Refractory Period & Conduction Speed Revision Notes | A Level AQA Biology

1. Why the Refractory Period Matters

Picture a fire alarm that, once triggered, stays silent for a fraction of a second before it can ring again. That enforced silence is not a flaw — it is what stops the alarm from producing one continuous, meaningless wail. Neurones work on exactly the same principle, and that enforced silence is called the refractory period.

After an action potential fires, the axon membrane enters a state where it temporarily cannot generate another action potential. This window of inexcitability is the refractory period, and it has two distinct phases that AQA examiners expect you to distinguish clearly.

During the absolute refractory period, the voltage-gated Na⁺ channels that drove depolarisation are inactivated — they have closed in a way that prevents them from reopening until they have transitioned from their inactivated state during membrane repolarisation. Recovery of these channels begins as the membrane repolarises and is completed when the resting potential is approached, but some recovery can start before full resting potential is reached. No amount of extra stimulation can override this; the channels are physically unable to respond.

During the relative refractory period, the membrane is hyperpolarised — more negative than the resting potential — because K⁺ channels are still open and potassium ions continue to leave the cell. A stimulus must overcome this extra negativity before threshold can be reached again.

2. Discrete Impulses and Frequency Limitation

The refractory period does two critical jobs simultaneously: it ensures impulses are discrete (separate from one another), and it sets an upper ceiling on how frequently impulses can be transmitted.

Because a region of membrane cannot fire again until it has fully recovered, each action potential is a distinct, all-or-nothing event. There is no possibility of two action potentials merging into one prolonged depolarisation — the refractory period physically separates them in time.

The refractory period also ensures that impulses travel in one direction only. The region of membrane behind the advancing action potential is still refractory, so depolarisation cannot spread backwards — it can only move forwards along the axon towards the next resting region.

Frequency limitation follows directly from this. If the refractory period lasts approximately 2 ms, the maximum theoretical frequency of impulse transmission is around 500 impulses per second (500 Hz). In practice, the relative refractory period means that sustained firing at maximum frequency requires unusually strong stimuli; most neurones fire at much lower frequencies under normal conditions.

This frequency ceiling is physiologically important. The nervous system encodes stimulus intensity not by varying the size of individual action potentials (they are all-or-nothing and therefore identical in amplitude) but by varying the frequency of impulses. A stronger stimulus produces a higher frequency of firing, up to the maximum set by the refractory period. The refractory period is therefore the mechanism that makes frequency coding possible.

The refractory period therefore underpins two fundamental properties of nervous communication: the discreteness of individual impulses and the frequency-based coding of stimulus intensity. Both are direct consequences of the ionic events at the membrane during and after an action potential.

3. Factors Affecting Conduction Speed — Myelination and Saltatory Conduction

Now that you understand how impulses are generated and separated, the next question is: how fast do they travel? Conduction speed varies enormously between neurones, and the first major factor is whether the axon is wrapped in a myelin sheath.

Myelin is a fatty, electrically insulating layer formed by Schwann cells wrapped repeatedly around the axon. This insulation prevents ion movement across the membrane along the myelinated sections, so depolarisation cannot occur there. Instead, ionic currents are forced to jump from one gap in the myelin to the next.

Because ion channels are concentrated at the nodes of Ranvier, depolarisation occurs only at these points. The action potential effectively leaps from node to node rather than propagating continuously along the entire membrane. This is saltatory conduction — from the Latin saltare, meaning to jump.

Saltatory conduction is far faster than continuous conduction for two reasons. First, the action potential skips large stretches of membrane, covering more distance per unit time. Second, fewer ion channels need to open and close along the length of the axon, which also reduces the metabolic cost of restoring ion gradients via the Na⁺/K⁺ pump.

To put numbers to this: unmyelinated C fibres conduct at roughly 0.5–2 m/s, whereas myelinated A fibres can conduct at 70–120 m/s. Myelination therefore increases conduction speed by a factor of up to 100-fold — a dramatic difference with real physiological consequences for reaction time and reflex speed.

4. Axon Diameter and Temperature

Myelination is not the only variable. Even among unmyelinated axons, conduction speed differs — and the key structural factor here is the diameter of the axon itself.

A wider axon has lower internal electrical resistance. Think of it like water flowing through a pipe: a wider pipe offers less resistance to flow, so the current moves more easily. In a wider axon, the local electrical currents that depolarise the next section of membrane spread further and faster, so the action potential advances more rapidly.

This is why the giant axons of squids — which can reach 1 mm in diameter — conduct at speeds of around 25 m/s despite being unmyelinated. Evolution has used two independent strategies to achieve fast conduction: increased diameter in invertebrates and myelination in vertebrates.

Temperature is the third major factor. Nerve conduction depends on the movement of ions through protein channels — a process driven by the kinetic energy of molecules. Higher temperatures increase molecular kinetic energy, so ion channels open and close faster, ion diffusion rates increase, and the Na⁺/K⁺ pump restores gradients more rapidly. The net effect is faster conduction.

Conversely, lower temperatures slow conduction. This is clinically relevant: cooling (such as with ice) can produce numbness by slowing nerve conduction, but local anaesthetics primarily work by blocking voltage-gated Na⁺ channels, preventing depolarisation. At very low temperatures, conduction can fail entirely — which is why extreme cold causes numbness and loss of sensation.

5. Applying the Concepts — Worked Reasoning

AQA questions on this topic often present unfamiliar scenarios and ask you to apply your understanding of conduction speed or the refractory period. The step-by-step walkthrough below models the reasoning process for a typical exam-style question.

Worked example: Applying Refractory Period to Calculate Maximum Impulse Frequency

A neurone has an absolute refractory period of 1 ms and a relative refractory period of 2 ms. (a) Calculate the maximum frequency of impulse transmission for this neurone. (b) Explain why the actual frequency under normal physiological conditions is likely to be lower than this maximum.

1. Total refractory period = absolute refractory period + relative refractory period = 1 ms + 2 ms = 3 ms — The minimum time between the start of one action potential and the start of the next is determined by the total refractory period. During the relative refractory period, a new action potential is theoretically possible, but only with a suprathreshold stimulus. For maximum frequency, we use the total refractory period as the minimum interval.
2. Maximum frequency = 1000 ms ÷ 3 ms = 333 impulses per second (to 3 significant figures) — There are 1000 ms in 1 second. Dividing by the minimum interval between impulses gives the maximum number of impulses that can occur in one second. [M1] for correct division setup; [A1] for correct answer with units.
3. During the relative refractory period, the membrane is hyperpolarised because K⁺ channels remain open. A stimulus must exceed the normal threshold to trigger a new action potential. Under normal physiological conditions, most stimuli are at or near threshold, so they cannot reliably trigger firing during the relative refractory period. — This part requires an 'explain' response: link the ionic state of the membrane (hyperpolarisation due to open K⁺ channels) to the functional consequence (higher stimulus needed, so firing is less frequent in practice). [B1] for hyperpolarisation; [B1] for linking to stimulus threshold requirement.

Answer: (a) Maximum frequency = 333 impulses per second. (b) During the relative refractory period, the membrane is hyperpolarised due to continued K⁺ efflux, so a suprathreshold stimulus is required to generate a new action potential; normal physiological stimuli are insufficient to achieve this consistently, so actual firing frequency is lower than the theoretical maximum.

6. Exam Technique and Common Mistakes

Questions on the refractory period and conduction speed appear regularly in AQA A Level Biology papers, often in the context of unfamiliar data or novel scenarios. The marks are almost always awarded for precise mechanistic reasoning, not vague generalisations.

When asked to explain why the refractory period produces discrete impulses, you must link the ionic mechanism to the functional outcome. A full-mark answer names the inactivated Na⁺ channels, states that the membrane cannot depolarise again until they recover, and then connects this to the separation of impulses in time. Stopping at 'the channels are closed' will not earn the explanation mark.

When asked to explain saltatory conduction, name the nodes of Ranvier explicitly and state that the action potential is generated only there. Saying the impulse 'skips along' without naming the nodes is too vague for AQA mark schemes.