Variation of the mineral density in cortical bone may serve to keep strain amplitudes within a physiological range.

Within-bone variation in mineral density could be functional. A heterogeneous mineral-density distribution might serve to maintain habitual amplitudes of bone strain within a non-harmful, i.e., physiological range. Regions of a bone that would be strained the most on the basis of architecture alone might have a higher mineral density to make them more stiff and resistant to strain. We hypothesised that the cortical bone of the rabbit mandible contains such a functional distribution of mineral density. We thereby expected similar mineral-density patterns in the mandibles of different individuals due to the shared masticatory function. Secondly, we hypothesised that the highest mineral densities occur in mandibular regions predicted to be exposed to the largest amplitudes of strain-when taking into account bone architecture only. Mineral-density maps of the cortical bone of rabbit mandibles were obtained using micro-computed tomography (µCT). The µCT scans of two rabbits were converted into finite-element models (FEMs). To predict mandibular deformation during biting, these models were loaded by muscle forces and reaction forces. The forces acted on the condyles and on either the incisal or molar bite point. The FEMs were assigned a homogeneous material stiffness to calculate the strain amplitudes that would occur when only the architecture of the mandibular bone would be of influence. We found the cortical bone-mineral density patterns to be similar in all six mandibles. The mineral density of the corpus was higher than that of the ramus. A second consistent feature of the mandibular mineral-density distribution was that the medial ridge of the temporal-muscle insertion groove contained more mineral than its surrounding regions. The strain amplitudes calculated with the FEMs were variable and did not feature clear corpo-ramal differences. However, specific mandibular bone sites calculated to be exposed to the largest amplitudes of strain, including the medial ridge of the temporal-muscle insertion groove, did correspond with high-mineral-density regions. We conclude that, in the rabbit mandible, the heterogeneous mineral-density distribution might serve to suppress bone-strain amplitudes in regions architecturally susceptible to the largest deformations during loading.

Changes in rabbit jaw-muscle activity parameters in response to reduced masticatory load.

Mechanical food properties influence the neuromuscular activity of jaw-closing muscles during mastication. It is, however, unknown how the activity profiles of the jaw muscles are influenced by long-term alterations in masticatory load. In order to elucidate the effect of reduced masticatory load on the daily habitual activity profiles of three functionally different jaw muscles, the electromyograms of the masseter, temporalis and digastric muscles were recorded telemetrically in 16 male rabbits between seven and 20 weeks of age. Starting at eight weeks of age the experimental animals were fed significantly softer pellets than the control animals. Daily muscle activity was quantified by the relative duration of muscle use (duty time), burst number and burst length in relation to multiple activity levels. The daily duty time and burst number of the masseter muscle were significantly lower in the experimental group than in the control group at 5% and 10% of the maximum activity during the two weeks following the change in food hardness. By contrast, altered food hardness did not significantly influence the activity characteristics of the temporalis and digastric muscles. The findings suggest that a reduction in masticatory load decreases the neuromuscular activity of the jaw-closing muscles that are primarily responsible for force generation during mastication. This decrease is most pronounced in the weeks immediately following the change in food hardness and is limited to the activity levels that reflect muscle contractions during chewing. These findings support the conclusion that the masticatory system manifests few diet-specific long-term changes in the activity profiles of jaw muscles.

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.

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.

Burst characteristics of daily jaw muscle activity in juvenile rabbits.

Muscle activation varies with different behaviors and can be quantified by the level and duration of activity bursts. Jaw muscles undergo large anatomical changes during maturation, which are presumably associated with changes in daily muscle function. Our aim was to examine the daily burst number, burst length distribution and duty time (fraction of the day during which a muscle was active) of the jaw muscles of juvenile male rabbits (Oryctolagus cuniculus). A radio-telemetric device was implanted to record muscle activity continuously from the digastric, superficial and deep masseter, medial pterygoid and temporalis during maturation week 9-14. Daily burst characteristics and duty times were determined for activations, including both powerful and non-powerful motor behavior. All muscles showed constant burst numbers, mean burst lengths and duty times during the recording period. Including all behavior, the temporalis showed significantly larger daily burst numbers (205,000) and duty times (18.2%) than the superficial and deep masseter (90,000; 7.5%). Burst numbers and duty times were similar for the digastric (120,000; 11.1%) and medial pterygoid (115,000; 10.4%). The temporalis and deep masseter showed many short low activity bursts (0.05 s), the digastric showed many long bursts (0.09 s). For activations during powerful behaviors the superficial masseter and medial pterygoid had the largest burst numbers and duty times. Both muscles showed similar burst characteristics for all activation levels. It was concluded that activation of the jaw muscles is differently controlled during powerful and non-powerful motor behaviors and the functional organization of motor control patterns does not vary from 9 to 14 weeks of age.

Long-term registration of daily jaw muscle activity in juvenile rabbits.

Understanding control of muscles during various tasks and their adaptive changes requires information on all motor behavior used throughout the day. The total duration of muscle activity depends on the magnitude of its activation and can change during maturation. Therefore, the purpose of this study was to examine the duration of muscle activity (i.e. duty time) exceeding various activity levels in maturing jaw muscles. A telemetric device was implanted into nine juvenile male New Zealand White rabbits to continuously record muscle activity during maturation weeks 9-14. Electrodes were inserted into digastric, superficial and deep masseter, medial pterygoid, and temporalis muscles. Duty times (expressed as a percentage of time) were calculated for activation exceeding different levels (5-90%) of EMG peak activity per 24-h period. At 10 weeks of age, for activation exceeding the 5% level, the duty time of the temporalis (20.0+/-5.2%) was statistically significantly higher than that of the medial pterygoid (11.2+/-1.5%), digastric (11.0+/-5.1%), superficial (12.6+/-5.6%), and deep masseter (8.6+/-5.5%). Duty times declined with increasing activity level. For activation exceeding the 40% level the duty times of the superficial masseter and medial pterygoid were significantly higher than those of the other muscles. During maturation none of the muscles showed a significant change in duty time. However, for activation exceeding the 5% level, the inter-individual variation in duty time decreased significantly for the digastric, and superficial and deep masseter.

Variation in daily masticatory muscle activity in the rabbit.

The daily use of masticatory muscles remains largely unclear, since continuous recordings were limited in space and time. This study's purpose was to use radio-telemetry to examine daily muscle use and its inter- and intra-individual variations. A telemetric device was implanted into the rabbit masseter, and the transmitted signals were digitally stored for 7 days. Muscle use was analyzed by calculation of the total time each muscle was activated above 5, 20, and 50% of the day's peak activity. Rabbits (n = 6) spent only 2% of the time chewing. Muscles were activated up to 20% of the total time at levels exceeding 5% of peak activity, and only about 0.5% of the time in forceful behaviors utilizing 50% of maximum contraction. It can be concluded that daily muscle use remained constant during succeeding days, but differed significantly among muscle regions and individuals.

Passive force characteristics of an architecturally complex muscle.

In architecturally complex muscles with large attachment areas, it can be expected that during movement different muscle regions undergo different amounts of length excursions. As a consequence, the amount of passive force produced by the regions will differ. Therefore, we tested the hypothesis that during movement the vector of the passive force of such a muscle, which defines the magnitude, position and orientation of the resultant force of the various regions, has no fixed position, between the muscle's center of origin and insertion. As a model for an architecturally complex muscle we used the masseter muscle. It was expected that during jaw opening anterior muscle regions are more stretched than posterior regions, leading to an anterior shift of the passive force vector. A three-component force transducer was used to measure both the position and magnitude of passive force in the masseter muscle of 9 rabbits. Forces were recorded during repeated cycles of stepwise opening and closure of the jaw. The muscle exhibited a clear hysteresis: passive force measured during jaw opening was larger than that during jaw closing. With an increase of the jaw gape there was an approximately exponential increase of the magnitude of the passive muscle force, while simultaneously the passive force vector shifted anteriorly. Moment arm length of passive force increased by about 100%. This anterior shift contributed substantially to the increase of the passive muscle moment generated during jaw opening. It can be concluded that in architecturally complex muscles the increase of the passive resistance moment which is associated with muscle lengthening might not only be due to an increase of the magnitude of passive muscle force but also to an increase of the moment arm of this force.

A longitudinal electromyographic study of the postnatal maturation of mastication in the rabbit.

At 2 weeks of age, infant rabbits show chewing movements that resemble those of the adult animal. Previous studies have shown that, at that stage, the accompanying masticatory motor pattern is clearly similar to the suckling motor pattern. As early as 4 weeks, chewing muscle activity is indistinguishable from the adult chewing motor pattern. These reports suggest that the adult chewing motor pattern is developed from the suckling motor pattern. In this study, the chewing motor pattern in the intermediate period (between 2 and 4 weeks of age) was investigated by means of fine-wire electromyography and jaw tracking. Maturation of masticatory movements was found to have two phases. Maximum gape increased in the first few days and was followed by strong development of transverse jaw excursions after the age of 17 days. The increase in jaw excursions was brought about by changes in motor behaviour and facilitated by the development of smooth occlusal surfaces. The changes in motor behaviour were: (1) the level of activity of the balancing-side muscles became more equal to that of the working side; (2) the timing of digastric muscle activity became asymmetrical at the age of 17 days; (3) the peak activity of masseter, temporalis, medial pterygoid and lateral pterygoid muscle portions was gradually shifted or prolonged into the power-stroke phase. It can be concluded that the masticatory contraction pattern shifts from one derived from the suckling contraction pattern at the age of 14 days to one almost similar to the adult chewing pattern at the age of 23 days.