Nucleus isthmi, pars semilunaris as a key component of the tectofugal visual system in pigeons

The avian isthmic nuclei are constituted by a group of structures reciproca lly connected with the tectum opticum and considered to play a role in the modulation of intratectal processes. Although the two larger isthmic nuclei , the n. isthmi pars parvocellularis (Ipc) and the n. isthmi pars magnocell ularis (Imc), have been studied in detail previously, the third and smalles t of this group, the n. isthmi pars semilunaris (SLu), has been largely neg lected. The present study demonstrates this isthmic component to be charact erized by a unique connectivity and immunohistochemical pattern: 1) SLu rec eives tectal afferents and projects back onto the outer retinorecipient tec tal layers; 2) it projects bilaterally onto the nucleus rotundus and thus m odulates the ascending tectofugal system; 3) in addition, previous studies have demonstrated SLu projections onto the lateral spiriform nucleus (SpL), which mediates basal ganglia output onto the tectum. In that SpL projects onto the deep layers of the tectum, SLu indirectly modulates descending tec tal output patterns. Taken together, the role of SLu goes far beyond a loca l modulation of intratectal processes. Instead, this isthmic structure is l ikely to play a key role in the topographically organized modulation of the ascending and, at least indirectly, also the descending projections of the optic tectum. J. Comp. Neurol. 436:153-166, 2001. (C) 2001 Wiley-Liss, Inc .