Myelin, saltatory conduction and demyelination
Myelin is a fatty insulating sheath around an axon, so the action potential appears to leap from one gap in the sheath to the next and the signal travels faster.
Remember
Myelin insulates the axon, so current spreads further and a new action potential only needs to be generated at each gap in the sheath; when myelin is lost, the current leaks away before it reaches the next gap, so the signal slows or fails.
Memory aid
myelin is the wall of a hosepipe, and demyelination punches holes in it so that water leaks out before it reaches the end
Demyelination is like holes in a hosepipe: the water leaks out before it reaches the end.
Myelin is a fatty insulating sheath wrapped around many axons (the long fibres of a neurone). It makes conduction, the travel of the action potential along the axon, faster. The sheath has small gaps in it, and these are called nodes of Ranvier. Ion channels are highly concentrated at the nodes, with very few along the myelinated stretches between them.
Depolarisation at one point spreads to the surrounding membrane as a local current, but the further it spreads, the weaker it gets. The length constant is the distance a voltage change travels before it falls to 37% of its original value, so a higher length constant means the charge spreads further. Myelin raises membrane resistance (how hard it is for current to leak across the membrane) and lowers membrane capacitance (the ability of the membrane to store charge).
Because the insulation keeps current from leaking away, the local current reaches the next node while it is still above threshold, and a new action potential is generated there. The signal seems to jump from node to node, which is called saltatory conduction (from the Latin for leaping). An unmyelinated axon generates the action potential at every point along its membrane, and that is slower.
In demyelination, the myelin sheath is damaged. Multiple sclerosis, an autoimmune condition, is one example. The damaged areas have low resistance and high capacitance, so the local current leaks away and fails to reach the next node, and conduction slows or fails.
Key facts
- Myelin
- a fatty insulating sheath wrapped around axons, interrupted by small gaps
- Nodes of Ranvier
- the small gaps in the sheath, where ion channels are highly concentrated
- Between the nodes
- a myelinated axon has very few ion channels along the membrane there
- Membrane resistance
- raised by myelin, so less current leaks across the membrane
- Membrane capacitance
- lowered by myelin, so the membrane stores less charge
- Length constant
- increased by myelin, so depolarisation spreads further along the axon
- Demyelination
- gives low resistance and high capacitance, so the local current fails to reach the next node
Exam traps
- Sodium channels spread evenly along the axon do not explain the faster conduction in a myelinated axon, because the channels are concentrated at the nodes and the myelinated stretches between them have very few; an even spread describes an unmyelinated axon.
- A narrower axon does not conduct faster, because it is harder for current to flow along it, whereas a wider axon conducts faster.
- Opening more potassium channels in the membrane between the nodes does not speed conduction, because it lowers membrane resistance, so current leaks away and depolarisation spreads less far.
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