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Maps in the brain, here, are defined as topologically equivalent - or homeomorphic, or iso-morphic - representations of some coherent sensory or motor domain, often a sheet, in the periphery. We shall stay on the sensory side. There, the somatosensory and the visual maps are those that received most attention. Our presentation deals with a distinct part of the cortical somatosensory map in the mouse: the end-station of the whisker-to-barrel pathway. Whiskers, or, rather, whisker follicles, planted in the animal’s muzzle, are sensory organs, provided with four types of receptors (Andres, 1966) that signal to the brain through 40 to 160 large-calibre nerve fibres (Welker and Van der Loos, 1986b). These fibres, gathered in the infraorbital nerve (Fig. 1; Dorfl, 1985), a part of the sensory division of the trigeminus, enter the brainstem to end on four nuclei of termination whose main thalamic destination is the contralateral ventrobasal nucleus; from there, axons reach the “barrel cortex”, characterized by the fact that, in layer IV, it contains multicellular units, “barrels” (Fig. 2, left). Each barrel contains about 2000 neurons (Lee and Woolsey, 1975), separated by cell-poor strips, “septa”. Barrels themselves have a cell-rich “side” and a cell-poor “hollow”. They are distributed in a pattern that is homeomorphic with respect to that of the whisker follicles (Fig. 3): in stereotyped fashion both sheets, the skin and the parietal cortex, display five rows of elements, each row with its own, defined number of such elements while, caudally, both patterns terminate with four “straddling” units. The uniqueness of the system, and its usefulness for the topic we shall discuss, lies in the fact that barrels, together forming a map of the whiskerpad, are visible (Woolsey and Van der Loos, 1970). The one-to-one relationship between vibrissal follicles and barrels has been borne out by electrophysiological (Welker, 1976; Simons, 1978; Nussbaumer and Van der Loos, 1985; Armstrong-James and Fox, 1987) and deoxyglucose (Melzer et al., 1985) studies. |
