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 few identified neurones. Here we summarise, with particular reference to the barrel cortex, the morphological and physiological properties of two synaptic connections, namely those between pairs of spiny neurones in layer 4 and pairs of pyramidal cells in layer 5. While layer 4 spiny neurones are the cortical input neurones that amplify and relay incoming excitation from the periphery, layer 5 pyramidal cells integrate neuronal activity Loth within and across cortical columns and subsequently distribute it to both cortical and subcortical brain regions.