Nerve Cell Action Potential • LITFL • BSCC Examination

Basic Science in Clinical Context Examination: 2 minutes long in 2 parts.

Exam candidate answering a question (under exam conditions)
Professor providing a more detailed explanation (with transcript)

Draw and describe a nerve cell action potential

Examinee response: Drawing and explanation in real-time video/audio

Examiner Explanation

Transcript

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