Search for:. Action Potential Learning Outcomes Explain the stages of an action potential and how action potentials are propagated. Practice Question The formation of an action potential can be divided into five steps, which can be seen in Figure 1. Figure 1. Action Potential. Show Answer Potassium channel blockers slow the repolarization phase, but have no effect on depolarization.
This video presents an overview of action potential. Now you should be able to understand that the refractory period for axons described in the section above has a very practical physiological purpose: it assures that action potentials move in one direction down the axon. When an action potential is generated at one node of Ranvier, the previous node is still in a refractory period. Although sodium ions entering at a node diffuse in both directions down the axon, the previously-activated node cannot generate an action potential.
This is key in assuring that an excitatory input to a neuron does not result in a reverberating series of action potentials. In contrast to myelinated axons, unmyelinated neurons must "refresh" the action potential in every successive patch of membrane. The ionic redistribution is restored to the resting state by the sodium-potassium pump, but this requires a large amount of energy.
This may be the reason why unmyelinated axons have a small diameter. If an unmyelinated axon was of large diameter, the surface area would be large and many voltage-gated channels would be needed on the surface. When an action potential occurred, the movement of ions would be large, and a tremendous amount of ATP would be needed to fuel the activity of the sodium-potassium pump to restore ionic balance. In general, the largest axons are also the most heavily myelinated, and propagate action potentials very rapidly.
The smallest axons are unmyelinated and propagate action potentials slowly. Although the action potentials of most neurons resemble those described above, some neurons have action potentials with different properties. As an example, cerebellar Purkinje cells produce complex spikes, which are very broad and complicated action potentials.
As shown to the left, the complex spike has a duration of almost 10 msec. This is because some of the voltage-gated sodium ion channels have recovered and the voltage-gated potassium ion channels are still open. The relative refractory period is the period of hyperpolarization after an action potential [2].
Action potentials in neurons are also known as "nerve impulses" or "spikes" [3] [4]. Jump to: navigation , search. Personal tools Log in. Namespaces Page Discussion.
Views Read View source View history. This page was last modified on 21 October , at This page has been accessed , times. Channels for cations positive ions will have negatively charged side chains in the pore.
Channels for anions negative ions will have positively charged side chains in the pore. Some ion channels are selective for charge but not necessarily for size. Some ion channels do not allow ions to freely diffuse across the membrane, but are gated instead. A ligand-gated channel opens because a molecule, or ligand, binds to the extracellular region of the channel Figure A mechanically-gated channel opens because of a physical distortion of the cell membrane.
Many channels associated with the sense of touch are mechanically-gated. For example, as pressure is applied to the skin, mechanically-gated channels on the subcutaneous receptors open and allow ions to enter Figure A voltage-gated channel is a channel that responds to changes in the electrical properties of the membrane in which it is embedded. Normally, the inner portion of the membrane is at a negative voltage. When that voltage becomes less negative and reaches a value specific to the channel, it opens and allows ions to cross the membrane Figure A leak channel is randomly gated, meaning that it opens and closes at random, hence the reference to leaking.
There is no actual event that opens the channel; instead, it has an intrinsic rate of switching between the open and closed states. Leak channels contribute to the resting transmembrane voltage of the excitable membrane Figure The membrane potential is a distribution of charge across the cell membrane, measured in millivolts mV.
The standard is to compare the inside of the cell relative to the outside, so the membrane potential is a value representing the charge on the intracellular side of the membrane based on the outside being zero, relatively speaking; Figure There is typically an overall net neutral charge between the extracellular and intracellular environments of the neuron.
However, a slight difference in charge occurs right at the membrane surface, both internally and externally. It is the difference in this very limited region that holds the power to generate electrical signals, including action potentials, in neurons and muscle cells. When the cell is at rest, ions are distributed across the membrane in a very predictable way.
The cytosol contains a high concentration of anions, in the form of phosphate ions and negatively charged proteins. With the ions distributed across the membrane at these concentrations, the difference in charge is described as the resting membrane potential.
The exact value measured for the resting membrane potential varies between cells, but mV is a commonly reported value. This voltage would actually be much lower except for the contributions of some important proteins in the membrane.
This may appear to be a waste of energy, but each has a role in maintaining the membrane potential. Resting membrane potential describes the steady state of the cell, which is a dynamic process balancing ions leaking down their concentration gradient and ions being pumped back up their concentration gradient. Without any outside influence, the resting membrane potential will be maintained. To get an electrical signal started, the membrane potential has to become more positive. Because sodium is a positively charged ion, as it enters the cell it will change the relative voltage immediately inside the cell membrane.
The resting membrane potential is approximately mV, so the sodium cation entering the cell will cause the membrane to become less negative. This is known as depolarization , meaning the membrane potential moves toward zero becomes less polarized. This is called repolarization , meaning that the membrane voltage moves back toward the mV value of the resting membrane potential. Repolarization returns the membrane potential to the mV value of the resting potential, but overshoots that value.
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