BSCC Physiology 010
Nerve Cell Action Potential
- The membrane potential is the difference in electrical potential between the interior and the exterior of the cell membrane. It is created by active processes- the most important is the NAKATPase pump which pumps 3 sodium ions out of the cell and 2 potassiums into the cell.
- There are also processes that involve selective ion channels and through these different processes you get a Resting Membrane Potential (RMP).
- At a RMP there is no net movement of ions across the membrane.
- In a neuron the RMP is -70mv. I.e. there is an electrical potential across the cell membrane, the inside is more negatively charge than outside the cell membrane.
- When the neuron cell membrane is exposed to a stimulus/current. The neuron becomes excited and the membrane potential decreases to -55mv where a threshold potential is reached. At this stage -55mV, the membrane will rapidly depolarise.
- This phase of rapid depolarisation is due to the sudden opening of the sodium channels and the consequent inpouring of Na ions. The membrane potential overshoots to a potential of +30 (or35) mv. At this level there is no further inward movement of NA into the cell, because of the decreased electrical gradient (more NA is now in the cell) (factor 1)
- There are 3 factors which cause the neuron to repolarise
- Factor 1 = the loss of the sodium electrical gradient
- Factor 2 = the electrical gradient is now reversed and the sodium channels close
- Factor 3 = the opening of potassium channels. This opening is slower and more prolonged than the opening of the sodium channels, resulting in complete repolarisation and reestablishment of the RMP
- Note that the potassium efflux produces a short period of after hyperpolarisation (goes below the RMP) but then quickly corrects to the RMP of -70mv.
- This brief period of local depolarisation of the membrane has effects on the surrounding neuron cell membrane allowing a net movement effect of a wave of depolarisation (and subsequent repolarisation) down the axon