Here's a breakdown:

* Myelin sheath: This is a fatty, white substance that wraps around the axons (the long, slender projections of neurons) like insulation around an electrical wire.

* Function: The myelin sheath acts as an insulator, preventing the loss of electrical signals as they travel down the axon. This insulation speeds up the transmission of nerve impulses, allowing for efficient communication between neurons.

* Composition: The myelin sheath is made up of specialized cells called Schwann cells in the peripheral nervous system and oligodendrocytes in the central nervous system. These cells wrap their membranes around the axon, forming multiple layers of myelin.

* Importance: The myelin sheath is crucial for proper brain function. Damage to the myelin sheath, as seen in diseases like multiple sclerosis, can lead to a range of neurological problems.

Let me know if you have any other questions!