The multiplicative neuron. Sigmoidal functions indicate nonlinear computations performed by various parts of the cell: gain modulation in spine heads, thresholded computations in small dendritic branchlets, and supralinearities at the main apical trunk and the cell body. The response of the cell is tuned by the overall effect of the network.

Nonlinear dendritic computations. (A) Summation of paired, single-pulse inputs in the apical dendrites of a CA1 pyramidal model cell. Simulations predict a sigmoidal modulation of combined excitatory postsynaptic potentials (EPSPs) within a branch (red symbols) and a linear summation of EPSPs between branches (green symbols). Red curves correspond to within-branch data for dendrites at 92 μm (short dashes), 232 μm (solid) and 301 μm (long dashes) from the soma. Due to differences in local compared with somatic responses, axis values for the red curves are scaled up by a factor of 10. (B) Experimental verification of predicted summation rules in basal dendrites of a layer V pyramidal neuron. Single-pulse stimulation of synapses in different branches results in linear summation at the cell body (green squares). When γ-aminobutyric acid (GABA)ergic inhibition is blocked, the summation of within-branch EPSPs is modulated by a sigmoidal nonlinearity (all other symbols). Reproduced with permission from Polsky et al (2004). (C) Schematic representation of a pyramidal neuron as a two-layer neural network. Radial oblique dendrites provide the first layer of the network, each performing individually thresholded computations as shown in (A) and (B). The outputs of this layer feed into the cell body, which constitutes the second layer of the network model. Adapted from Poirazi et al (2003b).

Top-down gain modulation in a CA1 pyramidal neuron model. In a CA1 pyramidal neuron, afferents from the entorhinal cortex (EC) synapse onto distal dendrites, whereas axons from CA3 neurons project to proximal dendrites. (A) Synaptic activation (1 Hz) of proximal dendrites induces a regular firing response at the cell body of the model cell. (B) Strong (theta-burst) stimulation of EC inputs coinciding with CA3 stimulation evokes calcium spikes in the distal dendrites, the occurrence of which initiates bursts of action potentials (APs), and facilitates the somatic response. (C) Temporally distant activation of EC with subthreshold bursts (400 ms after the CA3 input) prevents the initiation of somatic action potentials for several seconds, thus weakening the somatic response. Green dots, excitatory inputs; red dots, inhibitory inputs.

Eleftheria Kyriaki Pissadaki, Panayiota Poirazi (who is an EMBO Young Investigator) & Kyriaki Sidiropoulou