Myelin Sheath

Oligodendrocytes

wrap their cell processes around the axons in CNS

Where does the Oligodendrocytes produce myelin sheaths?

CNS (Brain and Spinal Cord)

Schwann cells

surround axons in the PNS

What does the Schwann Cells forms myelin sheath?

around axons in the PNS

Which type of axons are typically myelinated in PNS?

Thick axons

Which type of axons are typically unmyelinated in the PNS?

Thin axons

What is the purpose of the myelin sheath?

Assists in the unidirectional propagation of action potentials down the axons

How does the myelin sheath help with nerve signal transmission?

By forming an insulating layer around the axon to prevent electrical ion leakage

What effect does myelin have on the speed of action potentials?

It increases the conduction velocity.

What happens to conduction velocity (CV) when axons have thicker myelin sheaths?

Conduction velocity increases

Why do axons with thicker myelin sheaths conduct signals faster?

Because of saltatory conduction

Saltatory conduction describes the process by which an action potential travels down a myelinated axon. Which of the following best explains this process?

The action potential “skips” from one Node of Ranvier to another, increasing the speed of impulse transmission

Where are voltage-gated sodium channels (VGSCs) and voltage-gated potassium channels (VGPCs) located in a myelinated axon?

Only at the Nodes of Ranvier

What channels are present on the myelin sheath?

no channels are presented

What happens to ion movement in the regions covered by the myelin sheath in Saltatory Conduction?

There is no ion current or conductance through the myelin sheath

What role does the myelin sheath play in ion movement within the axon?

It keeps ions inside the axon and helps push the electrical signal down the axon

What occurs when the axon membrane at Point A reaches threshold? Saltatory Conduction

Voltage-gated sodium channels open and Na⁺ enters the cell

What role does the myelin sheath play after Na⁺ enters the axon at Point A? Saltatory Conduction

It prevents sodium from leaking out and pushes ions toward the next Node of Ranvier

Where do the Na⁺ ions move after entering the axon at Point A? Saltatory Conduction

Toward the next Node of Ranvier (Point B)

What happens about 1–2 ms after the voltage-gated sodium channels open at Point A? Saltatory Conduction

Voltage-gated sodium channels inactivate, and voltage-gated potassium channels open

What is the result of K+ leaving the cell at Point A? Saltatory Conduction

Repolarizing that part of the axon

What triggers the opening of voltage-gated sodium channels (VGSCs) at Point B? Saltatory Conduction

Depolarization to threshold caused by Na⁺ ions moving from Point A

What happens immediately after VGSCs open at Point B? Saltatory Conduction

Na⁺ rushes into the cell at Point B

How does the myelin sheath affect Na⁺ movement at Point B? Saltatory Conduction

It prevents Na⁺ from leaking and Na+ ions pushed toward the next Node of Ranvier (Point C)`

What occurs 1–2 ms after VGSCs open at Point B? Saltatory Conduction

VGSCs inactivate, and VGPCs open.

What is the result of K⁺ leaving the axon at Point B? Saltatory Conduction

Repolarizing of that part of the axon

Saltatory Conduction cycle will repeat down…

the entire axon