Reflexes elicitable in jaw muscles and their role during jaw function and dysfunction: a review of the literature. Part I: Receptors associated with the masticatory system

Excitability of the human trigeminal motoneuronal pool and interactions with other brainstem reflex pathways

We studied the properties of motoneurones and Ia-motoneuronal connections in the human trigeminal system, and their functional interactions with other brainstem reflex pathways mediated by non-muscular (Abeta) afferents. With surface EMG recordings we tested the recovery cycles of the heteronymous H-reflex in the temporalis muscle and the homonymous silent period in the masseter muscle both elicited by stimulation of the masseteric nerve at the infratemporal fossa in nine healthy subjects. In four subjects single motor-unit responses were recorded from the temporalis muscle. In six subjects we also tested the effect of the stimulus to the mental nerve on the temporalis H-reflex and, conversely, the effect of Ia input (stimulus to the masseteric nerve) on the R1 component of the blink reflex in the orbicularis oculi muscle. The recovery cycle of the H-reflex showed a suppression peaking at the 5-20 ms interval; conversely the time course of the masseteric silent period was facilitated at comparable intervals. The inhibition of the test H-reflex was inversely related to the level of background voluntary contraction. Single motor units were unable to fire consistently in response to the test stimulus at intervals shorter than 50 ms. Mental nerve stimulation strongly depressed the H-reflex. The time course of this inhibition coincided with the EMG inhibition elicited by mental nerve stimulation during voluntary contraction. The trigeminal Ia input facilitated the R1 component of the blink reflex when the supraorbital test stimulation preceded the masseteric conditioning stimulation by 2 ms. We conclude that the time course of the recovery cycle of the heteronymous H-reflex in the temporalis muscle reflects the after-hyperpolarization potential (AHP) of trigeminal motoneurones, and that the Ia trigeminal input is integrated with other brainstem reflexes.

[Gnathographic study of mandibular borderline movements and chewing patterns before and after surgical dysgnathia correction in retrognathism and prognathism]

Chewing patterns and longitudinal border movements of 16 patients with severe dysgnathia were examined preoperatively and up to 12 months postoperatively by means of sirognathographic records of surgical-orthodontic treatment results. Patients with a skeletal class-III malocclusion showed an increased range of movement compared to the preoperative situation, after a surgical mandibular setback performed by wire osteosynthesis, followed by an intermaxillary immobilisation of 6 weeks. Patients with a skeletal class-II malocclusion showed a considerably limited range of movement during the first postoperative registration after a surgical mandibular advancement. The preoperative situation had not been fully achieved even after 12 months. The postoperative sirognathographic records present distinct differences in functional adaptation between surgical mandibular advancement and mandibular setback.