Conduction Speed: Myelination and the Refractory Period
Myelin speeds conduction by reducing current leak and allowing action potentials to regenerate at nodes. Refractory periods limit firing frequency and prevent recently active membrane from immediately firing again.
What a learner can do afterwards
- Explains how myelin speeds passive current spread and where action potentials regenerate.
- Explains how axon diameter and temperature can affect conduction speed.
- Links refractory periods to impulse direction and maximum firing frequency.
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Myelin insulates an axon between gaps called nodes of Ranvier. It reduces current leak and lowers membrane capacitance, so less charge is needed to change membrane voltage. Current spreads passively between nodes. Voltage-gated sodium channels are concentrated at nodes, where action potentials regenerate. This is saltatory conduction. Without myelin, successive membrane regions regenerate the signal.
Greater axon diameter lowers internal resistance to current, usually speeding conduction when other conditions match. Moderate warming within a healthy axon's physiological range usually speeds channel gating and conduction. Excess heat can impair function, so this is not an unlimited warming rule. Myelin loss increases current leak and can slow or block conduction.
The absolute refractory period begins during a spike. Many sodium channels are inactivated, so another spike cannot start in that membrane region, even with a strong stimulus. During the following relative refractory period, a stronger stimulus can trigger another spike. Sodium channels are recovering, while continued potassium conductance and hyperpolarisation can keep excitability below normal.
A normal spike travels from its initiation region toward the terminals. Current spreads both ways, but recently active membrane behind it is refractory and cannot immediately fire again. Resting membrane ahead can respond. Direction is not a permanent property of the axon: stimulating the middle of an axon that is resting throughout can launch spikes in both directions.
For a theoretical maximum firing rate, divide 1000 milliseconds by the minimum interval between spike starts. If the interval is 4 milliseconds, 1000 / 4 = 250 spikes per second. Absolute refractoriness sets an upper limit. Relative refractoriness and other cell properties can lower the actual rate. The interval differs among neurones.
Myelin speeds current spread between nodes. Diameter and temperature affect speed. Refractory periods limit firing rate and prevent immediate re-excitation of recently active membrane.
Lesson, questions and flashcards written for this dataset. Factual background includes OpenStax chapters on action potentials and neuronal communication.
Readings and revised flashcards written for this dataset, with OpenStax chapters consulted for facts. Video references: Khan Academy. Video reuse terms have not been verified.
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How myelin changes conduction
An action potential depends on ion movement through membrane channels. In an unmyelinated axon, successive exposed membrane regions regenerate the signal. In a myelinated axon, insulation covers most of the membrane. The gaps between covered regions are nodes of Ranvier, where voltage-gated sodium channels are concentrated.
Myelin reduces current leak and lowers membrane capacitance. Less charge is needed to change the voltage, and local current can excite the next node. Sodium entry at that node regenerates the action potential. This pattern of node-to-node regeneration is called saltatory conduction. No physical object jumps across the covered region, and passive current spread takes time.
Comparing conduction speeds
Axon diameter is one factor in conduction speed. When temperature and myelination match, a wider axon usually conducts faster because it offers less internal resistance to local current.
Temperature affects channel gating. Moderate warming within a healthy axon's physiological range usually speeds conduction. This limited comparison does not mean that heating always helps: excessive heat can impair cell function.
Myelin damage changes another part of the system. Current leaks through membrane that was previously insulated. The current reaching the next node may bring it to threshold later, or may not reach threshold at all. Loss of myelin can therefore slow or block conduction.
Recovery, direction and firing rate
During the absolute refractory period, sodium-channel inactivation prevents another action potential in the same membrane region. During the following relative phase, a stronger stimulus can trigger another spike, although excitability has not fully recovered.
A normally travelling spike encounters resting membrane ahead and recently active, refractory membrane behind. This prevents immediate re-excitation behind it. The axon is not permanently restricted to one direction. Stimulating the middle of an entirely resting axon can launch spikes toward both ends.
To calculate a theoretical maximum rate, use a stated minimum interval between spike starts. An interval of 4 milliseconds gives 1000 / 4 = 250 spikes per second. Actual rates can be lower because refractory recovery is not the only limit on firing.
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