Propagation

Saltatory & Continuous Propagation

There are two types of propagation or conductionThe transmission of nerve impulses along neurons.: saltatory and continuous.  Propagation refers to how the action potentialA rapid, temporary electrical charge that travels along neurons, allowing signal transmission. moves down the axon once it has passed over the axon hillockCone-shaped region of the neuron where the axon originates and where action potentials begin..  This is an example of positive feedbackA control mechanism that amplifies a change instead of reversing it.. The ionsCharged atoms or molecules. flow into and out of the axon. These movements occur in the steps of an action potential. We have been generally talking about saltatory propagation previously in this class. Saltatory is a word that comes from the Latin root saltare which means to jump. The axon is covered by myelin sheaths that resemble little pillows. They are made of a lipid substance. We likened this to the insulation covering on electrical wires.  The myelin smothers the ion channelsProtein passages in the cell membrane that allow specific molecules to pass through. on the axon. It leaves exposed pieces of axon and ion channels in the nodes of Ranvier.  The action potential can still make the ion channels in the nodes of Ranvier open and close. It conducts the electricity. This allows an action potential to jump over each myelin sheathA fatty covering around axons that increases conduction speed.. It opens ion channels only on the nodes of Ronvey. By doing this, an action potential conducts very quickly. It opens only a limited number of ion channels. This fast conduction or saltatory propagation is preferred for action potentials that have to move a long distance. For example, the neuron that leaves your brain travels down your spinal cordThe central nervous system structure that relays signals between the brain and body.. It goes out your sciatic nerveThe largest nerve in the body, arising from the sacral plexus. to your big toe. This neuron is myelinated, because that is a very long distance for an action potential to travel.

Continuous propagation occurs on unmyelinated axons. It is a very slow type of conduction. This method is preferred for short distances, such as traveling from the left to the right side of the brain. There are no myelin sheaths on the axon and all of the ion channels are exposed. This means that an action potential will have to open each ion channel. Then it must close every ion channel to propagate down the axon. This takes a very long time since each individual ion channel has to be opened. This is called continuous propagation and is quite slow. This situation reminds me of shopping with my mother at Shoprite when I was 10. I would try to just walk on the tiles of a certain color. And I would try stepping only on those tiles. And I would try to just walk on the tiles on the floor of a certain color. So I would jump from tile to tile and that saltatory propagation. But then I would also sometimes try to step in each individual tile. That would be continuous propagation.

Speed of AP

The speed of an action potential definitely depends upon whether the axon is myelinated or not. That difference is substantial in terms of the speed of an action potential. These are cross-sections of axons. We know the axon on the left with the myelin has greater conduction. The axon on the right is unmyelinated. Or is that true? The diameter of an axon also makes a difference in terms of the speed of an action potential.  Larger axons are capable of conducting an action potential much faster than a small axon.

 This phenomenon simply has to do with the number of ion channels that are available on the Axel Lemma. Larger nerve fibers have a greater circumference a greater diameter and thus have more ion channels. This contributes to a quicker graded potential leading to thresholdThe minimum voltage needed to trigger an action potential. and a much quicker rise to positive 30 megavolts. In reality, nerve fibers in your peripheral nervous systemPNS All nervous tissue outside the CNS; includes nerves and ganglia. are classified by two aspects. These aspects are myelinationThe process by which glial cells wrap axons in myelin. and axon diameter. Examples of these fibers include the ulnar nervenerve from the brachial plexus that controls hand and forearm muscles. or the sciatic nerve. Large myelinated neuronsThe functional cells of the nervous system that transmit signals. such as those in your sciatic nerve conduct action potentials very quickly. We classify them as type a neuron. Small unmyelinated neurons have very small diameter axons. These neurons connect one part of your brain to another part in the right hemisphere. They are most likely unmyelinated since they’re very short. Although very short, they could conduct action potentials quite slowly. We call these type C fibers. B fibers are known as intermediate and of course are somewhere between those two.

Multiple Sclerosis

Multiple sclerosis is a disease that attacks the myelin sheaths onto axons. Some rogue B cell in a person’s bone marrow decides to attack the myelin and degrade it throughout the body. This turns saltatory propagated action potentials into continuous potentials. Now, an action potential travels from the brain to the muscle in the big toe. It must pass through more ion channels than it should. Many times the action potential just simply does not get to where it’s going. This is what accounts for the inability to activate muscles.  Multiple sclerosis often targets some of the cranial nervesNerves that arise from the brain and control head and neck functions. first. This leads to a loss of vision or hearing as some of the initial symptomsSubjective experiences reported by the patient (e.g., nausea, fatigue)..  The cranial nerves are myelinated.