Using computer models and experiments, researchers at the University of California, Davis, have identified the neurons and connections that are necessary for crayfish to swim.

"We can now pin down the essential components of the circuit," said Brian Mulloney, a professor of neurobiology, physiology and behavior at UC Davis.

The nervous system controlling locomotion is highly tuned and very stable across different groups of animals, Mulloney said. That makes crayfish a good model for much more complex nervous systems such as the human spinal cord.

New advances in the field were discussed in a session chaired by Mulloney at the Society for Neuroscience meeting in November 2002.

Crayfish swim by beating pairs of paddles called swimmerets on each body segment. The swimmerets move in sequence, starting at the back of the animal and moving forward. The movements of each segment keep a precise difference in timing, while varying in speed and force.

To keep those movements in the right sequence, the animal's nervous system has to integrate signals from each of these different segments as well as signals from the brain.

Mulloney's group, working with mathematicians Stephanie Jones at Harvard University and Frances Skinner at the University of Toronto, built mathematical models of the crayfish nervous system to see how they might work. They used those models to design experiments where they recorded impulses in crayfish nerves.

They showed that the swimmeret system is made up of eight modules of 70 neurons each. They found which neurons are necessary to complete the circuit, and what cells they connect to.

As the swimmerets beat, each module receives a stream of nerve impulses from the modules behind and in front of it. Signals from behind indicate a power stroke; those from the front represent a recovery stroke. Mulloney's team has found that those different messages converge on the same target neuron, which integrates them into a graded, non-spiking signal. This combined signal tells the module when to release neurotransmitters -- chemicals which change the timing and force of limb movement.

The same basic plan is likely found in insects and other animals, Mulloney said.