Masseteric nerve

Abstract: Rodents are defined by their unique masticatory apparatus and are frequently separated into three nonmonophyletic groups--sciuromorphs, hystricomorphs, and myomorphs--based on the morphology of their masticatory muscles. Despite several comprehensive dissections in previous work, inconsistencies persist as to the exact morphology of the rodent jaw-closing musculature, particularly, the masseter. Here, we review the literature and document for the first time the muscle architecture noninvasively and in 3D by using iodine-enhanced microCT. Observations and measurements were recorded with reference to images of three individuals, each belonging to one of the three muscle morphotypes (squirrel, guinea pig, and rat). Results revealed an enlarged superficial masseter muscle in the guinea pig compared with the rat and squirrel, but a reduced deep masseter (possibly indicating reduced efficiency at the incisors). The deep masseter had expanded forward to take an origin on the rostrum and was also separated into anterior and posterior parts in the rat and squirrel. The zygomaticomandibularis muscle was split into anterior and posterior parts in all the three specimens by the masseteric nerve, and in the rat and guinea pig had an additional rostral expansion through the infraorbital foramen. The temporalis muscle was found to be considerably larger in the rat, and its separation into anterior and posterior parts was only evident in the rat and squirrel. The pterygoid muscles were broadly similar in all three specimens, although the internal pterygoid was somewhat enlarged in the guinea pig implying greater lateral movement of the mandible during chewing in this species.

Abstract: The neural organization of the pig masseter, an architecturally and functionally compartmentalized muscle, was investigated by using dissection, glycogen depletion, evoked electromyography, and counts of axon numbers at various levels along the masseteric nerve. The masseteric nerve enters the muscle as two rostral branches, which also supply the zygomatico-mandibularis, and a more caudal main branch, which soon divides into four terminal nerves with variable distributions. Stimulation of filaments containing roughly 50 extrafusal motor axons resulted in glycogen depletion of 5-20% of the muscle fibers in very small subvolumes of the masseter; the affected subvolumes were delimited by perimysium. Electromyography after stimulation of various branches of the nerve confirmed the distributions deduced from anatomy and further indicated that axons do not branch between the rostral and main nerve branches but may occasionally do so among the more distal terminal branches of the main branch. The proximal trunk of the masseteric nerve contains about 3,500 myelinated fibers with a bimodal size distribution. Approximately 1,000 of the larger fibers were estimated to be extrafusal motor axons. Along the proximal trunk of the nerve, fibers were constantly rearranged; coupled with the observation that the locations of motor unit territories were usually not related to the position of the stimulated axons within the nerve, this suggests that the nerve trunk is not strictly ordered somatotopically.

Abstract: Summary Objective One-stage free-flap facial reanimation may be accomplished by using a gracilis transfer innervated by the masseteric nerve, but this technique does not restore the patient's ability to smile spontaneously. By contrast, the transfer of the latissimus dorsi innervated by the contralateral facial nerve provides the correct nerve stimulus but is limited by variation in the quantity of contraction. The authors propose a new one-stage facial reanimation technique using dual innervation; a gracilis muscle flap is innervated by the masseteric nerve, and supplementary nerve input is provided by a cross-face sural nerve graft anastomosed to the contralateral facial nerve branch. Methods Between October 2009 and March 2010, four patients affected by long-standing unilateral facial paralysis received gracilis muscle transfers innervated by both the masseteric nerve and the contralateral facial nerve. Results All patients recovered voluntary and spontaneous smiling abilities. The recovery time to voluntary flap contraction was 3.8 months, and spontaneous flap contraction was achieved within 7.2 months after surgery. According to Terzis and Noah's five-stage classification of reanimation outcomes, two patients had excellent outcomes and two had good outcomes. Conclusions In this preliminary study, the devised double-innervation technique allows to achieve a good grade of flap contraction as well as emotional smiling ability. A wider number of operated patients are needed to confirm those initial findings.

Abstract: The location of bulbar neurones with axons projecting to the ipsi- and contralateral trigeminal motor nucleus were investigated in cats anaesthetized with sodium pentobarbital. Wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) was injected in amounts of 5–24 nl. A volume-calibrated microelectrode was used for recording of evoked potentials and pressure injection of WGA-HRP. The injection site was guided by the position where a maximal antidromic response was evoked by electrical stimulation of the masseteric nerve. The survival time was 19–22 h. In preparations with the depot located in the masseteric subnucleus retrogradely stained neurones were found bilaterally in the borderzone of the trigeminal motor nucleus. Dense populations of stained neurones were observed ipsi- and contralaterally in the dorsal division of the main sensory trigeminal nucleus and the subnucleus-γ of the oral nucleus of the spinal trigeminal tract. Clusters of WGA-HRP-neurones were observed bilaterally in the lateral tegmental field at the level of the subnucleus-β of the oral nucleus of the spinal trigeminal tract, bilaterally dorsal to the facial nucleus and contralaterally adjacent to the hypoglossal nucleus. No stained neurones were found in the gigantocellular reticular nucleus. A group of stained neurones was located in the marginal nucleus of brachium conjunctivum and some were found in the raphe nuclei near obex. Cell profiles were of two types: medium-sized neurones with a triangular profile and 30–40 μm diameter, and fusiform neurones 10×50–70 μm. Convergence of descending cortical and trigeminal afferent inputs on interneurones located in the lateral borderzone of the trigeminal motor nucleus, i.e. the intertrigeminal area, is reported in the preceding paper.