The ascending projections of the nuclei of the descending trigeminal tract (nTTD) in the zebra finch (Taeniopygia guttata)

Abstract

In our traditional view of the avian somatosensory system, input from the beak and head reaches the telencephalon via a disynaptic pathway, involving projections from the principal sensory nucleus (PrV) directly to nucleus basorostralis (previously called nucleus basalis), whereas input from the rest of the body follows a trisynatic pathway similar to that in mammals, involving projections from the dorsal column nuclei to the thalamus, and thence to somatosensory wulst. However, the role of the nuclei of the descending trigeminal tract (nTTD) in this scenario is unclear, partly because their ascending projections have been examined in only one species, the mallard duck. Here we examine the ascending projections of the nTTD in the zebra finch, using in vivo injections of biotinylated dextran amine and verification of projections by means of retrograde transport of the beta subunit of cholera toxin. The results show a high degree of interconnectivity within the nTTD, and that these nuclei project to PrV. We also find a projection from nTTD to the contralateral thalamic nucleus uvaeformis, a multi-sensory nucleus connected to the song system. Furthermore, our finding of a projection from nTTD to the contralateral somatosensory thalamic nucleus dorsalis intermedius ventralis anterior (DIVA) is consistent with the well-known projection in mammals from nTTD to the ventrobasal thalamus, suggesting that the ascending trigeminal pathways in birds and mammals are more similar than previously thought.

Differential projections of the densocellular and intermediate parts of the hyperpallium in the pigeon (Columba livia)

“…A small ‘rostral Wulst’ is somatosensory in nature, whereas a much larger part, with which the present paper is particularly concerned, extends caudally from behind the rostral Wulst, and is visual in nature. Somatosensory input to the rostral Wulst targets IHA and is derived primarily from the body and limbs by way of the thalamus (Wild, ), although recent evidence also suggests some degree of input from the beak, also by way of the thalamus (Faunes & Wild, ). Although the rostral Wulst is generally regarded as a somato sensory structure, a pyramidal‐like tract originates from HA and innervates many structures throughout the neuraxis and extends as far as the upper spinal cord (Wild & Williams, ).…”

Innervation of the syrinx of the zebra finch (Taeniopygia guttata)

“…Vocalisation and respiration are highly integrated functions in songbirds (Hartley, ), and therefore it is likely that the projection from the syrinx to the nTS serves a function in both processes. Furthermore, the projection to nTTDi could reach the song system via nucleus uvaeformis (Uva) of the dorsal thalamus (Faunes & Wild, ), which mediates the projections from respiratory and vocal centres to the forebrain nuclei of the song system (Ashmore et al, ; Wild, ).…”

“…The procedure for brainstem injections is described in an accompanying article (Faunes & Wild, ). Briefly, the animals were anesthetised as described above and fixed in the stereotaxic frame with the beak angled down with respect to the horizon so that the confluence of the mid‐sagittal and cerebellar sinuses (“Y” point) was 0.3 mm caudal to inter‐aural zero.…”

“…Furthermore, the projection to nTTDi could reach the song system via nucleus uvaeformis (Uva) of the dorsal thalamus (Faunes & Wild, 2017b), which mediates the projections from respiratory and vocal centres to the forebrain nuclei of the song system (Ashmore et al, 2008;Wild, 1994).…”

A newly identified trigeminal brain pathway in a night-migratory bird could be dedicated to transmitting magnetic map information

“…Apart from cerebellar projections, SpV has been shown to form a feedback loop projecting to the PrV complex [17,18]. It is thus reasonable to assume that the only region to send trigeminally perceived magnetic information to the forebrain is the PrV complex [17][18][19][20]. Therefore, PrVv is a prime candidate to relay trigeminally perceived magnetic information to the forebrain.…”