Brain myths and facts
One of my favorite science writers, Carl Zimmer, investigates a surprising claim about brain power in his latest blog entry: You're a Dim Bulb (And I mean that in the best possible way). The claim is that a normally functioning brain only uses about 10 watts, which of course is much less power than your standard 75 watt light bulb consumes. It turns out that "10 watts" is a little low, but in the right ballpark. I decided to do my own calculations (details below*) and came up with 16 watts. Dim bulb indeed!
Then again, maybe the comparison isn't fair. Incandescent light bulbs are notoriously inefficient -- at least 90% of the energy they consume is wasted as heat. According to this site, a 23 watt compact fluorescent bulb produces as much light as a 75 watt incandescent bulb, and lasts over 10 times as long. But I digress.
Carl also refers to the best known brain myth of all, the hopeful notion that we use only 10% of our brain, suggesting that we all have huge reservoirs of untapped mental potential. I'm sure we all have untapped potential, but as Carl notes, the 10% myth is just plain wrong. We use essentially all of our brain (although, at any given moment, perhaps only 1% of its neurons are active). Check out these fine web pages if you're not convinced:
- The average adult human brain weights about 1400 grams (3 lbs.), or about 2% of total body weight. These are just averages - there can be considerable variation in both brain mass and body mass.
- Although it represents only 2% of the body's mass, the brain consumes about 20% of the energy used by the entire body at rest. That's over twice as much energy as the heart uses.*
- The number of neurons in the human neocortex is around 20 billion. That number is larger than the age of the universe in years (13.7 billion), but smaller than the number of stars in our galaxy (200-400 billion).
- The total number of synapses (connections between neurons) in the neocortex is estimated to be more than 160 trillion. That works out to an average of about 8000 synapses per neuron. Obviously the cartoons of a "typical neuron" in biology textbooks are a little oversimplified!
* Here are some more specific details for the curious. According to Elia (1992), the metabolic rate of the brain is 240 kcal/kg/day, and the metabolic rate of the heart is 440 kcal/kg/day. Although you can see that heart tissue is more metabolically active than brain tissue, the heart as a whole is smaller than the brain as a whole, so the heart ends up consuming less energy than the brain.
For example, a 1400-gram brain burns about 336 kilocalories per day (16 watts), while the heart, weighing in at 330 grams, burns 145 kcal/day (7 watts). Note that kilocalories are equivalent to the "calories" that weight watchers keep track of. Also note that 240 kcal/kg/day is a bit higher than the brain metabolic rate assumed by Bill Leonard in Carl Zimmer's blog. I'm not sure what to make of the discrepancy.
Elia, M. (1992) "Organ and tissue contribution to metabolic rate." In: Energy Metabolism: Tissue Determinants and Cellular Corollaries. Edited by Kinney and Tucker. Raven Press, Ltd. New York. pp.61-77.