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A study of jaw reflexes of the awake cat during mastication and locomotion.

This paper reports on experiments on the jaw opening reflex carried out while awake unrestrained cats were eating or walking on a treadmill. It is shown that the jaw opening reflex response to low intensity stimulation diminished in all phases of the masticatory cycle. The response to higher threshold afferents, however, is phase modulated so that the largest responses occur during the jaw closing phase. This is in accord with the need for increased protection of the soft tissues in this phase of the movement. In contrast, the digastric reflex amplitude does not change when the cat passes from quiet standing to walking on the treadmill. Evidence is presented that the electromyographic activity of the jaw closing muscles increases during upwards movement of the head during walking and decreases as the head falls. These data support the hypothesis that the myotatic reflex in the elevator muscles plays a role in stabilizing the mandible during locomotion.

Afferent Pathways↗

Mandibular molar teeth and the development of mastication in the miniature pig (Sus scrofa).

The only components of the orofacial complex that are fully developed as soon as they establish function are the occlusal surfaces of teeth. It is usually assumed that the occlusal surface of the molar teeth influences the orientation of the power stroke in spite of the fact that the data supporting this claim are lacking. The purpose of this longitudinal study was therefore to determine whether or not this hypothetical form-function relationship existed during development and whether or not it was related to growth. Serial dorsoventral and lateral radiography, and dorsoventral cineradiography were performed during natural feeding of 5 Hanford miniature pigs (Sus scrofa) of both genders between their 8th and 18th weeks. Sequences of power strokes were analyzed frame by frame and compared between sessions, and related to the position of the fourth primary mandibular molar (dm4). The changes in direction of the power stroke and in position of the dm4 relative to the midline were subsequently related to growth. The results of this study indicate that changes in orientation of the power stroke, though significant only on the balancing side, occur independently of the repositioning working and balancing side mandibular molars, as well as of skeletal growth. The null hypothesis that the mandibular primary molars influence the transverse orientation of the power stroke during growth is therefore refuted.

Animals↗

Motion of the human mandibular condyle during mastication.

It has been reported that loading to the mandible during closing movement makes the condylar path move more in the superior direction than that during the free closing movement. In this study, the hypothesis was tested that the displacement of the condyle on the chewing side is greater in the direction of the mandibular fossa than that on the non-chewing side. Using a six-degrees-of-freedom jaw movement recording system, we recorded condylar motion in 12 healthy adults without TMD, during the chewing of a large hard gummy jelly. The maximum displacements at the condyle on the chewing side from the maximum intercuspation (CO) position were significantly larger in the superior and medial directions at the initial stage and in the posterior direction at all stages (0.5 mm, 0.5 mm, and 0.6 mm, respectively) than those on the non-chewing side (0.0 mm, 0.1 mm, and 0.1 mm, respectively). This suggests that, in healthy adults, the condyles at CO are located in a position such that excessive load is not applied to the temporomandibular joint when there are the aforementioned displacements.

Adult↗