The complexity of receptive fields of periodontal mechanoreceptive neurons in the postcentral area 2 of conscious macaque monkey brains.

The representation of the oral structures in area 2 of the postcentral somatosensory cortex was studied in conscious macaque monkeys by recording single-neuron activities. A total of 58 penetrations were made in the oral region of five hemispheres in three animals and 707 neurons were isolated. The receptive field characteristics were identified for 480 neurons. Among them, 62 neurons along 21 penetrations responded to mechanical tooth stimulation (periodontal mechanoreceptive neurons). The overwhelming majority (81%, 50/62) of periodontal mechanoreceptive neurons had receptive fields on several teeth in either jaw. Moreover, six had receptive fields on corresponding maxillary and mandibular teeth. Thirty-seven percent (23/62) of periodontal mechanoreceptive neurons also had receptive fields on other oral structures surrounding the teeth, such as gingiva (16/23), lip (10/23), and tongue mucosa (1/23). Among them, four neurons had receptive fields on both the gingiva and lip. These receptive field features were readily interpreted as a combination of the regions stimulated simultaneously during food intake. We therefore speculated that these periodontal mechanoreceptive neurons in area 2 may be the prerequisite neural substrate for the eventual oral stereognosis that will take place in the neighboring association cortices. The coexistence of periodontal mechanoreceptive neurons with simple and complex receptive fields, or small and large receptive fields in the oral region of the postcentral area 2 suggests that this region could be the stage for the integration of sensory information from the periodontal ligament and from other oral structures.