Action potential, the brief (about one-thousandth of a second) reversal of electric polarization of the membrane of a nerve cell (neuron) or muscle cell. In the neuron an action potential produces the nerve impulse, and in the muscle cell it produces the contraction required for all movement. Sometimes called a propagated potential because a wave of excitation is actively transmitted along the nerve or muscle fibre, an action potential is conducted at speeds that range from 1 to 100 metres (3 to 300 feet) per second, depending on the properties of the fibre and its environment.
Before stimulation, a neuron or muscle cell has a slightly negative electric polarization; that is, its interior has a negative charge compared with the extracellular fluid. This polarized state is created by a high concentration of positively charged sodiumions outside the cell and a high concentration of negatively charged chloride ions (as well as a lower concentration of positively charged potassium) inside. The resulting resting potential usually measures about −75 millivolts (mV), or −0.075 volt, the minus sign indicating a negative charge inside.
In the generation of the action potential, stimulation of the cell by neurotransmitters or by sensory receptor cells partially opens channel-shaped protein molecules in the membrane. Sodium diffuses into the cell, shifting that part of the membrane toward a less-negative polarization. If this local potential reaches a critical state called the threshold potential (measuring about −60 mV), then sodium channels open completely. Sodium floods that part of the cell, which instantly depolarizes to an action potential of about +55 mV. Depolarization activates sodium channels in adjacent parts of the membrane, so that the impulse moves along the fibre.
If the entry of sodium into the fibre were not balanced by the exit of another ion of positive charge, an action potential could not decline from its peak value and return to the resting potential. The declining phase of the action potential is caused by the closing of sodium channels and the opening of potassium channels, which allows a charge approximately equal to that brought into the cell to leave in the form of potassium ions. Subsequently, protein transport molecules pump sodium ions out of the cell and potassium ions in. This restores the original ion concentrations and readies the cell for a new action potential.
The Nobel Prize for Physiology or Medicine was awarded in 1963 to Sir A.L. Hodgkin, Sir A.F. Huxley, and Sir John Eccles for formulating these ionic mechanisms involved in nerve cell activity.
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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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