Recovery of the jaw-opening reflex after lesions of the lingual nerve in the rat

Unilateral lingual nerve transection alters jaw-tongue coordination during mastication in pigs

During chewing, movements and deformations of the tongue are coordinated with jaw movements to manage and manipulate the bolus and avoid injury. Individuals with injuries to the lingual nerve report both tongue injuries due to biting and difficulties in chewing, primarily because of impaired bolus management, suggesting that jaw-tongue coordination relies on intact lingual afferents. Here, we investigate how unilateral lingual nerve (LN) transection affects jaw-tongue coordination in an animal model (pig, Sus scrofa). Temporal coordination between jaw pitch (opening-closing) and 1) anteroposterior tongue position (i.e., protraction-retraction), 2) anteroposterior tongue length, and 3) mediolateral tongue width was compared between pre- and post-LN transection using cross-correlation analyses. Overall, following LN transection, the lag between jaw pitch and the majority of tongue kinematics decreased significantly, demonstrating that sensory loss from the tongue alters jaw-tongue coordination. In addition, decrease in jaw-tongue lag suggests that, following LN transection, tongue movements and deformations occur earlier in the gape cycle than when the lingual sensory afferents are intact. If the velocity of tongue movements and deformations remains constant, earlier occurrence can reflect less pronounced movements, possibly to avoid injuries. The results of this study demonstrate that lingual afferents participate in chewing by assisting with coordinating the timing of jaw and tongue movements. The observed changes may affect bolus management performance and/or may represent protective strategies because of altered somatosensory awareness of the tongue.NEW & NOTEWORTHY Chewing requires coordination between tongue and jaw movements. We compared the coordination of tongue movements and deformation relative to jaw opening-closing movements pre- and post-lingual nerve transection during chewing in pigs. These experiments reveal that the timing of jaw-tongue coordination is altered following unilateral disruption of sensory information from the tongue. Therefore, maintenance of jaw-tongue coordination requires bilateral sensory information from the tongue.

The effect of unilateral lingual nerve injury on the kinematics of mastication in pigs

OBJECTIVE

This study evaluates the effect of unilateral lingual sensory loss on the spatial and temporal dynamics of jaw movements during pig chewing.

DESIGN

X-ray Reconstruction of Moving Morphology (XROMM) was used to reconstruct the 3-dimensional jaw movements of 6 pigs during chewing before and after complete unilateral lingual nerve transection. The effect of the transection were evaluated at the temporal and spatial level using Multiple Analysis of Variance. Temporal variables include gape cycle and phase durations, and the corresponding relative phase durations. Spatial variables include the amplitude of jaw opening, jaw yaw, and mandibular retraction-protraction.

RESULTS

The temporal and spatial dynamics of jaw movements did not differ when chewing ipsilateral versus contralateral to the transection. When compared to pre-transection data, 4 of the 6 animals showed significant changes in temporal characteristics of the gape cycle following the transection, irrespective of chewing side, but the specific response to the lesion was highly dependent on the animal. On the other hand, in affected individuals the amplitude of jaw movements was altered similarly in all 3 dimensions: jaw opening and protraction-retraction increased whereas jaw yaw decreased.

CONCLUSION

The variable impact of this injury in this animal model suggests that individuals use different compensatory strategies to adjust or maintain the temporal dynamics of the gape cycle. Because the amplitude of jaw movements are more adversely affected than their timing, results suggest that maintaining the tongue-jaw coordination is critical and this can come at the expense of bolus handling and masticatory performance.

Jaw-stretch reflex is weaker in patients after orthognathic surgery

OBJECTIVES

The jaw-stretch reflex (JSR) was studied in both patients and healthy participants in order to investigate the possible long-term impact of orthognathic surgery on the motor function of the masticatory system.

DESIGN

JSR was measured in patients before surgery (PC), 1year after surgery (PS) and in healthy controls (HC) (N=31 in each group). JSR was evoked by a standardized stretch device and recorded bilaterally from masseter and anterior temporalis muscles using surface electromyography (EMG).

RESULTS

The peak-to-peak amplitude (which was normalized to pre-stimulus EMG activity) of JSRs in PC and PS were significantly smaller than in HC (P<0.001; P<0.001). The onset latency in PS was significantly longer compared with HC (P<0.05). The duration of JSR in PS was significantly longer than in HC and PC (P<0.001; P<0.05).

CONCLUSION

Patients with dentofacial deformities are characterized by reduced JSR amplitude. The delayed onset and elongated duration of JSR might be potential indicators of a long-term surgical impact on the motor function of the masticatory system.

Objective evaluation of iatrogenic lingual nerve injuries using the jaw-opening reflex

The extent of reflex inhibition of masseteric electromyographic activity, after an electrical stimulus applied to lingual mucosa, was used as a test of the ability of the lingual nerve to conduct nerve impulses and this was compared with the results of standard clinical tests. Two groups of subjects were assessed: healthy subjects (n=10) and patients with lingual nerve injuries (n=17). The patients were tested 8-9 weeks after their injury and retested 6 months later when they were retrospectively allocated to either a temporary injury or a permanent injury group. The group measure of reflex inhibition after stimulation of the tongue on the opposite side to the injury was no different from the same measure in controls, whereas two-point discrimination did differ. Group measures of inhibition and of subjective function after stimulation on the side of the injury were significantly different from controls whereas light touch and two-point discrimination were not. There was good agreement between quantified masseteric inhibition and subjective function, but it was not possible at 8-9 weeks after the injury to differentiate between those that would recover and those that would be permanent.