Box 1. Action potentials in neurons
From the following article:
Neuroscience: Spikes too kinky in the cortex?
Boris Gutkin and G. Bard Ermentrout
Nature 440, 999-1000 (20 April 2006)
doi:10.1038/440999a
The membranes of all cells have a potential difference across them, as the cell interior is negative with respect to the exterior (a). In neurons, certain stimuli can reduce this potential difference by opening sodium-ion channels in the membrane. For example, neurotransmitters interact specifically with ligand-gated sodium-ion channels. So sodium ions flow into the cell, reducing the voltage across the membrane.
Once the potential difference reaches a threshold voltage, the reduced voltage causes hundreds of voltage-gated sodium channels in that region of the membrane to open briefly. Sodium ions flood into the cell, completely depolarizing the membrane (b). This opens more voltage-gated ion channels in the adjacent membrane, and so a wave of depolarization courses along the cell — the action potential.
As the action potential nears its peak, the sodium channels close, and potassium channels open, allowing ions to flow out of the cell (the hyperpolarizing current) to restore the normal potential of the membrane (c).
Because action potentials are an all-or-nothing response, occurring only once the threshold voltage is reached, the strength of the stimulating signal will not produce a larger 'spike' in the neuron. Strong stimuli will instead produce a series of action potentials — so it is the frequency, number and timing of the spikes that encode neural information.
B.G. & G.B.E.
