The primary somatosensory barrel cortex processes tactile vibrissae information, allowing
rodents to actively perceive spatial and textural features of their immediate surroundings.
Each whisker on the snout is individually represented in the neocortex by an anatomically
identifiable 'barrel' specified by the segregated termination zones of thalamocortical
axons of the ventroposterior medial nucleus, which provide the primary sensory input
to the neocortex. The sensory information is subsequently processed within local synaptically
connected neocortical microcircuits, which have begun to be investigated in quantitative
detail. In addition to these local synaptic microcircuits, the excitatory pyramidal
neurons of the barrel cortex send and receive long-range glutamatergic axonal projections
to and from a wide variety of specific brain regions. Much less is known about these
long-range connections and their contribution to sensory processing. Here, we review
current knowledge of the long-range axonal input and output of the mouse primary somatosensory
barrel cortex. Prominent reciprocal projections are found between primary somatosensory
cortex and secondary somatosensory cortex, motor cortex, perirhinal cortex and thalamus.
Primary somatosensory barrel cortex also projects strongly to striatum, thalamic reticular
nucleus, zona incerta, anterior pretectal nucleus, superior colliculus, pons, red
nucleus and spinal trigeminal brain stem nuclei. These long-range connections of the
barrel cortex with other specific cortical and subcortical brain regions are likely
to play a crucial role in sensorimotor integration, sensory perception and associative
learning.
