What brain structure is involved in the formation of myelin?

The vertebrate nervous system.

Explanation:

Myelin sheaths are produced by glial cells: Schwann cells in the peripheral nervous system and oligodendrocytes in the central nervous system.

Dendrite

Explanation:

A dendrite (tree branch) is where a neuron receives input from other cells. Dendrites branch as they move towards their tips, just like tree branches do, and they even have leaf-like structures on them called spines.