The spatial organization of the anatomical structures along the trigeminal
afferent pathway of the rat conserves the topographical order of the recept
or sheath: The brainstem barrelettes, thalamic barreloids, and cortical bar
rels all reflect the arrangement of whiskers across the mystacial pad. Alth
ough both the amount of innervation in the mystacial pad and the size of co
rtical barrels were shown previously to exhibit increasing gradients toward
the ventral and caudal whiskers, whether similar gradients existed in the
brainstem and thalamus was not known. Here, the authors investigated the si
ze gradients of the barreloids in the ventral posteromedial nucleus of the
rat thalamus. Because the angles used to cut the brain were crucial to this
study, the optimal cutting angles were determined first for visualization
of individual barreloids and of the entire barreloid field. Individual barr
eloids, arcs, and rows as well as entire barreloid fields were clearly visu
alized using cytochrome oxidase staining of brain slices that were cut with
the optimal cutting angles. For the first five arcs (including straddlers)
, the length of barreloids increased in the direction of dorsal-to-ventral
whiskers and of caudal-to-rostral whiskers. These gradients reveal an inver
se relationship between the size of barreloids and whiskers (length and fol
licle diameter) along arcs and rows. The largest barreloids in the ventral
posteromedial nucleus were those that represent whiskers C2-C4, D2-D4, and
E2-E4, which are neither the largest nor the most innervated whiskers in th
e mystacial pad. This implies that the extended representation is not merel
y a reflection of peripheral innervation biases and probably serves an as y
et unknown processing function. J. Comp. Neurol. 429:372-387, 2001. (C) zoo
l. Wiley-Liss, Inc.