247 Votes by Paul King (Computational Neuroscientist, Redwood Center fo...), Aaron Kucyi (PhD Student in Neuroscience), Shan Kothari, and 244 more.
Before explaining this, I want you to try a quick experiment for me: hold your hand in front of you, fingers straight and pointing upward. Now, flex your index finger and justyour index finger. Did your middle finger flex, too? Maybe your ring finger even twitched a little? Try flexing just your ring finger. Unless you're a piano or string instrument player, it's unlikely that you were very successful at doing so.
The reason why that happens is closely related to why you can't control your individual toes. Stick with me.
This sexy beast is the motor homunculus:
He's built to reflect the relative area in the motor cortex that is devoted to controlling specific muscle groups. Notice how overrepresented the hands, lips, and eyes are and how underrepresented the arms, legs, and feet are?
Here's the motor cortex in the brain:
Basically, the more motor cortical area devoted to a region, the greater and finer the voluntary control over those muscles groups that we have.
Originally this map was created by Canadian neurosurgeon Wilder Penfield in 1937. Penfield pioneered brain surgery on awake patients. He would use a small electrical stimulator to map out different parts of the brain, which is still done by neurosurgeons to this day. The logic was simple: stimulate a part of the motor cortex and watch which parts of the body twitched. This gives a mapping between brain and body, and what he found was a clear topography in the motor cortex.
Journalist and science writer Mo Costandi wrote an amazing history of Penfield here that's well worth reading, by the way:
http://scienceblogs.com/n
As I said in my answer to:
Are all the wrinkles on a brain's cortex the same across people?
Penfield used a highly invasive means to map out the motor homunculus. But it turns out we have some pretty cool modern technology with which we study the motor cortex non-invasively: Transcranial Magnetic Stimulation (TMS).
TMS induces an electrical current using a rapidly changing magnetic field
The reason why that happens is closely related to why you can't control your individual toes. Stick with me.
This sexy beast is the motor homunculus:
He's built to reflect the relative area in the motor cortex that is devoted to controlling specific muscle groups. Notice how overrepresented the hands, lips, and eyes are and how underrepresented the arms, legs, and feet are?
Here's the motor cortex in the brain:
Basically, the more motor cortical area devoted to a region, the greater and finer the voluntary control over those muscles groups that we have.
Originally this map was created by Canadian neurosurgeon Wilder Penfield in 1937. Penfield pioneered brain surgery on awake patients. He would use a small electrical stimulator to map out different parts of the brain, which is still done by neurosurgeons to this day. The logic was simple: stimulate a part of the motor cortex and watch which parts of the body twitched. This gives a mapping between brain and body, and what he found was a clear topography in the motor cortex.
Journalist and science writer Mo Costandi wrote an amazing history of Penfield here that's well worth reading, by the way:
http://scienceblogs.com/n
As I said in my answer to:
Are all the wrinkles on a brain's cortex the same across people?
...neurosurgeons will perform electrical stimulation mapping of awake people if they have to remove any brain tissue near what they call "eloquent cortex"... [because] [t]he only way even an experienced surgeon can be sure that specific brain area in a specific person is motor, or speech, or sensory, is via this mapping technique... This is because, although gross neuroanatomical features are generally conserved across people, there can be a huge range of variation.
Penfield used a highly invasive means to map out the motor homunculus. But it turns out we have some pretty cool modern technology with which we study the motor cortex non-invasively: Transcranial Magnetic Stimulation (TMS).
TMS induces an electrical current using a rapidly changing magnetic field
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