Introduction

Modeling the brain requires some a priori description of its functional computing elements. The sophistication of this description depends on the goal of the model, but it is clear that describing single neurons as simple ‘leaky-integrators’ with a non-linear threshold has limited utility in exploring the capability of the neural net for processing information. In fact, cortical pyramidal neurons are functionally quite complicated, both in terms of how their complex geometry determines their electrotonic structure, and in terms of the active currents that modulate the linear response of these cells.

Over the past several years investigators have uncovered a plethora of currents in one type of cortical pyramidal neuron, the hippocampal pyramidal cell. The currents, which presumably are mediated by distinct ion channels, include the classical fast sodium current (INa), a persistent sodium current (INaP) (French & Gage, 1985), a delayed-rectifier potassium current (IDR) (Segal & Barker, 1984), a calcium current (ICa) (Halliwell, 1983), a slow calcium current (ICaS) (Johnston et al., 1980), a fast transient calcium-mediated potassium current (Ic) (Brown & Griffith, 1983), an after-hyperpolarization calcium-mediated potassium current (IAHP) (Lancaster & Adams, 1986), a muscarine-inhibited potassium current (IM) (Halliwell & Adams, 1982), a transient potassium current (IA) (Gustafsson et al., 1982), a chloride current (ICI(V)) (Madison et al., 1986), and a possibly mixed carrier anomalous rectifier current (IQ) (Halliwell & Adams, 1982). Each current has a unique time-and voltage-dependence, and some currents are sensitive to constituents in the extra-or intra-cellular environment, for example free Ca++ or muscarinic agonists.