Action potential

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An action potential is a message in the form of an electrical impulse caused by a rapid change in a cell's membrane potential.

When a stimulus reaches the threshold at the axon hillock, an action potential is generated.

An action potential relies on many protein channels.  In a neurone, the Potassium leak channel and Sodium-Potassium pump maintain the resting potential. The voltage gated sodium channel and the voltage gated potassium channel are involved in the progression of an action potential along the membrane.

The action potential progression can be separated into several steps;

  1. Voltage channels are closed and the Potassium (K+) leak channel and the sodium (Na+) pump maintain the resting membrane potential of -70 mV. The Sodium/Potassium Pump (ATPase) is responsible for maintaining the membrane potential at -70mv, the protein actively pumps three sodium ions out of the cell and pumps two potassium ions into the cell.
  2. The neurone becomes stimulated. The voltage gated sodium channels begin to open and the membrane potential begins to slowly depolarises and sodium enters the cell down its concentration gradient. All the voltage gated Sodium channels open when the membrane potential reaches around -55 mV and there's a large influx of Sodium, causing a sharp rise in voltage. As the potential nears +30mV, the rate of depolarisation slows down as the voltage gated Sodium channels become saturated and inactivate, preventing further sodium ions from entering the cell.
  3. Voltage gated potassium channels open, and potassium leaves the cell down its concentration gradient. The depolarisation of the cell stops and repolarisation can occur through these voltage gated Potssium channnels.
  4. Voltage gated sodium channels are completely deactivated and potassium floods out through the voltage gated potassium channels,
  5. Voltage gated potassium channels are slow to close, and therefore hyperpolarisation occurs. This is where the membrane potential drops below the resting potential of -70 mV as potassium continues to leave.
  6. Once the voltage gated potassium channels close, the resting state can be re-established through the Potassium leak channel and Sodium pump.

The action potential travels along the neurone's axon via current loops in order to reach the axon terminal.

An action potential is a transient, electrical signal, which is caused by a rapid change in resting membrane potential (-70 mV). This occurs when the threshold potential (-55 mV) is reached, this causes a rapid opening in the voltage gated sodium channels leading to a influx of sodium into the cell.  The threshold potential also causes a slow opening of voltage gated potassium channels leading to the eflux of potassium out of the cell. This causes the cell to depolarise, meaning the inside of the cell is now positive compared to the outside.

The action potenial starts in the axon hilock as there is a high density of voltage gated sodium channels here, it is also where graded potentials need to reach the threshold potential to cause a action potential. If the graded potential do not reach the supratheshold level, then an Action Potenitial is not triggered and the graded potenital is known as subthreshold[1]. Above threshold, increase in the strength of a stimulus will not increase the size of the corresponding action potential. The strength of a stimulus, or the size of a graded potential, is indicated by frequency of action potentials travelling along a neurone.

The action potential travels via current loops. In myelinated axons its jumps from node of ranvier to node of ranvier, this is known as saltatory conduction.

The higher the density of the myelin sheaths and higher the membrane resistance of the myelinated axon, the faster the axon potential can travel.

The point at which the membrane of an axon is depolarised causes a local circuit to be set up between the depolarised region and the region either side of it. This causes the resting at regions either side to become depolarised also. In this way the action potential sweeps along the axon.

The refractory period prevents the action potential from travelling backwards. There are two types of refractory periods, the absolute refractory period and the relative refractory period. The absolute refractory period is when the membrane cannot generate another action potential no matter how large the stimulus is, this is because the voltage- gated sodium ion channels are inactivited. The relative refactory period is when the membrane can produce another action potential if the stimulus is larger than normal, this is because some of the voltage-gated soduim ion channels have recoveredand the voltage-gated potassium ion channels are still open. The relative refractory period is the period of hyperpolarisation after an action potential [2].

Action potentials in neurons are also known as "nerve impulses" or "spikes" [3][4].

References

  1. http://www.ncbi.nlm.nih.gov/books/NBK11104/
  2. The McGill Physiology Virtual Lab, Refractory Period. Available at: http://www.medicine.mcgill.ca/physio/vlab/cap/refract.htm (Last accessed 9.11.13)
  3. Silverthorn, D. (2012). Human physiology. 5th ed. San francisco Claifornia: Pearson Education, p.261.
  4. http://www.ncbi.nlm.nih.gov/books/NBK11104/


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