Periodically, some sodium ions leak across the membrane and into the cell. Remember from the Cell membrane Module that the phospholipid bilayer contains protein channels to help in the transport of ions and large molecules across the membrane. This is one way the membrane in semi-permeable.
These transmembrane channels consist of leak channels like pores , chemically gated channels, voltage gated channels, etc. When we say that sodium leaks through the membrane, this happens through the protein leak channels.
When an ion can move across the membrane due to electrical changes, this is due to the opening of voltage-gated channels. Figure 3. Cell Membrane and Transmembrane Proteins. The cell membrane is composed of a phospholipid bilayer and has many transmembrane proteins, including different types of channel proteins that serve as ion channels. There are two forces acting on the sodium ions to move them into the cell, the force of diffusion and the electrical force.
There is a high concentration of sodium outside the cell and a low concentration of sodium inside the cell. This concentration gradient would tend to move sodium ions into the cell.
There is also an electrical gradient at work. The positively charged ions outside the cell repel one another. There are more positive ions outside the cell than inside the cell. The sodium ions tend to want to move into the cell along this electrical gradient. Although the membrane is relatively impermeable to sodium ions, they do tend to slip across the membrane periodically along their diffusion and electrical gradients.
As these positive ions move into the cell, the membrane potential would become more and more positive if something didn't correct the problem. The resting membrane potential however is maintained at mV even though these positive ions keep slipping into the cell.
This is because there is a sodium-potassium pump at work within the cell membrane. This pump actively and continually moves sodium ions out of the cell and potassium ions into the cell.
For every three sodium ions it pumps out, it pumps in two potassium ions. This sodium-potassium pump is able to maintain the resting membrane potential at mV even though the sodium ions leak into the cells occasionally. When the cell is not being stimulated, it is said to be resting.
However, the resting membrane potential can change as a result of stimulation. There are two types of changes in membrane potential, action potentials and graded potentials. Action potentials are rapid changes in membrane potential that involve the entire cell. The action potential is the nerve impulse. Graded potentials are small changes in membrane potential. Graded potentials may lead to action potentials or they may inhibit action potentials. Let's look at action potentials first.
Action Potentials are big changes in membrane potential. Remember in a resting cell, there are more positive ions outside the cell and fewer positive ions inside the cell. During an action potential, this situation is reversed.
More positive ions are found inside the cell than outside the cell during an action potential. The inside of the cell actually becomes positive with respect to the outside.
The membrane is said to depolarize. The membrane then repolarizes and returns to normal after a short period of hyperpolarization. Action potentials work according to the all-or-none principle. This means either an action potential happens or it doesn't happen, there is no in between.
It's sort of like being pregnant. One is either pregnant or not pregnant, there is no in between. Or think about firing a gun, it either fires or it doesn't fire. It is not possible to barely shoot a bullet from the barrel of a gun. It comes out with complete force or not at all. Like firing a gun, action potentials have a threshold. To fire a gun, the trigger must be pulled. Triggers on some guns can be pulled back a little bit without the gun firing.
However, once the trigger is pulled to a certain point, the hammer falls and the bullet fires. With action potentials, there is a threshold level that must be reached before the action potential will fire. This means the magnitude of the stimulation on the cell must be large enough to create and action potential. If the stimulus strength is not at or above the threshold level, an action potential will not be fired. The threshold level for many neurons is mV. This means that the membrane must depolarize to mV or an action potential will not be fired.
A stimulus may be strong enough to cause the membrane to depolarize to the threshold level or it may not be strong enough. In order for an action potential to fire, threshold must be reached.
Figure 4. Graph of Action Potential. Plotting voltage measured across the cell membrane against time, the action potential begins with depolarization, followed by repolarization, which goes past the resting potential into hyperpolarization, and finally the membrane returns to rest. The above figure shows how the membrane potential might change over time. The membrane was stimulated two times, but not to the threshold level. Therefore, an action potential was not fired.
However, the third time the membrane was stimulated; the membrane did reach its threshold level. The result was an action potential. The action potential has three phases; the depolarization phase , the repolarization phase , and the after-hyperpolarization phase.
As you can see, the action potential is a rapid change in membrane potential. The inside of the cell actually became more positive than the outside of the cell. The threshold level may at first seem like some magical level at which the neuron fires an action potential. However, there is nothing magical too it at all. The threshold level is simply the point at which a very important positive feedback loop kicks in.
Remember, positive feedback loops lead to the extreme. The cell experiences a change in one direction and responds by promoting a change in the same direction. Once the threshold level is reached enough sodium ions were allowed in from the ligand-gated channels opening that the membrane potential gets up to mV , special voltage-gated sodium gates in the cell membrane open up.
When these gates open, sodium ions stream into the cell along their electrical and diffusion gradients because there was less inside than outside the cell. As sodium ions move into the cell, the inside of the cell becomes more positive it depolarizes. This depolarization of the membrane causes even more sodium gates to open. When these additional sodium gates open, more sodium ions move into the cell.
The negative resting membrane potential is created and maintained by increasing the concentration of cations outside the cell in the extracellular fluid relative to inside the cell in the cytoplasm. The negative charge within the cell is created by the cell membrane being more permeable to potassium ion movement than sodium ion movement. In neurons, potassium ions are maintained at high concentrations within the cell while sodium ions are maintained at high concentrations outside of the cell.
The cell possesses potassium and sodium leakage channels that allow the two cations to diffuse down their concentration gradient. However, the neurons have far more potassium leakage channels than sodium leakage channels.
Therefore, potassium diffuses out of the cell at a much faster rate than sodium leaks in. Because more cations are leaving the cell than are entering, this causes the interior of the cell to be negatively charged relative to the outside of the cell. The actions of the sodium potassium pump help to maintain the resting potential, once established.
As more cations are expelled from the cell than taken in, the inside of the cell remains negatively charged relative to the extracellular fluid. It should be noted that chlorine ions Cl — tend to accumulate outside of the cell because they are repelled by negatively-charged proteins within the cytoplasm.
Improve this page Learn More. Skip to main content. Consequently, it is necessary to understand thoroughly their properties. To answer the questions of how action potentials are initiated and propagated, we need to record the potential between the inside and outside of nerve cells using intracellular recording techniques. The potential difference across a nerve cell membrane can be measured with a microelectrode whose tip is so small about a micron that it can penetrate the cell without producing any damage.
When the electrode is in the bath the extracellular medium there is no potential recorded because the bath is isopotential. If the microelectrode is carefully inserted into the cell, there is a sharp change in potential.
The reading of the voltmeter instantaneously changes from 0 mV, to reading a potential difference of mV inside the cell with respect to the outside. The potential that is recorded when a living cell is impaled with a microelectrode is called the resting potential, and varies from cell to cell. Here it is shown to be mV, but can range between mV and mV, depending on the particular type of nerve cell.
In the absence of any stimulation, the resting potential is generally constant. It is also possible to record and study the action potential. The electrode records a resting potential of mV. The cell has also been impaled with a second electrode called the stimulating electrode.
This electrode is connected to a battery and a device that can monitor the amount of current I that flows through the electrode. Changes in membrane potential are produced by closing the switch and by systematically changing both the size and polarity of the battery.
If the negative pole of the battery is connected to the inside of the cell as in Figure 1. This result should not be surprising. The negative pole of the battery makes the inside of the cell more negative than it was before. A change in potential that increases the polarized state of a membrane is called a hyperpolarization.
The cell is more polarized than it was normally. Use yet a larger battery and the potential becomes even larger. The resultant hyperpolarizations are graded functions of the magnitude of the stimuli used to produce them.
Now consider the case in which the positive pole of the battery is connected to the electrode Figure 1. When the positive pole of the battery is connected to the electrode, the potential of the cell becomes more positive when the switch is closed Figure 1.
Such potentials are called depolarizations. The polarized state of the membrane is decreased. Larger batteries produce even larger depolarizations. Again, the magnitude of the responses are proportional to the magnitude of the stimuli. However, an unusual event occurs when the magnitude of the depolarization reaches a level of membrane potential called the threshold. A totally new type of signal is initiated; the action potential.
Note that if the size of the battery is increased even more, the amplitude of the action potential is the same as the previous one Figure 1. The process of eliciting an action potential in a nerve cell is analogous to igniting a fuse with a heat source.
A certain minimum temperature threshold is necessary. Temperatures less than the threshold fail to ignite the fuse. Temperatures greater than the threshold ignite the fuse just as well as the threshold temperature and the fuse does not burn any brighter or hotter. If the suprathreshold current stimulus is long enough, however, a train of action potentials will be elicited. In general, the action potentials will continue to fire as long as the stimulus continues, with the frequency of firing being proportional to the magnitude of the stimulus Figure 1.
Action potentials are not only initiated in an all-or-nothing fashion, but they are also propagated in an all-or-nothing fashion. An action potential initiated in the cell body of a motor neuron in the spinal cord will propagate in an undecremented fashion all the way to the synaptic terminals of that motor neuron. Again, the situation is analogous to a burning fuse. Once the fuse is ignited, the flame will spread to its end.
The action potential consists of several components Figure 1.
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