The composite neuron: a realistic one-compartment Purkinje cell model suitable for large-scale neuronal network simulations.

We present a simple method for the realistic description of neurons that is well suited to the development of large-scale neuronal network models where the interactions within and between neural circuits are the object of study rather than the details of dendritic signal propagation in individual cells. Referred to as the composite approach, it combines in a one-compartment model elements of both the leaky integrator cell and the conductance-based formalism of Hodgkin and Huxley (1952). Composite models treat the cell membrane as an equivalent circuit that contains ligand-gated synaptic, voltage-gated, and voltage- and concentration-dependent conductances. The time dependences of these various conductances are assumed to correlate with their spatial locations in the real cell. Thus, when viewed from the soma, ligand-gated synaptic and other dendritically located conductances can be modeled as either single alpha or double exponential functions of time, whereas, with the exception of discharge-related conductances, somatic and proximal dendritic conductances can be well approximated by simple current-voltage relationships. As an example of the composite approach to neuronal modeling we describe a composite model of a cerebellar Purkinje neuron.

Spontaneous calcium transients in automatic boutons and varicosities.

Spontaneous multiquantal events are recorded at many different boutons and varicosities for which there is evidence that the receptor patch at these individual synapses is saturated by the transmitter unit. In order to reconcile these observations, a model is considered in which calcium release from a ryanodine channel within a nerve terminal can reach adjacent active zones in single synapses in sufficient concentration to occasionally trigger exocytosis from adjacent zones synchronously, giving rise to multiquantal spontaneous events. It is shown that the spatial and temporal distribution of calcium concentration at the active zone after a spontaneous opening of a ryanodine channel can predict the amplitude and time course of observed calcium-activated potassium channel currents. Similar calcium transients are sufficient to give rise to multiquantal events. Such events suggest a multi hypothesis for secretion.