[TA] a group of cells located immediately adjacent to the posterior commissure at the mesencephalon-diencephalon junction; may be divided into pars ventralis [TA] (ventral subdivision [TA]), pars dorsalis [TA] (dorsal subdivision [TA]), and pars interstitialis [TA] (interstitial subdivision [TA]). SYN: nucleus commissurae posterioris [TA].