Backward cortical projections to primary somatosensory cortex in rats extend long horizontal axons in layer I

Abstract

We have studied the origin and extent of axons within layer I of the primary somatosensory cortex (SI) of rats by using retrograde and anterograde tracers with an emphasis on reciprocal connections to layer I of SI from ipsilateral cortical areas that are the target of SI projections. Small crystals of 1,1',dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanine perchlorate (DiI) labeled horizontal axons projecting in all directions within layer I, which extended for up to 4 mm with numerous terminal branches. Applications of horseradish peroxidase, Diamidino yellow, or fast blue to the pial surface of SI labeled a characteristic pattern of neurons below the application site that excluded neurons in layer IV of the barrel fields, unless the dye penetrated deeper than layer II. This provided a control for the effective depth of the layer I dye applications. Retrograde transport from layer I of SI was traced to the primary motor area, the lateral parietal areas, including the secondary somatosensory (SII) and agranular insular cortex ipsilaterally, as well as the homotopic areas of SI contralaterally. Injections of the anterograde tracer dextran amine at the same site as the SI surface application labeled dense fiber terminations in middle layers of these same secondary areas in the primary motor cortex (MI) or SII in the midst of cells labeled by retrograde transport from layer I of SI. Injections of dextran amine into these secondary cortical areas labeled fibers that coursed through deep layers to SI, where they ascended to layer I. These reciprocal corticocortical inputs to SI were concentrated in layer I, where they branched and extended horizontally across several SI barrels.

Boosting of Action Potential Backpropagation by Neocortical Network ActivityIn Vivo

“…The distri- bution of these afferents over the dendritic tree is not uniform. For instance, long-range feedback connections from secondary somatosensory areas arrive principally via synapses in L1 (Cauller and Connors, 1994;Cauller et al, 1998). Because up states are presumably driven by a subset of the many synaptic inputs onto a neuron, one might expect up state amplitudes to differ in different parts of the neuron.…”

Bilateral Integration of Whisker Information in the Primary Somatosensory Cortex of Rats

“…This categorization allowed us to test the possibility that homologous bilateral stimuli would result in stronger attenuation of test-evoked response probability than attenuation resulting from heterologous bilateral stimuli. This hypothesis was based on previous experiments that demonstrated an anatomical and functional homotopy between the whisker barrel cortices (White and DeAmicis, 1977;Pidoux and Verley, 1979;Olavarria et al, 1984;Welker et al, 1988;Koralek et al, 1990;Cauller et al, 1998). To the contrary, no significant differences were observed between homologous and heterologous bilateral stimuli for C IC responses throughout the time course of recovery.…”

“…Comparisons between these separate sources of tactile input would then allow the animal to successfully detect the width of an aperture, or the orientation of an obstacle. A possible anatomical substrate for the integration of bilateral whisker information within the SI is provided by the approximately homotopic interconnection of the whisker barrel fields via the corpus callosum (White and DeAmicis, 1977;Olavarria et al, 1984;Koralek et al, 1990;Cauller et al, 1998) (for nonhomotopic connections, see Welker et al, 1988). Furthermore, field potential recordings provided evidence, more than two decades ago, for the existence of callosally mediated ipsilateral whisker responses in the rat SI (Pidoux and Verley, 1979).…”

The Role of Dendritic Signaling in the Anesthetic Suppression of Consciousness

“…While bottom-up fibers terminate predominantly in cortical layer 4 of the target area, top-down fibers target mainly layer 1, [30][31][32][33][34] where they contact primarily apical dendrites of pyramidal neurons located in layers 2, 3, and 5. Traditionally, dendrites were thought to be passive conduction elements: the excitatory and inhibitory potentials induced at their synapses would be relayed to the cell soma simply via electrotonic spread, diminishing exponentially along the way.…”

“…It is by now well established that there are reciprocal anatomical connections at all stages of sensory processing for at least the visual, [30][31][32]65 auditory, 33,65 and somatosensory 34,65,66 modalities. In fact, area V1 receives top-down projections from more areas than it sends bottom-up projections to, and there appears to be a general predominance of topdown over bottom-up connectivity along the ventral visual pathway.…”