New insights into the brain’s motor cortex

Researchers found that the brain’s primary motor cortex is structured differently than thought before. Kateryna Kon / Shutterstock

The brain’s primary motor cortex is a thin band of nerve cells and circuits that extends from the top of the head and downward on both sides. It’s long been thought to have a straightforward role as the brain’s command center for voluntary muscle movements, sending out signals that trigger movements in specific body parts. Complex movements were believed to be coordinated by other areas of the brain.

A century ago, a systematic study involved electrically stimulating different human brain areas during surgery to see the resulting body movements. The study revealed a well-organized “body map.” This research led to a familiar drawing of the cortical band that’s taught in biology classrooms to this day. The drawing shows the motor cortex as an orderly continuum from head to toe. One end primarily controls face-related movements, followed by regions that control the hands and then the feet.

In recent decades, studies have questioned this linear body map in the motor cortex. For instance, studies in nonhuman primates suggested that the map may be divided into concentric zones instead of a continuous row. So digits like fingers might be in the middle, surrounded by areas for the wrists, elbows, then shoulders. Other studies found that large portions of the primary motor cortex seemed to prompt no muscle movements at all, but revealed connections to brain areas controlling other functions.

To learn more, a team led by Drs. Evan Gordon and Nico Dosenbach at Washington University in St. Louis used an advanced imaging method called precision fMRI to reexamine the function and organization of the human motor cortex. The technique involved repeatedly scanning each person’s brain over many hours to create high-resolution brain maps for each individual. Results appeared in Nature on April 19, 2023.

The team began by analyzing the brains of seven healthy adults who were resting or performing simple movements, such as closing a hand, flexing a foot, swallowing, or winking. Brains were also scanned while planning movements or simultaneously moving different body parts.

Results showed that the regions controlling the face, hands, and feet were generally located where expected. But interspersed among these regions were three thinner areas that did not seem to control specific muscles or movement. Rather, they were functionally linked to each other and to brain regions that may play roles in goal-oriented action, arousal, pain, and physiological control. The authors named these functionally linked regions the somato-cognitive action network (SCAN).

Additional fMRI studies suggested that the familiar motor regions that control specific movements are structured in a concentric rather than linear fashion, as had been seen in nonhuman primates. The concentric rings that control movements of the foot, hand, and mouth are separated from each other by the integrated SCAN regions.

Overall, the researchers propose that the motor cortex includes two distinct and interactive systems. One includes the well-known regions that provide precise movement control. The other is the SCAN system that helps coordinates complex movements.

“We pulled together a lot of different data in addition to our own observations, and zoomed out and synthesized it, and came up with a new way of thinking about how the body and the mind are tied together,” Dosenbach says.

—by Vicki Contie