16.8
Nervous Tissue: Myelin
The myelin sheath is a multilayered lipid and protein covering that insulates the axon of a neuron, enhancing the speed of nerve impulse conduction. A…
Myelin is a protective, multilayered covering around axons, composed of lipids and proteins, that provides electrical insulation.
In the PNS, individual Schwann cells repeatedly coil around short segments of an axon. The inner layers of this wrapping form the myelin sheath, while the outer portion, the neurolemma, contains the Schwann cell's nucleus and cytoplasm.
Two adjacent myelinated regions, or internodes, are separated by exposed areas of the axon, known as nodes of Ranvier. Because myelin is insulating, a nerve impulse jumps along an axon from one node to the next, speeding up the propagation of nerve impulses.
In the CNS, oligodendrocytes send out flat, cellular processes that wrap around axons. Several oligodendrocytes contribute to the myelination of one axon, and one oligodendrocyte myelinates parts of many axons.
Myelinated axons in the CNS also have nodes of Ranvier and internodes but no neurolemma.
Unmyelinated PNS and CNS axons are thin and lack myelin sheaths. In the PNS, a single Schwann cell sometimes encloses many unmyelinated axons without coiling around them.
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Q1: What is myelin and what does it do for nerve impulses?
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.
Q2: How do Schwann cells and oligodendrocytes differ in myelinating 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.
Q3: What are nodes of Ranvier and internodes?
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.
Q4: Why is the neurolemma important for axon regeneration?
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.
Q5: What are unmyelinated axons and how do Schwann cells handle them?
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.
Q6: How does myelination affect nerve response speed in infants versus adults?
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.
Q7: What is the structural composition of the myelin sheath?
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.