The action potential

Along the axon the nerve message is neither chemical nor mechanical: it is an electrical signal, and it follows very particular rules.

The resting potential

At rest, the neuron's membrane is polarised: the inside is negative relative to the outside.

   outside     + + + + + + + + + + + +
   ────────────────────────────────────  membrane
   inside      - - - - - - - - - - - -

            resting potential ≈ -70 mV

This difference is maintained actively, by pumps that expel sodium ions and bring in potassium. It is permanent work, and costly: it accounts for a large share of the 20 % of energy consumed by the brain. A resting neuron is working.

The action potential

If a stimulus exceeds a certain threshold, the membrane flips abruptly:

   potential (mV)
    +40 |          /\
        |         /  \
      0 |        /    \
        |       /      \
    -55 |......./........\.....  threshold
    -70 |______/          \____________  rest
        +-------------------------------> time
         0    1    2    3    4  (ms)
        stimulus  depolarisation  repolarisation

Three phases: depolarisation (the inside becomes positive), repolarisation, then a brief dip below the resting potential before recovery. The whole thing lasts about 1 to 2 milliseconds.

The all-or-none law

This is the most counter-intuitive property:

   stimulus BELOW threshold  ->  NOTHING at all
   stimulus ABOVE            ->  a FULL action potential
   very strong stimulus      ->  an IDENTICAL action potential, no bigger

Amplitude therefore encodes no information: all action potentials look alike, whether from a caress or a burn.

Frequency coding

How then can a weak stimulus be told from an intense one? By the number of action potentials per second.

   weak stimulus     |    |       |     |         ~ 10 per second
   medium stimulus   | |  | |  | | |  |           ~ 50 per second
   intense stimulus  ||||||||||||||||||||         ~ 100 per second

Information is thus coded in frequency, not amplitude. This is a digital-style code: each pulse is identical, only their rhythm carries the message. The nervous system solved, hundreds of millions of years ago, the problem electronics rediscovered in the 20th century — digital signals resist noise far better than analogue ones.

One limit follows: the refractory period after each action potential caps the rate at about 1000 pulses per second. That is a neuron's maximum frequency.

Summary

• At rest the membrane is polarised at about -70 mV, actively maintained.
• Beyond a threshold, an action potential fires: depolarisation then repolarisation, in 1 to 2 ms.
• All-or-none law: the action potential is identical whatever the stimulus intensity.
• Information is coded in frequency, not amplitude.
• The refractory period caps the rate at about 1000 pulses per second.