In a behaving animal, the brain processes information from sensory inputs, and communicates its intentions to muscles via the pattern of activity in descending projection neurons. In vertebrates, these cells transmit their motor command to the local networks of the spinal cord, which in turn initiate and coordinate muscle contraction. A basic question in the field of motor control is how different actions are represented by the pattern of projection neruon activity. Do all neurons participate in every behavior, or are individual actions controlled by dedicated subsets? The translucent brain of zebrafish allows us to look at the activity of all these neurons, and provides answers to these questions.
We first identified visual stimuli that drive particular swimming patterns in the fish. Zebrafish track moving patterns to maintain a stable position in moving water. They swim to catch up with stripes that move from their tail to their head, and turn left or right to orient themselves with stripes that move left or right. We then imaged calcium responses to investigate which descending neurons were activated by each stimulus. We find that small groups of neurons in the fish control different behaviors, revealing that the brain’s control system is composed of surprisingly organized pathways, connecting sensory stimuli to behavioral responses. This description of the functional organization of the zebrafish motor system provides a framework for identifying the complete circuit underlying a vertebrate behavior.
Read more in Nature Neuroscience