Princeton University, New Jersey
A neuroscientist explores the energy efficiency of the brain.
Considering its substantial processing capacity, the human brain consumes remarkably little power — about as much as an idling laptop computer. So I was interested to learn that action potentials — the electrical ‘spikes’ that are the fundamental units of neuronal activity — are likewise remarkably energy efficient (H. Alle et al. Science 325, 1405–1408; 2009).
During a spike, the voltage across a neuron’s membrane is reversed when sodium ions flow into the cell and potassium ions move out. This reversal spreads as a wave down the neuron’s axon towards its terminals, where it triggers synaptic transmission to other neurons.
Henrik Alle of the Max Planck Institute for Brain Research in Frankfurt, Germany, and his colleagues recorded charge movements at axon terminals in mammalian hippocampal neurons. They found that sodium and potassium ions flow at largely non-overlapping times, with more than 75% of all charge contributing unopposed to the rise or fall of a spike.
Such efficiency comes as a surprise. These axons outperform the much-studied squid giant axon by a factor of three. If the findings apply to other mammalian neurons, brain tissue may support more firing than suspected. The authors suggest that synaptic transmission may dominate the energy budget of brain tissue.
These results have implications for functional magnetic resonance imaging, which measures increases in blood oxygenation in the brain as an indicator of neural activity. What causes the blood-oxygen boost is unknown: suggested triggers include synaptic transmission and action potentials. This paper is evidence for the former, because energy-intensive events such as synaptic signalling are more likely to be oxygen-hungry and to stimulate blood flow. The idea is supported by other recent evidence — a wonderful convergence.