                               INTRODUCTION

This simulation is a recreation of the three neuron model of the tritonia CPG
described in Chapter 6 (by Peter Getting) of "Methods in Neuronal
Modeling", edited by Christof Koch and Idan Segev.  In order to fully
understand this simulation, you should be familiar with this chapter.

The central pattern generator responsible for escape swimming in the mollusc
tritonia consists of three classes of interneurons.  In the simulation,
these are represented by three neurons, a dorsal swim interneuron (DSI), a
ventral swim interneuron (VSI), and a cerebral cell (C2).  The model was
produced to test the hypothesis that the alternating bursts of firing
produced by the DSI and VSI arise from mutually inhibitory connections
beween the DSI and VSI, paralleled by a delayed excitatory connection from
the DVI to the VSI via the C2 cell.

The cells are all single-compartment models of a soma with synaptically
activated channels.  (The one exception to this is the vsi neuron, which
also has a Hodgkin-Huxley K channel.)  The default values of the various
channel parameters are those given in Appendix 6.B of the Getting chapter.
In this simulation, you will be able to easily modify their values.  Thus,
you will be able to perform computational "experiments", rather than simply
duplicating the results of the original simulation.

No action potentials are generated in the soma, but a spike generator is
used to fire a spike when the soma Vm exceeds a threshold.  As described in
Appendix 6.B, this produces a threshold which exponentially decays as a
function of the time since the last spike.  The spike generator then feeds
an axon (essentially a delay line) which is connected to various channels.
The axon connections to the synaptically activated channels all have been
given unit weights and no delays.

The connections are essentially as described in K&S, Ch. 6.
The connections are:

connections from dsi:
    to       dsi channel dsiNa1
    to       dsi channel dsiNa2
    to       dsi channel dsiK1 
    to       dsi channel dsiK2 
    to       c2 channel dsiNa1 
    to       c2 channel dsiNa2 
    to       vsi channel dsiK  
connections from c2:
    to        dsi channel c2Na 
    to        dsi channel c2K  
    to        c2 channel c2K1  
    to        c2 channel c2K2  
    to        vsi channel c2Na 
connections from vsi:
    to       dsi channel vsiK  
    to       c2 channel vsiK1  
    to       c2 channel vsiK2  
    to       vsi channel vsiNa 
    to       vsi channel vsiK1 
    to       vsi channel vsiK2 

In addition, there is a "ramp" input to dsi channel rampNa.  (Unlike all the
other cell inputs, this has a synaptic weight of 1.5)  In fact, the "ramp"
isn't really a ramp, but is a single pulse of unit amplitude which starts at
5000 msec and ends at 5900 msec.
