Synaptic efficacy and reliability of excitatory connections between the principal neurons of the input (layer 4) and output layer (layer 5) of the neocortex

A prerequisite for the understanding of how a cortical column functions is a description of small and defined neuronal circuits consisting of only a f ew identified neurones. Here we summarise, with particular reference to the barrel cortex, the morphological and physiological properties of two synap tic connections, namely those between pairs of spiny neurones in layer 4 an d pairs of pyramidal cells in layer 5. While layer 4 spiny neurones are the cortical input neurones that amplify and relay incoming excitation from th e periphery, layer 5 pyramidal cells integrate neuronal activity Loth withi n and across cortical columns and subsequently distribute it to both cortic al and subcortical brain regions.