Cortical and thalamic connections of the representations of the teeth and tongue in somatosensory cortex of new world monkeys

Abstract

Connections of representations of the teeth and tongue in primary somatosensory cortex (area 3b) and adjoining cortex were revealed in owl, squirrel, and marmoset monkeys with injections of fluorescent tracers. Injection sites were identified by microelectrode recordings from neurons responsive to touch on the teeth or tongue. Patterns of cortical label were related to myeloarchitecture in sections cut parallel to the surface of flattened cortex, and to coronal sections of the thalamus processed for cytochrome oxidase (CO). Cortical sections revealed a caudorostral series of myelin dense ovals (O1-O4) in area 3b that represent the periodontal receptors of the contralateral teeth, the contralateral tongue, the ipsilateral teeth, and the ipsilateral tongue. The ventroposterior medial subnucleus, VPM, and the ventroposterior medial parvicellular nucleus for taste, VPMpc, were identified in the thalamic sections. Injections placed in the O1 oval representing teeth labeled neurons in VPM, while injections in O2 representing the tongue labeled neurons in both VPMpc and VPM. These injections also labeled adjacent part of areas 3a and 1, and locations in the lateral sulcus and frontal lobe. Callosally, connections of the ovals were most dense with corresponding ovals. Injections in the area 1 representation of the tongue labeled neurons in VPMpc and VPM, and ipsilateral area 3b ovals, area 3a, opercular cortex, and cortex in the lateral sulcus. Contralaterally, labeled neurons were mostly in area 1. The results implicate portions of areas 3b, 3a, and 1 in the processing of tactile information from the teeth and tongue, and possibly taste information from the tongue.

Brain Activations in Response to Vibrotactile Tooth Stimulation: a Psychophysical and fMRI Study

“…Thus the results of this study clearly indicate that periodontal afferents from the first upper left incisor project to both contra-and ipsilateral SI cortices via the trigeminothalamic tract and that the projection from the contralateral ascending pathway is dominant. This is in agreement with earlier findings based on recording somatosensory-evoked magnetic fields during intraoral tactile stimulation (excluding the teeth) in humans (Tamura et al 2008) and from neuronal tracing studies (teeth and tongue) in new world monkeys (Iyengar et al 2007). Previous studies have reported locations in the SI cortex for intraoral structures including the lip, hard palate, gingiva, and tongue (Murayama et al 2005;Nakahara et al 2004;Nevalainen et al 2006), tooth dental pulp (Kubo et al 2008), and tooth tactile stimulation (Miyamoto et al 2006), but this is the first report of SI activations to psychophysically controlled vibrotactile stimulation of periodontal afferents.…”

Cortical network for representing the teeth and tongue in primates

“…The results are incompletely analyzed and only reported in the form of an abstract (Iyengar et al, 2002). Yet a few conclusions are well supported.…”

The Ventral Primary Somatosensory Cortex of the Primate Brain: Innate Neural Interface for Dexterous Orofacial Motor Control

“…This "hand-face border" can be detected as a myelin-light septum in brain sections cut parallel to the cortical surface. Further laterally, myelin-dense ovals were shown to indicate anatomical modules that correspond to representations of different orofacial structures in both New [16,17] and Old World monkeys [13,15]. In New World monkeys, the myelin-dense ovals are arranged in a rostrocaudal direction and extend to the ventral frontal lobe.…”

“…Dense neural interconnections between representations of different orofacial structures have been documented in both New [17] and Old World monkeys [15]. For example, a recent neuroanatomical study of macaques [15] demonstrated that the tongue representation received dense projections from regions representing the lower and upper teeth and tissue lining the inside of the cheek and lips.…”

“…The bidirectional neural interconnection between the vSI and the ventral primary motor area (vMI) was established in neuroanatomical studies of New [17] and Old World monkeys [15,32]. In one study on the vMI, most neurons respond to light tactile stimulation rather than stimulation to deep tissues, such as the muscle and joint, which suggests the particular importance of tactile input in motor control [33].…”