A central pattern generator (CPG for short) is a structure that causes rhythmic movement. In living creatures, this movement can be anything from the alternating side-to-side bending of the body in a swimming eel to the coordinated movements of an ant's six legs to the regular gait of a walking person. Although CPGs are usually discussed in terms of biological entities, they can also be instantiated in non-biological, moving beings (robots). The only real requirement is that they produce continuous rhythmic movement after an initial stimulus. There are numerous models of central pattern generators, but they tend to have a setup involving three separate nodes interacting with each other. In this writeup, I will be discussing one specific model, written about by D.W.Tank1. The model looks something like this:

___                       ___
/  A   \-------t-------/  B   \
\      \                    /      /
\      \              /      /
t      \          /      t
\      \___/      /
\    /   C   \    /
\  \___/  /

In this diagram, each of the hexagons, labeled A, B, and C, represents a node of the CPG -- in the case of a biological being, a neuron. The inner, non-labeled connecting lines represent excitatory connections in the directions of A to B, B to C, and C to A. The outer connecting lines, each labeled with a t, represent inhibitory connections that create time delays in the excitatory connections.

When the first component, A, is excited, it inhibits C and both inhibits B briefly and then excites B. This brief inhibition of B creates a time delay that is necessary in order for the signal to propagate from one component to the next. (Otherwise, the nodes would all be trying to excite and inhibit each other at the same time and the system would go haywire.) Once B is stimulated, it inhibits A, then goes on to act on C as A just acted on B: it inhibits C briefly, then excites it. The pattern continues, going from A to B to C to A to B to C, and so on, indefinitely until an outside connection completely inhibits one of the nodes. The continuous A-B-C pattern causes the entire CPG to send bursts of current out at regular intervals.

In the case of a biological being, the CPG sends these bursts of current down a path to a motor neuron, which causes the appropriate muscle to contract. However, a single muscle can only move in one direction; it cannot expand again without the help of an opposing muscle that contracts and pulls the first muscle back. So, in order to create rhythmic movement, there must be more than one muscle and, consequently, more than one CPG.

In an eel, for example, both the right side and the left side are lined with muscles that are all attached to CPGs. As noted previously, each CPG is attached to a controlling node that can excite or inhibit it. When a CPG is sending out its burst of current, in this case, its controlling node will simultaneously inhibit the controlling nodes of the adjacent CPGs and of the CPG directly across from it. In this way, a given muscle will never be trying to contract at the same time as the ones across from or next to it, and, if timed correctly, a smooth wave of motion will pass down the body of the eel. One exception, in which the adjacent muscles will contract at the same time as a given muscle, is the fast start response that many fish use to get away from predators; but that's a story for a different node.

1Tank, D.W. (1989). What details of neural circuits matter? Trends in Neurosciences, 1, pp. 67-79.

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