B. Hellmann et al., Nucleus isthmi, pars semilunaris as a key component of the tectofugal visual system in pigeons, J COMP NEUR, 436(2), 2001, pp. 153-166
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
.