Rat somatosensory (SmI) cortex: II. Laminar and columnar organization of noxious and non-noxious inputs.

Abstract

The laminar and columnar organization of the rat SmI cortex was investigated under halothanenitrous oxide anaesthesia during microelectrode penetrations perpendicular to the cortical surface, using histological reconstruction of the electrode tracks. Neurons were studied in terms of receptive field properties (see companion paper) and laminar distribution. Neurons driven by non-noxious cutaneous stimulation and others by non-noxious deep stimulation, applied on contralateral receptive fields, could be recorded in the same penetration. Modality specific penetrations were observed in 48% of cases only. This finding suggests that there is little submodality segregation in the rat SmI cortex. Receptive fields successively observed in a given penetration were usually overlapping. “Modality-specific” columns were less often observed than “place-specific” columns during radial penetrations. Neurons driven by non-noxious stimulation as well as neurons driven by noxious stimulation could be recorded successively in the same penetration. Neurons driven by non-noxious cutaneous stimulation were found in layers II to V and were rare in layer VI. In contrast, nociceptive specific neurons were found almost exclusively in layers Vb and VI and convergent neurons in layer V. The size of the receptive fields was a function of the laminar position of the neurons: receptive fields were, on the average, small in layer IV, larger in layer II–III and V and even larger in layer VI. The somatotopic organization of the receptive fields of neurons driven by noxious stimulation was different from that of neurons driven by non-noxious stimulation. Such a difference was due in part to the larger size of the receptive fields of the neurons driven by noxious stimulation and also to their different topographical organization. These results provide new data concerning the functions of the rat SmI cortex. Namely they show that the cortical processing of somatic non-noxious and noxious inputs is based on laminar as well as columnar patterns of organization.