Evidence for Functional Partitioning of the Rabbit Digastric Muscle

Abstract

The rabbit digastric muscle has a single belly that opens and retracts the mandible. It does not contain connective tissue partitions, and all fibers arise from the same tendon and insert into a single broad site. Historically, it was assumed that the muscle functioned as a single unit. Since we had preliminary evidence that this might not be the case, we carried out five small studies in rabbits. First, we showed that electromyographic (EMG) activity varies between recording sites within the muscle during the masticatory cycle induced by repetitive stimulation of the sensorimotor cortex. We found that EMG activity in the caudal region sometimes began before the anterior EMG during mastication when the jaw swung to the side of the muscle, but the two regions became active at the same time during other patterns. We next showed that separate branches of the mylohyoid nerve enter the anterior, intermediate and caudal regions of the digastric. However, a separate study showed that the motor endplates were distributed across a continuous sheet, consistent with a single anatomical partition. We then stimulated single nerve branches to deplete glycogen. By comparing the optical density of fibers labeled by the periodic acid-Schiff method for glycogen, we were able to show that the three branches innervate separate regions of the muscle. Finally, we applied either FluoroGold or Fast Blue dyes to the central cut ends of the branches to label the cell bodies of the three pools of motoneurons. These were found within the middle and caudal thirds of the trigeminal motor nucleus, but there appeared to be no spatial separation of the three pools or double labeling of cells. We conclude that the digastric muscle contains two and possibly three functional subregions. The fact that the motoneurons are intermingled suggests that the distribution of motor commands to the three pools is not based on their location.

Heterogeneous activation of the medial pterygoid muscle during simulated clenching

“…Furthermore, the results agree with data found for the other masticatory muscles, 7,8,[21][22][23] and they confirm the heterogeneous activation capability as a general control behaviour for all masticatory muscles. This capacity does not exclude a homogeneous activation, 37 but it might be an additional control mechanism to meet the requirements for fine motor skills.…”

“…[17][18][19][20] Thus far, the phenomenon of differential activation has been substantiated for the masseter, 8,21 the lateral pterygoid, 22 and the digastric muscles. 23 Evidence for the temporalis is conflicting, because the heterogeneous activation has been demonstrated by some groups, 7 but not by others. 24 The goal of this study was to investigate if a differential activation can also be detected for the medial pterygoid.…”

Observation of the Relationship between the Shape of Skeletal Muscles and Their Nerve Distribution Patterns: A Transparent and Microanatomic Study

“…Tsuruyama et al 32 found three primary nerve branches that arose from the mylohyoid nerve in the digastric muscle of the rabbit. The most proximal branch (posterior) innervated the posterior region of the digastric.…”

Electromyographic activity during the reflex pharyngeal swallow in the pig: Doty and Bosma (1956) revisited

“…While task unit differences are relatively common in the muscle system under study (34,55,56,62), it was surprising that different task units should have been activated in what is conventionally viewed simply as reflex emptying of the valleculae. Although the reflex pharyngeal swallow was largely isolated by the process of decerebration, we occasionally elicited additional rhythmic tongue and/or jaw activity.…”