Compartmental models of electrotonic structure and synaptic integration in an identified neurone.

A three-compartment model of the electrotonic structure of an identified motoneurone, the median gastric (m.g.) neurone of the stomatogastric ganglion of the spiny lobster (Panulirus interruptus) was constructed, based on the passive response of the cell to a step of injected current. While its structure is only remotely related to that of the cell, the model is able to predict the passive response of the cell to any wave form of injected current. The shape of the m.g. neurone provided the basis for the development of a multicompartment model of the cell from the simple compartment model. Unlike the three-compartment model, the multicompartment model has a structure that corresponds closely to that of the cell while it retains the ability to predict the passive response of the cell to any wave form of injected current. The multicompartment model was used to analyse the electrotonic structure and synaptic integration of the cell. The axon acts as a current sink, causing steady-state voltage attenuation between the tips of different dendrites and the integrating segment to range between 26 and 89%. Steady-state voltage attenuation in the distal direction is 2% or less. Synaptic inhibition of m.g. by Interneurone 1 was simulated with simultaneously activated conductance-increase synapses located on all dendritic end-compartments of the model. Inhibitory post-synaptic potential (i.p.s.p.) wave forms recorded in the cell soma were duplicated in the soma compartment when the synaptic conductance change in each of the twenty-eight end-compartments was set equal to 5 nS for 8 ms. I.p.s.p. wave forms in dendritic end-compartments were 30% larger than the soma compartment i.p.s.p., while i.p.s.p.s in the integrating segment compartment were intermediate in size. Charge from a 92 mV, 1 ms action potential in the model axon was passively conducted from axonal compartments to the soma compartment of the model, where it reproduced the attenuated, broadened voltage wave forms of action potentials recorded in the cell soma. Passive spread of charge from an axonal action potentials to terminal dendritic compartments evoked potentials there that were 30% larger and faster than the corresponding soma compartment potential.