Corticofugal projection patterns of whisker sensorimotor cortex to the sensory trigeminal nuclei.

Jared B Smith,Glenn D R Watson,Shubhodeep Chakrabarti,Kevin D Alloway,Cornelius Schwarz

doi:10.3389/fncir.2015.00053

Abstract

The primary (S1) and secondary (S2) somatosensory cortices project to several trigeminal sensory nuclei. One putative function of these corticofugal projections is the gating of sensory transmission through the trigeminal principal nucleus (Pr5), and some have proposed that S1 and S2 project differentially to the spinal trigeminal subnuclei, which have inhibitory circuits that could inhibit or disinhibit the output projections of Pr5. Very little, however, is known about the origin of sensorimotor corticofugal projections and their patterns of termination in the various trigeminal nuclei. We addressed this issue by injecting anterograde tracers in S1, S2 and primary motor (M1) cortices, and quantitatively characterizing the distribution of labeled terminals within the entire rostro-caudal chain of trigeminal sub-nuclei. We confirmed our anterograde tracing results by injecting retrograde tracers at various rostro-caudal levels within the trigeminal sensory nuclei to determine the position of retrogradely labeled cortical cells with respect to S1 barrel cortex. Our results demonstrate that S1 and S2 projections terminate in largely overlapping regions but show some significant differences. Whereas S1 projection terminals tend to cluster within the principal trigeminal (Pr5), caudal spinal trigeminal interpolaris (Sp5ic), and the dorsal spinal trigeminal caudalis (Sp5c), S2 projection terminals are distributed in a continuum across all trigeminal nuclei. Contrary to the view that sensory gating could be mediated by differential activation of inhibitory interconnections between the spinal trigeminal subnuclei, we observed that projections from S1 and S2 are largely overlapping in these subnuclei despite the differences noted earlier.

Highlights

The trigeminal sensory nuclei constitute the first synaptic station of sensory processing in the whisker-to-barrel pathway conveying tactile information from the mystacial vibrissae to the primary somatosensory cortex (S1)
We quantified the corticofugal projections to the trigeminal sensory nuclei by combining anterograde and retrograde tracing approaches
We found an overall similarity between the innervation patterns of S1 and S2 corticofugal projections with a few notable and important differences

Summary

Introduction

The trigeminal sensory nuclei constitute the first synaptic station of sensory processing in the whisker-to-barrel pathway conveying tactile information from the mystacial vibrissae to the primary somatosensory cortex (S1). A recent study based on electrical cortical stimulation suggested that corticofugal projections alter the trigeminal response profiles by selectively gating the ascending lemniscal pathway during whisking and non-whisking conditions (Furuta et al, 2010). According to this hypothesis, descending corticofugal projections from S1 barrel cortex and S2 exert differential effects on the neurons in Pr5 that give rise to the lemniscal pathway. Anatomical data on the precise origin of corticofugal projections to different trigeminal nuclei and their pattern of termination have never been quantified

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Results

Conclusion