Why does the increase in extracellular potassium depolarize the neurons
Chemical gradients for ions cause them to diffuse, and that creates electrical gradients. The resting membrane potential is essentially based on potassium gradients: Potassium ions diffuse out of the cell through "potassium channels" in the cell membrane. In the process, there is (almost) a potassium equilibrium potential: Potassium leaves the cell until the electrical gradient (positive on the outside, negative on the inside) balances the chemical gradient.
This mechanism applies to all ions - for each there is an equilibrium potential (according to its distribution). The concentration pattern and availability of the respective ion channels are decisive for the charging (polarization) of the membrane.
If, for example, the membrane becomes permeable to sodium (opening of the sodium channels by a stimulus), Na flows+ into the cell, which is thereby discharged (depolarized) - before the sodium channels close again and the membrane repolarizes itself through changed ion currents. The time course of the potential fluctuation is referred to as the action potential. During this time, the cell cannot be excited again - it is refractory.
An action potential propagates through ongoing excitation of a membrane that is still unexcited. The speed of this "wildfire" depends on the structure of the electrical field: In nerve fibers, whose myelin sheaths only release narrow constrictions with a bare membrane surface, the electrical field of the transmembrane stimulation current is concentrated here. The stimulation current quickly exceeds the threshold and the process is repeated from one ring to the next - the action potential advances by leaps and bounds (saltatory excitation conduction).
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