Differential topography of the bilateral cortical projections to the whisker and forepaw regions in rat motor cortex

Abstract

Whisker and forelimb movements in rats have distinct behavioral functions that suggest differences in the neural connections of the brain regions that control their movements. To test this hypothesis, retrograde tracing methods were used to characterize the bilateral distribution of the cortical neurons that project to the whisker and forelimb regions in primary motor (MI) cortex. Tracer injections in each MI region revealed labeled neurons in more than a dozen cortical areas, but most labeling was concentrated in the sensorimotor areas. Cortical projections to the MI forepaw region originated primarily from the primary somatosensory (SI) cortex in the ipsilateral hemisphere. In contrast, most projections to the MI whisker region originated from the MI whisker region in the contralateral hemisphere. Tracer injections in the MI whisker region also revealed a higher proportion of labeled neurons in the claustrum and in the posterior parietal cortex. Injections of different tracers into the MI whisker and forepaw regions of some rats revealed a topographic organization of neuronal labeling in several sensorimotor regions. Collectively, these findings indicate that the MI whisker and forepaw regions receive different sets of cortical inputs. Whereas the MI whisker region is most strongly influenced by callosal projections, presumably to mediate bilateral coordination of the whiskers, the MI forepaw region is influenced mainly by ipsilateral SI inputs that convey somatosensory feedback.

Brain-wide map of efferent projections from rat barrel cortex

“…We found substantial amounts of labeled fibers in the contralateral S1, bilaterally in the primary motor cortex (M1) and the secondary somatosensory cortex (S2), and to a lesser extent the secondary motor cortex (M2; Figures 2A–D), in agreement with earlier observations (Donoghue and Parham, 1983; Reep et al, 1990; Fabri and Burton, 1991a; Wright et al, 1999; Hoffer et al, 2003; Alloway et al, 2004, 2008; Hoffer et al, 2005; Colechio and Alloway, 2009; Smith and Alloway, 2013). The amount of forelimb related projections to motor areas was consistently higher than whisker related projections, relative to the size of the injection sites (Table 1).…”

Molecular and Cellular Approaches for Diversifying and Extending Optogenetics

“…We first validated this concept in the rat (Figure 2D) by devising a strategy to selectively introduce eNpHR3.0 into those primary motor cortex (M1) microcircuits that are involved in corticocortical connections with primary sensory cortex (S1) (Colechio and Alloway, 2009). To do this, we injected the previously described Cre-dependent AAV, now conditionally expressing eNpHR3.0 into motor cortex, and injected a novel WGA-Cre-expressing AAV (AAV2-EF1α-mCherry-IRES-WGA-Cre) remotely into primary somatosensory cortex.…”

Anatomical Pathways Involved in Generating and Sensing Rhythmic Whisker Movements

“…There are several indirect routes from wM1 to the lateral facial nucleus, for example via SC (see Superior Colliculus) or the pontomedullary RF (see Pontomedullary Reticular Formation) and wS1 (see Somatosensory Cortex as a Premotor Area). In addition, wM1 is involved in several feedback loops, including reciprocal connections with wS1 (Aronoff et al, 2010), thalamus (Cicirata et al, 1986; Colechio and Alloway, 2009), and loops involving the basal ganglia (see Basal Ganglia), the cerebellum (see The Cerebellar System), and the claustrum (see Bilateral Coordination of Whisker Movements). Finally, wM1 projects to the deep mesencephalic nucleus, the periaqueductal gray, and the red nucleus (Alloway et al, 2010).…”