Layer 2/3 (L2/3) and L5 pyramidal neurones receive ascending sensory input onto the basal and proximal regions of their dendritic arbours. The distal regions of the apical dendrites receive input from other sources, including projections from accessory sensory thalamic nuclei and from higher-order sensory regions. A cellular mechanism for associating these superficial-and deeper-layer inputs has been described in L5 neurones. This results from two cellular properties: active backpropagation of a somatically initiated action potential (AP) and a second AP initiation zone in the apical tuft. Here we consider whether L2/3 neurones have comparable dendritic properties. We studied L2/3 neurones in parasagittal slices (animals anaesthetised with halothane and decapitated) and in intact animals under urethane anaesthesia (2 g kg^-1; killed by cervical dislocation). We used whole-cell recording and calcium imaging to study apical dendrites of L2/3 pyramidal neurones in primary somatosensory cortex in acute slices (widefield imaging) and urethane-anaesthetized rats (intraperitoneal injection; imaging using 2-photon microscopy).

Both in vitro and in vivo, action potential (AP) amplitude declined gradually from the soma (to half the somatic amplitude at approximately 250 µm from the soma for deep layer 2/3 neurones in vitro). Calcium transients induced by single somatic APs were visible at (and often distal to) the principal bifurcation in all neurones, both in vitro and in vivo, but rarely in far distal L1 branches. Active AP backpropagation was confirmed in vitro by blocking sodium channels with TTX, which decreased both dendritic depolarization and the resulting calcium transient following injection of an AP-like waveform at the soma (under two-electrode voltage clamp).

L2/3 neurones also have a distal initiation zone since a dendritically initiated regenerative potential could be induced in the majority of neurones by dendritic current injection in vitro. A substantial dendritic calcium transient accompanied this dendritically initiated event. Pairing of a somatic action potential with subthreshold dendritic depolarization could also trigger a distal event. To date we have been unable to perform an equivalent experiment in vivo. However, a distal dendritic calcium transient of comparable amplitude to that observed during a dendritic event could be evoked by brief, high-frequency bursts of action potentials induced by somatic current injection. Burst-induced calcium transients were observed both in vitro and in vivo, indicating that dendritically initiated events may also occur in vivo in L2/3 pyramidal neurones.

These active dendritic properties provide a mechanism which enables L2/3 pyramidal neurones to act as cellular coincidence detectors that associate superficial- and deeper-layer inputs to the neocortex.