ORGANIZATION OF SOMATOSENSORY CORTEX IN MONOTREMES - IN SEARCH OF THEPROTOTYPICAL PLAN

The present investigation was designed to determine the number and int ernal organization of somatosensory fields in monotremes. Microelectro de mapping methods were used in conjunction with cytochrome oxidase an d myelin staining to reveal subdivisions and topography of somatosenso ry cortex in the platypus and the short-billed echidna. The neocortice s of both monotremes were found to contain Four representations of the body surface. A large area that contained neurons predominantly respo nsive to cutaneous stimulation of the contralateral body surface was i dentified as the primary somatosensory area (SI). Although the overall organization of SI was similar in both mammals, the platypus had a re latively larger representation of the bill. Furthermore, some of the n eurons in the bill representation of SI were also responsive to low am plitude electrical stimulation. These neurons were spatially segregate d from neurons responsive to pure mechanosensory stimulation. Another somatosensory field (R) was identified immediately rostral to SI. The topographic organization of R was similar to that found in SI; however , neurons in R responded most often to light pressure and taps to peri pheral body parts. Neurons in cortex rostral to R were responsive to m anipulation of joints and hard taps to the body. We termed this field the manipulation field (M). The mediolateral sequence of representatio n in M was similar to that of both SI and R, but was topographically l ess precise. Another somatosensory field, caudal to SI, was adjacent t o SI laterally at the representation of the face, but medially was sep arated from SI by auditory cortex. its position relative to SI and aud itory cortex, and its topographic organization led us to hypothesize t hat this caudal field may be homologous to the parietal ventral area ( PV) as described in other mammals. The evidence for the existence of f our separate representations in somatosensory cortex in the two specie s of monotremes indicates that cortical organization is more complex i n these mammals than was previously thought. Because the two monotreme families have been separate for at least 55 million years (Richardson , B.J. [1987] Aust. Mammal. 11:71-73), the present results suggest eit her that the original differentiation of fields occurred very early in mammalian evolution or that the potential for differentiation of soma tosensory cortex into multiple fields is highly constrained in evoluti on, so that both species arrived at the same solution independently. ( C) 1995 Wiley-Liss, Inc.