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

Abstract

The avian nucleus lentiformis mesencephali (LM) is a visual structure involved in the optokinetic response. The LM consists of several morphologically distinct cell types. In the present study we sought to determine if different cell types had differential projections. Using retrograde tracers, we examined the morphology and distribution of LM neurons projecting to the vestibulocerebellum (VbC), inferior olive (IO), dorsal thalamus, nucleus of the basal optic root (nBOR), and midline mesencephalon. From injections into the latter two structures, small LM cells were labeled. More were localized to the lateral LM as opposed to medial LM. From injections into the dorsal thalamus, small neurons were found throughout LM. From injections into the VbC, large multipolar cells were found throughout LM. From injections into IO, a strip of medium-sized fusiform neurons along the border of the medial and lateral subnuclei was labeled. To investigate if neurons project to multiple targets we used fluorescent retrograde tracers. After injections into IO and VbC, double-labeled neurons were not observed in LM. Likewise, after injections into nBOR and IO, double-labeled neurons were not observed. Finally, we processed sections through LM for glutamic acid decarboxylase (GAD). Small neurons, mostly in the lateral LM, were labeled, suggesting that projections from LM to nBOR and midline mesencephalon are GABAergic. We conclude that two efferents of LM, VbC and IO, receive input from morphologically distinct neurons: large multipolar and medium-sized fusiform neurons, respectively. The dorsal thalamus, nBOR, and midline mesencephalon receive input from small neurons, some of which are likely GABAergic.

Neural specialization for hovering in hummingbirds: Hypertrophy of the pretectal nucleus lentiformis mesencephali

“…The LM consists of medial and lateral subnuclei, but we simply grouped these together, because the distinction between the two was difficult in some specimens. There are a variety of sizes of neurons in LM, including extremely large multipolar cells that project to the cerebellum (Gamlin and Cohen, 1988b;Pakan et al, 2006). Toward the rostral pole, LM is bordered laterally by the optic tectum (TeO) and medially by the nucleus laminaris precommissuralis (LPC), a nonretinorecipient nucleus consisting of a thin group of small basophilic cells.…”

“…Caudally, the LM is bordered by the retinorecipient rostral tectal gray (GT), which appears continuous with layer 5 of TeO. The GT consists of loosely packed small cells (Gamlin and Cohen, 1988a), but the large multipolar cells seen in LM are few or absent from GT (Pakan et al, 2006). More caudally, the rostral GT becomes divided into dorsal and ventral components (GTd, GTv) separated by the caudal GT (GTc), which appears continuous with layer 8 of TeO.…”

Processing of visual signals related to self-motion in the cerebellum of pigeons

“…The mcIO receives a projection from the ipsilateral LM (Clarke, 1977). This projection is mainly directed to the caudal mcIO (Wylie, 2001; Pakan et al, 2010) and arises from a distinct group of medium-sized fusiform cells found in a thin strip along the border of the medial and lateral subnuclei of LM (LMm, LMl) (Gamlin and Cohen, 1988; Wylie, 2001; Pakan et al, 2006). We have referred to this region as the intercalated nucleus of LM (LMi).…”

“…First, LM and nBOR project to the medial column of the inferior olive (mcIO), which in turn provides climbing fiber input to the vestibulocerebellum (VbC; folia IXcd and X) (blue pathway) (Clarke, 1977; Brecha et al, 1980; Gamlin and Cohen, 1988; Arends and Voogd, 1989; Wylie et al, 1997, 1999, 2007, 2008; Lau et al, 1998; Crowder et al, 2000; Wylie, 2001; Winship and Wylie, 2003; Pakan et al, 2005, 2006, 2010; Pakan and Wylie, 2006, 2008; Winship et al, 2006; Iwaniuk et al, 2009). Second, LM and nBOR project directly to IXcd of the VbC as mossy fibers (Brauth and Karten, 1977; Gamlin and Cohen, 1988; Wylie and Linkenhoker, 1996; Pakan et al, 2006, 2010; Wylie et al, 2007, 2008; Iwaniuk et al, 2009). Third, LM projects to folia VI–VIII, an area known as the “oculomotor cerebellum” (for review see Voogd and Barmack, 2006), where there is interaction with local motion inputs from a tecto-pontine system (Pakan et al, 2006).…”

“…Second, LM and nBOR project directly to IXcd of the VbC as mossy fibers (Brauth and Karten, 1977; Gamlin and Cohen, 1988; Wylie and Linkenhoker, 1996; Pakan et al, 2006, 2010; Wylie et al, 2007, 2008; Iwaniuk et al, 2009). Third, LM projects to folia VI–VIII, an area known as the “oculomotor cerebellum” (for review see Voogd and Barmack, 2006), where there is interaction with local motion inputs from a tecto-pontine system (Pakan et al, 2006). Each of the pathways is discussed below.…”