Projections of the nucleus of the basal optic root in pigeons (Columba livia): A comparison of the morphology and distribution of neurons with different efferent projections

Abstract

The avian nucleus of the basal optic root (nBOR) is a visual structure involved in the optokinetic response. nBOR consists of several morphologically distinct cell types, and in the present study, we sought to determine if these different cell types had differential projections. Using retrograde tracers, we examined the morphology and distribution of nBOR neurons projecting to the vestibulocerebellum (VbC), inferior olive (IO), dorsal thalamus, the pretectal nucleus lentiformis mesencephali (LM), the contralateral nBOR, the oculomotor complex (OMC) and a group of structures along the midline of the mesencephalon. The retrogradely labeled neurons fell into two broad categories: large neurons, most of which were multipolar rather than fusiform and small neurons, which were either fusiform or multipolar. From injections into the IO, LM, contralateral nBOR, and structures along the midline-mesencephalon small nBOR neurons were labeled. Although there were no differences with respect to the size of the labeled neurons from these injections, there were some differences with the respect to the distribution of labeled neurons and the proportion of multipolar vs. fusiform neurons. From injections into the VbC, the large multipolar cells were labeled throughout nBOR. The only other cases in which these large neurons were labeled were contralateral OMC injections. To investigate if single neurons project to multiple targets we used paired injections of red and green fluorescent retrograde tracers into different targets. Double-labeled neurons were never observed indicating that nBOR neurons do not project to multiple targets. We conclude that individual nBOR neurons have unique projections, which may have differential roles in processing optic flow and controlling the optokinetic response.

Novel, continuous visual motion induces c-fos expression in the avian optokinetic nuclei and optic tectum

“…We conjecture that the indirect inputs from the contralateral eye are important to compute the abnormal or novel condition produced by the continuous whole-field rotation. Support for this conjecture comes from the finding that there are neurons in the nBOR that receives input from the contralateral nBOR (Brecha et al, 1980;Wylie et al, 1997Wylie et al, , 2007, and many nBOR neurons show binocular responses which, by combining appropriate directions of visual stimulation in the two eyes, explicitly encode rotations and translations (Wylie et al, 1990b(Wylie et al, , 1999. The fact that monocular stimulation of nondeprived chicks produced strong c-fos expression, also indicates that changes in the signals coming from the unstimulated eye are relevant to define a novel situation.…”

“…In addition to directional selectivity, LM and the nBOR neurons can be classified according to morphology (Bre- , 1980;Gamlin and Cohen, 1988), immunochemistry (Zayats et al, 2003), physiology and connectivity (Brecha et al, 1980;Wylie et al, 2007). How these properties coalesce in functional groups is presently unclear but, presumably, neurons showing the same directional responses could differ in other aspects.…”

“…Alternatively, they may be neurons projecting to certain targets or receiving specific connections. For example, if c-fos expression is related to synaptic plasticity, it is plausible that this expression occurs only in those that project to the inferior olive (Brecha et al, 1980;Wylie et al, 2007) and/or those that project to the vestibular nuclei (37% of all cells, according to Peduzzi and Crossland, 1983), the nuclei directly driving the optokinetic nystagmus (OKN) behavior. Interestingly, Wylie (2001) and have reported that LM neurons projecting to the inferior olive are located in a thin strip very much alike to the c-fos-stained strip of cells shown in Figs.…”

“…How these properties coalesce in functional groups is presently unclear but, presumably, neurons showing the same directional responses could differ in other aspects. For example, in both LM and nBOR, directional neurons can be further subdivided into fast and slow responsive neurons (Wylie and Crowder, 2000;Crowder et al, 2003), and evidence suggests that each subgroup projects to specific cerebellar divisions, likely playing a differential function regarding to the optokinetic response (for a thorough discussion see Wylie et al, 2007).…”

“…These nuclei (which we will refer to collectively as the optokinetic nuclei) project to several different targets, including the vestibular nuclei and cerebellum and drive stabilizing eye movements such as the optokinetic response (McKenna and Wallman, 1985a;Wylie et al, 2007;. Thus the optokinetic nuclei respond to simple visual stimuli and have a clear function.…”