Tracking neural activity in behaving hydra - C. Dupre and R. Yuste

Full caption for Movie S2:
Tracking neurons during behavior, Related to Figure 1
One hydra is placed between two coverslips separated by a 100µm spacer and imaged with a fluorescence dissecting microscope with an exposure time of 100ms. The position of the 620 neurons was tracked manually over 20 seconds. Each circle marks one neuron, and the color corresponds to the functional group. Green = RP1, Red = RP2, Blue = CB, light blue = CB2, yellow = others, including subtentacle network, white = nematocytes. 1 second in the movie corresponds to 5 seconds in real time. Scale bar = 100 µm.

All the neurons that are firing at any given frame are bright and the other neurons are completely dark. In the movie, the first thing we notice is that there are large groups of neurons that are coactive, which means that they fire during the same frame. What is interesting is that these groups are stable: all the neurons that are coactive in one frame will be coactive anywhere else in the movie.

The contraction burst behavior is a sequence of contractile pulses that reduce the animal to a tight, stubby ball. There is a specific group of coactive neurons that fire during each of these pulses, and for this reason we decided to call this group the contraction bursts neurons. They could be motor neurons that cause contraction of all the muscle fibers that they innervate, but they could also be sensory neurons that respond to the contractions. In any case, we can associate the activity of this group of coactive neurons with the behavior of contraction bursts.

Next, there are two groups of coactive neurons that fire in the absence of any obvious behavior. What allows us to distinguish between the two is the fact that only one of them has neurons in the tentacles, and not the other one. We decided to call them rhythmic potentials 1 and 2 because we believe that they correspond to the rhythmic potentials that had been discovered 40 years ago with extracellular recordings.