Cortical and Subcortical Projections from Granular Insular Cortex Receiving Orofacial Proprioception

We have recently revealed that the proprioceptive signal from jaw-closing muscle spindles (JCMSs) is conveyed to the dorsal part of granular insular cortex rostroventrally adjacent to the rostralmost part of secondary somatosensory cortex (dGIrvs2) via the caudo-ventromedial edge (VPMcvm) of ventral posteromedial thalamic nucleus (VPM) in rats. However, it remains unclear to which cortical or subcortical structures the JCMS proprioceptive information is subsequently conveyed from the dGIrvs2. To test this issue, we injected an anterograde tracer, biotinylated dextranamine, into the electophysiologically identified dGIrvs2, and analyzed the resultant distribution profiles of labeled axon terminals in rats. Labeled terminals were distributed with an ipsilateral predominance. In the cerebral cortex, they were seen in the primary and secondary somatosensory cortices, lateral and medial agranular cortices and dorsolateral orbital cortex. In the basal ganglia, they were found in the caudate putamen, core part of accumbens nucleus, lateral globus pallidus, subthalamic nucleus, and substantia nigra pars compacta and pars reticulata. They were also observed in the central amygdaloid nucleus and extended amygdala (the interstitial nucleus of posterior limb of anterior commissure and the juxtacapsular part of lateral division of bed nucleus of stria terminalis). In the thalamus, they were seen in the reticular nucleus, ventromedial nucleus, core VPM, parvicellular part of ventral posterior nucleus, oval paracentral nucleus, medial and triangular parts of posterior nucleus, and zona incerta as well as the VPMcvm. These data suggest that the JCMS proprioceptive information through the dGIrvs2 is transmitted to the emotional ‘limbic’ regions as well as sensorimotor regions.