Recently someone asked "Why can't I control my individual toes?".
This was a really fun answer to write, so I thought I'd share it here.
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 just your 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 that's well worth reading, by the way.
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.
Pictures speak volumes:
As do videos. Watch the TMS disrupt conscious motor control:
The stimulation of the motor cortex by the TMS sends a "fake" signal to the hands, causing muscular contractions. (MEP is "motor evoked potential": the electrical signal recorded from the hand muscles.)
By combining TMS with individual MRIs, we can get relatively fine mapping between TMS and the brain.
In 1995 a group of neuroscientists published a paper called, "Modulation of muscle responses evoked by transcranial magnetic stimulation during the acquisition of new fine motor skills" in the Journal of Neurophysiology.
In this paper, they showed that they could map the amount of motor cortical territory devoted to specific fingers.
The authors had their subjects train on a piano and mapped motor cortex finger representation before and after training. Here's what they found:
Over the course of 5 days, as subjects learned the one-handed, five-finger exercise through daily 2-h manual practice sessions, the cortical motor areas targeting the long finger flexor and extensor muscles enlarged, and their activation threshold decreased.
Thus, they demonstrated that even adults show cortical plasticity after some simple muscle training. That is, piano practice caused the amount of brain devoted to voluntary muscular control grow.
So although you may not currently be able to flex your ring finger or control your individual toes, there's no reason that you can't learn how!
Amputees, for example, can learn to be quite dexterous with their toes.
Here's a man writing perfectly well:
And a woman demonstrating how she bathes:
So to (finally!) answer your question: you can't control your toes because you haven't practiced, and therefore the "toe" area of your motor cortex is too small to allow fine control.
Exercise your brain's plasticity and practice writing with your toes! Get back to me after a few weeks of practice and let me know how it went.
(Note, some of the above images probably came from my friend and zombie collaborator Timothy Verstynen many moons ago during our PhDs.)
Pascual-Leone A, Nguyet D, Cohen LG, Brasil-Neto JP, Cammarota A, & Hallett M (1995). Modulation of muscle responses evoked by transcranial magnetic stimulation during the acquisition of new fine motor skills. Journal of Neurophysiology, 74 (3), 1037-45 PMID: 7500130
Penfield, W., & Boldrey, E. (1937). Somatic motor and sensory representation in the cerebral cortex of man as studied by electrical stimulation Brain, 60 (4), 389-443 DOI: 10.1093/brain/60.4.389