Do animals give much thought to voluntary behavior? Before you or I reach for a cup of coffee, we make a conscious—even if barely so—decision to do it. But what about tiny-brained crustaceans? How does a crayfish decide to be on its way?
Researchers in Japan are trying to crack this neural code, known in the neurophysiology field as "readiness." Just before an animal starts to move, it fires off a so-called readiness discharge, a spike in brain activity that drops off once the legs get to walking.
But scientists haven't been able to get a good look at the specific neural mechanisms behind the action. A new study, published online April 14 in Science, has drilled down to the cellular level to see just what is happening in this split-second process—in the crayfish.
Katsushi Kagaya and Masakazu Takahata, both of the Department of Biological Sciences at Hokkaido University, placed a crayfish (Procambarus clarkii) on top of a spherical treadmill that only moved to accommodate the animal's walking, stopping when it stopped. In the meantime, sensors recorded the crustacean's neuronal activity.
Kagaya and Takahata found that just 45 individual neurons spiked right before the crayfish started walking on its own—whether it was walking forward or backward. Some of the readiness discharge neurons were those that were also active while the crayfish was walking and others that were involved in the stopping process, suggesting that "the main synaptic activation for voluntarily initiated walking of crayfish takes place in the medial protocerebrum," the area of the arthropod brain that also receives visual input and information from other organs. This means that the neural signals the animal employs to get going are "organized and activated by presynaptic brain neurons, not by endogenous mechanisms," they explained in their study.
The findings from these plodding crustaceans' cerebrums might seem circumlocutory, but, the researchers contend, they provide a new model for how these key early actions are organized in the brains of many animals.