Cortical organization in shrews: Evidence from five species

Cortical organization was examined in five shrew species. In three species, Blarina brevicauda, Cryptotis parva, and Sorex palustris, microelectrode r ecordings were made in cortex to determine the organization of sensory area s. Cortical recordings were then related to flattened sections of cortex pr ocessed for cytochrome oxidase or myelin to reveal architectural borders. A n additional two species (Sorex: cinereus and Sorex longirostris) with visi ble cortical subdivisions based on histology alone were analyzed without el ectrophysiological mapping. A single basic plan of cortical organization wa s found in shrews, consisting of a few clearly defined sensory areas locate d caudally in cortex. Two somatosensory areas contained complete representa tions of the contralateral body, corresponding to primary somatosensory cor tex (S1) and secondary somatosensory cortex (S2). A small primary visual co rtex (V1) was located closely adjacent to S1, whereas auditory cortex (A1) was located in extreme caudolateral cortex, partially encircled by S2. Area s did not overlap and had sharp, histochemically apparent and electrophysio logically defined borders. The adjacency of these areas suggests a complete absence of intervening higher level or association areas. Based on a previ ous study of corticospinal connections, a presumptive primary motor cortex (M1) was identified directly rostral to S1. Apparently, in shrews, the solu tion to having extremely little neocortex is to have only a few small corti cal subdivisions. However, the small areas remain discrete, well organized, and functional. This cortical organization in shrews is likely a derived c ondition, because a wide range of extant mammals have a greater number of c ortical subdivisions. (C) 1999 Wiley-Liss, Inc.