Action potential

• The membrane starts at resting potential (-65 mV)
• The initial stimulus causes a depolarising generator potential at the receptor
• The generator potential is created when the stimulus reaches a critical value called the threshold potential (approximately -55 mV)
• Voltage-gated sodium ion channel proteins rapidly open when the threshold potential is reached
• Sodium ions diffuse down their electrochemical gradient into the neurone, depolarising it further
• The neurone eventually becomes overall positive inside and overall negative outside, reaching a peak membrane potential of +40 mV
• This triggers the closing of voltage-gated sodium ion channel proteins and the opening of voltage-gated potassium ion channel proteins
• Potassium ions diffuse down their electrochemical gradients out of the neurone, causing repolarisation
• The loss of positive charge during repolarisation causes the inside of the neurone to become overall negative again
• The membrane becomes hyperpolarised as so many potassium ions leave
• The voltage-gated potassium ion channel proteins close after hyperpolarisation
• Resting potential is eventually restored by the Na+/K+ pump protein and leak channel proteins
• Action potential overly simplified:

  -65 mV 🡪 Na+ flood in 🡪 +40 mV 🡪 K+ flood out 🡪 -65 mV
• • An action potential is only generated when threshold is reached;
  • For a given neurone, if an action potential is generated, it will always be of the same size.
• The stronger the stimulus, the greater the frequency of action potentials.
• Threshold potential can vary from one neurone to another.

  If threshold is lower, action potentials will be activated by a weaker stimulus.

  The brain can then detect the relative numbers of neurones of different thresholds that have been activated and use this to determine stimulus strength.