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3.5.7Inquiry Process

Nerve Conduction Velocity — IB Biology HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 138 of 160

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Part 1 of 5Saltatory Conduction

3.5.7 Nerve Conduction Velocity
1
Myelin insulates the axon, so ions cannot cross the membrane where it is wrapped.
2
Depolarisation can only happen at the nodes of Ranvier, where sodium and potassium channels are concentrated.
3
Local currents flow quickly inside the axon from one node to the next.
4
The impulse therefore appears to jump from node to node, which is called saltatory conduction.
5
This is much faster and uses less ATP than continuous conduction because fewer ions cross the membrane.

Part 2 of 5Factors Affecting Speed

Myelination
  • Myelinated axons can conduct at about 100 m/s, but unmyelinated axons of similar size are usually near 1 m/s.
Axon diameter
  • Wider axons have less resistance to local current flow inside, so impulses travel faster, as in the giant axons of squid.
Temperature
  • Warmer conditions speed up ion movement and channel opening, so conduction is faster until proteins start to denature.
Disease
  • In multiple sclerosis the myelin sheath is damaged by an immune attack, slowing or blocking conduction.
Axon typeTypical speedExample use
Unmyelinated, narrow0.5 to 2 m/sSlow pain, autonomic control
Myelinated, wideUp to about 100 m/sMotor control of muscles

Part 3 of 5Measuring Velocity

Measuring Velocity
Formula
  • Conduction velocity equals the distance between two recording points divided by the time taken for the impulse to travel it.
Units
  • Distance is given in metres and time in seconds so velocity is in m/s; convert from mm and ms if needed.
Method
  • Place two electrodes a known distance apart on the nerve and measure the delay between the two spikes on an oscilloscope.
Clinical use
  • Slowed conduction in nerve tests helps to diagnose demyelinating disease and nerve compression.

Part 4 of 5Worked Example

Question

  • A nerve impulse takes 5 ms to travel 0.45 m along a myelinated axon.
  • Calculate the velocity, and the time taken to cover the same distance at 1.5 m/s in an unmyelinated axon.

  1. Convert time5 ms = 0.005 s.
  2. Velocity = distance ÷ time = 0.45 ÷ 0.005 = 90 m/s.
  3. Time unmyelinated = 0.45 ÷ 1.5 = 0.30 s = 300 ms, which is 60 times longer.

Answer

  • 90 m/s in the myelinated axon;
    • the unmyelinated axon needs 0.30 s (300 ms).

Part 5 of 5Exam Checklist & Tips

3.5.7 – Key Points for the Exam

  • Saltatory conduction Jumping of an impulse between nodes of Ranvier along a myelinated axon.
  • Node of Ranvier Gap in the myelin sheath where the axon membrane is exposed.
  • Conduction velocity Distance travelled by an impulse per unit time.
  • Demyelination Loss of myelin, as in multiple sclerosis, which slows impulses.

Common Mistakes

  • Thinking the impulse is a current flowing through the myelin; the sheath insulates.
  • Forgetting to convert ms to s before dividing.
  • Claiming that myelin speeds conduction by widening the axon; it works by insulation and node-to-node jumping.

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Physics 0625 & 9702 SUVAT Mastery Graph Interpretation

Constant Acceleration Equations (SUVAT) & Slopes

v = u + at  |  s = ut + ½at2  |  v2 = u2 + 2as  |  s = ½(u + v)t

  • Displacement-Time Graph: Gradient = Velocity (curve = accelerating).
  • Velocity-Time Graph: Gradient = Acceleration; Area under curve = Distance travelled / Displacement.
  • Free Fall in vacuum: a = g ≈ 9.81 m/s2 downwards (air resistance negligible).