Nerve impulses (A-level only) — AQA A-Level Biology
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Nerve impulses (A-level only) explained
Nerve impulses (A-level only) covers 6 specification points, including The structure of a myelinated motor neurone.
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and The establishment of a resting potential in…. Open a point below for its explanation and revision guidance.
The structure of a myelinated motor neurone.
A motor neurone carries impulses from the central nervous system to an effector. Its cell body, containing the nucleus and much rough endoplasmic reticulum for making proteins such as neurotransmitter enzymes, lies in the spinal cord, and many short dendrites carry impulses towards it. One long axon carries impulses away, ending in synaptic knobs at the effector. Schwann cells wrap repeatedly around the axon, and the many layers of their plasma membrane form the myelin sheath, which is rich in lipid and acts as an electrical insulator. Between adjacent Schwann cells are short gaps, the nodes of Ranvier, where the axon membrane is exposed to tissue fluid and voltage-gated sodium ion channels are concentrated. Depolarisation can occur only at these nodes, so the action potential jumps from node to node by saltatory conduction, which is faster than conduction along an unmyelinated axon.
The establishment of a resting potential in terms of differential membrane permeability, electrochemical gradients and the movement of sodium ions and potassium ions.
At rest the inside of the axon is about 70 mV negative relative to the outside. Two things create this. First, sodium-potassium pumps in the axon membrane actively transport three sodium ions out for every two potassium ions taken in, using ATP; this alone moves net positive charge outwards and builds steep electrochemical gradients, with sodium ions concentrated outside and potassium ions inside. Second, the membrane is differentially permeable: potassium ion channels are open at rest while the voltage-gated sodium ion channels are shut, so the membrane is far more permeable to potassium ions than to sodium ions. Potassium ions therefore diffuse back out down their concentration gradient faster than sodium ions leak in, leaving the inside negative.
Changes in membrane permeability lead to depolarisation and the generation of an action potential. The all-or-nothing principle.
A stimulus makes the axon membrane more permeable to sodium ions: some voltage-gated sodium ion channels open, sodium ions diffuse in down the electrochemical gradient, and the inside becomes less negative. If enough sodium ions enter to lift the potential to the threshold of about minus 55 mV, positive feedback opens all the remaining voltage-gated sodium ion channels, sodium ions flood in and the membrane depolarises to roughly plus 40 mV, so the inside becomes positive. That is the action potential. At the peak the sodium ion channels close and voltage-gated potassium ion channels open, so potassium ions diffuse out and the membrane repolarises, overshooting briefly into hyperpolarisation before the sodium-potassium pump restores the resting potential.
The passage of an action potential along non-myelinated and myelinated axons, resulting in nerve impulses.
An action potential is not a substance that travels; it is regenerated over and over along the membrane. In a non-myelinated axon, the sodium ions that enter at the depolarised point spread sideways inside the axon as local currents, depolarising the adjacent patch of membrane to threshold, which opens its sodium ion channels and produces a fresh action potential there. This repeats continuously along the whole length of the membrane, so conduction is relatively slow. In a myelinated axon the sheath insulates most of the membrane, so ions cannot cross except at the nodes of Ranvier. Local currents therefore run from one node to the next and the action potential is regenerated only at the nodes, which is saltatory conduction, the impulse appearing to jump.
The nature and importance of the refractory period in producing discrete impulses and in limiting the frequency of impulse transmission.
Immediately after an action potential the membrane cannot produce another one. During the absolute refractory period the voltage-gated sodium ion channels are inactivated, so no stimulus, however strong, can depolarise that patch of membrane. During the relative refractory period that follows, the channels have reset but the membrane is hyperpolarised because potassium ion channels close slowly, so only a stronger than normal stimulus reaches threshold. Two consequences are examinable. First, action potentials are discrete: each is a separate event that cannot merge with or overlap the one before, and because the membrane behind the wave is refractory the impulse can travel only forwards. Second, the refractory period sets a ceiling on frequency, so a shorter refractory period allows a higher maximum frequency of impulses.
Factors affecting the speed of conductance: myelination and saltatory conduction; axon diameter; temperature.
Three factors affect nerve impulse conductance speed. Myelination provides electrical insulation. In myelinated neurones, depolarisation only occurs at the nodes of Ranvier, allowing the action potential to jump from node to node via saltatory conduction. This is much faster than continuous depolarisation along a non-myelinated axon. Axon diameter also affects speed; a wider axon, like the squid giant axon, offers less resistance to the flow of local currents in the cytoplasm, so depolarisation spreads faster. Finally, higher temperatures increase the kinetic energy of ions. This leads to a faster rate of passive diffusion of Na⁺ and K⁺ through voltage-gated channels, increasing conduction speed up to an optimum before membrane proteins denature.
Your focus
- Label the axon, Schwann cell, myelin sheath, node of Ranvier and cell body on a diagram of a myelinated motor neurone.
- Explain how the layered membranes of a Schwann cell insulate the axon and restrict depolarisation to the nodes.
- Distinguish a motor neurone from a sensory neurone by the position of the cell body and the direction of conduction.
Show all 18 objectives
- Explain how moving three sodium ions out for every two potassium ions in contributes to a negative interior.
- Describe the differential permeability of the axon membrane at rest and predict what happens if potassium ion channels are blocked.
- Explain why a respiratory inhibitor such as cyanide causes the resting potential to decay.
- Annotate a trace of an action potential with the ion movements responsible for each phase.
- Explain, using threshold, why a stimulus below threshold produces no action potential at all.
- Explain how the nervous system codes a stronger stimulus when every action potential is the same size.
- Explain how local currents depolarise the next region of membrane to threshold in a non-myelinated axon.
- Compare conduction in myelinated and non-myelinated axons in terms of how much membrane depolarises.
- Explain why the refractory period, rather than the myelin sheath, makes propagation one-way.
- Explain why an action potential cannot travel backwards along the stretch of axon it has just passed along.
- Calculate the maximum frequency of impulses from a given refractory period and say what that means for coding stimulus intensity.
- Distinguish the absolute from the relative refractory period in terms of the state of the sodium ion channels and the membrane potential.
- Explain why destroying myelin, as happens in multiple sclerosis, produces slower responses to a stimulus.
- Predict and justify the effect of increasing axon diameter on the speed of conduction.
- Explain how temperature affects conduction speed through the kinetic energy of ions and protein denaturation.
Nerve impulses (A-level only) exam tips
Marking Points
- Identify the axon, or its membrane, as the process carrying impulses away from the cell body towards the effector.
- Identify the Schwann cell, or the myelin sheath formed from its layered plasma membranes, wrapped around the axon.
- Name the nodes of Ranvier as gaps in the myelin sheath where the axon membrane is exposed to tissue fluid.
- State that the cell body contains the nucleus and lies in the central nervous system, with dendrites carrying impulses towards it.
- Link the myelin sheath to electrical insulation of the axon membrane, restricting depolarisation to the nodes.
- Explain that depolarisation occurring only at nodes allows saltatory conduction, so the impulse jumps from node to node and travels faster.
- one mark for the membrane being more permeable to potassium ions and less permeable to sodium ions
- one mark for sodium ions being actively transported out of the axon and potassium ions in
- one mark for stating that this active transport is carried out by a sodium-potassium pump and requires ATP
- one mark for potassium ions diffusing out down the concentration gradient, leaving the inside of the axon negative
- one mark for quoting a resting potential of about minus 70 mV, with the inside negative relative to the outside
- one mark for sodium ion channels opening so the membrane becomes more permeable to sodium ions
- one mark for sodium ions diffusing into the axon down the electrochemical gradient
- one mark for depolarisation, the inside becoming positive, once the threshold is reached
- one mark for potassium ion channels opening and potassium ions diffusing out, causing repolarisation
- one mark for the all-or-nothing point, that below threshold no action potential is produced and above threshold every action potential is the same size
- one mark for local currents, meaning sodium ions spreading along the inside of the axon to the next region of membrane
- one mark for the next region being depolarised to threshold so that a new action potential is generated there
- one mark for depolarisation occurring along the whole length of the membrane in a non-myelinated axon
- one mark for myelin preventing ion movement across the membrane except at the nodes of Ranvier
- one mark for saltatory conduction, the action potential jumping from node to node, giving faster transmission
- State that during the absolute refractory period the membrane cannot be depolarised again because the voltage-gated sodium ion channels are inactivated.
- State that during the relative refractory period a stronger than normal stimulus can trigger an action potential, because the membrane is hyperpolarised.
- Explain that the impulse travels in one direction only, because the membrane behind the action potential is refractory.
- Explain that action potentials are discrete, so they are separated and cannot overlap or merge.
- Explain that the refractory period limits the frequency of impulses, giving a maximum number per second.
- Calculate a maximum frequency from a given refractory period, for example a 2 ms refractory period gives a maximum of about 500 impulses per second.
- Myelination provides electrical insulation, allowing saltatory conduction where the action potential jumps between nodes of Ranvier.
- In non-myelinated axons, depolarisation must occur across the entire membrane length, resulting in slower conduction.
- A greater axon diameter reduces resistance to the flow of ions in the cytoplasm, increasing the speed of conduction.
- Higher temperatures increase the kinetic energy of ions, leading to a faster rate of passive diffusion of Na⁺ and K⁺ through voltage-gated channels.
- Temperatures above the optimum cause voltage-gated channel proteins to denature, preventing action potentials and stopping conduction.
Examiner Tips
- 💡On a labelling question, check that your label line points precisely to the structure named; a correct term on the wrong structure gains nothing.
- 💡Give a function alongside every name, since structure questions on neurones usually carry a function mark as well as a label mark.
- 💡Add an arrow for the direction of the impulse if you are asked to annotate a diagram.
- 💡Always write ions after sodium and potassium; leaving the word out is the single commonest way to lose marks here.
- 💡Give both halves of the explanation, the pump and the differential permeability; one alone rarely scores full marks.
- 💡If asked how the resting potential is maintained rather than established, stress that the pumping is continuous.
- 💡Work from the graph: label minus 70 mV, the minus 55 mV threshold, the plus 40 mV peak and the return to rest, then attach an ion movement to each section.
- 💡Code the intensity of a stimulus as the frequency of action potentials, never their amplitude.
- 💡Say voltage-gated when writing about an axon, and stretch-mediated only when writing about a Pacinian corpuscle.
- 💡Use the word regenerated; it separates a describing answer from an explaining answer.
- 💡When comparing the two axon types, make the comparison explicit in both directions rather than describing each separately.
- 💡Link speed back to how much of the membrane has to be depolarised, not simply to the presence of myelin.
- 💡Three separate ideas live here: one direction only, discrete impulses, and limited frequency. Write all three even for a two-mark question.
- 💡If a refractory period is given in milliseconds, convert it into a maximum frequency; that calculation is a common application mark.
- 💡Link the frequency ceiling back to how stimulus intensity is coded, which ties this statement to the all-or-nothing principle.
- 💡In questions about multiple sclerosis or demyelination, explicitly state that saltatory conduction is lost and depolarisation must occur over the whole membrane.
- 💡When evaluating data on conduction speeds, always make comparative statements (e.g., 'axon A has a wider diameter than axon B, so offers less resistance').
Common Mistakes
- Treating the myelin sheath as a separate substance coating the axon, when it is the many layers of the Schwann cell's own plasma membrane wrapped around it.
- Calling the dendrites axons, which reverses the described direction of conduction.
- Drawing nodes of Ranvier as breaks in the axon itself rather than gaps in the sheath around it.
- Describing the myelin sheath as protein when it is largely lipid.
- Confusing a neurone with a nerve: a nerve is a bundle of many axons (nerve fibres), not a bundle of whole neurones.
- writing sodium or potassium rather than sodium ions or potassium ions, which caps the marks available
- saying the pump moves equal numbers of each ion, which removes the reason for a net charge difference
- calling the outward movement of potassium ions active transport, when it is facilitated diffusion through open channels
- stating that the outside of the axon is negative, so the sign of the potential is the wrong way round
- claiming the membrane is completely impermeable to sodium ions rather than less permeable
- describing a stronger stimulus as producing a bigger or stronger action potential, which examiners ignore because the size is fixed
- writing sodium rather than sodium ions, which caps the marks available
- saying sodium ions are actively transported into the axon during depolarisation, when they diffuse in through open channels
- leaving threshold out altogether, so the all-or-nothing idea has nothing to hang on
- confusing repolarisation with hyperpolarisation, or claiming the pump is what repolarises the membrane
- saying the same action potential physically travels along the axon rather than being regenerated at each point
- claiming the impulse jumps over the axon in a myelinated neurone, instead of between nodes on the same axon
- forgetting that a non-myelinated axon still conducts, just more slowly
- attributing one-way travel to the myelin sheath rather than to the refractory period behind the wave
- describing local currents as an electric current in a wire rather than as ion movement
- Saying the refractory period stops the impulse travelling backwards because of the myelin sheath, rather than because the membrane behind is refractory.
- Describing the refractory period as the time taken to reach threshold rather than the time during which threshold cannot be reached.
- Claiming that a sufficiently strong stimulus can override the absolute refractory period; it cannot, because the sodium ion channels are inactivated.
- Confusing a limit on the frequency of action potentials with a limit on their size, which is fixed by the all-or-nothing principle.
- Using the word discrete without saying what it means here, namely that each action potential is a separate event that cannot merge with the next.
- Stating myelin speeds up the impulse without explicitly naming saltatory conduction or the nodes of Ranvier. Correction: Always specify that the impulse jumps between nodes of Ranvier via saltatory conduction.
- Explaining demyelinating diseases (like multiple sclerosis) as the destruction of the axon itself. Correction: Explain that loss of myelin means depolarisation must now occur along the whole membrane, slowing the impulse.
- Claiming higher temperatures speed up conduction because the sodium-potassium pump works faster. Correction: The pump maintains resting potential; faster conduction is due to increased kinetic energy and faster passive diffusion of Na⁺ and K⁺ through channels.