Myelin Sheath

What are myelin sheaths?

Insulating structures that are composed of lipoprotein myelin

Which axons are myelinated?

Surround thicker axons of the body

Therefore which axons and unmyelinated?

Thin axons

Difference between myelinated and unmyelinated axons?

Structure

• Layers

• Nodes of Ranvier

Speed of impulse transmission

Location

Structure:

• MY:

  • Have concentric layers of plasma membrane

  • Have nodes of Ranvier

• UN:

  • Schwann cells do surround axon, but not in layers

  • Nodes of Ranvier are absent

Speed of impulse transmission

• MY: Fast

• UN: Slow

Location:

• MY: In most cells

• UN:

  • Autonomic nervous system

  • Sensory fibers

  • Dendrities that only carry graded potentials

What is the structure of myelin?

Each segment consist of plasma membrane of a supporting cells rolled in concentric layers around axon

What are the functions of myelin sheaths?

Form an insulating layer that

• Prevents leakage of electrical current from axon

• Increases speed of impulse conduction

• Makes impluse propagation more energy efficient

Why are they good electrical insulators?

Because plasma membrane of myelinated cells have less protein

What is consist in the external material and what is it called?

Called the neurilemma and consist of:

• Nucleus

• Most of cytoplasm

How does myelin sheath make electrical impulses quicker?

Produces Nodes of Ranvier because myelin sheath do not touch each other, forming gaps

• Allows electrical impulses to jump between Nodes of Ranvier

Myelin sheath produced by in:

CNS

PNS

In CNS, MS produced by: Oligodendrocytes

In PNS, MS produced by: Schwann cells

Myelin Sheath in PNS

Developed during

Structure

Developed during: Fetal peirod and 1 year of postnatal life

Structure: Concentric layers of Schwann cell plasma membrane wrap the axon in packed coil of membranes

Myelin Sheath in CNS

Structure

Structure: Each oligodendrocyte has multiple processes that coil around several different axons

Structural differences between myelin sheath in CNS and PNS

Which one forms faster

Which one has nodes that are spaced more

Faster formation: CNS

Nodes are spaced more: PNS

What is neurolemma?

Outer nucleated cytoplasmic layer which encloses myelin sheath

What is white matter?

Regions of the brain that contain dense collections of myelinated fibers

What is grey matter?

Regions of the brain that contain mostly nerve bodies and unmyelinated fibers