16.8
Tessuto nervoso: mielina
La guaina mielinica è un rivestimento multistrato lipidico e proteico che isola l'assone di un neurone, migliorando la velocità di conduzione dell'imp…
La mielina è un rivestimento protettivo multistrato attorno agli assoni, composto da lipidi e proteine, che fornisce isolamento elettrico.
Nel SNP, le singole cellule di Schwann si avvolgono ripetutamente attorno a brevi segmenti di un assone. Gli strati interni di questo involucro formano la guaina mielinica, mentre la porzione esterna, il neurolemma, contiene il nucleo e il citoplasma della cellula di Schwann.
Due regioni mielinizzate adiacenti, o internodi, sono separate da aree esposte dell'assone, note come nodi di Ranvier. Poiché la mielina è isolante, un impulso nervoso salta lungo un assone da un nodo all'altro, accelerando la propagazione degli impulsi nervosi.
Nel SNC, gli oligodendrociti emettono processi cellulari piatti che avvolgono gli assoni. Diversi oligodendrociti contribuiscono alla mielinizzazione di un assone e un oligodendrocita mielizza parti di molti assoni.
Gli assoni mielinizzati nel SNC hanno anche nodi di Ranvier e internodi, ma nessun neurolemma.
Gli assoni del SNP e del SNC non mielinizzati sono sottili e privi di guaine mieliniche. Nella PNS, una singola cellula di Schwann a volte racchiude molti assoni non mielinizzati senza avvolgersi attorno ad essi.
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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.