Myelination and Saltatory Conduction

Speed matters. A gazelle escaping a cheetah cannot afford to wait 500 ms while nerve impulses meander from its retina to its leg muscles. Evolution has produced an elegant solution: myelinate the axon, insulating it so that action potentials leap from gap to gap rather than propagating continuously. This lesson explores how myelination is achieved, how it accelerates conduction, and why it matters clinically. It covers OCR A-Level Biology A specification 5.1.3(d).

Key Definitions:

• Myelin sheath — a multilayered wrap of Schwann cell membrane that insulates an axon.
• Schwann cell — a glial cell of the peripheral nervous system that forms myelin.
• Nodes of Ranvier — short gaps (~1 μm) between adjacent Schwann cells where the axon membrane is exposed.
• Saltatory conduction — the "jumping" of action potentials from one node of Ranvier to the next, greatly increasing conduction velocity.

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The Schwann Cell: An Extraordinary Wrap

In the peripheral nervous system, a single Schwann cell wraps itself around a short segment of axon (~1 mm). It does this by extending a flap of plasma membrane, then rotating around the axon many times — up to 100 layers in the thickest axons. Because each Schwann cell lies alongside only one small stretch of axon, thousands are needed to myelinate a long motor neurone.

Between each pair of neighbouring Schwann cells is a small exposed region of axon membrane called a node of Ranvier, typically 1–2 μm long and spaced roughly every 1 mm. At the node, voltage-gated Na⁺ and K⁺ channels are densely packed; elsewhere along the axon, under the myelin, these channels are absent because the membrane is physically insulated.

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How Myelin Insulates

Myelin is rich in lipid (about 80% lipid, 20% protein) and is therefore an excellent electrical insulator. Current flow across the axon membrane (i.e. ion movement) can only occur where the membrane is exposed — at the nodes of Ranvier. Under a myelinated segment, no ion movement takes place. This has two consequences:

1. The action potential cannot regenerate along myelinated segments because there are no voltage-gated channels and no ion flow.
2. Current leaks only at the nodes, so the charge carried by Na⁺ influx at one node spreads passively down the axon to the next node, causing it to depolarise.

The upshot is that the action potential effectively jumps from node to node, reaching the next node in a fraction of the time it would take to propagate continuously.

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Conduction Speed — Myelinated vs Unmyelinated

Type of axon	Typical diameter	Typical speed	Example
Unmyelinated (small)	1 μm	0.5–2 m s⁻¹	Many sympathetic post-ganglionic neurones
Unmyelinated (giant squid)	500 μm	~25 m s⁻¹	Escape reflex of squid
Myelinated (mammalian)	5–20 μm	10–120 m s⁻¹	Motor neurones, sensory neurones

Two features determine conduction speed:

• Axon diameter — larger axons have lower internal resistance to current flow, so the action potential propagates more quickly. This is a physics relationship: like a larger water pipe carrying more flow.
• Myelination — as described above, the most effective way to accelerate conduction without needing a massive axon.

Myelination allows mammalian neurones to achieve extraordinary speed with a diameter of just 10–20 μm — a huge saving in space and metabolic cost compared with the giant axons needed in molluscs.

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Why It Is Called Saltatory

"Saltatory" comes from the Latin saltare, meaning "to leap". Action potentials do not literally teleport — they appear at each node in sequence, one after another, regenerated by voltage-gated Na⁺ channels. Between nodes, the electrical disturbance travels passively and very quickly, because it is a simple electrical event rather than a series of ion channels opening.

flowchart LR
    A[Node 1<br/>Action potential] -->|Passive current flow under myelin| B[Node 2<br/>Depolarises to threshold]
    B -->|New action potential| C[Node 3<br/>Depolarises to threshold]
    C -->|...| D[Next node]

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Metabolic Efficiency