Study Challenges Established Belief on Neural Control of Motor Units

There is a longstanding belief that the cerebral cortex controls motor units in the spinal cord collectively in a somewhat rigid fashion for voluntary movements to take place. However, a recent study shows that this may not always be the case.

A Neuron

The cortex, which is located in the brain’s outer region, communicates to the spinal cord to enable humans and other mammals to perform voluntary movements, including running and jumping. It was thought that motor units acted rigidly to enable these movements or activities. Scientists believed the same set of neurons is used for all.

In this research, however, scientists found that the flexibility of motor units in the spinal cord might have grossly been misjudged in previous studies. Their results were published in the journal Nature Neuroscience.

Neural control of motor units

Neuroscientists found in previous studies that the cerebral cortex did not seem to control motor neurons individually. Rather, it issues “common instructions” to a set of motor units for an activity to take place.

Also, evidence from experiments suggests that the motor units act in a rigid way.

“Consider a muscle in your leg, such as a quadricep, which straightens the leg and is used when running, cycling or skiing,” Mark M. Churchland, a researcher involved in the study, explained. “That muscle is excited by many (let’s say 1,000) motor neurons that live in your spinal cord. A longstanding belief in our field is that those 1,000 neurons act together, in concert, in a very inflexible way.”

What this means is that the same number of neurons would be required to generate a particular force no matter the activity. If a specific force requires that 500 neurons be activated, for instance, the same 500 would always be used regardless of the activity, explained Churchland.

It was thought that using the same number of neurons would make things easier than if different numbers were used.

However, researchers in this study wanted to probe the theory that the same set of neurons acts together in a rigid way for voluntary movements. This is because this premise failed to account for individual motor neuron differences.

Also, studies have shown that motor neurons have specialized functions. In other words, they excel in a certain activity compared to others.

“Some neurons excite muscle fibers that contract slowly and tend to be fatigue resistant,” said Churchland. “Other neurons excite different muscle fibers that contract quickly but also fatigue quickly. If the brain was working in an ‘optimal’ way, it would care about this. In other words, better to use the first kind of neurons during yoga and the second kind when jumping.”

Challenging a longstanding belief

This was not the first time that the possibility of different sets of neurons being used has been suggested. A number of neuroscientists had thought about this but the idea has mostly been discounted.

The common belief remains that motor units act in tandem in a largely rigid way.

Churchland and his colleagues wanted to explore the possibility that different neurons are activated based on the specific activity. They did this with the aid of an isometric task that they developed. The task involved the tightening or contraction of certain muscles or muscle groups.

The research team used this to record the motor unit activity of a rhesus macaque while it swiftly altered its movements and behaviors. It managed to observe more neurons at once than had ever been done while the same movements were taking place.

The scientists in the study found that the human brain, body, and muscles are a lot more flexible than previously thought. According to them, our brains not only determine the force to use for an activity but also what neurons would do this better. Movements can efficiently be optimized to suit situations.

Having disproved a seemingly deep-rooted belief, this work could lead to great things in neuroscience research. The findings may help in getting a better understanding (through future work) of the role of the brain and the spinal cord’s motor neurons in voluntary movements.