If we didn't know which way was up, life would be very disorientating. While humans rely on a series of fluid filled canals and mineralised structures in the inner ear for balance, aquatic animals such as crayfish depend on organs at the base of each antennule called statocysts which respond to body tilting. Scientists already knew that an animal's behaviour affects how the nervous system responds to signals from the statocyst, but they had not measured this before in freely moving animals. Noriyuki Hama, Yoshikazu Tsuchida and Masakazu Takahata from Hokkaido University, Japan, took on the challenge, measuring the response of the nervous system to tilting in freely moving crayfish (p. 2199).
The team relied on an innovative and relatively new technique –optical telemetry – so that they could record from the crayfishes'nervous systems while they scurried around their tanks. They placed wire electrodes shaped like hooks around one of two identical large nerves which connect the crayfish's brain to the rest of the nervous system on each side of the body. Using this stable recording method, they could pick up activity in many of the individual neurons found in the nerve. Having fastened the electrodes securely into position they connected them to an infrared wireless transmitter which the crayfish carried on their backs. Four receivers placed at the corners of the tank picked up the signals and transferred them to a computer.
To investigate how the nervous system responds to tilting the team measured the activity of three neurons, which transmit feedback from the statocyst, as the crayfish sauntered across the floor of their tank, which was tilted to 10°. They found that not only did the activity of the neurons differ depending on which way the crayfishes' bodies tilted, but also on how they were behaving at the time. The first neuron, C1, was more active when the crayfish walked up and down the slope than then they were resting. Recording from the nerve on the left side of the body, they found that when the crayfish walked across the slope such that the left side of the body was lower than the right, C1 was more active. It was less active when the right side of the body was tilted lower. However, this response stopped when the abdomen was extended, but not when it was flexed under the body,showing that the part of the nervous system signalling the abdomen's position probably influences the activity of C1.
The second and third neurons, A and B, responded differently than C1. Again recording from the left nerve, A fired when the body was tilted down to the left when the animal was at rest but stopped responding to the tilt when the crayfish ambled along. Flexing or extending the abdomen didn't have any effect either. In contrast, B responded to tilting to the right, both at rest and during walking. The activity was greater when the abdomen was held extended, but not when it was flexed, opposite to C1's response.
So, perhaps not surprisingly, the nervous system responds to statocyst feedback in a varied and complex way in freely moving animals. Not only are neurons receiving input from the statocysts about body tilting, but also sensory input from the rest of the body and information about how the legs and abdomen are moving, which all work together to keep the crayfish upright.