In connection with the transmission of signals in nerves and muscle fibers, the threshold potential is the potential difference at which an action potential is triggered. Values of the membrane potentials below the threshold potential are called "subliminal", those above the threshold "supra-threshold".
While the resting membrane potential (around −70 mV) is a membrane potential that is essentially determined by potassium and is therefore close to the value for the potassium equilibrium potential (around −90 mV), the fast voltage-activated sodium channels responsible for an action potential have an activation threshold of about −50 mV.
In order to open these ion channels for sodium , the cell membrane must be depolarized to at least this threshold value . Depending on the cell type and conditions, this happens either through the action of neurotransmitters that cause post- synaptic local depolarization ( EPSP ), or through a generator potential ( receptor potential in sensory cells), or electrotonically on the axon hill or through a transmitted action potential on the axon .