Muscle spindles in the jaw-closing muscles, which are innervated by trigeminal 
mesencephalic neurons (MesV neurons), control the strength of occlusion and the 
position of the mandible. The mechanisms underlying cortical processing of 
proprioceptive information are critical to understanding how sensory information 
from the masticatory muscles regulates orofacial motor function. However, these 
mechanisms are mostly unknown. The present study aimed to identify the regions 
that process proprioception of the jaw-closing muscles using in vivo optical 
imaging with a voltage-sensitive dye in rats under urethane anesthesia. First, 
jaw opening that was produced by mechanically pulling down the mandible evoked 
an optical response, which reflects neural excitation, in two cortical regions: 
the most rostroventral part of the primary somatosensory cortex (S1) and the 
border between the ventral part of the secondary somatosensory cortex (S2) and 
the insular oral region (IOR). The kinetics of the optical signal, including the 
latency, amplitude, rise time, decay time and half duration, in the S1 region 
for the response with the largest amplitude were comparable to those in the 
region with the largest response in S2/IOR. Second, we visualized the regions 
responding to electrical stimulation of the masseter nerve, which activates both 
motor efferent fibers and somatosensory afferent fibers, including those that 
transmit nociceptive and proprioceptive information. Masseter nerve stimulation 
initially excited the rostral part of the S2/IOR region, and an adjacent region 
responded to jaw opening. The caudal part of the region showing the maximum 
response overlapped with the region responding to jaw opening, whereas the 
rostral part overlapped with the region responding to electrical stimulation of 
the maxillary and mandibular molar pulps. These findings suggest that 
proprioception of the masseter is processed in S1 and S2/IOR. Other sensory 
information, such as nociception, is processed in a region that is adjacent to 
these pulpal regions and is located in the rostral part of S2/IOR, which 
receives nociceptive inputs from the molar pulps. The spatial proximity of these 
regions may be associated with the mechanisms by which masseter muscle pain is 
incorrectly perceived as dental pain.