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Received October 6, 2004
Revised November 5, 2004
Accepted after revision November 9, 2004
1 The School of Pharmacy
* To whom correspondence should be addressed. E-mail: joanne.watts{at}ulsop.ac.uk.
The cerebral cortex is pivotal in information processing and higher brain function and its laminar structure of 6 distinct layers, each in receipt of a different constellation of inputs, makes it important to identify connectivity patterns and distinctions between excitatory and inhibitory pathways. The "feedforward" projections from layer 4 to 3 and from 3 to 5 target pyramidal cells and to lesser degrees interneurones. "Feedback" projections from layer 5 to 3 and from 3 to 4, on the other hand, mainly target interneurones. Understanding the microcircuitry may give some insight into the computation and information processing performed in this brain region. A major afferent input to primary sensory regions of the cortex is from the thalamus. Ascending thalamocortical input arrives in layer 4 (and layer 6) whence it is relayed to layer 3 and from layer 3 onto layer 5 and from 5 to 6 (Gilbert, 1983; Gilbert & Wiesel, 1985; Bode-Greuel et al, 1987). The afferent input is excitatory but rapidly induces disynaptic inhibition which limits the excitation in a spatial and temporal fashion (Porter et al, 2001). The majority of the input to the neocortex, however, comes from the cortex. This corticocortical communication consists of both "feedforward" and "feedback" projections and much information has been gleaned from studies of the visual cortex in particular. In cat longer-distance, inter-areal "feedforward" projections from primary sensory regions to the posteromedial lateral suprasylvian cortical area target layers 3 and 4 (Lowenstein & Somogyi, 1991), forming an input analogous to the primary sensory input to layer 4 in V1, while "feedback" projections target layers 1, 3 and 6 (Rockland & Drash, 1996). This simplified view has generated much interest in elucidating which excitatory and inhibitory cells target each other and whether there is any inter-laminar selectivity. Dual and triple intracellular recordings with biocytin labelling were used to explore small circuits of synaptically connected neurones in slices of adult rat and cat neocortex.
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