Know Your Brain: The Motor Cortex — What Moves You

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.

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