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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.
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