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You may have heard of an action potential before, but how does it actually travel down an axon? In this blog post, we’ll take a look at the science behind action potentials and how they help our bodies to function.
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Introduction
Your body is made up of cells, and each cell is full of electrical activity. This activity is necessary for the cell to function properly. One type of electrical activity that occurs in cells is an action potential.
Action potentials are bursts of electrical activity that travel down axons (the long, thin parts of cells that transmit information). They are caused by changes in the voltage across the cell membrane (the thin layer that surrounds the cell).
When an action potential starts, there is a sudden change in voltage across the cell membrane. This change in voltage causes ion channels (specialized proteins in the cell membrane) to open. This allows ions (atoms with a charge) to flow into or out of the cell. The movement of these ions creates an electrical current that travels down the axon.
As the action potential travels down the axon, it causes ion channels to open and close. This causes a change in voltage, which propagates (or spreads) the action potential along the axon.
The action potential eventually reaches the end of the axon, where it triggers a release of chemicals (neurotransmitters). These neurotransmitters bind to receptors on other cells and cause a change in voltage across their cell membranes. This can either excite or inhibit these cells, depending on the type of neurotransmitter that is released.
The Structure of an Axon
In order for an action potential to occur, certain conditions must be met. The cell must be polarized, meaning that the inside of the cell has a negative charge in comparison to the outside of the cell. This is accomplished by the selective permeability of the cell membrane, which allows certain ions to enter or leave the cell. For example, when a nerve cell is at rest, there is a high concentration of potassium ions (K+) inside the cell, and a high concentration of sodium ions (Na+) outside the cell. This creates a voltage difference across the cell membrane, called the resting potential.
When an action potential is triggered, there is a rapid change in this voltage difference. First, the gates that control ion flow across the cell membrane open, allowing K+ ions to flow out of the cell. This makes the inside of the cell more positive, and starts to offset the voltage difference. At the same time, other gates open that allow Na+ ions to flow into the cell. As more and more Na+ flows in, eventually there is enough charge inside thecell to equalizethe voltage acrossthe membrane, andthe action potentialis said to “peak.” Finally,the Sodium-Potassiumpump kicks inand helpsrestorethe originalconcentration gradientof these importantions.
The Mechanism of an Action Potential
When a neuron is at rest, the inside of the cell is more negative than the outside. This is due to the presence of more negatively-charged ions, such as chloride, inside the cell. There is a difference in electrical potential across the cell membrane, known as the resting membrane potential.
When a neuron receives a signal (in the form of an electrical impulse), this causes certain channels in the cell membrane to open. This allows positively-charged ions, such as sodium, to flow into the cell. This change in ion concentration causes a change in electrical potential across the cell membrane, known as an action potential.
The action potential then travels down the length of the axon (the long extension of the neuron), causing ion channels to open and close as it goes. When it reaches the end of the axon, it triggers the release of neurotransmitters (chemical signals) from tiny sacs called synaptic vesicles. These neurotransmitters cross the synapse (the gap between two neurons) and bind to receptors on the next neuron, causing ion channels to open or close and thus inhibiting or exciting that neuron.
The Propagation of an Action Potential
When a neuron is at rest, the combination of negatively charged proteins inside the cell and positively charged ions outside creates a difference in voltage across the cell membrane. This difference is called the resting potential, and for most neurons it is about –70 mV. A neuron’s plasma membrane has special proteins called ion channels that selectively control the flow of ions across the cell membrane. When the channels are open, ions flow down their concentration gradients, moving from areas of high concentration to low concentration. This movement changes the voltage across the neuron’s plasma membrane.
There are two types of ion channels—voltage-gated and ligand-gated—that differ in how they open and close. Voltage-gated channels open or close in response to changes in voltage across the cell membrane, whereas ligand- gated channels open or close in response to specific molecules (called ligands) that bind to them.
When a neuron is at rest, most of its ion channels are closed. However, some channels are opened by default, allowing a small amount of current to flow constantly. This leakage current happens because there is always a slight difference in voltage between the inside and outside of the cell. In addition to leakage current,voltage-regulated sodium channels are also open at rest. These keep a balanced distribution of ions on both sides of the cell membrane by allowing sodium ions (Na+) to enter the cell down their electrochemical gradient.
The movement of these Na+ ions into the cell causes depolarization, which is a decrease in voltage (a less negative value). When this happens, more voltage-regulated Na+ channels opened until there are enough open to reach thresholdvoltage—the point where an action potential is generated.
The Refractory Period
During an action potential, the membrane potential of the neuron rapidly rises and then falls. This rise and fall is caused by the movement of ions across the cell membrane. The ions that are most important for action potentials are sodium (Na+) and potassium (K+). When an action potential is triggered, Na+ channels in the cell membrane open and allow Na+ to flow into the cell. This causes the inside of the cell to become more positive than the outside. At the same time, K+ channels open and allow K+ to flow out of the cell, which makes the inside of the cell less positive than the outside.
Factors Affecting the Propagation of an Action Potential
There are several factors that affect the propagation of an action potential down an axon, including the diameter of the axon, the myelin sheath, and the presence of voltage-gated sodium channels.
The diameter of the axon is important because it affects how quickly an action potential can travel. A wider axon will have a larger cross-sectional area, which means that there is more room for ions to flow and that the action potential can travel more quickly. A narrower axon will have a smaller cross-sectional area, which means that there is less room for ions to flow and that the action potential will travel more slowly.
The myelin sheath is also important because it acts as an insulator. This means that it prevents ions from flowing across the membrane, which slows down the propagation of an action potential. The myelin sheath also increases the refractory period, which is the time after an action potential during which another one cannot be generated. This helps to prevent action potentials from spreading uncontrollably.
Finally, voltage-gated sodium channels are important because they allow for the rapid influx of sodium ions during an action potential. Without these channels, action potentials would not be able to propagate down an axon.
The Speed of an Action Potential
How fast does an action potential travel down an axon? The speed of an action potential is very fast, but it depends on the type of axon. The largest and fastest axons can conduct action potentials at speeds up to 120 m/s. However, most axons conduct action potentials at speeds between 1 and 100 m/s.
The Importance of an Action Potential
An action potential is an electrical signal that travels down the length of an axon. This signal is generated by the movement of ions across the cell membrane, and it is what allows nerve cells to communicate with one another.
The action potential is a very important part of the nervous system, as it is responsible for transmitting information from one area of the body to another. Without action potentials, we would not be able to move, think, or feel.
Conclusion
Action potentials propagate along axons by a process of depolarization nationally followed by repolarization. The cell body of the neuron contains the nucleus and much of the cytoplasm. The dendrites are long, thin structures that protrude from the cell body and receive input signals from other neurons. The axon is a long, thin process that also protrudes from the cell body and transmits output signals to other neurons.
References
-“Action Potential.” Khan Academy. N.p., n.d. Web. 26 Apr. 2017.
-Bennett, Margaret, and Colin Abraham. “The Myelin Sheath.” The Myelin Sheath. N.p., 26 Apr. 2010. Web. 26 Apr. 2017
-Bennett, Margaret C., and Colin J. Abrahams. “Saltatory Conduction.” Saltatory Conduction. PhysiologyWeb, 01 Jan. 1970. Web. 26 Apr
