Sunday, January 13, 2013

Asylum Walk (15)

The notoriously prodigious energy demands of the human brain—including 20% of the body’s oxygen, delivered by the 100 ml of fresh blood bathing every 100 g of brain tissue every minute; a trillion glia (nutritive cells) supporting 100 billion neurons connecting a quadrillion synapses—invite the question of what all this energy is for.

Deep anaesthesia reduces the energy consumption of the brain by only 50%. Half of the brain’s energy is needed to maintain ion gradients across neuronal membranes at rest, without ever firing off an action potential. Half of the brain’s energy, in other words, goes to keeping the brain ready to respond to the environment. The total set of possible responses, including the small subset of purposive conscious thought, presumably consumes most of the remaining half.

This leads to strange inferences. In terms of energy demands on the brain, there may not be much difference between Charles Darwin sitting at his desk pondering On the Origin of Species and a cable subscriber sitting on his couch watching The Planet of the Apes; in fact, the balance of costs may tip against the latter, owing to the amount of expensive visual processing involved. In some ways it costs less to be a genius.

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