When a neurone is stimulated an action potential is generated by a change in ion concentrations across the cell membrane. The resting membrane potential (the potential of charge across the membrane when the cell membrane is not being stimulated) is -60mV. When a neurone is stimulated sodium ion channels open causing sodium ions to move into the neurone, down the electrochemial gradient (as there are more sodium ions outside the cell compared to inside the cell). As sodium ions have a positive charge, when they enter the neurone, they cause the membrane potential to become more positive. If the number of sodium ions that enter the cell, cause the membrane potential to exceed the threshold potential, of approximately -50mV, this will cause voltage gated sodium ion channels to open, so there will be a large influx of sodium ions and the membrane potential will become more positive, and reach +4-mV. The process of sodium ions entering the neurone, is called depolarisation. Once the membrane potential has reached +40mV, the sodium ions channels close and potassium ions channels open, in order for the process of repolarisation to occur, so that the neurone does not remain stimulated once the stimulus has passed. This means that pottasium ions move out of the neurone, down the electrochemial gradient. As potassium ions also have a positive charge, when they leave the neurone, they cause the membrane potential to become more negative, and it decreases back to the original resting membrane potential. Potassium ions are slow to close, leading to hyperpolarisation, where the membrane potential goes below the resting membrane potential of -60mV for a very short period of time. When the membrane is at rest, the sodium-potassium ions pumps actively transport sodium ions out of the cell, and potassium ions into the cell (against their concentration gradients) so that the ions are in the correct positions, as they were originally before the neurone was stimulated.
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