The force-velocity relation of the rabbit digastric muscle

In 30 animals, the digastric was made to pull actively against a slide loaded by a servo-controlled linear motor. Force and velocity were recorded at the end of active shortening to the in-situ (jaw-closed) muscle length. Passive and active force-length relations were also determined in 17 of the rabbits. The empirical force-velocity data were fitted to a hyperbolic equation. The average speed of muscle shortening at zero load was 14.67 cm/s. Mean maximum isometric force at in-situ length (P0) was 1267 g, and the mean ratio a/P0 was 0.18. The average time-to-peak twitch tension was 31.8 ms under isometric conditions. In-situ muscle-belly length was about 3 per cent less than optimum length for isometric force. Maximum muscle force was positively correlated with animal size, but maximum velocity showed no relation to force or length. The estimated maximum speed of sarcomere shortening was 26 micron/s, which is slightly slower than in fast limb muscles of the cat, and may indicate the presence of both histochemical type I and II fibres. The isometric force after shortening had ceased was less than P0, and was correlated with the velocity during shortening. This depression of isometric force may result from an alteration of the excitation-coupling system during activation. These observations suggest a role for the digastric in the rapid acceleration and deceleration of the mandible near the jaw-closed position during opening and closing.     

Anatomical and electromyographic studies of the digastric muscle

The techniques and sites for EMG recordings from the digastric muscles are controversial. To re-evaluate old techniques for recording from the digastric muscles, especially the posterior bellies, the morphology of the muscles was studied by conventional dissections and by examination of specimens sectioned in the frontal and the horizontal planes. Based on these anatomical findings, recording sites and approaches to them were developed for the anterior and posterior bellies of the digastric muscles. EMG recordings from the two bellies of the muscle were obtained from five healthy subjects. The EMG recordings were ranked according to muscle activity level and the activity within single muscles and between muscles compared using the Wilcoxon signed rank test. The anterior and posterior bellies had synchronized activity in all mandibular movements but were silent or had negligible activity with the mandible in the rest position, when the head was rotated, and while clenching. Both bellies had marked to very marked activity during jaw opening, and moderate to marked activity during protrusion, retrusion and lateral movements. During swallowing the anterior and posterior bellies had patterns characterized by bursts of activity of high amplitude and short duration. The two bellies were not, however, always synchronously active.     

The effects of tenotomy on the morphology of the rabbit digastric muscle

The digastric muscles of 17 New Zealand White rabbits were subjected to tenotomy. A sham operation was performed on six animals. Groups of three or four animals were killed at one hour, one day, 10 days, 30 days, and 60 days after the tenotomy. There was evidence of tendon regeneration by 10 days, and by 30 days the tendon appeared normal macroscopically. The tendon was normal in microscopic appearance in the animals examined at 60 days. Muscle belly length and fascicle length decreased after the tenotomy, and the angle of pinnation increased. Sarcomere lengths underwent a transient decrease within one hour of the tenotomy, but then were as long as or longer than those in the sham-operated group. A biomechanical analysis suggests that the net result of the morphological changes produced by the tenotomy leads to a reduction in force capability of only about 12%. The shorter belly and fascicle lengths, however, may diminish the effective range over which effective force can be exerted.     

Muscle spindles in the digastric muscle of the rabbit

The digastric muscle of the rabbit was studied for the presence of muscle spindles. One of the 19 adult digastric muscles we examined contained in excess of 12 spindles scattered throughout the belly. No spindles were found in the contralateral muscle, or in any of the digastric muscles from other animals. Although jaw depressor muscles of most mammals contain few, if any, muscle spindles, their occasional presence suggests that these muscles have a potential for spindle formation.     

Histochemical fibre composition of the human digastric muscle

The histochemical muscle-fibre composition of the anterior and posterior belly of the human digastric muscle was analysed in young male adults. Both bellies, of differing embryological origin and supplied by different cranial nerves, showed a fibre composition similar to that of normal limb and trunk muscles. Type I, type IIA and type IIB fibres each occupied about one-third of the total fibre population and were evenly distributed in a mosaic pattern. About 1 per cent of fibres were type IIC and ATPase (pH 9.4) intermediate fibres. Thus, there were major differences between the anterior belly of digastric and the likewise trigeminal nerve innervated masticatory muscles with respect to both histochemical profile and size and distribution of various types of fibre. The observations suggest that the fibre pattern of the digastric is not primarily related to its specific nervous supply but its special functional demands. The predominance of type II fibres indicates a capacity for fast acceleration and speed in mandibular movements. The disparity in fibre-type profile between the digastric and the jaw elevator muscles might be related to changing demands during evolution. Civilized diets need no heavy mastication and, while the requirements upon the jaw elevators have thus changed, the functional demands on the jaw openers would have remained unchanged.     

The effects of digastric muscle tenotomy on jaw opening in the rabbit

The digastric and geniohyoid muscles of the rabbit both produce jaw-opening torque. Anatomic and biomechanical analysis, and electromyography of normal chewing, are not wholly adequate in determining the roles of these two synergists. Cinematographic and electromyographic records of pellet and carrot chewing were obtained before and after tenotomy of both digastric muscles. After tenotomy, jaw opening occurred more slowly and maximum gape was reduced for both foods. However, the overall frequency of chewing was unchanged, and the jaw muscles did not change their contraction patterns. Changes in opening speed and amount of gape result from loss of functional digastric muscles, not fully compensated for by the synergistic geniohyoids. The changes in opening speed and maximum gape are consistent with a biomechanical analysis which predicts a maximal contribution to jaw-opening torque by the geniohyoid muscle of about 25 per cent at the start of opening, and a substantial reduction of this torque in the course of the opening movement.     

Physiologic and histologic measurements of the rabbit digastric muscle

Isometric length-tension curves were established for digastric muscles of 14 male New Zealand White rabbits under pentobarbital anaesthesia. Maximum tetanic tension was about 10 N and occurred at muscle lengths slightly less than the probable maximum physiologic length. Tetanic tension rose on the ascending limb and fell on the descending limb of the length-tension curve in equal amounts, making the length-tension curve appear symmetrical. Twitch contractions were fast; time to peak tension about 20 ms. Resting sarcomere lengths at optimum length averaged 3.0 μm, which is longer than the 2.4 μm expected on the basis of maximum thick and thin filament overlap.     

Muscle spindles in the human anterior digastric muscle.

The occurrence of muscle spindles in the anterior digastric muscle in man was investigated and the fibre calibre spectrum of the corresponding nerve was determined. After removal at autopsy from five individuals of both sexes (aged 23--73), the muscles were stained with Weigert's iron hematoxylin-van Gieson stain and the nerves according to the Alzheimer - Mann - H?ggqvist method. Altogether 12 spindles were found in five out of ten muscles. Only muscles from one individual were devoid of spindles. This sparsity or absence was supported by analyses of fibre calibre spectra. The small number of spindles and the fact that they do not occur in all muscles or in all individuals, suggest that they are not an essential source of sensory information.     

Electromyographic analysis of the sternohyoid muscle and anterior belly of the digastric muscle in jaw movements

SUMMARY The sternohyid muscle and the anterior belly of the left digastric muscle were studied electro-myographically in 20 young adult volunteer individuals. A surface monopolar electrode and a needle monopolar electrode, inserted into the muscle mass 1.0 cm apart were employed. The most significant action of the two muscles was found in the opening of the jaw, during which the sternohyoid muscle presented an isotonic contraction, allowing for displacements of the hyoid bone. They also acted on those movements that included one of the jaw depression components, such as protrusion, lateral movements to either side, and retrusion. They were inactive when the jaw was in the resting position. Both muscles operated simultaneously most of the time, but a synchronization of their actions could not be demostrated.     

Physiological properties of motor units in the human anterior digastric muscle

The activity of the single motor units in the human anterior digastric muscle was recorded in 7 subjects with a monopolar fine wire electrode. The force generated by jaw opening was simultaneously recorded with a mechanical transducer. The results obtained were as follows: (1) Increasing the force gradually, a few motor units were recruited orderly and the firing rate of each unit reached the peak frequency rapidly. These findings suggest that the control of the muscle force caused by jaw opening movements might depend more on the orderly recruitment of the motor units than on the rate modulation. (2) The contraction time, measured by the spike-triggered averaging technique varied from 25.6 to 132.4 ms. It was suggested that the anterior digastric muscle consisted of both the fast twitch motor units and the slow twitch motor units. (3) The motor units that have a higher threshold force tended to have a larger twitch tension and have a shorter twitch contraction time than that of the lower threshold motor units. This means that the anterior digastric muscle has a relatively simple role during the active jaw opening movements. (4) Fatigue can be estimated as the change in the firing rate. The motor units of a higher threshold force tended to be fatigue during the continuous jaw opening movements.     

The cause of the difference in the submental region: aberrant muscle bundles of the anterior belly of the digastric muscle

The aberrant bundles' presence in the anterior belly of the digastric muscle is important in terms of causing asymmetry in the submental region, getting confused with some pathologic cases, radiologic examination, and aesthetic facial surgery. To provide data, aberrant bundles in the submental region were investigated in 30 cadaver heads. During the dissection of the submental region, origin, insertion, shape, and bilaterality of the anterior bellies of the digastric muscles and the aberrant bundles were investigated. The 20 heads with aberrant bundles were classified into two types based on the muscle arrangement: digastric fossa type and crossover type. The aberrant bundles, which did not cross the median line, were classified as being of the digastric fossa type, whereas those that crossed the line were of the crossover type. Fifteen of the heads contained bundles of the unilateral type and five heads contained the crossover type. In three heads, digastric fossa and crossover types coexisted. In this study, a wide range for incidence in the submental region was observed of variations. Some cases were not described in the classification of the previous studies of this muscle. It is also possible that the incidences may vary as a result of the ethnic differences of the populations studied. Bilaterality was frequently observed in this study. Anatomic variations of the anterior bellies of the digastric muscle can easily be confused with the pathologic conditions in ultrasonography, computed tomography, and magnetic resonance imaging; therefore, it is necessary to recognize that variants of the anterior belly of the digastric muscle occur to avoid confusion when diagnosis shows abnormal lesions in the floor of the mouth and submental region. Additionally, the possible occurrence of such anomalies should be remembered during the surgical procedures involving the submental region.     

Reflex responses to taps in various directions from the human digastric muscle

Reflex responses to standardized solenoid chin taps were studied on 21 subjects with electromyographic (EMG) recordings from two jaw muscle antagonists, the masseter and the digastric. Taps were delivered downward and upward as parallel as possible to the masseter fibre direction and also backwards at right angles to these directions. Taps were delivered during isometric masseter and digastric activity as well as during relaxed postural position. Reflex excitation of the digastric muscle with a latency of 25–35 ms was recorded during all three situations after taps in all three directions. When this response was superimposed on ongoing digastric isometric activity after downward and upward taps, it was followed by a period of inhibition (mean duration 31 ms) and directly followed by a second EMG burst (mean latencies 73 and 75 ms, respectively). Responses were significantly (p < 0.001) more often obtained during digastric background activity than during postural position and clench. Upward and downward taps were equally efficient in evoking the responses, significantly (p < 0.001) more so than backward taps. The concurrent recordings of the masseter EMG imply the possibility of a reciprocal interplay between the two antagonists. The results accord with reports of the capability of the digastric muscle to produce reflex responses despite lack of anatomically-defined muscle spindles.     

Short electromyographic bursts in the rabbit digastric muscle during the jaw-closing phase.

Masseter and digastric muscle activities and jaw movement trajectories were recorded in freely moving rabbits during eating. The patterns in these trajectories and activities were similar to those described in previous studies on restrained animals. Although the duration of masticatory sequences, which started with food intake followed by grinding movements and ended by swallowing, varied, the total number of chewing cycles in a chain of masticatory sequences was consistent (1043 +/- 51, mean +/- SD; n = 5, for chow pellets) among the animals tested. When animals ate hard foods, extra bursts in the digastric electromyograms occurred frequently in the jaw-closing phase. The digastric activities were rather short (6.1 +/- 1.0 ms; n = 100) and the amplitude of these digastric short bursts (DSBs) was much larger (1.69 +/- 0.81 mV; n = 100) than in the opening phase (0.56 +/- 0.33 mV; n = 100), which actually depressed the jaw. When a soft food (bread) was tested, this activity was not observed. The proportion of occurrences of the DSB in a chewing cycle was high at the slow-closing phase, indicating that the DSBs were due to tooth contacts during food crushing. Of 1035 chewing cycles examined in the five animals, 124 were associated with a DSB and 415 cycles with a masseter inhibitory period (MIP). The proportion of the occurrences of the MIP was significantly larger than that of the DSBs. Of 124 DSBs, 85 (68.5 per cent) coincided with an MIP. Four were not associated with clear MIPs, although there was masseter activity at the time of the DSBs. The other 35 DSBs were out of phase with the masseter bursts, although still in a closing phase. The durations of the MIPs accompanied by a DSB were significantly longer than those not so associated. The DSB may be a reflex response mediated by periodontal mechanoreceptors when the upper and lower teeth come together while chewing hard food. The reflex arc for the DSB may be independent of that for the MIP, and the threshold for the DSB may be higher.     

The anatomy and electrical activity of the platysma muscle

Dissection of seven cadavers showed that the platysma muscle may cover large parts of the masseter muscle. The platysma may thus be a significant source of artefact activity when recording the masseteric activity with surface electrodes. This is illustrated by a patient-case. The electrical activity of the platysma muscle was studied during jaw movements in normal, healthy subjects. Activity was regularly recorded during the latter half of large vertical jaw opening movements and, in five of the seven subjects, in the ipsilateral platysma muscle also during combined lateral-vertical jaw opening movements. The platysma may, therefore, in some individuals, have a functional role during the opening phase of chewing cycles when this has a marked lateral component.     

Occurrence of a muscle spindle in the anterior digastric muscle of a monkey (Macaca fascicularis).

The muscles from 5 monkeys were examined for spindles. One spindle only was observed, in the middle of the muscle. As spindles in this muscle are absent in various mammals, these receptors cannot be an essential source of sensory information in the anterior digastric muscle, or play a major role in mandibular function.     

Electromyographic analysis of the sternohyoid muscle and anterior belly of the digastric muscle in head and tongue movements

summary The sternohyoid muscle and anterior belly of the left digastric muscle were electromyographically studied in 20 young adult volunteer individuals. A surface monopolar electrode and a needle monopolar electrode, inserted into the muscle mass 1.0 cm apart were employed. The muscles acted during the following movements of the tongue; protraction, lateral movements to either side, and placement of the tip of the tongue on the hard and soft palate. It was in the latter movement that the most significant action potentials of the sternohyoid muscle were observed, which coincided with a major displacement of the hyoid bone. Both muscles studied do not take part in the kinesiology of the head.     

Deformation/displacement of posterior digastric and sternocleidomastoid muscles during posterior digastric muscle palpation using magnetic resonance imaging and image processing procedure

Objective: As the process of palpation is basically subjective, the scientific evidence for masticatory muscle palpation, especially from an anatomical point of view, is scarce. Our previous study concerning the quantification of masseter muscle deformation during palpation using magnetic resonance imaging and image analysis procedure showed that masseter muscles are deformed and their fascia must be stretched during palpation (Ishikawa et al., Int J Prosthodont, in press). The aim of this study was to further evaluate the deformation/displacement of posterior digastric and sternocleidomastoid muscles during posterior digastric muscle palpation using almost the same procedure as in the previous study. Subjects and methods: Subjects were 10 male volunteers with an average age of 26·8 years. MR images were taken using a 1·5 T scanner (MAGNEX 150 SHIMAZU Co. Ltd, Kyoto, Japan). Images were obtained on the axial plane with a spin‐echo, T₁‐weighted technique using a head coil. At first, non‐compressed condition (control) was taken, then the continuously compressed condition of the bilateral posterior digastric muscle region using round plastic balls was taken. By superimposing a compressed image on a non‐compressed image, deformation/displacement was subjectively evaluated and measured using free image analysing software (NIH‐image). Results and conclusion: Deformation of the bilateral posterior digastric muscle was difficult to identify, however, the average amount of displacement in medial direction was 9·0 ± 5·3 mm. Although we tried to compress posterior digastric muscles, sternocleidomastoid muscles were additionally deformed and displaced. The amount of medial displacement was 9·3 ± 4·7 mm. The values contained in our results exhibited fairly substantial variance. These findings suggest that masticatory muscle palpation is subjective and inconsistent.