In the mouse, each layer 4 barrel is composed of a central region of high density neuropil, containing the clustered VPM axonal arborizations surrounded by a cell-dense wall of layer 4 neurons that orient their dendritic and axonal arborizations into one specific barrel (Woolsey et al., 1975). Optical stimulation has been used to study the functional projection from VPM thalamus to barrel cortex, revealing prominent VPM glutamatergic input onto neurons located in layer 4, layer 5B and layer 6 (Bureau et al., 2006; Petreanu et al., 2009; Cruikshank et al., 2010). Thalamocortical POM
neurons also project to the primary somatosensory barrel cortex, terminating densely in layer 1 and layer 5A. Functional characterization of this projection has revealed a prominent POM input onto layer 5A barrel cortex neurons (Bureau et al., Selleckchem Veliparib 2006; Petreanu et al., 2009). In the rat, several subdivisions and additional parallel pathways have been characterized from the principal trigeminal and spinal trigeminal nuclei via different subdivisions of the VPM and POM thalamus (Deschenes, 2009). It has been suggested that Y-27632 purchase these parallel pathways in the rat process different aspects of whisker sensorimotor information (Yu et al., 2006).
However, in the mouse, little is currently known about the differential sensory information signalled by VPM and POM neurons, and further experimental work focusing on these issues will be of great importance. Progress has also been made toward defining the synaptic circuits within mouse S1 barrel cortex through simultaneous whole-cell recordings (Lefort et al., 2009) and glutamate uncaging Loperamide (Bureau et al., 2006; Xu & Callaway, 2009), providing complementary data to that obtained in rat (Lübke & Feldmeyer, 2007; Schubert et al., 2007). These studies have revealed several prominent synaptic pathways
for processing sensory information within a cortical barrel column (defined as the entire thickness of the cortex from layer 1 to layer 6 and laterally bounded by the extent of the layer 4 barrel). Specific investigation of the microcircuits in the C2 barrel column revealed that excitatory neurons in layer 4 dominate synaptic connectivity within this barrel column (Lefort et al., 2009). Layer 4 neurons signal to neurons located in all other cortical layers and they are therefore able to robustly transmit to the entire barrel column the tactile information received via the dense layer 4 innervation by VPM. Other prominent neocortical signalling pathways in the mouse C2 barrel column are from supragranular to infragranular layers, with an interesting elevated reciprocal connectivity between layer 2 and layer 5A (Bureau et al., 2006; Lefort et al., 2009). In vivo recordings from mouse barrel cortex neurons are beginning to shed light on how these neocortical microcircuits operate functionally during behavior (Crochet & Petersen, 2006; Poulet & Petersen, 2008; Gentet et al., 2010).