THINGS TO LEARN TODAY: MYELIN SHEATH AND ACTION POTENTIAL

At what stage of Fetal Development is MYELINATION seen?

-Myelination is the production of the myelin sheath -it is seen at the 14th Week of Fetal Development

How does SALTATORY CONDUCTION contribute to impulse propagation, and why?

-Saltatory conduction is the propagation of Action Potentials along myelinated axons from one node of Ranvier to the next node, Increasing the Conduction Velocity of Action Potentials -The uninsulated nodes of Ranvier are the only places along the axon where IONS are Exchanged across the axon membrane, regenerating the action potential between regions of the axon that are insulated by myelin, unlike electrical conduction in a simple circuit -As sodium rushes into the node it creates an ELECTRICAL FORCE which pushes on the ions already inside the axon. -This rapid conduction of electrical signal reaches the next node and creates another action potential, thus refreshing the signal. In this manner, saltatory conduction allows electrical nerve signals to be propagated long distances at high rates without Any Degradation of the signal -IN ADDITION to increasing the speed of the nerve impulse, the myelin sheath also helps in reducing energy expenditure over the axon membrane as a whole, because the amount of Sodium and Potassium ions that need to be pumped to bring the concentrations back to the resting state following each action potential is Decreased

How does the Myelin Sheath Increase the Speed of Impulse Propagation?

-in myelinated fibers, impulses propagate by "Saltatory Conduction" -Myelin DECREASES CAPACITANCE and INCREASES ELECTRICAL RESISTANCE across the cell membrane -this results in the prevention of electrical current loss from the axon

Demyelination is a hallmark of what category of diseases?

-it is a hallmark of some NEURODEGENERATIVE AUTOIMMUNE DISEASES -Examples: Multiple sclerosis, leukodystrophy, Canavan Disease -may be partially explained by the overproduction of Cytokines

How is the ACTION POTENTIAL regenerated by the existence of the Myelin Sheath and Nodes of Ranvier?

-myelinated fibers lack VOLTAGE-GATED ION CHANNELS, except at the NODES OF RANVIER, where they are found abundantly -this lack of voltage-gated ion channels results in significantly reducing Sodium leakage into the Extracellular fluid, and maintains a strong separation of charge between the intracellular fluid (ICF) and the ECF -This increases Sodium's ability to travel along the axon freely, but as it diffuses along the Axolemma (cell membrane) it is Decremental by nature and thus eventually cannot trigger the Voltage-gated sodium channels once it becomes too weak -Since the Nodes of Ranvier are exposed to the ECF and contain large amounts of voltage-gated sodium channels, they allow enough sodium into the axon to regenerate the ACTION POTENTIAL -ESSENTIALLY: each time the action potential reaches a node of Ranvier, it is restored to its original action potential

What is the MYELIN SHEATH?

-the Myelin Sheath is a fatty white substance that surrounds the Axon of a neuron, and acts as a DIELECTRIC (electrically Insulating) material -it is essential for the proper functioning of the nervous system, and is an outgrowth of a type of Glial Cell

What is the FUNCTION and PURPOSE of the Myelin Sheath?

-the main purpose of the myelin sheath is to INCREASE the SPEED at which Impulses propagate along the myelinated fiber

Can regeneration of Myelinated nerve fiber occur?

-yes, but regeneration is not always ensure perfect regeneration of the nerve fiber, and they don't always reconnect with their appropriate muscle fibers

What are the two key COMPONENTS of Myelin?

1. CHOLESTEROL 2. SPHINGOMYELIN

Summarize the THREE ways that the Myelin Sheath affects the speed of an Action Potential?

1. It acts as an insulator around a neuron axon, thereby focusing the propagation of the action potential along the axis of the axon 2. The action potential "leaps" from one node of Ranvier to the next, and so on, faster than the action potential can propagate a wave along an unmyelinated axon of the same diameter 3. The regions along a myelinated axon depolarize locally and successively, thus allowing an action potential to travel along an axon using less energy, which in turn allows the neuron to repolarize more quickly, and thus be ready to conduct the next action potential sooner, thereby increasing the overall speed of information transmission