Monday, March 19, 2012

Nerve Anatomy of the Hand: A Fascinating Connection

Once you've mastered the basic bone anatomy of the hand and figured out the various nerve-related hand diseases, you may start wonder just how our hands appear to contain 5 distinct digits with various degrees of freedom yet often operates as a unified system, without much conscious effort. The NYTimes recently had a piece discussing just this phenomenon:
You may think you’re pretty familiar with your hands. You may think you know them like the back of your hand. But as the following exercises derived from the latest hand research will reveal, your pair of bioengineering sensations still hold quite a few surprises up their sleeve.  
• Make a fist with your nondominant hand, knuckle side up, and then try to extend each finger individually while keeping the other digits balled up tight. For which finger is it extremely difficult, maybe even impossible, to comply?  
 • Now hold your hand palm up, fingers splayed straight out, and try curling your pinky inward without bending the knuckles of any other finger. Can you do it? 
• Imagine you’re an expert pianist or touch-typist, working on your chosen keyboard. For every note or letter you strike, how many of your fingers will move?  
 • You’re at your desk and, without giving it much thought, you start reaching over for your water bottle, or your pen. What does your hand start doing long before it makes contact with the desired object? 
And a high-five to our nearest nonhuman kin:  
 • What is the most important difference between a chimpanzee’s hands and our own? (a) the chimpanzee’s thumbs are not opposable; (b) the chimpanzee’s thumbs are shorter than ours; or (c) the chimpanzee’s thumbs are longer than ours.  
 On, then, to the answer key.
To find out the answers, check out the entire article here. As the article notes, an enormous amount of functional anatomy is packed into the volume of each hand. This is reflected in the homunculus, the visual representation of how the motor skills of the body map to the brain cortex:

Homunculus map of the brain
Source: The Masks We Wear

As you can see, the hand takes up a disproportionately large area compared to its percentage of body volume. The only other body part which compares is the face. Clinically, this means that if you see a patient with hand complaints, you should be extra-sensitive for finding subtle changes from normal. While a full discussion of a careful hand exam is beyond the scope of this post, being aware of some of the links between digits as discussed in the NYT article is helpful to understand the relationships between the digits, especially if someone has suffered hand trauma.

As the article notes, loss of the thumb for example can translate into a loss of 50% of the hand function. Since we use our hands constantly in our daily life, such hand injuries can be highly disabling. Not only are they disabling though: if managed improperly, hand injuries can be incredibly painful. While it's easy for us and our patients to take our hands for granted, careful study will give you, um, a leg up when dealing with hand-related health issues.

Updated 2015-12-25

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