Action Potential
An action potential represents a change in electrical potential from the resting potential of the neuronal cell membrane, and is a series of electrical and underlying chemical changes that travel down the length of a neural cell (neuron). The neural impulse is created by the controlled development of action potentials that sweep down the body (axon) of a neural cell.
There are two major control and communication systems in the human body, the endocrine system and the nervous system. In many respects, the two systems compliment each other. Although long duration effects are achieved through endocrine hormonal regulation, the nervous system allows nearly immediate control, especially regulation of homeostatic mechanisms (e.g., blood pressure regulation).
The neuron cell structure is specialized so that at one end, there is a flared structure termed the dendrite. At the dendrite, the neuron is able to process chemical signals from other neurons and endocrine hormones. If the signals received at the dendritic end of the neuron are of a sufficient strength and properly timed, they are transformed into action potentials that are then transmitted in a "one-way" direction (unidirectional propagation) down the axon.
In neural cells, electrical potentials are created by the separation of positive and negative electrical charges that are carried on ions (charged atoms) across the cell membrane. There are a greater number of negatively charged proteins on the inside of the cell, and unequal distribution of cations (positively charged ions) on both sides of the cell membrane. Sodium ions (Na+) are, for example, much more numerous on the outside of the cell than on the inside. The normal distribution of charge represents the resting membrane potential (RMP) of a cell. Even in the rest state there is a standing potential across the membrane and, therefore, the membrane is polarized (contains an unequal distribution of charge). The inner cell membrane is negatively charged relative to the outer shell membrane. This potential difference can be measured in millivolts (mv or mvolts). Measurements of the resting potential in a normal cell average about 70mv.
The standing potential is maintained because, although there are both electrical and concentration gradients (a range of high to low concentration) that induce the excess sodium ions to attempt to try to enter the cell, the channels for passage are closed and the membrane remains almost impermeable to sodium ion passage in the rest state.
The situation is reversed with regard to potassium ion (K+) concentration. The concentration of potassium ions is approximately 30 times greater on the inside of the cell than on the outside. The potassium concentration and electrical gradient forces trying to move potassium out of the cell are approximately twice the strength of the sodium ion gradient forces trying to move sodium ions into the cell. Because, however, the membrane is more permeable to potassium passage, the potassium ions leak through he membrane at a greater rate than sodium enters. Accordingly, there is a net loss of positively charges ions from the inner part of the cell membrane, and the inner part of the membrane carries a relatively more negative charge than the outer part of the cell membrane. These differences result in the net RMP of -70mv.
The structure of the cell membrane, and a process termed the sodium-potassium pump maintains the neural cell RMP. Driven by an ATPase enzyme, the sodium potassium pump moves three sodium ions from the inside of the cell for every two potassium ions that it brings back in. The ATPase is necessary because this movement or pump of ions is an active process that moves sodium and potassium ions against the standing concentration and electrical gradients. Equivalent to moving water uphill against a gravitational gradient, such action requires the expenditure of energy to drive the appropriate pumping mechanism.
When a neuron is subjected to sufficient electrical, chemical, or in some cases physical or mechanical stimulus that is greater than or equal to a threshold stimulus, there is a rapid movement of ions, and the resting membrane potential changes from -70mv to +30mv. This change of approximately 100mv is an action potential that then travels down the neuron like a wave, altering the RMP as it passes.
The creation of an action potential is an "all or none" event. Accordingly, there are no partial action potentials. The stimulus must be sufficient and properly timed to create an action potential. Only when the stimulus is of sufficient strength will the sodium and potassium ions begin to migrate done their concentration gradients to reach what is termed threshold stimulus and then generate an action potential.
The action potential is characterized by three specialized phases described as depolarization, repolarization, and hyperpolarization. During depolarization, the 100mv electrical potential change occurs. During depolarization, the neuron cannot react to additional stimuli and this inability is termed the absolute refractory period. Also during depolarization, the RMP of -70mv is reestablished. When the RMP becomes more negative than usual, this phase is termed hyperpolarization. As repolarization proceeds, the neuron achieves an increasing ability to respond to stimuli that are greater than the threshold stimulus, and so undergoes a relative refractory period.
The opening of selected channels in the cell membrane allows the rapid movement of ions down their respective electrical and concentration gradients. This movement continues until the change in charge is sufficient to close the respective channels. Because the potassium ion channels in the cell membrane are slower to close than the sodium ion channels, however, there is a continues loss of potassium ion from the inner cell that leads to hyperpolarization.
The sodium-potassium pump then restores and maintains the normal RMP.
In demyelinated nerve fibers, the depolarization induces further depolarization in adjacent areas of the membrane. In myelinated fibers, a process termed salutatory conduction allows transmission of an action potential, despite the insulating effect of the myelin sheath. Because of the sheath, ion movement takes place only at the Nodes of Ranvier. The action potential jumps from node to node along the myelinated axon. Differing types of nerve fibers exhibit different speed of action potential conduction. Larger fibers (also with decreased electrical resistance) exhibit faster transmission than smaller diameter fibers.
The action potential ultimately reaches the presynaptic portion of the neuron, the terminal part of the neuron adjacent to the next synapse in the neural pathway. The synapse is the gap or intercellular space between neurons. The arrival of the action potential causes the release of ions and chemicals (neurotransmitters) that travel across the synapse and act as the stimulus to create another action potential in the next neuron.
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