When humans and other mammals perform voluntary movements, a number of neural processes occur. The cerebral cortex, the outer region of the brain, sends signals to motor units (such as neurons) in the spinal cord, which in turn activate individual muscles.
Past neuroscience research has suggested that cerebral cortex does not control individual motor units, but sends “general instructions” to a pool of motor units. In addition, experimental data support the hypothesis that these motor units are recruited in a rigid rather than an adaptive fashion.
Researchers at Columbia University recently conducted a study to see if these two widely accepted ideas about the neural processes behind voluntary movements are reliable. Their findings, published in Neuroscience of natureshow that cortical control of motor units may be much more flexible than past research has suggested.
“Think of the muscles in your leg, like the quadriceps muscle, which straightens the leg and is used when you run, bike or ski,” Mark M. Churchland, one of the researchers who led the study, told MedicalXpress. “This muscle is excited by many (say 1000) motor neurons that live in your spinal cord. It is a long-held belief in our field that these 1,000 neurons acting together, in concert, is very inflexible.”
If motor neurons acted in tandem and rigidly, the same number of neurons would always be required to be activated to produce a certain force. This number of neurons will remain the same regardless of the specific activity the person or animal is engaged in.
“So, let’s say a certain force requires the activation of 500 neurons, the same 500 neurons will always be used to generate that force, whether you’re running, cycling, jumping, doing a yoga chair pose, etc. “, — Churchland. explained.
“Of course it doesn’t have to be that way, as you can use one set of the total when you hold the chair pose and another set of 500 when you jump, but the idea is that it would make life more difficult. it was thought that it would be better, easier and clearer if you always used the same 500.’
While this long-held belief in the neural activity involved in voluntary movements seems simple and straightforward, it does not take into account the differences between individual motor neurons. In fact, neurons are now known to have specialized functions, meaning they are better at some things than others.
“Some neurons excite muscle fibers that contract slowly and tend to be resistant to fatigue,” Churchland said. “Other neurons fire differently muscle fibers which contract quickly but also tire quickly. If the brain was functioning “optimally” it would take care of this. In other words, it is better to use the first type of neurons during yoga, and the second – when jumping.
While some neuroscientists have suggested that the cerebral cortex activates different neurons depending on the type of physical activity we engage in, this has until now been widely seen as a controversial hypothesis. The general consensus remained that the neural processes involved in human and mammalian movement are not so flexible, favoring simplicity over efficiency.
To test whether this was indeed true, Churchland and colleagues developed a new isometric task (ie, a task that involves tensing and contracting specific muscles or muscles groups). This task allowed them to reliably record motor unit activity in a rhesus macaque as it rapidly changed its movement patterns and behaviors.
“We monitored motoneuron activity by recording from muscles (where motoneurons send their projections),” Churchland said. “We did this in a much greater variety of movements than had been studied before, and we observed more neurons simultaneously (all at the same time, during the same set of movements) than had been achieved before.”
The results collected by the researchers are very interesting because they refute the established opinion that engine neurons are activated together in a tight fashion. In the future, understanding this research may lead to new and important discoveries about how the brain and neurons in the spinal cord really support volunteer movements.
“We discovered that our brain, body and muscles are capable of much more flexibility than we previously thought,” added Churchland. “They can optimize traffic for different situations much more effectively than people thought. This means that in some ways our brains have a more complex job than we thought. Instead of just deciding which force to use, the brain must also figure out which neurons should generate that force.”
Naja J. Marshall et al. Flexible neural control of motor units, Neuroscience of nature (2022). DOI: 10.1038/s41593-022-01165-8
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