The barrelettes—architectonic vibrissal representations in the brainstem trigeminal complex of the mouse. Normal structural organization

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Abstract
The organization of the brainstem trigeminal complex (BTC) of the mouse is described, with emphasis on the normal organization of the vibrissal representations. Thionin staining for Nissl substance was employed to reveal the cytoarchitecture. Cytochrome oxidase histochemistry was used to reveal the chemoarchitecture. Golgi impregnation methods, in combination with thionin staining, were used to examine the neuronal dendritic morphology within a defined cytoarchitectonic context. An in vitro horseradish peroxidase labelling method was used to study the distribution and morphology of primary trigeminal afferent terminals within the BTC. The BTC consists of four distinct subnuclei: principalis (nVp), oralis (nVo), interpolaris (nVi), and caudalis (nVc). The present study shows that these sub-nuclei can be distinguished from each other on the basis of several anatomical criteria, including the distribution and density of neuronal size classes, histochemical staining intensity, morphology and orientation of neuronal dendrites, and size and texture of primary afferent terminal arbors. Anatomical manifestation of vibrissal representations within the BTC can be descried in nVp, nVi, and nVc, but not in nVo. Within the three subnuclei where they are found, anatomical vibrissal representations are composed of architectural subunits that form an overall pattern homeomorphic to the pattern of vibrissae on the face of the animal. Each sub-unit forms a cylindrical tube running in a rostrocaudal orientation within the BTC. These sub-units will be called barrelettes. Cytologically, each barrelette consists of cell-dense “sides” surrounding a practically cell-free “hollow” Individual sub-units are separated by narrow, cell-free “septa” Histochemically, each subunit is manifested as a discrete patch of positive-staining reaction products. Differential interference contrast optics shows that these patches correspond precisely to the barrelette hollows. Evidence is presented to show that the barrelettes are the functional units for the processing of vibrissal sensory information. Terminal arborizations of individual primary afferents seem to be confined to the hollow of single barrelettes. The majority of neurons that form the sides of a barrelette have bitufted dendritic arbors, which project predominantly into the barrelette hollow, although a minority of neurons, particularly in nVi and nVc, also extend part of their dendritic arbors into adjacent barrelette hollows. The barrelette hollows are thus the principal neuropil region in which primary afferents and their target neurons interact. Contacts are made mainly between en passant varicosities and terminal boutons on primary afferent collaterals and dendritic spines and shafts of second order neurons. The confinement of primary afferent terminal arbors and second order neuronal dendrites within the hollow of a single barrelette is presumably the anatomical basis for the abundance of single-whisker units seen in electrophysiological recordings. Neurons with dendrites extending into the hollow of adjacent barrelettes may represent the physiologically multiwhisker units. Barrelettes may be a highly adaptive configuration that facilitate the transfer and integration of neural signals within a confined amount of neural tissues while still maintaining the topological arrangement of peripheral sense organs. Quantitative analyses show that, in nVp, the proportion of neural tissue allotted to the representation of each mystacial vibrissa is directly related to the innervation density of the vibrissal follicle: larger barrelettes correspond to more densely innervated vibrissae. There is no further change in this proportion between the barrelettes in nVp and the whisker barrels in the cortex.