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.
