Action Potential
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Direct answer
An action potential is a rapid, self-propagating reversal of the membrane potential that sweeps along an excitable cell. In a large nerve fibre the potential rises from a resting level of about -70 mV to roughly +30 mV and returns to rest within a millisecond, driven by sequential opening and inactivation of voltage-gated sodium channels followed by potassium efflux. It obeys the all-or-none law — once threshold is reached, its amplitude is independent of stimulus strength.
What you must remember
- Resting state: about -70 mV in neurons, maintained by the sodium-potassium pump and potassium leak channels; threshold lies near -55 mV.
- Depolarisation: stimulus opens voltage-gated sodium channels, and sodium influx drives the potential towards +30 mV; the upstroke is regenerative because opening of a few channels depolarises the membrane further.
- Repolarisation: sodium channels inactivate and slower potassium channels open, so potassium efflux restores the negative resting potential, often overshooting into after-hyperpolarisation.
- Absolute refractory period: sodium channels are inactivated, so no stimulus, however strong, can excite the fibre; this enforces one-way propagation and caps the firing frequency.
- Relative refractory period: follows repolarisation, when a stronger-than-normal stimulus can fire the cell because potassium conductance is still elevated.
- Cardiac action potential: phase 0 depolarisation, phase 1 initial repolarisation, phase 2 plateau maintained by slow calcium influx, phase 3 repolarisation and phase 4 rest; the long plateau gives cardiac muscle an absolute refractory period spanning almost the whole contraction, preventing tetanus.
- Conduction: saltatory conduction jumps from node to node in myelinated fibres, greatly increasing velocity; local anaesthetics block sodium channels and abolish conduction.
Common confusion
Students confuse the cardiac plateau with the nerve spike — only cardiac muscle and smooth muscle have a calcium-mediated plateau that prolongs the action potential to 200 to 300 milliseconds. The two refractory periods are also swapped: absolute means no response is possible at all, relative means a bigger stimulus is needed. A final error is calling the upstroke a passive spread — it is an active, regenerative process, while passive electrotonic spread merely brings the next patch to threshold.
Exam-focused takeaway
In theory, draw and label the nerve action potential with ionic events, then compare it with the cardiac ventricular potential using phase numbers, and finish with refractory periods and saltatory conduction. In viva, expect why cardiac muscle cannot be tetanised, the effect of hyperkalaemia and hypokalaemia, and which channels local anaesthetics and tetrodotoxin block. In practicals, identify the phases on a cardiac action potential trace and distinguish fast-response from pacemaker tissue by shape.
Frequently asked questions
What are the phases of a nerve action potential?
Resting state at about -70 mV, a depolarisation upstroke to about +30 mV driven by sodium influx, repolarisation by sodium inactivation and potassium efflux, and a brief after-hyperpolarisation before rest returns.
What is the all-or-none law?
Once the threshold of about -55 mV is reached, the action potential fires with a constant amplitude regardless of a stronger stimulus. Greater stimulus strength is coded by frequency of firing, not amplitude.
What is the difference between absolute and relative refractory periods?
In the absolute period no stimulus can produce a response because sodium channels are inactivated. In the relative period, a stronger than normal stimulus can excite the fibre while potassium conductance is still high.
Why cannot cardiac muscle be tetanised?
Its plateau creates an absolute refractory period lasting almost as long as the contraction, so a second stimulus cannot tetanise the muscle.
What is saltatory conduction?
In myelinated fibres, the action potential jumps from one node of Ranvier to the next because the myelin insulates the internodal membrane. This makes conduction faster and metabolically cheaper.