whiskers of the face - are caudal and lateral. Although the whiskers make up a relatively small portion of the animal, they dominate the somatotopic map. Barrels of the major facial whiskers The barrels that correspond to the major facial whiskers (mystacial vibrissae) are contained within the posteromedial barrel subfield (PMBSF). The barrels here are the largest and most elliptical in shape and have a striking topographical organization that is identical to that of the whiskers; they are organized into 5 rows of 4-7 large whiskers that run close to parallel with the bridge of the nose. The organisation of the mystacial vibrissae and corresponding barrels is so consistent that there is a naming convention to identify each whisker in rats and mice. Rows are designated A to E from top to bottom, and columns of whiskers within each row are numbered from back to front. The first four rows also have an additional whisker behind column 1, which is designated with a lower case letter or a Greek letter (α, β, γ, or δ). These four whiskers are also called straddlers. Anatomy and connectivity of the barrels The barrels of the barrel cortex were named because the densities of cells resembled barrels, that is, they are collected into cylindrical shapes that are narrowed at the top and bottom. The centre of the barrel is designated the hollow, and the spaces between the barrels are the septa (singular: septum) Sensory information flows from whisker follicles to barrel cortex via the trigeminal nerve nuclei and the thalamus. Barrel like divisions can be seen in some, but not all parts of the trigeminal nuclei (where they are called barrelets) and the thalamus (where they are called barreloids). The trigeminal nerve carries afferent fibres from the follicles into the brainstem where they connect to neurons in four different trigeminal nerve nuclei: principal, interpolar, oral, and caudal. Projections from the trigeminal nuclei to the thalamus are split into pathways designated lemniscal, extralemniscal, and paralemniscal. In the lemniscal pathway, axons from the principal trigeminal nucleus cross over the midline and project to “barreloids” in the thalamus, specifically in the dorsomedial section of the ventroposterior medial nucleus (VPMdm). Neurons in VPMdm project mainly to barrels in layer 4 of primary somatosensory cortex (S1). In the extralemniscal pathway, neurons of the interpolar nucleus project to the ventrolateral section of the ventroposterior medial nucleus (VPMvl). Neurons in VPMvl