Functional organization of the direct and indirect projection via the reticularis thalami nuclear complex from the motor cortex to the thalamic nucleus ventralis lateralis

Abstract

The projection systems which arise from the motor cortex to reach the nucleus ventralis lateralis (VL) were investigated in the rat. They included a direct as well as an indirect projection via the reticularis thalami nuclear complex (RT). The investigation was performed in two steps: i) the former concerned the projection to the VL as well as to the RT from individual cortical foci electrophysiologically identified by the motor effects evoked by electrical stimulation; the second step concerned the projection from the RT to functionally defined regions of the VL. The direct projection from the motor cortex to the VL is somatotopically arranged. The projection reciprocates the fiber system directed from the VL to the motor cortex. Thus cortical zones controlling the motor activity of the proximal segments of the limbs project onto the regions of the VL that project back to these same cortical areas. With regard to cortical zones controlling the motor activity of the distal segments of the limbs, they not only project to the region of the VL specifically related to them, but also to the region of the VL associated with the cortical areas responsible for movements of the proximal parts of the same limb. In that case fiber terminals were more dense in the VL region controlling the proximal segment than in the region controlling the distal segment of the same limb. This organization suggests that proximal adjustments may be automatically provided by the motor activity of the distal segments of the same limb. The motor cortex projects to the rostral region of the RT with a precise topographical organization. In particular, the projection shows a dorsoventral organization in the RT in relation to the caudorostral body representation in the motor cortex. The projection which arises from the rostral region of the RT also reaches the VL with a topographical arrangement. It discloses a rostrocaudal organization in the VL in relation to a dorsoventral displacement in the RT. Comparing the projection from the motor cortex to the RT and that from this nuclear complex to the VL it was shown that the regions of the VL and their receptive cortical areas were associated with the same regions of the RT. It was therefore concluded that the motor cortical projection to the VL relayed by the RT is somatotopically organized. In both direct and relayed pathways the projections from "hind-" and "forelimb" motor area are segregated, whereas the "head" projection overlaps, at least partially, the "forelimb" terminal field.(ABSTRACT TRUNCATED AT 400 WORDS)

Prefrontal Projections to the Thalamic Reticular Nucleus form a Unique Circuit for Attentional Mechanisms

“…In contrast to the detailed information on the central and caudal TRN sectors, little is known about its anterior part, which includes a motor sector, connected with motor cortex and ventrolateral thalamic nuclei, and the rostral pole, associated with the limbic cortex and anterior, midline, and intralaminar thalamic nuclei (Cicirata et al, 1990;Gonzalo-Ruiz and Lieberman, 1995;Lozsadi, 1995;Tai et al, 1995;Kolmac and Mitrofanis, 1997;Xiao and Barbas, 2004). We now provide evidence that the anterior quarter of TRN constitutes the prefrontal sector, overlapping with the motor and limbic sectors.…”

“…In rats, cats, rabbits, and primates, cortical and thalamic afferents in TRN are topographically organized (Crabtree and Killackey, 1989;Cicirata et al, 1990;Conley and Diamond, 1990;Cornwall et al, 1990;Crabtree, 1992aCrabtree, ,b, 1996Crabtree, , 1998Crabtree, , 1999Lozsadi, 1994Lozsadi, , 1995Ilinsky et al, 1999). Along the rostrocaudal axis, centroposterior loci in TRN are respectively linked with the somatosensory, visual, and auditory cortices and their associated thalamic nuclei.…”

“…The noradrenergic and cholinergic systems, which have been implicated in the sleep-wake cycle, arousal, and attentional mechanisms (McCormick, 1989;Steriade et al, 1993a;Hasselmo, 1999;Aston-Jones et al, 1999), heavily innervate TRN (Cicirata et al, 1990;Asanuma, 1992;Heckers et al, 1992;Kolmac and Mitrofanis, 1998), and their input could be modulated by the widespread prefrontal projections. These neurotransmitter systems and dopamine also innervate robustly thalamic nuclei connected with prefrontal cortices.…”

Organization of projections from the anterior pole of the nucleus reticularis thalami (NRT) to subdivisions of the motor thalamus: Light and electron microscopic studies in the Rhesus monkey

“…It is possible that the lateral NRT sectors adjacent to the VA and VL generate more focal projections to these nuclei. These lateral NRT regions also receive the afferent input from the motor and premotor cortices (Jones, 1975;Kü nzle, 1976;Cicirata et al, 1990) unlike the anterior pole that is connected mainly with limbic and prefrontal cortical areas (Tourtellotte et al, 1990;Lozsadi, 1994). Indeed, the functional roles of different NRT regions projecting to the same motor nuclei may be different and can be mediated by different types of NRT connectivity.…”

Circuits for Multisensory Integration and Attentional Modulation Through the Prefrontal Cortex and the Thalamic Reticular Nucleus in Primates

“…The anterior TRN includes a motor sector, connected with motor cortex and the ventrolateral thalamic nucleus, and the rostral pole, associated with the limbic cortex and the anterior, midline and intralaminar thalamic nuclei / 24,65,91,102,164,179 /. Recent studies have provided evidence that the anterior quarter of the rhesus monkey TRN constitutes the prefrontal sector, overlapping extensively with the motor and limbic sectors / 179,184 / (Fig.…”