The molecular homunculus: How does the brain organize and orchestrate basic human movements? An analysis by Dosenbach and Fox

How does our brain control everyday actions, such as grabbing coffee or telling a joke? A century of experimental neurology testifies that a thin strip of motor cortex running sideways down the outer surface of the brain translates voluntary actions (the decision to take a sip, for instance) into muscle movements. This strip is usually thought of as representing a continuous map of the human body, with separate sections each containing neurons that contact the spinal cord to operate a particular group of muscles. The brain’s higher regions act as puppeteers, orchestrating movements by creating certain sequence of activity in the motor cortex map. The cortex is composed of two separate systems, one of which is devoted to bodily movements, according to a study written in Nature.

The idea of the homunculus was first mentioned in the late 19th century when researchers discovered that stimulation of the primary motor cortex causes body parts to twitch. Later work found that some body parts, such as the hands, feet and mouth, took up a disproportionate amount of space in the primary motor cortex compared with the rest of the body. The motor homunculus was the first publication that translated to “little man” in Latin.

A new theory was proposed by a neuroscientist and her colleague based on data from a group of amputees. They suggested that there are two systems in the primary motor cortex: one for motor commands and another for muscle synergies that enable coordinated movement.

The researchers discovered the same regions in large data sets from previously scanned individuals. They found out that newborns aren’t able to control their movements exactly because they don’t have a brain network yet, which supports the theory that the network coordinates complex action plans.

So Dosenbach’s team was puzzled when they began seeing hints of a very different organization. Data from high-resolution functional magnetic resonance neuroimaging of individual brains was used for the clues.

The findings could lead to changes in therapy for disorders of the primary motor cortex caused by stroke or injury. Many of the brain areas currently targeted for neurostimulation, a technique used to rehabilitate people with movement disorders, came from trial and error, Fox says. The new homunculus could help explain how and why certain targets work, and identify new or better targets.

The Primary Motor Cortex of Wilder Penfield reveals two interleaved systems for the control of the movements of his finger and his foot

It has to plan a trajectory to the cup, control dozens of muscles, make adjustments based on feedback from the eyes and fingers, and maintain its focus on the goal: a tasty jolt of caffeine.

He says that even simple movements need nuanced control. You have to control the heart rate. It’s important that you control your blood pressure. You have to control so called fight and flight responses.”

The authors of the study say that the motor cortex depicted in Textbooks still doesn’t correspond to the questions “what am I going to do today?” and “what is the region that controls your finger?”

Evan Gordon, the first author of the study, says that the data from the magnetic resonance shows that there’s more than one system. It was there, even though we did not perceive it due to the things we learned in the first neuroscience class.

The chair of neuroscience at the University of Pittsburgh thinks this is a fundamental change in how we view the motor cortex.

The strip of brain tissue is called the primary motor cortex. As its name suggests, this area is considered the main source of signals that control voluntary movements.

The view goes back to the 1930s, when Canadian sputnik Wilder Penfield started to map the brains of his patients with electrical currents. Ultimately, Penfield identified segments that would reliably cause a foot, finger, or the tongue to move.

The data from the magnetic resonance device suggested there were important areas between Penfield’s sections. The cortex had a lot of connections, but not muscles. The connections lead to places in the brain that control internal organs, including the heart and lungs.

Gordon was initially skeptical about what he was seeing. He wondered: “Is this just something weird about the data we have collected or is this present in other people?”

But if these segments of brain tissue weren’t for controlling muscles, what were they doing? To find out, the team turned to their lead scientist: Nico Dosenbach.

Dosenbach had to perform complicated tasks such as rotating his hand in one direction while rotating his foot in the other. These tasks required his brain to plan his movements before carrying them out.

“We found that these regions in the motor cortex were more active during this planning phase and that’s what really pointed us in the right direction,” Gordon says.

There are two interleaved systems. So right below an area controlling the fingers, for example, the team would find an area involved in “whole body integrative action.”

Gordon says they found proof that the ribbon of motor cortex had alternating areas, one for controlling a specific muscle and the other for keeping track of the entire body.

A system that weaves together movement and mental states could explain how our posture affects our mood, or why exercise makes us feel better.

How you move can have a significant impact on how you feel. And how you feel is going to have an impact on how you move,” Strick says. My mother told me to stand up straight and I would feel better. And maybe that’s true.”