Monosynaptic Connections between Pairs of L5A Pyramidal Neurons in Columns of Juvenile Rat Somatosensory Cortex

Layer 5 (L5) of somatosensory cortex is a major gateway for projections to intra− and subcortical brain regions. This layer is further divided into 5A and 5B characterized by relatively separate afferent and efferent connections. Little is known about the organization of connections within L5A of neocortical columns. We therefore used paired recordings to probe the anatomy and physiology of monosynaptic connections between L5A pyramidal neurons within the barrel columns of somatosensory cortex in acute slices of ˜3−week−old rats. Post hoc reconstruction and calculation of the axodendritic overlap of pre− and postsynaptic neurons, together with identification of putative synaptic contacts (3.5 per connection), indicated a preferred innervation domain in the proximal dendritic region. Synaptic transmission was reliable (failure rate