[TA] a median, unpaired, ovoid cell group at the base of the midbrain tegmentum between the cerebral peduncles; it receives the retroflex fasciculus from the habenula and projects to the raphe region (raphe nuclei) and periaqueductal gray substance of the midbrain. SYN: nucleus interpeduncularis [TA], ganglion isthmi, Gudden ganglion, intercrural ganglion, interpeduncular ganglion.