Most people recognise that jumpy feeling when they're startled, and fish are no different. Caught by surprise, they react in one of two ways, either curling round into a C shape to make a quick get away, or sweeping into an S with a tail flick to flee. The C-start is so predictable that it has become a standard of motor neural control, with a single neuron triggering the response. Everyone thought that the S response was just a variation on the C theme, with an extra flick of the tail. But when Melina Hale started measuring muscle activity while startling the fish into an S-start, she realised that the fish were using a different neural circuit to the C-start(p. 2005), making the S-start very intriguing from her `comparative' perspective.
When Hale began this study, the neural control of C- and S-starts seemed to be pretty well understood. Catch a fish off guard, and the Mauthner neuron in the brain fires a single action potential that forces all the muscles down one side of the fish to contract abruptly, bending the fish's body into a C shape. It seemed natural to assume that the same neural controls were driving the S-start, while the tail drifted passively to convert the C into an S.
But fish don't keep these rapid reactions just for making a fast get away;they also use them for ambushing prey. Hale wondered whether the neural controls behind the feeding response were the same as the escape response. Because it isn't easy to directly measure neural activity as the fish spring into action, she recorded muscle activity in the hope that it might tell her whether there were differences in the neural control of feeding versus fleeing.
Hale fitted electromyography electrodes to a fish that could happily twist into S and C shapes, ready to start collecting data. But as soon as the fish recovered from surgery, she noticed something was wrong. The fish wouldn't feed with the electrodes in place, so she couldn't test the difference between feeding and startle responses. But the fish still produced C and S starts when she started them, so she changed direction and decided to compare both startle responses directly. After collected EMG data for C and S startle responses Hale realised that they were completely different! Maybe there was more to the S start than she had thought.
Using a high-speed camera, she began filming the fish as she measured their muscle activity so that she could accurately correlate each muscular spasm with the fish's movements. Instead of seeing a single massive contraction down one side of the fish, she measured contractions on both sides. Hale knew that it was unlikely that the Mauthner neuron could trigger a response like this. Some other neural circuit was responsible for this behaviour. Hale remembers that this was `incredibly exciting'. She realised that her discovery allows her to do the ultimate comparative analysis as the S-start provides a simple system to compare to the model C-start neural circuit.
Ultimately, the goal is to identify the nerves involved in the S-start circuit. Meanwhile, Hale is optimistic that some well-known neurons could turn up in the circuit, but the Mauthner neuron probably won't.