Chapter 5 Synaptic Integration by Electro - Diffusion in Dendritic Spines TERRENCE

Many vertebrate and invertebrate neurons receive synaptic inputs on spines (Coss and Perkel, 1985). In hippocampal pyramidal cells, almost all synapses found on spines are excitatory (Harris and Stevens, 1989), but in the cat primary visual cortex, 7% of synapses on spines are inhibitory (Beaulieu and Colonnier, 1985), which comprise almost one-third of the total number of inhibitory synapses on a pyramidal cell. It has been suggested that shunting inhibition on a spine could perform a selective AND-NOT-like operation (Koch and Poggio, 1983b). The nonlinear effects would be localized to the spine, making it an effective computational module. In this chapter, we compare the predictions made by the cable model with those made by the electro-diffusion model for this problem. The conduction of action potentials in axons can be accurately modeled by the Hodgkin-Huxley equation, which is based on the cable equation (Hodgkin and Huxley, 1952). The integration of postsynaptic signals in dendrites has also been studied with analytic solutions to passive cables (Rall, 1977), and recently, several investigators have used the cable model to examine the possibility of more complex signal processing in dendrites with complex morphologies, multiple synaptic inputs, and passive or excitable membranes (Shepherd et al., 1985;