When the membrane potential of the axon hillock of a neuron reaches threshold, a rapid change in polarity occurs that moves along the axon in the form of an action potential. This moving change in polarity has several stages:
- The depolarization, also called the rising phase, is caused when positively charged sodium ions (Na+) suddenly rush through open sodium channels into a neuron.The membrane potential of the stimulated cell undergoes a localized change from-65 millivolts to 0 in a limited area. As additional sodium rushes in, the membrane potential actually reverses its polarity so that the outside of the membrane is negative relative to the inside. During this change of polarity the membrane actually develops a positive value for a moment (+40 millivolts). The change in voltage stimulates the opening of additional sodium channels, which are called voltage-gated ion channels.
- The repolarizaton, or falling phase, is caused by the closing of sodium ion channels and the opening of potassium ion channels releasing positively charged potassium ions (K+) from the neuron when potassium gates open. Again, these are opened in response to the positive voltage--they are voltage-gated. This expulsion acts to restore the localized negative membrane potential of the cell; a level of about -65 or -70 mV is typical for nerves.
- Many more potassium channels have been opened than are required and not all close when the membrane potential returns to normal, causing an undershoot or afterhyperpolarization. This will persist until the membrane permeability to potassium returns to normal.
- The refractory phase which can be divided into an absolute refractory period during which it is impossible to evoke another action potential, and then a relative refractory period, during which a stronger-than-usual stimulus is required. After the sodium channels close, they become inactive and cannot be opened again, regardless of the membrane potential (absolute refractory), until they transition to an active state . As more sodium channels return to active states the cell may depolarize, but a fraction of potassium channels remain open hyperpolarizing the cell, making it harder to depolarize to threshold. The absolute refractory period is responsible for the unidirectional propagation of action potentials.
The action potential generated at the axon hillock propagates as a wave along the axon. The currents flowing inwards at a point on the axon during an action potential spread out along the axon, and depolarize the adjacent sections of its membrane. The absolute refractory period keeps the direction of propagation unidirectional. In order to enable fast and efficient transduction of electrical signals in the nervous system, certain neuronal axons are covered with myelin sheaths. Myelin is a multilamellar membrane that wraps the axon in segments separated by intervals known as nodes of Ranvier . Myelin is produced by Schwann cells--specialized cells found exclusively in the peripheral nervous system--and by oligodendrocytes found exclusively in the central nervous system. Myelin prevents ions from entering or leaving the axon along myelinated segments. However, the current is carried by the cytoplasm, which is sufficient to depolarize the first or second subsequent node of Ranvier. Instead, the ionic current from an action potential at one node of Ranvier provokes another action potential at the next node; this apparent "hopping" of the action potential from node to node is known as saltatory conduction. The myelin sheath and nodes of Ranvier in combination, help in reducing energy expenditure at the area of depolarization. Thus, the amount of sodium/potassium ions that need to be pumped to bring the concentration back to normal, or repolarize, is decreased. The conduction in myelinated fibers is hundreds of times faster since the action potentials only occur at the nodes of Ranvier. The myelinated fibers allow for transmission of signals quickly and efficiently.