Know Your Brain: The Motor Cortex — What Moves You

(Editor’s note: This article is from the Summer 2017 issue of Brain World magazine. If you enjoy this article, please support us with a print or digital subscription!)


Watching a sunset is one of those experiences that nearly all people of all walks of life enjoy — the slow burning of flames over a thick strata of cloud, turning them pink and burgundy as the evening slowly approaches; a sight especially beautiful over a lake or a beach. It’s one of those vacation shots you’ll always feel obligated to take.

Fortunately, there’s not much to worry about these days when it comes to grabbing pictures with your smartphone, as most cameras come equipped with an autocontrast feature, making it possible to take high-quality pictures despite having to deal with a wide array of lighting conditions — such as a darkened sky contrasting with the brightness of the beach sand, or the harsh shadows on trees in the far background. The autocontrast works by increasing the camera’s sensitivity in dim light, while simultaneously decreasing the sensitivity to nearby intense light — keeping exposure in the sweet spot.

Our neurons work similarly to this autocontrast feature, adjusting our visual cortex to light — that’s why we’re able to take in so much great detail with our eyes. The same occurs with those neurons receptive to sound and touch in a process known as dynamic range adaptation. The process may also occur among neurons sensitive to direction of movement — information processed by the brain’s motor cortex.

If the visual cortex acts as a camera — revealing the important visual information we need to take in — then think of the motor cortex as a sort of cursor for the body. Located in the cerebral cortex, it is responsible for the planning, control, and execution of each of our voluntary movements. It is best understood divided into three areas, each serving a specific function.

THE SUM OF ITS PARTS

The primary motor cortex, found in the dorsal portion of the brain’s frontal lobe, is largely involved in planning and carrying out each of your steps. It contains massive pyramid-shape neurons known as Betz cells, the largest found in the central nervous system — measuring 100 micrometers long in diameter. Acting as the upper motor neurons, these Betz cells send out axons (connections not unlike telephone wires), which reach down the spinal cord by means of the corticospinal tract. The spinal cord contains gray matter, with anterior horn cells that the Betz cells connect with. In turn, the horn cells synapse directly with connecting muscle — sending the signals to move our jaws as we eat, or to lift our arms as we walk.

To the front of the primary motor cortex is the premotor cortex, also responsible for many aspects of motor control, such as the preparation for movement. You may not give it much thought, but before you can walk across the room, you must rely on your premotor cortex to guide you, as this component processes the visual information absorbed by your brain — and often deals with sound-processing as well, particularly when you’re navigating your own room late at night. It also allows you to approximate the space between your chair and the cup of coffee on your desk when you reach for it. And it is also through the premotor cortex that you depend on much of your ability to learn — by watching and understanding the actions of others before you imitate them yourself.

The supplementary motor area is connected directly to the spinal cord with a number of proposed functions, serving as a primary output region of the cortical motor system. It is believed that this is where your movement plans are generated — such as that decision to flee in the face of impending danger. When you carry out movements step by step, proceeding with caution on an icy surface, for example, your supplementary motor area is lighting up. It also works as a coordinator for both sides of the body during more complicated tasks — such as when you’re driving and need the use of both hands.

IN EVERYDAY USE

The motor cortex also relies partially on the use of the posterior parietal cortex, which too controls aspects of motor planning and may play a role in using multisensory information to determine movements. A recent study conducted in March at Johns Hopkins University also underscored a crucial relationship that the motor cortex shares with the cerebellum when it comes to learning and timing specific movements. The study involved 32 participants who had to touch a target on a screen using a mouse that moved only at 30-degree angles. Participants performed the task using either their hand or foot, and magnetic stimulation revealed that between trials, “without first training the right hand, the subjects’ ability to complete the task improved from the baseline measurements, showing that the learning transferred from the foot” — recalled researcher Pablo Celnik. Connectivity changed in the brain between both regions as the test subjects performed their task — learning to do one task by hand meant that they could more easily learn to do the same task with their feet. Celnik hopes that one day further exploration of this connection will incite breakthroughs in physical therapy — restoring movement in damaged limbs.

For many years, the brain was thought to only contain one cortical field related to movement. It was Alfred Walter Campbell, better known in history as Australia’s first neurologist, who studied the cortex under a microscope and from its unusual architecture determined that there might be two fields responsible for movement — a primary and a central intermediate. Today, as our methods have advanced into what many hail as the golden age of neuroscience — we’ve moved on to computer models and functional MRI scanners to monitor brain activity throughout the motor cortex when dealing with movement.

In one such experiment conducted in April by the University of Pittsburgh, test subjects were placed in a virtual-reality setting, where they performed tasks in both 2-D and 3-D layouts. Their brains adjusted to the new maps accordingly as they were introduced to each environment, as signaled by the motor-cortical neurons.

There’s a pretty good incentive for understanding what’s going on inside the motor cortex, too. As researcher Steven Chase, assistant professor at the Center for Neural Basis of Cognition, explains: “Our brain has an amazing ability to optimize its own information processing by changing how individual neurons represent the world. If we can understand this process as it applies to movements, we can design more precise neural prostheses.”

In the near future, for example, robotic arms could be constructed to closely model the intended movement of the patient, based on a closer understanding of how their motor cortex functions while moving — aiding in the manufacturing of artificial parts that you might almost feel you were born with.

Yet, researchers are already exploring a future of possibilities that goes far beyond artificial limbs. In spring 2017, Facebook revealed a technology that would allow people to transmit messages from their brains at up to 100 words per minute — using a brain-to-computer interface. The technology may not sound like reality, but in fact has its roots in a Stanford-based research effort that placed electrode arrays in a patient’s brain to record signals transmitted by the motor cortex — allowing persons with paralysis to type out words.

Describing his hope for the near future, Facebook CEO Mark Zuckerberg posted: “Our brains produce enough data to stream four high-definition movies every second. The problem is that the best way we have to get information out into the world — speech — can only transmit about the same amount of data as a 1980s modem.”

You might fear the unthinkable — but Facebook’s intent is to develop a noninvasive system for transmitting speech, one that would not record our random thoughts, but that could be used as a type of speech prosthetic; the ultimate evolution of speech turned into texting. The human brain streams its data at the rate of 1 terabit per second, while basic speech is transmitted at the rate of 40 to 60 bits per second.


And Facebook is hardly the only interested party. Elon Musk launched the company Neuralink to further explore electrode-array technology and is hoping to one day merge our minds with the machine. It’s still a long way off — but as the technology advances, so could a myriad cures for neurological and neurodegenerative disorders.

(Editor’s note: This article is from the Summer 2017 issue of Brain World magazine. If you enjoy this article, please support us with a print or digital subscription!)

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