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The detailed fibre architecture of the rat medial pterygoid muscle, including the courses and attachment points of muscle fibres, was investigated histologically in 10 micron thick serial sections in the horizontal, coronal and parasagittal planes. Four extramuscular tendinous sheets (external aponeuroses) and four intramuscular tendinous sheets (internal aponeuroses) were found. Three of the internal aponeuroses were arranged parallel to the rostro-caudal axis; the other was oblique to the rostro-caudal axis. Muscle fibres were located between internal aponeuroses, between external aponeuroses, between internal and external aponeuroses, between an internal aponeurosis and the periosteum, and between an external aponeurosis and the periosteum. The courses of muscle fibres were divided into three main groups: vertical, rostro-caudal and medio-lateral. Eight compartments were distinguished. The multiple movement of the medial pterygoid muscle seemed to be due to this compartmentalization. These findings suggested that the rat medial pterygoid muscle, like the rat masseter muscle, is mainly composed of multipennate muscles and compartmentalized into many muscle-fibre bundles running in different directions.
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An electromyographic investigation of the lateral pterygoid muscles was undertaken to determine their activity relative to isolated and functional movements of the mandible. Daerally in the lateral and medial pterygoid muscles. Surface electrodes were placed bilaterally on the skin over the temporalis and masseter muscles. Our data strongly suggest that separate roles for the superior and inferior heads of the lateral pterygoid cannot be supported electromyographically. In addition, the lateral pterygoid is active in protrusive movements, including an incisor clench, and not active in retrusion or a molar clench. Both lateral pterygoid muscles initiate depression of the mandible and the contralateral muscle initiates a lateral transversion. The ipsilateral activity present is not sufficient to be an active stabilizer of the temporomandibular joint. In the chewing cycle, the lateral pterygoids are bilaterally active, both in alternation with and overlapping the elevating musculature. The sequence of the lateral pterygoid activity in the chewing cycle is led by the ipsilateral lateral pterygoid muscle.
Although dysfunction of the lateral pterygoid muscle could contribute to the pain associated with TMJ disorders, it has been demonstrated through the use of dissections and lateral head radiographs that it is not possible to palpate the lateral pterygoid muscle directly by conventional clinical techniques without applying pressure through the overlying superficial head of the medial pterygoid muscle. The possibility of confusing temporal muscle hypersensitivity with that of another muscle in this region is suggested.
The relationships of the lateral pterygoid muscle within the infratemporal fossa were observed by conventional dissections and by examination of specimens sectioned in the horizontal and frontal planes. The following less well-known features were noted. At the origins of the superior and inferior heads there are regions in which the fibres are interlaced or closely overlapped by fibres of either the temporalis muscle or the medial pterygoid muscle. Fibres of the superior head insert not only into the meniscus of the temporomandibular joint, but also into the pterygoid fovea at the neck of the mandibular condyle. Specimens sectioned through the origin of the inferior head of the muscle show internal tendon lamellae consistent with a pennate structure. Electromyographic (EMG) activity was recorded in five healthy subjects using concentric needle and fine-wire electrodes. Strong to very strong activity was consistently observed in the superior head during clenching and tooth gnashing. The inferior heads were silent or had negligible to slight activity most of the time during ipsilateral movements or clenching, but were co-activated bilaterally, with strong to very strong activity during jaw opening, protrusion, swallowing, tooth gnashing and during passive retrusion. They showed marked activity unilaterally during contralateral movements.
EMG recordings of the left and right masseter and lateral pterygoid muscles were repeated three times in 15 young adults to test the reproducibility of the signals. Two tests were made on the same day (morning and afternoon) and one test three days later (morning). Needle and hook electrodes were used for the masseter muscle and needle electrodes for the lateral pterygoid muscle. The superficial areas of recording were tattooed on the skin. An intra-oral approach was selected for the lateral pterygoid muscle. The subjects were asked to perform three definite tasks for the masseter muscle (intercuspid occlusion, maximum protrusion, and centric relation with a bite opening of 3mm) and two for the pterygoid muscle (maximum opening and maximum protrusion). The EMG signals were directly integrated, and root mean squares of the AC components were computed. The selection of the signals to discard possible artefacts was performed by displaying EMG and RMS outputs on a six-channel UV oscillograph. The selected RMS values were submitted to different analyses of variance to define additive and non-additive models of effects. The results may be summarized as follows: The between-day sessions showed more variation than the within-day sessions. The variations related to the method accounted for a very high percentage of the total variance (48.9%). The recordings with needle electrodes displayed a large percentage of the individual variation (masseter, 31.2%; lateral pterygoid muscle, 65.9%) and significantly lower values for the variations related to the session or the side of recording. The one-side recordings were more suitable for distinguishing the biological variations than were the bilateral ones.(ABSTRACT TRUNCATED AT 250 WORDS)
The purpose of this investigation was to study the morphology of the human lateral pterygoid muscle and its attachment to the temporomandibular joint. A total of twenty-five lateral pterygoid muscles, twenty for macroscopic and five for microscopic study, were obtained with their temporomandibular joints from eighteen adult cadavers. The results were as follows: 1) The lateral pterygoid muscles observed in this study were classified into three types based on the number of their heads; 65% of the macroscopic specimens had two heads (superior and inferior), 20% three heads (superior, inferior and medial), and 15% a single head. 2) The fascicular architecture of the single-headed specimens was quite simple and so the orientation of their fasciculi was parallel or radial to the line of pull. In their course the fasciculi of the two and three-headed specimens showed a relatively complicated architecture, having a large number of crossover fibers between each head. 3) In all specimens observed macroscopically, the uppermost fasciculi were inserted into the articular disc and capsule of the temporomandibular joint and the rest were inserted into the pterygoid fovea of the neck of the mandible, although no clear boundary could be found between them on their lateral surfaces. The bulk of the fasciculi which were inserted into the articular disc relative to the total amount of the lateral pterygoid muscles was estimated as being in the range of one fifth to one tenth. 4) From the microscopical examination of the specimens the cross-sectional area of the fasciculi which were inserted into the articular disc was about 3 mm2 at their attachment and this corresponded to 30% of the superior head and 3% of total amount of the lateral pterygoid muscle. 5) The fasciculi attached to the articular disc were derived from the superior and medial head of the lateral pterygoid muscle and from the temporalis muscle. In two cases of the twenty examined macroscopically, the fasciculi derived from the inferior head of the lateral pterygoid muscles were inserted into the articular disc.
Location of the neurons in the trigeminal mesencephalic nucleus innervating stretch receptors of the lateral pterygoid muscle and the mode of their synaptic connection on the lateral pterygoid motoneurons of the guinea pig were studied physiologically as well as morphologically, in comparison with the trigeminal mesencephalic neurons innervating muscle spindles in the superficial masseter muscle, with the following results: stimulation of the caudal half of the trigeminal mesencephalic nucleus evoked monosynaptic excitatory postsynaptic potentials in the ipsilateral lateral pterygoid motoneurons. Stimulation of the lateral pterygoid nerve directly evoked spike potentials in the neurons located in the caudal half of the ipsilateral trigeminal mesencephalic nucleus, which responded with increased firing to stretch, and with silent period to twitch, of the ipsilateral lateral pterygoid muscle. Averaging of intracellular potentials of the lateral pterygoid motoneurons with extracellular spike potentials of these trigeminal mesencephalic neurons revealed excitatory postsynaptic potentials after a monosynaptic latency, but no inhibitory postsynaptic potentials. Injection of horseradish peroxidase into the lateral pterygoid muscle labeled 15-20 cells in the caudal half of the ipsilateral trigeminal mesencephalic nucleus, while 174-228 cells retrogradely labeled by horseradish peroxidase were found throughout the whole rostrocaudal extent of the ipsilateral trigeminal mesencephalic nucleus following injection of horseradish peroxidase into the masseter muscle. It was concluded that neurons in the caudal half of the trigeminal mesencephalic nucleus send their peripheral processes to stretch receptors, presumably muscle spindles, in the ipsilateral lateral pterygoid muscle and that their central processes have excitatory synapses on ipsilateral lateral pterygoid motoneurons, thus comprising the afferent limb of a monosynaptic stretch reflex arc of the lateral pterygoid muscle of the guinea pig.
This anatomic study examines the attachment of the lateral pterygoid muscle to the capsule and disc of the temporomandibular joint. The anatomy of the temporomandibular joint and its surroundings, in particular the insertion of the superior head of the lateral pterygoid muscle, was studied by dissection and conventional histologic techniques. The material consisted of 16 cadaver specimens from individuals 60 years or older. The results showed that only a part of the superior head of the lateral pterygoid muscle is attached to the anterior portion of the capsule, which, in turn, is firmly attached to the disc, giving the impression that the muscle and the disc are directly connected. All specimens showed attachment of the superior head of the lateral pterygoid muscle to the anterior medial portion of the capsule, but they showed varying degrees of attachment to the lateral aspect of the temporomandibular joint capsule. The remaining part of the superior head of the lateral pterygoid muscle attached to the mandibular condyle. Serial sectioning in no instance showed direct insertion into the disc of the fibers of the superior head of the lateral pterygoid muscle.
Physiological evidence is presented for the presence of stretch reflexes in the lateral pterygoid (Pt) muscle of the guinea pig. The central reflex effects of excitation of Pt stretch reflex afferents were also investigated. Passive lateral jaw displacement, which resulted in stretch of the Pt muscle on the side of jaw movement and stretch of the zygomatico-mandibularis (Z) muscle on the side contralateral to the movement, evoked increased EMG activity in these muscles. Stimulation of the trigeminal mesencephalic nucleus (mes V) evoked monosynaptic reflexes in both the Pt and Z nerves. Tonic stretch of the Pt muscle facilitated the monosynaptic reflex in the Pt nerve evoked by stimulation of mes V. Tonic vibration of the Pt muscle facilitated the mes V evoked monosynaptic reflex in the nerves to the ipsilateral Pt and contralateral Z muscles. Conversely, tonic vibration of the Z muscle facilitated the monosynaptic reflex evolved by mes V stimulation in the contralateral Pt and ipsilateral Z nerve. The results support the view that muscle spindles exist in the Pt and Z muscles and that there is a monosynaptic stretch reflex for both the Pt and Z muscles with cell bodies located in the mes V nucleus. It was also shown that the ipsilateral Pt muscle and the contralateral Z muscle act as synergists in the production of lateral jaw movements and that the organization of the stretch reflexes originating from the Pt and Z muscles support their synergistic action.
This muscle has some physiological properties resembling those of jaw-opening muscles. Biomechanical analysis showed that, because of the anatomical peculiarities of the feeding apparatus in caviid rodents, the action of the muscle changes from closing to opening as the mandibular condyles are protruded in the groove-like mandibular fossa. This unusual changing function may be useful in producing the ventral, medial and anterior power stroke of mastication.
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