Essay On Action Potential Notes

Action potential

An action potential is a rapid electrical impulse where the resting membrane potential is increased by roughly 100mv. This is due to the rapid changes in membrane permeability to sodium and potassium ions. An action potential is only formed if the threshold value is exceeded which is why it is known as an all or none response. Action potentials only occur in electrically excitable cells as these cells possess voltage activate channels which open in response to the depolarisation of cells. These cells are neurons which use electrical signals for nervous conduction and myocytes which use electrical signals for muscle contraction.

The formation of an action potential depends on the following things

1. gating (opening and closing) of potassium and sodium channels which changes the permeability properties. The gating of these channels depends on the membrane potential and time

2. intracellular concentrations and extracellular concentrations of sodium, potassium,calcium ions

3. membrane properties which include, resistance

During an action potential, membrane potential rapidly rises and becomes more positive and creates spikes which propagate long distances along nerve/muscle. Conduction allows info from sensory organs to brain.

How action potential rises?

The events of an action potential were studied on a giant axon of the squid. This is because the axons of squid are unmyelinated, have large diameters (500 to 1000 micrometers) and the potentials are easily measured from the inside. It is also possible to measure the intracellular and extracellular concentration of sodium ions. When electrodes were placed in the inside fibre to excite it, there was a rapid increase in concentration of sodium ions inside the cell. To confirm that the sodium ions were causing the action potential, the concentration of sodium ions in the extracellular solution were decreased by replacing some sodium ions with sucrose; this consequently reduced the amplitude of the action potential.

The first stage of the action potential is the ‘initial depolarisation’ in which the membrane potential becomes more positive. This occurs due to a stimulus such as electrical stimulation, mechanical compression or application of chemicals. In chemical stimulation substances like acetylcholine stimulates uptake of sodium ions into nerve cells. The depolarisation of the cell is detected by voltage gated sodium channels which open in response to the membrane becoming more positive. However an action potential is only triggered when sufficient sodium ion channels are opened and the threshold value of the cell is exceeded.

Threshold

The reason why a small positive increase in the membrane potential doesn’t stimulate an action potential is because at resting potential the permeability to potassium ions are greater than the permeability to sodium ions. Therefore the effluxes of potassium ions are greater than the influx of sodium ions even if the permeability to sodium ions is gradually increased. Therefore there is a net outward current. For depolarisation that triggers an action potential there must be a net inward current in which the influx of sodium ions are greater than efflux of potassium ions. The threshold value for any cell is reached at a membrane potential when the influx of sodium ions is equal to the efflux of potassium ions. When the influx of sodium ions is greater, an action potential is triggered. The threshold value depends on the number of voltage sensitive sodium ion channels in membrane. This is highest in the axon membrane leaving the cell body, which means that a small depolarisation is enough to trigger an action potential.

The threshold value of a particular cell can be calculated by doing experiments. In the axon of the squid, intracellular probes (labelled a and b) were placed within the axon 10cm apart. When a small amount of positive charge was injected into the axon at point a , the membrane potential measured at probe a become more positive whereas the membrane potential at point b didn’t change. The reason that there was no change in membrane potential at point b was because the extra positive charge leaks out of the leakage channels and charges don’t travel far down the fibre.

However when the depolarisation within the cell was increased by injecting a larger positive current the membrane potential at b was the same as a. This signifies that the action potential has been created because one of the properties of an action potential is that amplitude remains constant. It is at this membrane potential where an action potential is formed.

Excitation

The next stage of the action potential is excitation. Once the membrane potential has reached the threshold value a large number of voltage activated sodium ion channels open. As a result there is...

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