Researchers at MIT have produced one of the most complete pictures yet of how a nervous system creates purposeful behavior. They studied the tiny worm C. elegans, which has only 302 neurons in its entire brain, while it moved toward pleasant smells or away from unpleasant ones.
The worms were placed in dishes with odor spots. Using special microscopes, the researchers recorded the activity of more than 100 neurons as the worms navigated. The results showed that the worms do not move randomly. Instead, they perform well-timed turns at sensible angles, suggesting they actively guide themselves along odor gradients.
The study identified a clear sequence of activity involving about ten key neurons. Some neurons detect the odor, others plan the turn, some switch the worm into reverse movement, and others carry out the actual turn before the worm moves forward again. One neuron, called SAA, plays an especially important role by linking odor detection to movement planning and correctly predicting the direction of the turn.
Tyramine organizes the full sequence of brain activity
The entire process is coordinated by a chemical called tyramine, which acts as a neuromodulator. Neuromodulators are substances that change how neurons communicate with each other. In this case, tyramine released from one specific neuron helps other neurons change their activity at the right moment so the worm can complete its turn smoothly.
When the researchers blocked tyramine, the worms’ navigation became much less organized and the orderly sequence of brain activity largely disappeared. This shows that tyramine is essential for switching the brain from one stage of behavior to the next.
The findings provide a rare full view of a complete sensorimotor arc — the entire pathway from sensing something in the environment to producing an appropriate action — inside a living nervous system. The work helps scientists understand the basic principles of how brains turn sensations into purposeful movements, which may offer insights into more complex nervous systems, including our own.
This study is published in Nature Neuroscience.