AQA · A-Level · Biology

    Course version: 7402 · Version from Sept 2016

    Changes in membrane permeability lead to depolarisation and the generation…

    Full specification wording

    Nerve impulses (A-level only): 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…

    Checked against the AQA 7402 specification (opens in a new tab) on .

    • Point 3 of 6
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    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.

    What earns the marks

    • Sodium ion channels opening so the membrane becomes more permeable to sodium ions.
    • Sodium ions diffusing into the axon down the electrochemical gradient.
    • Depolarisation, the inside becoming positive, once the threshold is reached.
    • Potassium ion channels opening and potassium ions diffusing out, causing repolarisation.
    • The all-or-nothing point, that below threshold no action potential is produced and above threshold every action potential is the same size.

    Common mistakes and how to put them right

    • 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.

    Exam tips

    • 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.

    Try the lesson

    The start of the Nerve impulses (A-level only): Changes in membrane permeability lead to depolarisation and the generation of an action potential. The all-or-nothing principle. lesson, made from checked teaching notes for this specification point.

    Have a go first

    Which change in membrane permeability causes depolarisation during an action potential?

    Listen

    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. A strong answer earns marks for these points. sodium ion channels opening so the membrane becomes more permeable to sodium ions. sodium ions diffusing into the axon down the electrochemical gradient. depolarisation, the inside becoming positive, once the threshold is reached. potassium ion channels opening and potassium ions diffusing out, causing repolarisation. Exam tip: 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.

    See it

    A vertical flowchart. A stimulus makes the membrane more permeable to sodium ions, which leads to voltage-gated sodium ion channels opening. Sodium ions then diffuse in down the electrochemical gradient, and the inside becomes less negative. A decision point asks whether the threshold of about minus 55 mV is reached. If no, there is no action potential, following the all-or-nothing principle. If yes, positive feedback opens all remaining sodium ion channels, sodium ions flood in and the membrane depolarises to roughly plus 40 mV. Then the sodium ion channels close and voltage-gated potassium ion channels open, potassium ions diffuse out and the membrane repolarises, followed by brief hyperpolarisation, and finally the sodium-potassium pump restores the resting potential.
    It shows how a stimulus leads through sodium ion entry and threshold to an action potential, then repolarisation, hyperpolarisation and restoration of the resting potential.

    Key words

    Say what each word means before you turn the card.

    threshold

    Try answering aloud or in your head before turning the card.

    depolarisation

    Try answering aloud or in your head before turning the card.

    repolarisation

    Try answering aloud or in your head before turning the card.

    all-or-nothing principle

    Try answering aloud or in your head before turning the card.

    See every definition
    threshold
    The potential of about minus 55 mV that enough sodium ions must lift the inside to, so that positive feedback opens all the remaining voltage-gated sodium ion channels.
    depolarisation
    The membrane becoming depolarised to roughly plus 40 mV, so the inside becomes positive, once the threshold is reached.
    repolarisation
    The membrane repolarising when the sodium ion channels close at the peak and voltage-gated potassium ion channels open, so potassium ions diffuse out.
    all-or-nothing principle
    Below threshold no action potential is produced, and above threshold every action potential is the same size.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A vertical flowchart. A stimulus makes the membrane more permeable to sodium ions, which leads to voltage-gated sodium ion channels opening. Sodium ions then diffuse in down the electrochemical gradient, and the inside becomes less negative. A decision point asks whether the threshold of about minus 55 mV is reached. If no, there is no action potential, following the all-or-nothing principle. If yes, positive feedback opens all remaining sodium ion channels, sodium ions flood in and the membrane depolarises to roughly plus 40 mV. Then the sodium ion channels close and voltage-gated potassium ion channels open, potassium ions diffuse out and the membrane repolarises, followed by brief hyperpolarisation, and finally the sodium-potassium pump restores the resting potential.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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