Figure 3. Activation of nuclear functions by action potentials.

Excitatory synaptic activity (arrowheads) generates local changes in membrane conductance, which causes the opening of voltage gated calcium channels (VGCCs). A rise in intracellular Ca ²⁺ triggers the local activation of Ca ²⁺-dependent signalling pathways thereby, coupling membrane depolarisation to intracellular signalling. a) When neurons receive weak excitatory inputs (small arrowheads), the signal spread is small (thin solid arrows) and the threshold to trigger action potentials is not reached, thus somatic L-VGCCs remain closed. b) When neurons receive strong synaptic inputs (big arrowheads), dendritic spikes can efficiently spread (thick solid arrows) in the forward direction and facilitate the initiation of action potentials at the axonal initial segment. Action potentials are backpropagated to the soma and dendrites (dashed arrows), locally generate intracellular Ca ²⁺-dependent signalling cascades. Ca ²⁺ entry in the soma through L-VGCCs will promote the activation of protein effectors that regulate the transcription of plasticity-related genes. In this diagram, the local signal intensity representing both the electrical activity of the membrane and its coupled intracellular signalling is coded by colour. Drawings are adapted from a reconstructed biocytin-filled layer V neuron in the rat cortex (courtesy of B. Chieng and J. Bertran-Gonzalez).