Change of digastric muscle length in feeding rabbits

The functional role of the rabbit digastric muscle during mastication

Muscle spindle abundance is highly variable in vertebrates, but the functional determinants of this variation are unclear. Recent work has shown that human leg muscles with the lowest abundance of muscle spindles primarily function to lengthen and absorb energy, while muscles with a greater spindle abundance perform active-stretch-shorten cycles with no net work, suggesting that muscle spindle abundance may be underpinned by muscle function. Compared with other mammalian muscles, the digastric muscle contains the lowest abundance of muscle spindles and, therefore, might be expected to generate substantial negative work. However, it is widely hypothesised that as a jaw-opener (anatomically) the digastric muscle would primarily function to depress the jaw, and consequently do positive work. Through a combination of X-ray reconstruction of moving morphology (XROMM), electromyography and fluoromicrometry, we characterised the 3D kinematics of the jaw and digastric muscle during feeding in rabbits. Subsequently, the work loop technique was used to simulate in vivo muscle behaviour in situ, enabling muscle force to be quantified in relation to muscle strain and hence determine the muscle's function during mastication. When functioning on either the working or balancing side, the digastric muscle generates a large amount of positive work during jaw opening, and a large amount of negative work during jaw closing, on average producing a relatively small amount of net negative work. Our data therefore further support the hypothesis that muscle spindle abundance is linked to muscle function; specifically, muscles that absorb a relatively large amount of negative work have a low spindle abundance.

Comparative study of the digastric and the stylohyoid muscles between wild boars (Sus scrofa scrofa) and domestic swine (Sus scrofa domesticus): revisiting the gross anatomy

Considering Suidae Familie as a perfect and viable experimental biomedical model for research applied to human medicine, it has been sought to describe the comparative anatomy of the digastric and the stylohyoid muscles between boars and domestic swine. Heads of Sus scrofa scrofa and Sus scrofa domesticus were dissected. The digastric muscle presented only one muscle belly as anatomical component of a tendinous origin in the jugular process of the occipital bone, and muscle insertion in the midventral edge of the caudal two thirds of the body of the mandible. Thus, its function is fundamentally associated with the lowering and the retracting of the mandible which, by the way, can deliver greater muscle power at lesser energy expense. For the stylohyoid muscle, the tendinous origin was in the laterocaudal edge of the dorsal third of the stylohyoid bone. The muscle insertion - primarily, was in the lateral and caudal edges from the mid third portion up to the ventral extremity of the thyrohyoid bone, and secondarily as a laterolateral aponeurotic blade which would unite, in a bilateral manner, an insertion that was common to the sternohyoid, the geniohyoid, and the mylohyoid muscles in a median ventral region. This morphology were similar to the two specimens studied expanding the information available, which were completely unknown for the suidae until the moment.

Whole muscle length-tension relationships are accurately modeled as scaled sarcomeres in rabbit hindlimb muscles

An a priori model of the whole active muscle length-tension relationship was constructed utilizing only myofilament length and serial sarcomere number for rabbit tibialis anterior (TA), extensor digitorum longus (EDL), and extensor digitorum II (EDII) muscles. Passive tension was modeled with a two-element Hill-type model. Experimental length-tension relations were then measured for each of these muscles and compared to predictions. The model was able to accurately capture the active-tension characteristics of experimentally-measured data for all muscles (ICC=0.88 ± 0.03). Despite their varied architecture, no differences in predicted versus experimental correlations were observed among muscles. In addition, the model demonstrated that excursion, quantified by full-width-at-half-maximum (FWHM) of the active length-tension relationship, scaled linearly (slope=0.68) with normalized muscle fiber length. Experimental and theoretical FWHM values agreed well with an intraclass correlation coefficient of 0.99 (p<0.001). In contrast to active tension, the passive tension model deviated from experimentally-measured values and thus, was not an accurate predictor of passive tension (ICC=0.70 ± 0.07). These data demonstrate that modeling muscle as a scaled sarcomere provides accurate active functional but not passive functional predictions for rabbit TA, EDL, and EDII muscles and call into question the need for more complex modeling assumptions often proposed.

EMG of the digastric muscle in gibbon and orangutan: functional consequences of the loss of the anterior digastric in orangutans

Unlike all other primates, the digastric muscle of the orangutan lacks an anterior belly; the posterior belly, while present, inserts directly onto the mandible. To understand the functional consequences of this morphologic novelty, the EMG activity patterns of the digastric muscle and other potential mandibular depressors were studied in a gibbon and an orangutan. The results suggest a significant degree of functional differentiation between the two digastric bellies. In the gibbon, the recruitment pattern of the posterior digastric during mastication is typically biphasic. It is an important mandibular depressor, active in this role during mastication and wide opening. It also acts with the anterior suprahyoid muscles to move the hyoid prior to jaw opening during mastication. The recruitment patterns of the anterior digastric suggest that it is functionally allied to the geniohyoid and mylohyoid. For example, although it transmits the force of the posterior digastric during mandibular depression, it functions independent of the posterior digastric during swallowing. Of the muscles studied, the posterior digastric was the only muscle to exhibit major differences in recruitment pattern between the two species. The posterior digastric retains its function as a mandibular depressor in orangutans, but is never recruited biphasically, and is not active prior to opening. The unique anatomy of the digastric muscle in orangutans results in decoupling of the mechanisms for hyoid movement and mandibular depression, and during unilateral activity it potentially contributes to substantial transverse movements of the mandible. Hypotheses to explain the loss of the anterior digastric should incorporate these functional conclusions.

Videofluorographic analysis of tongue movement in the rabbit (Oryctolagus cuniculus)

The movement of the entire tongue and intermolar eminence during mastication is described in the domestic rabbit (Oryctolagus cuniculus). Tongue movement and jaw position were analyzed videofluorographically from separate lateral and dorso-ventral views in six rabbits. Metallic markers were inserted into the tongue so that its movement was visible on the fluorographic image. Frame-by-frame analysis of the videofluorographic tape recordings demonstrates that tongue movement in all animals was identical in direction during each part of the chewing cycle. In the lateral view the forepart of the tongue moves down and forward during the opening stroke, whereas the intermolar eminence moves up and forward to appose the palate. During the closing stroke, as the tip of the tongue moves up and back, the intermolar eminence lowers from the palate and retracts. During the power stroke the forepart of the tongue is at its most elevated and retruded position, while the intermolar eminence is its lowest and most retruded. The dorso-ventral view showed that lateral movement of the tongue and mandible are highly synchronous. The intermolar eminence decreases in width during the power stroke, possibly twisting to place or keep food on the teeth. An anterior to posterior undulating movement of the entire tongue occurs throughout the chewing cycle. As the intermolar eminence elevates to appose the palate during the opening stroke, it may replace the bolus on the teeth on the chewing side. The intermolar eminence also appears to be twisting during the closing and power strokes to place or maintain food on the teeth.

Morphological and videofluorographic study of the hyoid apparatus and its function in the rabbit (Oryctolagus cuniculus)

The anatomy of the hyoid apparatus and positional changes of the hyoid bone during mastication and deglutition are described in the New Zealand White rabbit (Oryctolagus cuniculus). A testable model is constructed to predict the range of movement during function of the hyoid, a bone entirely suspended by soft tissue. Frame-by-frame analysis of a videofluorographic tape confirms the accuracy of the prediction through observation of hyoid bone excursion during oral behavior. During chewing, translation of the hyoid bone is diminutive and irregular, lacking a clearly discernible path of excursion. However, some movements of the hyoid occur with regularity. During fast opening, anterodorsal movement of the hyoid is interrupted with an abrupt posteroventral depression when the bolus is moved posteriorly toward the cheek teeth by the tongue. This clockwise rotation (when viewed from the right side) of the hyoid accompanies jaw opening and is reversed (posteroventral movement) for the jaw closing sequence. Lateral movements of the hyoid may be slightly coupled to mandibular rotation in the horizontal plane. The findings suggest that the hyoid bone maintains a relatively static position during the dynamics of chewing. The primary function would be to provide a stable base for the movements of the tongue. Another possible function would be to control the position of the larynx within the pharyngeal cavity. Some characteristic features of the rabbit hyoid apparatus may be consequential to relatively erect posture and a saltatory mode of locomotion.

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.

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.

Active length-tension relation and the effect of muscle pinnation on fiber lengthening

The active length-tension relation was determined for the left digastric muscle of seven New Zealand White rabbits anesthetized with pentobarbital. Measurements of muscle length and fiber architecture were made from photographs of resting and actively contracting muscle. There was a marked difference between length-tension curves based upon resting as compared to active muscle length. The active length-tension relation had a longer descending limb than ascending limb, whereas the length-tension relation based on passive muscle length tended to be symmetrical around optimum length. On the average, muscle fibers lengthened 0.77 mm for each 1 mm of extension of the muscle belly. Since the rabbit digastric muscle is unipinnate, this suggests that pinnation serves to enhance the range of muscle excursion in this muscle.