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6.2.5Organisms Respond to Changes in their Environments (A Level only)

Speed of Impulse Conduction — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 8 parts Lesson 20 of 44 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
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Lesson summary

6.2.5 Speed of Impulse Conduction — AQA A Level Biology 7402 notes: Explain how myelination, axon diameter and temperature affect the speed of conduction.In myelinated neurones the myelin sheath insulates the axon, so ions can cross the membrane only at the nodes of Ranvier.Actio…

  • saltatory conduction
  • wider axon
  • kinetic energy
  • Exam-style question 1
  • Exam-style question 2
  • Exam-style question 3

Part 1 of 7Saltatory conduction

In myelinated neurones the myelin sheath insulates the axon, so ions can cross the membrane only at the nodes of Ranvier.

Saltatory conduction
Action potentials therefore appear to jump from node to node, which is called saltatory conduction (from the Latin saltare, to leap).
Local circuits of Na+ ions inside the axon carry the depolarisation from one node to the next, and the next node is brought to threshold.
As action potentials do not need to occur along the whole length of the axon, conduction is much faster: up to about 100 m s-1, compared with about 0.5 to 2 m s-1 in non-myelinated axons.
It also saves energy, because fewer ions enter and so fewer need to be pumped back after each impulse.

Part 2 of 7Axon diameter

A wider axon conducts faster than a narrow one.

1
Larger axons have lower internal resistance to the flow of ions inside the axon,
  • so local circuits spread further and depolarise the next region more quickly.
2
There is also less leakage of ions across the membrane relative to the volume of cytoplasm, because the ratio of surface area to volume is smaller.
3
Invertebrates such as squid, which lack myelin, have evolved giant axons about 1 mm wide for rapid escape responses.
4
Vertebrates achieve high speeds with narrow axons by using myelin, which allows a large number of axons to be packed into a nerve.

Part 3 of 7Temperature

Higher temperature increases the kinetic energy of ions, so diffusion through channels is faster, and it increases the rate of the enzyme-controlled processes that supply ATP for the sodium–potassium pump.

Temperature
Conduction speed therefore increases with temperature, up to an optimum.
At high temperatures the proteins that form channels and pumps denature, so conduction fails.
This is why the speed of conduction in cold-blooded animals depends strongly on the environmental temperature,
  • and why a limb that is very cold can lose sensation.

Part 4 of 7Summary table

FactorEffect on speedReason
MyelinationIncreasesSaltatory conduction; depolarisation only at nodes
Greater axon diameterIncreasesLower resistance to ion flow, less leakage
Higher temperatureIncreases (up to an optimum)Faster ion diffusion, more energy for pumps
Longer internodal distanceIncreases, up to a limitFewer depolarisation events along the axon

Part 5 of 7Using data

When given conduction speed data for different neurones, compare myelinated and non-myelinated types of similar diameter to isolate the effect of myelin.

1
Remember that conduction speed is distance divided by time, so check the units, particularly mm and ms.
2
If a graph of speed against temperature rises and then falls sharply, say that speed increases because ions diffuse faster,
  • then falls because the proteins in the membrane denature.
3
In multiple sclerosis the immune system damages myelin,
  • causing slower and sometimes failed conduction, with symptoms such as muscle weakness, numbness and loss of coordination.
4
This links structure to function and to disease.

Part 6 of 7Evaluating the evidence for saltatory conduction

Early evidence for saltatory conduction came from recordings along single myelinated axons.

1
When electrodes are placed along an axon, action potentials are detected only at the nodes,
  • and the time to travel between nodes is very short compared with the time spent at each node.
2
Local anaesthetic applied to a single node can be bypassed if the neighbouring node is close enough,
  • but when several adjacent nodes are blocked conduction fails, showing that the local current has a limited reach.
3
If the myelin is experimentally removed, the conduction speed falls and may fail altogether, as in demyelinating disease.
4
Such evidence supports the explanation that the sheath forces the current to flow between nodes.
5
In an evaluation question, link each observation to this mechanism and comment on whether it applies to non-myelinated axons.

Note that fish and amphibians have myelinated axons too, but their conduction speeds are lower than mammals at the same temperature, partly because they are cold-blooded. Always compare like with like.

A final point is that conduction speed is measured in the nervous system clinically: a nerve conduction test applies a stimulus at one site and records the response at another, and a slowed value suggests demyelination or nerve damage.

Part 7 of 7Exam-style questions

  1. Explain why impulses are conducted faster along a myelinated axon than a non-myelinated axon. [3]3 marks

  2. An impulse takes 4.0 ms to travel 0.30 m along a motor neurone. Calculate its speed in m s-1. [2]2 marks

  3. Explain why conduction speed in a frog neurone increases when the temperature is raised from 5 °C to 20 °C. [2]2 marks

Key terms

TermMeaning
Saltatory conductionJumping of impulses between nodes of Ranvier in myelinated axons
Node of RanvierUnmyelinated gap in the sheath where depolarisation occurs
Conduction speedDistance travelled by an impulse per unit time
Axon diameterWidth of the axon, affecting resistance to ion flow
Multiple sclerosisAutoimmune disease in which myelin is damaged

Avoid these mistakes

1
Saying the impulse jumps because myelin has gaps; the nodes are the gaps.
2
Thinking a myelinated axon has a larger action potential; the size is the same.
3
Ignoring unit conversion between ms and s in speed calculations.
4
Stating that higher temperature always increases speed; at very high temperatures proteins denature.
5
Describing myelin as a conductor; it is an insulator.

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