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8.1.5Unit 6: Practical Skills in Biology II

Myelination and Saltatory Conduction — IAL Edexcel Linear Revision Notes & Topical Questions

5 min read 5 parts Lesson 39 of 48 Unit quiz

📚 IAL Edexcel Linear Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
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Part 1 of 5Structure of Myelinated Axons

Myelin sheath
  • Myelin sheath Formed by Schwann cells wrapped around the axon, it is a fatty layer that electrically insulates the membrane.
Ion movement
  • Ion movement Sodium and potassium ions cannot cross the membrane under the myelin, so depolarisation cannot occur there.
Nodes of Ranvier
  • Nodes of Ranvier Gaps between Schwann cells where the axon membrane is exposed and has many voltage-gated channels.
Action potentials
  • Action potentials They are generated only at the nodes, one after another along the axon.

Part 2 of 5Saltatory Conduction

Saltatory means jumping.

1
An action potential at one node depolarises the membrane there, and sodium ions enter the axon.
2
The sodium ions diffuse along the axon cytoplasm beneath the myelin as local circuits.
3
When they reach the next node they depolarise its membrane to threshold.
4
Voltage-gated channels open at that node, and a new action potential forms.
5
The impulse therefore appears to jump from node to node, which is called saltatory conduction.

Part 3 of 5Comparison and Speed

Comparison and Speed
Myelinated
  • Myelinated Impulse jumps between nodes, so conduction is much faster than in an unmyelinated axon of the same diameter.
Unmyelinated
  • Unmyelinated Depolarisation has to occur along the whole length of the membrane, so conduction is slow.
Energy
  • Energy Fewer sodium ions need to be pumped back since depolarisation only occurs at the nodes.
Importance
  • Importance Fast conduction matters for quick responses, such as in reflexes.
FeatureMyelinatedUnmyelinated
DepolarisationAt nodes onlyAlong whole axon
SpeedFastSlow
InsulationSchwann cell myelinNone

Part 4 of 5Worked Example

Question

  • An impulse travels 0.80 m along a myelinated axon in 8.0 ms, and the same distance along an unmyelinated axon in 0.80 s.
  • Calculate the speed in each and compare them.

  1. Speed = distance ÷ time.
  2. Myelinated0.80 ÷ 0.0080 = 100 m s⁻¹.
  3. Unmyelinated0.80 ÷ 0.80 = 1.0 m s⁻¹.
  4. Ratio = 100 ÷ 1.0 = 100.

Answer

  • 100 m s⁻¹ and 1.0 m s⁻¹;
    • the myelinated axon is 100 times faster.

Part 5 of 5Exam Checklist & Tips

8.1.5 – Key Points for the Exam

  • Saltatory conduction Conduction in which the impulse jumps between nodes of Ranvier.
  • Local circuit A flow of ions inside the axon between active and resting regions.
  • Schwann cell A cell that wraps around an axon to form the myelin sheath.
  • Node of Ranvier A gap in the myelin where action potentials occur.

Common Mistakes

  • Saying the action potential travels through the myelin; it cannot depolarise there.
  • Believing that myelin makes the action potential larger.
  • Writing that the nodes are made of myelin.
  • Stating that saltatory conduction needs no sodium ions.

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