaw muscles were isolated for immunohistochemical and PCR analyses. In PD model rats, the following findings of the digastrics muscles verify that compared to the control group: (i) the higher duty time exceeding 5% of the peak activity level, (ii) the higher expression of the mRNA of myosin heavy chain type I, and (iii) the tendency for fast to slow fibre-type transition. With respect to the masseter muscle, there were no significant differences in all analyses. In conclusion, PD leads to the changes in the jaw behaviours, resulting in a PD-specific chewing and swallowing dysfunctions.

Influence of botulinum toxin on rabbit jaw muscle activity and anatomy.

INTRODUCTION: Muscles can adapt their fiber properties to accommodate to new conditions. We investigated the extent to which a decrease in muscle activation can cause an adaptation of fiber properties in synergistic and antagonistic jaw muscles. METHODS: Three months after the injection of botulinum toxin type A in one masseter (anterior or posterior) muscle changes in fiber type composition and fiber cross-sectional areas in jaw muscles were studied at the microscopic level. RESULTS: The injected masseter showed a steep increase in myosin type IIX fibers, whereas fast fibers decreased by about 50% in size. Depending on the injection site, both synergistic and antagonistic muscles showed a significant increase in the size of their fast IIA fibers, sometimes combined with an increased number of IIX fibers. CONCLUSION: Silencing the activity in the masseter not only causes changes in the fibers of the injected muscle but also leads to changes in other jaw muscles.

The role of masticatory muscles in the continuous loading of the mandible.

Muscles are considered to play an important role in the ongoing daily loading of bone, especially in the masticatory apparatus. Currently, there are no measurements describing this role over longer periods of time. We made simultaneous and wireless in vivo recordings of habitual strains of the rabbit mandible and masseter muscle and digastric muscle activity up to ∼25 h. The extent to which habitually occurring bone strains were related to muscle-activity bursts in time and in amplitude is described. The data reveal the masseter muscle to load the mandible almost continuously throughout the day, either within cyclic activity bouts or with thousands of isolated muscle bursts. Mandibular strain events rarely took place without simultaneous masseter activity, whereas the digastric muscle only played a small role in loading the mandible. The average intensity of masseter-muscle activity bouts was strongly linked to the average amplitude of the concomitant bone-strain events. However, individual pairs of muscle bursts and strain events showed no relation in amplitude within cyclic loading bouts. Larger bone-strain events, presumably related to larger muscle-activity levels, had more constant principal-strain directions. Finally, muscle-to-bone force transmissions were detected to take place at frequencies up to 15 Hz. We conclude that in the ongoing habitual loading of the rabbit mandible, the masseter muscle plays an almost non-stop role. In addition, our results support the possibility that muscle activity is a source of low-amplitude, high-frequency bone loading.

Variations in habitual bone strains in vivo: long bone versus mandible.

Little is known about the similarities and dissimilarities between daily in vivo strain histories of different bones, other than the generally accepted view that most bones need daily loading to maintain their mass. Similarities in daily strain histories might uncover a common basic mechanical stimulus for homeostatic bone maintenance, whereas dissimilarities might explain why bones respond differently to changes in their environment. We compared the daily strain histories of two different bones from the rabbit: the tibiofibula and the mandible. Bone strain was recorded wirelessly in freely moving animals up to 57 h. Habitual strain amplitudes, rates, and frequencies were compared. The exponential decrease in occurrence of greater strain amplitudes was similar in long and mandibular bone. Also similar was the exponentially decreasing incidence of higher strain rates. Mandibular-bone strains distinguished themselves from long-bone strains, however, through the presence of a plateau in the occurrence of compressive strains with amplitudes between 200 and 450 microstrain (µÎµ) and in the occurrence of compressive-strain rates between 5000 and 10,000 µÎµ/s. The frequency spectrum of the mandibular-bone strain history contained peaks at 4.4 Hz and 8.9 Hz, which were absent in the long-bone strain spectrum. We hypothesise that the exponentially decreasing incidence of larger strain amplitudes and rates might outline the minimum amount of mechanical stimuli needed for bone maintenance. Furthermore, the strong presence of rhythmicity in mandibular strains might provide an important clue in confirming or improving the anabolic character of cyclic-loading regimes employed in clinical settings.

The daily habitual in vivo strain history of a non-weight-bearing bone.

Daily mechanical loading strongly influences the architecture and composition of bone tissue. Throughout the day, the amplitudes, rates, frequencies, and the dispersion over time of these loads vary. Nevertheless, most experimental and descriptive studies on the aforementioned relationship consider only cyclic loading and, in addition, focus on weight-bearing bones. A more complete assessment of the daily loading of bone might lead to a better understanding of the natural everyday stimulus for bone maintenance or adaptive responses. In the present study, we measured the daily habitual strain history of the non-weight-bearing mandible bone in the rabbit. Long-term continuous strain recordings were made using an implantable telemetry device able to read out bone-bonded strain gauges. The lateral surface of the rabbit mandibular corpus was chosen as the bone surface of interest. During the recordings, which lasted up to 33 h, the rabbits (N=7) were able to move unrestrictedly in their cages, performing their habitual behaviours. Analysis of the recordings revealed that the measured bone surface was subjected to 2.9 (+/-1.4)x10(3) strain events per hour of which 1.8 (+/-1.0)x10(3) had amplitudes < or =10 microstrains (muvarepsilon). Larger strain amplitudes occurred less often and principal strains fell within the range of -517 (+/-118) muvarepsilon to 298 (+/-81) muvarepsilon. Strain rates never exceeded 10,000 muvarepsilon/s and only 8.9% (+/-7.2%) of the habitual strain rates were higher than 1000 muvarepsilon/s. Strain frequency spectra displayed clear peaks at 4-5 and 9 Hz. The wirelessly recorded daily strain history of the rabbit mandible featured peak strain amplitudes resembling those of other mammalian mandibles, but much smaller than those found in many long-bone strain measurements.

A fully implantable telemetry system for the long-term measurement of habitual bone strain.

Long-term in-vivo recordings of habitual bone strain in freely moving animals are needed to better understand the everyday mechanical loading environment responsible for bone-tissue maintenance. However, wireless methods to make such recordings are scarce. We report on the successful customisation of a commercially available voltage transmitter hooked-up to a strain-gauge rosette, its subcutaneous implantation in rabbits, and the quality of the implant's strain-gauge recordings. Continuous wireless recordings of a completely operational strain-gauge rosette glued to the mandibular surface of a freely moving rabbit could be made up to 33h. The resolution of the system was 1.5 microstrains/bit. The noise in the signal was 4.5 microstrains. To facilitate the automatic counting of bone-strain events in the retrieved data, and to calculate their peak amplitude, a novel approach is presented. The described technique enables the quantification of the daily bone-strain history defining the architecture and composition of bone tissue, and can help to further elucidate the strain parameters which influence bone tissue.

Postnatal development of fiber type composition in rabbit jaw and leg muscles.

We examined the difference in fiber type composition and cross-sectional areas during postnatal development in male rabbit jaw muscles and compared these with changes in leg muscles. The myosin heavy chain (MyHC) content of the fibers was determined by immunohistochemistry. No fiber type difference was found between the jaw muscles in 20-week-old rabbits. However, the way this adult fiber type composition was reached differed between the muscles. The deep temporalis, medial pterygoid, and superficial masseter displayed an increase in alpha fibers during early and a decrease during late postnatal development. Other jaw muscles displayed an increase in alpha fibers during early development only. In contrast, alpha fibers were not found in the soleus, in which fiber type changes were completed at week 4. The gastrocnemius muscle did not change its fiber type composition. Initially, fibers in jaw-opening muscles had larger cross-sectional areas than in other muscles, but they increased less during development. Although there were no large differences in the fiber type composition of muscles in young adult rabbits, large differences were found in the jaw muscles, but not in the leg muscles, during development. In part, these developmental changes in fiber percentages within the jaw muscles can be explained by functional modifications in this muscle group. In the present study, the deep temporalis, medial pterygoid, and superficial masseter showed the most dramatic percent changes in fibers during postnatal development.

Regional differences in fiber characteristics in the rat temporalis muscle.

The behavioral differences in muscle use are related to the fiber type composition of the muscles among other variables. The aim of this study was to examine the degree of heterogeneity in the fiber type composition in the rat temporalis muscle. The temporalis muscle was taken from 10-week-old Wistar strain male rats (n = 5). Fiber types were classified by immunohistochemical staining according to their myosin heavy chain content. The anterior temporalis revealed an obvious regional difference of the fiber type distribution, whereas the posterior temporalis was homogeneous. The deep anterior temporalis showed a predominant proportion of type IIA fibers and was the only muscle portion displaying slow type fibers (< 10%). The other two muscle portions, the superficial anterior and posterior temporalis, did not differ significantly from each other and contained mainly type IIB fibers. Moreover, the deep anterior temporalis was the only muscle portion showing slow type fibers (< 10%). In the deep portion, type IIX fibers revealed the largest cross-sectional area (1943.1 +/- 613.7 microm(2)), which was significantly (P < 0.01) larger than those of type IIA and I + IIA fibers. The cross-sectional area of type IIB fibers was the largest in the remaining two muscle portions and was significantly (P < 0.01) larger than that of type IIX fibers. In conclusion, temporalis muscle in rats showed an obvious heterogeneity of fiber type composition and fiber cross-sectional area, which suggests multiple functions of this muscle.

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.

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.

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 alpha 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.

Daily activity of the rabbit jaw muscles during early postnatal development.

Early postnatal development of the jaw muscles is characterized by the transition from suckling to chewing behavior. As chewing develops the jaw closing muscles become more powerful compared with the jaw openers. These changes are likely to affect the amount of daily muscle activity. Therefore, the purpose of this study was to characterize for a jaw opener (digastric) and jaw closer (masseter) the total duration of daily muscle activity (i.e. the duty time), and the daily burst numbers and lengths during early postnatal development. Using radiotelemetry the activity of these muscles was recorded in 10 young New Zealand White rabbits between three and eight weeks of age. Fiber-type composition was analyzed at eight weeks of age by determining the myosin heavy chain content of the fibers. During postnatal development both muscles showed no significant decrease or increase in their daily activity. However, the interindividual variation of the duty time and burst number significantly decreased. There were no significant differences between the digastric and masseter except for the most powerful activities at eight weeks of age, where the masseter showed a significantly higher duty time and burst number than the digastric. The masseter contained a higher number of slow-type fibers expressing myosin heavy chain-I and myosin heavy chain-cardiac alpha than the digastric. The present results suggest that the amount of jaw muscle activation is already established early during postnatal development, before the transition from suckling to chewing behavior. This amount of activation seems to be related to the number of slow-type fibers.

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.

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.

Myosin heavy-chain isoform composition of human single jaw-muscle fibers.

Diversity in muscle contractile properties is based on the variability of contractile properties of single muscle fibers which in turn is related to the presence of different myosin heavy-chain (MyHC) isoforms. Human jaw muscles are featured by many hybrid fibers expressing more than one MyHC isoform. The purpose of this study was to determine the proportion of each isoform within these fibers for evaluation of the fiber's capacity of producing a large diversity in contractile properties. Electrophoretic separation of MyHC isoforms was performed on 218 single fibers of the temporalis and digastric muscles. Of these fibers, 100 were classified as hybrid fibers. Most hybrid fibers co-expressed MyHC-IIA and -IIX (n = 62); a smaller number co-expressed MyHC-I and -IIA (n = 14), MyHC-I and -IIX (n = 12), and MyHC-I, -IIA, and -IIX (n = 12). The proportions of the individual MyHC isoforms in the hybrid fibers varied highly, suggesting a large range of contractile properties among these fibers.

Myosin heavy chain composition in human masticatory muscles by immunohistochemistry and gel electrophoresis.

In this study we compared the immunohistochemically quantified fiber type area with the myosin heavy chain (MyHC) contents of a bundle of fibers from a human masticatory muscle. The total cross-sectional areas were determined immunohistochemically for the three major fiber types (I, IIA, and IIX) in bundles of fibers (n=42) taken from the anterior and posterior belly of the human digastric muscle (n=7). The relative MyHC contents of the same fiber bundles were determined electrophoretically (MyHC-I, -IIA, and -IIX; anterior, 32%, 35%, and 33%; posterior, 39%, 42%, and 19%) and compared with the immunohistochemical data (MyHC-I, -IIA, and -IIX; anterior, 32%, 31%, and 37%; posterior, 39%, 45%, and 15%). No significant differences were seen in the mean fiber type distribution between the two techniques; the correlation coefficient ranged from 0.71 to 0.96. The correlation coefficient was higher for MyHC type I and MyHC type IIX than for MyHC type IIA. The MyHC contents of single fibers taken from the posterior belly indicated that many fibers in this belly co-express MyHC-IIA and MyHC-IIX. Despite the presence of these hybrid fibers, the correspondence between both methods was relatively large.