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Between the neurolemma and the axis cylinder is the medullated sheath, composed of a fatty substance known as myelin.

The axis cylinder is the essential conducting structure of the nerve, while the neurolemma and the myelin act as insulating agents.

Each nerve fibre consists of a number of nerve fibrils collected into a central bundle—the axis cylinder—which is surrounded by an envelope, the neurolemma or sheath of Schwann.

This medullated sheath is interrupted at the nodes of Ranvier, and in each internode is a nucleus lying between the myelin and the neurolemma.