1. The behavior of the mathematical model for the lateral pyloric (LP) neuron of the crustacean stomatogastric ganglion (STG) developed in the previous paper was further studied. 2. The action of proctolin, a neuromodulatory peptide that acts directly on the LP neuron, was modeled. The effect of the proctolin-activated current (i(proc)) on the model neuron mimics the effects of proctolin on the isolated biological LP neuron. The depolarization and increased frequency of firing seen when i(proc) is activated are associated with changes in the relative contributions of the delayed rectifier (i(d)) and the Ca2+-activated outward current (i(o(Ca))) to the repolarization phase of the action potential. 3. The effects of turning off the A-current (i(A)) in the model were compared with those obtained by pharmacologically blocking i(A) in the biological neuron. i(A) appears to regulate action-potential frequency as well as postinhibitory rebound activity. 4. The role of i(A) on the rhythmic activity of the cell was studied by modifying several of its parameters while periodically activating a simulated synaptically activated conductance, i(syn). 5. The effects of manipulations of the maximal conductances (ḡ) for i(d) and i(o(Ca)) were studied. i(d) strongly influences action-potential frequency, whereas i(o(Ca)) strongly influences action-potential duration. 6. Modifications of the maximal conductance of the inward Ca2+ current (i(Ca)) were compared with the effects of blocking i(Ca) in the real cell. 7. The role of the hyperpolarization-activated inward current (i(h)) during ongoing rhythmic activity was assessed by periodically activating i(syn) while modifying i(h).
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