NOVA Online | Secrets of the Mind | A Map of the Motor Cortex

The motor cortex located on the left side of the brain controls movement on the right side of the body.

A Map of the Motor Cortex

Tap the index finger of your right hand. It's a simple maneuver, yet carrying it out requires a vastly complicated series of actions. First, the image of the words on the screen (telling you to tap your finger) enters your eyes and strikes the retinas. The retinas then convert the image into electrical impulses. These impulses are sent to your brain. Your brain "sees" the words and gives meaning to them. Your brain then decides whether or not to carry out what it has read. If it decides yes, your brain's motor cortex, a small area that exists on the outer part of your brain, calls for messages to be sent through your spinal cord and down your arm to the muscles that control the finger. Only then does the finger move.

The best view of the brain's motor cortex is from a brain that has been sliced in half. The image to the left displays a brain that has been sliced lengthwise, dividing the front and back halves, viewed from the back.

The cartoon-like drawings in the illustration show how much of the brain's motor cortex is devoted to controlling specific body parts. The reason that some body parts are depicted larger than others (for example, the hand is larger than the shoulder) is that there are more muscles controlling those areas.

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