Nervous Tissue: Myelin

Myelin is a protective, multilayered covering composed of lipids and proteins that insulates axons. This insulation allows nerve impulses to jump along the axon from one node of Ranvier to the next, significantly speeding up the propagation of action potentials compared to unmyelinated axons.

In the PNS, individual Schwann cells wrap around short segments of a single axon to form myelin. In the CNS, one oligodendrocyte myelinates parts of many different axons. Additionally, Schwann cells produce a neurolemma containing the cell nucleus and cytoplasm, while oligodendrocytes lack this outer layer around axons.

Nodes of Ranvier are exposed gaps in the myelin sheath where the axon membrane is uninsulated. Internodes are the myelinated regions between adjacent nodes. This segmented structure allows nerve impulses to jump rapidly from node to node rather than traveling continuously along the entire axon.

The neurolemma is the outer layer of the myelin sheath in the PNS containing the Schwann cell's nucleus and cytoplasm. After injury, the neurolemma forms a tube that guides and stimulates axon regrowth. This structure is absent in the CNS, which limits neurogenesis and regeneration of nervous tissue in the brain and spinal cord.

Unmyelinated axons lack myelin sheaths and are thin, conducting impulses more slowly than myelinated axons. In the PNS, a single Schwann cell encloses many unmyelinated axons without coiling around them. Unmyelinated axons exist in both the PNS and CNS but lack the insulating protection of myelin.

Myelination continues from birth through maturity, progressively increasing the speed of nerve impulse conduction. Infants, whose myelination is still incomplete, exhibit slower and less coordinated responses to stimuli than older children or adults. This developmental process is crucial for normal nervous system function.

The myelin sheath consists of multiple layers of glial cell membrane wrapped around an axon. In the PNS, Schwann cell membranes form the inner myelin layers, while the outer portion containing the cell nucleus and cytoplasm is the neurolemma. This lipid and protein structure provides electrical insulation essential for rapid nerve impulse transmission.