Biology 30 - Nervous System - The cerebrum of the brain is considered as "grey matter" because the ...

I think you're right about the first one:

All axons in the peripheral nervous system are surrounded by a continuous living sheath of Schwann cells, known as the neurilemma. The axons of the central nervous system lack a neurilemma. Some axons in the peripheral nervous system and the central nervous system are surrounded by a myelin sheath. In the peripheral nervous sytem, this insulating covering is formed by successive wrappings of the cell membrane of Schwann cells; in the central nervous system it is formed by oligodendrocytes. Axons smaller than two micrometers in diameter are usually unmyelinated, and those larger are mostly myelinated. Myelinated axons conduct impulses faster than those than are inmyelinated. Each Schwann cell wraps only about a millimeter of axon, leaving gaps of exposed axon between the Schwann cells. These gaps in the myelin sheath are called thye nodes of Ranvier. The wrappings of Schwann cells provide insulation around the axon, leaving the nodes of Ranvier exposed to produce nerve impulses. When an axon is cut, the distal portion of the axon that was severed from the cell body degenerates and is phagocytosed by Schwann cells. The Schwann cells, surrounded by the basement membrane then for a regeneration tube as the part of axon that is connected to the body begins to grow and exhibit amoeboid movement.