In the previous slides, we've discussed how an action potential is generated at one point of the axon. Next, we'll look at how an action potential is passed on to the next segment of the axon. While a graded potential decays as it travels farther from the site of stimulation, an action potential does not get weaker with distance, because the action potential is reborn or regenerated and the action potential is regenerated. at each point that it passes along the length of the axon. This process is known as the propagation of action potentials.
As a consequence of the propagation of action potentials, an axon is able to maintain the strength of the message it is conveying all the way to the tips of the axon, which happens to also be consistent with the premise of the all-or-none law. Action potentials typically begin at the axon hillock of a neuron, before traveling along the axon and finally reaching the terminal buttons. Perhaps you're now wondering, how does the axon know which direction the message is supposed to be transmitted to? That is to say, how can an axon be smart enough to know that the message needs to be sent towards the presynaptic terminals? The answer is...
The action potential only travels in one direction because the area that has just undergone an action potential is in refractory period and is therefore unable to produce another action potential. But does that mean that an action potential that begins at the axon hillock cannot travel the opposite direction towards the soma and dendrites? Not necessarily. The message does travel to the soma and dendrites.
through a process known as backpropagation. However, the soma and dendrites will not produce an action potential like the axon does. Instead, the soma and dendrites will only passively receive the information, basically registering the fact that some electrical event is presently occurring in the nearby axon.
Please watch the following video to further enhance your understanding of the propagation of an action potential. An action potential, depicted as a red band, is propagated in one direction along the axon. During an action potential, the inside of the cell membrane becomes positive with respect to the outside.
An action potential generates local currents that tend to depolarize the membrane immediately adjacent to the action potential. When depolarization caused by the local currents reaches threshold, a new action potential is produced adjacent to the original one. Action potential propagation occurs in one direction because the recently depolarized area of the membrane is in absolute refractory period and cannot generate an action potential. In the thinnest axons, the speed of action potential propagation is approximately 1 meter per second.
Meanwhile, in large diameter axons, the speed of action potential propagation is much faster, reaching up to 30 meters per second. While this speed may seem fast enough for humans, but it's actually rather slow for larger animals like a blue whale or taller animals like giraffes. To further increase the speed of action potential propagation, humans and other vertebrate animals have a special mechanism for information transmission that is made possible by the myelin sheath.
On myelinated axons, the speed of conduction of nerve impulses can reach as high as 120 meters per second because action potentials need to occur only in unmyelinated parts of the axon known as the nodes of Ron VA. In a myelinated axon, the action potential would jump from one node of Ron VA to the next node of Ron VA through a process called saltatory conduction, which significantly increases the speed at which the information is conveyed. The word saltatory itself is derived from the Latin word saltare, Which means to jump.
Remember though, the word is saltatory and not salutatory. Finally, before we end. In today's lecture, we will see how the saltatory conduction happens from one node of Ranvier to the next node of Ranvier.
As you can see in this picture, a myelinated axon does not have the myelin sheath covering every single part of it. Instead, myelinated segments alternate with unmyelinated segments. Each unmyelinated segment, that is each node of Ranvier, is approximately one micrometer wide. Because some parts of the axon are covered in myelin, there are no sodium and potassium channels in the membrane of these myelinated parts and no action potential can occur there. As you can see in the picture, in picture A, when an action potential occurs at the first node of RonvA, sodium ions enter the axon thereby initiating an action potential.
Once inside, the sodium ions then travel to the next node of Ron VA, carrying with them their positive charge. This then results in the action potential being regenerated at the next node of Ron VA. This process, through which action potentials need to only occur at nodes of Ron VA instead of at each point along the axon, not only results in a faster transmission of information, But it also conserves energy because sodium ions only need to be admitted at each unmyelinated node. This concludes today's lecture. You've been introduced to several important concepts throughout the lecture, including the resting potential, the action potential, and the propagation of action potentials.
I strongly encourage you to read your textbook if you have not already done so. or revisit the book to strengthen your comprehension of today's material. Thank you for your attention.