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Generation of Action Potentials.
Step 1: Depolarisation of the threshold.
* The stimulus that initiates an action potential is a ( graded depolarisation, graded repolarisation ) large enough to open the channels.
* A graded depolarisation brings an ( area of excitable membrane, the entire cell membrane ) to threshold ( (-60mV), (-90mV) )
Step 2: Activation of Sodium Channels and Rapid Depolarisation.
* The ( Voltage-Gated, mechanically-gated ) sodium channels open (sodium channel activation)
* Sodium Ions, driven by ( electrical attraction, diffusion ) and the chemical gradient, flood into the cell
* The transmembrane potential goes from -60mV (the threshold level) to ( +30mV, -90mV ) (the sodium equilibrium potential)
* The membrane cannot respond to any other stimulation during this time, the ( Refectory Period, Refractory Period )
Step 3: Inactivation of Sodium Channels and Activation of Potassium Channels.
* The Voltage-Gated sodium channels ( close, remain open ) (sodium channel inactivation occurs) at +30mV
* The ( Voltage-Gated, chemically gated ) potassium channels are now open, and positively charged potassium ions ( diffuse out, are actively transported into ) of the cell, shifting the membrane potential back toward ( resting levels, +60mV ).
* ( Repolarisation, Hyperpolarisation ) begins.
Step 4: Closing of Potassium Channels
* The Voltage-Gated sodium channels regain their normal properties
* The membrane is now capable of generating another action potential if a larger-than-normal stimulus is provided
* The Voltage-Gated potassium channels begin closing at ( -70mV, +30mV ). Because they do not all close at the same time, potassium loss continues and a ( temporary hyperpolarisation, ongoing hypopolarisation ) occurs, bringing the membrane to approximately ( -90mV, +30mV ).
* At the end of the relative Refractory Period, all Voltage-Gated channels have closed and the membrane is back to it's resting state.
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