Motor neurons are the cells the brain uses to command muscles to act. Scientists typically thought of them as simple connections, much like the cables that link computers with their accessories. Now, in fly studies, researchers at Columbia's Zuckerman Institute have discovered that single motor neurons can each direct an insect’s body to move in far more complex ways than previously thought. Their findings were published in Nature.
"This is one of the first times scientists have analyzed in 3D what single motor neurons do while the body moves naturally," said Stephen Huston, PhD, an associate research scientist at Columbia’s Zuckerman Institute, and the study’s corresponding author. "You can't understand how the brain makes the body move without knowing what each motor neuron does, just as you can't understand how a puppeteer makes a marionette move without understanding what the puppet strings do."
Single Motor Neurons
Motor neurons are the final links through which the brain controls the body’s motions, from a flick of a finger to a blink of an eye. Despite this pivotal duty, researchers are only now beginning to uncover the role that single motor neurons play in movement. Measuring the activity of individual neurons in moving animals has proven to be experimentally difficult. Now advances in laboratory techniques have made it possible for researchers to manipulate single motor neurons in fruit flies as the insects move freely.
Utilizing advanced laboratory techniques and artificial intelligence, the team was able to stimulate single motor neurons and observe the resultant movements. Researchers discovered that activating each motor neuron could make the head rotate in a variety of ways, some even in opposite directions from each other, depending on the starting posture of a fly’s head.
Unexpected Complexity
"Most neurons act in concert as a population, so we didn't expect to see much or even any head movement at all when we activated just one motor neuron at a time," Dr. Huston said. At most, the scientists had expected that each single motor neuron was hardwired to produce one simple motion — for instance, making the head turn left 10 degrees. Instead, through computational analysis later performed at the Zuckerman Institute, the researchers discovered that activating each motor neuron could make the head rotate in a variety of ways, some even in opposite directions from each other, depending on the starting posture of a fly's head.
"I was really excited about how specifically we could activate individual neurons to drive these motions," said Benjamin Gorko, a PhD student in the molecular, cellular, and developmental biology department at the University of California, Santa Barbara, and the study's first author. Much like a digital thermostat
The thermostat-like model suggests that when the brain wants to move the body a specific way, it cannot simply stimulate the same set of motor neurons each time and expect the same result. Instead, the brain must calculate which motor neurons to activate based on sensory data it receives about the body’s current posture. The researchers hope that understanding what a fruit fly's brain does on the cellular level can provide a better understanding of diseases that affect the motor system. Next the researchers want to investigate how other kinds of neurons in the fly, such as those in the visual system, interact with motor neurons to control movement.
StepUp Note
All of our purposeful movements require us to tell ourselves what to do and when to do it. This research shows how fruit flies learn to become better decision-makers through practice. It shows how a single neuron can participate in a variety of different movement decisions. In the classroom, StepUp exercises give students daily practice in independent decision-making: telling themselves what to do, and when to do it.
Note by Nancy W Rowe, MS, CCC/A
Reported from Columbia Zuckerman Institute