The Action Potential is the elctrical signal that accompanies the mechanical contraction of a cell when stimulated by an electrical current.(neural or external). It is caused by the flow of sodium cation, pottasium cation, chloride anion and other ions accross the cell membrane. The action potential is the basic component of all bioelectrical signals. It provides information on the nature of physiological activity at the single cell level. Recording of an Action Potential requires the isolation of a single cell, and microelectrodes with tips of the order of a few micrometers to stimulate the cell and record the response.
Resting Potential: Nerve and Muscle cells are encased in a semipermeable membrane that permits selected substances to pass through while others are kept out. Body fluids surrounding cells are conductive solutions containing charged atoms known as ions. In their resting state, membranes of excitable state readily permit the entry of K+ and Cl- ions, but effectively block the entry of Na+ ions (the permeability for K+ is 50-100 times that of Na+). Various ions seek to establish a balance between the inside and the outside of a cell according to charge and concentration. The inability of Na+ to penetrate a cell membrane results in the following:
Depolarization: Whwn a cell is exicted by ionic currents or an external stimulus, the membrane changes its characteristics and begin to allow Na+ ions constitutes an ionic current, which further reduces the membrane barrier to Na+ ions. This leads to an avalanche effect: N+ ions rush into the cell. K+ ions try to leave the cell as they were in higher concentration inside the cell in the preceding resulting state, but cannot move as fast as the Na+ ions. The net result is that the inside of the cell becomes positive with respect to the outside due to an imbalance of K+ ions. A new state of equillibrium is reached after the rush of Na+ ions stops. The change represents the beginning of the action potential, with a peak value of about 20+ mV for most of the cells. An excited cell displaying an action potential is said to be depolarized,; the process is called Depolarization.
Repolarization: After a certain period of being in the depolarized state the cell becomes polarized again and returns to its potential via a process known as repolarization. Repolarization occurs by processes that are analogous to those the of depolarization, except that instead of Na+ ions, the principal ion involved for repolarization are K+ ions. Membrane depolarization , while increasing the permeability for Na+ ions, also increases the permeability of the membrane for K+ ions via a specific class of ion channels known as voltage dependent K+ channels. Although this may appear paradoxial at first glance, the key to mechanism for repolarization lies in the time dependence and voltage dependence of the membrane permeability changes for K+ ions compared with that for Na+ ions. The permeability changes for K+ during depolarization occurs considerably more slowly than those for NA+ ions, hence the initial depolarization is caused by an inrush of Na+ ions. However, the membrane permeability changes for Na+ spontaneously decrease near the peak of depolarization, whereas those for K+ions are beginning to increase. Hence, during repolarization, the predominant membrane permeability is for K+ ions. Because K+ concentration is much higher inside the cell than outside there is a net efflux of K+ from the cell, which makes the inside more negative thereby effecting repolarization back to the resulting potential.
It should be noted that the voltage-dependent K+ permeability change is due to a distinctly different class of ion channels than those that are responsible for setting the resting potential. A mechanism known as Na+-K+ pump extrudes Na+ ions in exchange for transporting K+ back into cell. However, this transport mechanism carry very little current in comparison with ion channels, and therefore makes a minor contribution the repolarization process. The Na+-K+ pump is essential for resetting the Na+-K+ balance of the cell, but the process occurs on a longer scale than the duration of an action potential.
Nerve and muscle cells repolarize rapidly, with an action potential duration of about one ms. Heart muscle repolarize slowly with an action potention of duration of 150-300 ms.
The action potential is always same for a given cell, regardless of the methods of excitation or the intensity of stimulus beyond a threshold: this is known as the all-or-none or all-for-nothing phenomenon. After an action potential, there is a period which a cell cannot respond to any new stimulus, known as refractory period(about 1 ms in nerve cells). This is followed by a relative refractory period(sevral ms in nerve cells), when another action potential may be triggered by a much stronger stimulus than in the normal situation.
Propagation of an action potential: An action potential propagates along a muscle fiber or anunmyelinated nerve fiber as follows : Once initiated by a stimulus, the action potential propagates along the whole length of the fibre without decresein amplitude by progressive depolarization of membrane. Current flows from a depolarized region through the intra-cellular fluid to adjacent inactive regions, thereby depolarizing them. Current also flows through the extra-cellular fluids, through the depolarized membrane, and back into intracellular space, completeing the local circuit. The energy required in conduction is supplied by the fiber itself.
Resting Potential: Nerve and Muscle cells are encased in a semipermeable membrane that permits selected substances to pass through while others are kept out. Body fluids surrounding cells are conductive solutions containing charged atoms known as ions. In their resting state, membranes of excitable state readily permit the entry of K+ and Cl- ions, but effectively block the entry of Na+ ions (the permeability for K+ is 50-100 times that of Na+). Various ions seek to establish a balance between the inside and the outside of a cell according to charge and concentration. The inability of Na+ to penetrate a cell membrane results in the following:
- Na+ concentration inside the cell is far less than the outside.
- The outside of the cell is more positive than the inside.
- To balance the charge, additional K+ ions enter the cell, causing higher K+ concentration inside the cell than outside.
- Charge balance cannot be reached due to differences in membranes permeability for the various ions
- A state of equillibrium is established with a potential difference, with the inside of the cell being negative with respect to the outside.
Depolarization: Whwn a cell is exicted by ionic currents or an external stimulus, the membrane changes its characteristics and begin to allow Na+ ions constitutes an ionic current, which further reduces the membrane barrier to Na+ ions. This leads to an avalanche effect: N+ ions rush into the cell. K+ ions try to leave the cell as they were in higher concentration inside the cell in the preceding resulting state, but cannot move as fast as the Na+ ions. The net result is that the inside of the cell becomes positive with respect to the outside due to an imbalance of K+ ions. A new state of equillibrium is reached after the rush of Na+ ions stops. The change represents the beginning of the action potential, with a peak value of about 20+ mV for most of the cells. An excited cell displaying an action potential is said to be depolarized,; the process is called Depolarization.
Repolarization: After a certain period of being in the depolarized state the cell becomes polarized again and returns to its potential via a process known as repolarization. Repolarization occurs by processes that are analogous to those the of depolarization, except that instead of Na+ ions, the principal ion involved for repolarization are K+ ions. Membrane depolarization , while increasing the permeability for Na+ ions, also increases the permeability of the membrane for K+ ions via a specific class of ion channels known as voltage dependent K+ channels. Although this may appear paradoxial at first glance, the key to mechanism for repolarization lies in the time dependence and voltage dependence of the membrane permeability changes for K+ ions compared with that for Na+ ions. The permeability changes for K+ during depolarization occurs considerably more slowly than those for NA+ ions, hence the initial depolarization is caused by an inrush of Na+ ions. However, the membrane permeability changes for Na+ spontaneously decrease near the peak of depolarization, whereas those for K+ions are beginning to increase. Hence, during repolarization, the predominant membrane permeability is for K+ ions. Because K+ concentration is much higher inside the cell than outside there is a net efflux of K+ from the cell, which makes the inside more negative thereby effecting repolarization back to the resulting potential.
It should be noted that the voltage-dependent K+ permeability change is due to a distinctly different class of ion channels than those that are responsible for setting the resting potential. A mechanism known as Na+-K+ pump extrudes Na+ ions in exchange for transporting K+ back into cell. However, this transport mechanism carry very little current in comparison with ion channels, and therefore makes a minor contribution the repolarization process. The Na+-K+ pump is essential for resetting the Na+-K+ balance of the cell, but the process occurs on a longer scale than the duration of an action potential.
Nerve and muscle cells repolarize rapidly, with an action potential duration of about one ms. Heart muscle repolarize slowly with an action potention of duration of 150-300 ms.
The action potential is always same for a given cell, regardless of the methods of excitation or the intensity of stimulus beyond a threshold: this is known as the all-or-none or all-for-nothing phenomenon. After an action potential, there is a period which a cell cannot respond to any new stimulus, known as refractory period(about 1 ms in nerve cells). This is followed by a relative refractory period(sevral ms in nerve cells), when another action potential may be triggered by a much stronger stimulus than in the normal situation.
Propagation of an action potential: An action potential propagates along a muscle fiber or anunmyelinated nerve fiber as follows : Once initiated by a stimulus, the action potential propagates along the whole length of the fibre without decresein amplitude by progressive depolarization of membrane. Current flows from a depolarized region through the intra-cellular fluid to adjacent inactive regions, thereby depolarizing them. Current also flows through the extra-cellular fluids, through the depolarized membrane, and back into intracellular space, completeing the local circuit. The energy required in conduction is supplied by the fiber itself.
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