Effects of incisor extraction on jaw and tongue motor representations within face sensorimotor cortex of adult rats

Abstract

Loss of teeth is associated with changes in somatosensory inputs and altered patterns of mastication, but it is unclear whether tooth loss is associated with changes in motor representations within face sensorimotor cortex of rats. We used intracortical microstimulation (ICMS) and recordings of cortically evoked muscle electromyographic (EMG) activities to test whether changes occur in the ICMS-defined motor representations of the left and right jaw muscles [masseter, anterior digastric (LAD, RAD)] and tongue muscle [genioglossus (GG)] within the cytoarchitectonically defined face primary motor cortex (face-M1) and adjacent face primary somatosensory cortex (face-S1) 1 week following extraction of the right mandibular incisor in anesthetized (ketamine-HCl) adult male Sprague-Dawley rats. Under local and general anesthesia, an "extraction" group (n = 8) received mucoalveolar bone surgery and extraction of the mandibular right incisor. A "sham-extraction" group (n = 6) received surgery with no extraction. A "naive" group (n = 6) had neither surgery nor extraction. Data were compared by using mixed-model repeated-measures ANOVA. Dental extraction was associated with a significantly increased number of sites within face-M1 and face-S1 from which ICMS evoked RAD EMG activities, a lateral shift of the RAD and LAD centers of gravity within face-M1, shorter onset latencies of ICMS-evoked GG activities within face-M1 and face-S1, and an increased number of sites within face-M1 from which ICMS simultaneously evoked RAD and GG activities. Our novel findings suggest that dental extraction may be associated with significant neuroplastic changes within the rat's face-M1 and adjacent face-S1 that may be related to the animal's ability to adapt to the altered oral state.

Memory-related gene expression profile of the male rat hippocampus induced by teeth extraction and occlusal support recovery

“…Analysis using positron emission tomography and magnetic resonance imaging has shown that chewing increases neuronal activity in various regions of the cerebral cortex. 6 It has reported that dental extraction may be associated with significant neuroplastic changes within the rat's face primary motor cortex and adjacent face primary somatosensory cortex that may be related to the animal's ability to adapt to the altered oral state 7 and the maturation of network function in the oral somatosensory cortex is impaired by tooth loss during the developmental period. 8 Epidemiological studies have revealed that tooth loss is a risk factor for senile dementia.…”

Neuroplasticity of face sensorimotor cortex and implications for control of orofacial movements

“…Unilateral transection of the lingual nerve supplying sensory innervation to the rat's tongue results in significant time-dependent changes of the anterior digastric and genioglossus representations within face MI, 1-4 weeks later [103]. In addition, extraction of a mandibular incisor in rats is associated 1 week later with significant neuroplastic changes within the contralateral face MI that are manifested as a significant expansion of the ICMS-defined anterior digastric motor representation (Table 1) [8,31]. Trimming both rat mandibular incisors also induces face MI neuroplastic changes but this is reflected as a significant reduction in the anterior digastric representation in face MI that can be restored if the incisors are allowed to erupt back into occlusion [8].…”

“…1). Short-train ICMS can also evoke jaw and tongue movements when applied to the face SI or face MI in subprimates [26][27][28][29][30][31]. On the other hand, longtrain ICMS can evoke semiautomatic movements such as mastication, swallowing and facial whisking from the face MI and face SI of monkeys and subprimates [19][20][21][22][32][33][34].…”

“…While it has been reported that partial trigeminal deafferentation is not associated 16 days later with any significant changes in the more lateral so-called cortical masticatory area (CMA) in rabbits [102], recent ICMS studies have provided evidence of face MI neuroplasticity following intraoral manipulations. These recent studies were driven in part by clinical observations that adaptive behaviours occur when the dental occlusion is modified or intraoral (e.g., lingual) nerves damaged, and in some cases maladaptive behaviours may result [8,31,81,103]. Unilateral transection of the lingual nerve supplying sensory innervation to the rat's tongue results in significant time-dependent changes of the anterior digastric and genioglossus representations within face MI, 1-4 weeks later [103].…”

“…2). There is a small orofacial motor representation within the rat face SI, and this motor representation is also subject to neuroplastic changes since mandibular incisor extraction is associated 1 week later with a significant increase in the representation of the anterior digastric muscle [31,81].…”