Nerve impulses and synaptic transmission - TCE Biology (Tasmania)
“Neural pathways consist of cells that transport nerve impulses from sensory receptors to neurons and on to effectors: the passage of nerve impulses involves transmission of an action potential along a nerve axon, and synaptic transmission by neurotransmitters and signal transduction”
A neuron at rest has a resting potential of about -70 mV (inside negative), maintained by the sodium-potassium pump and potassium leak channels. A stimulus that reaches threshold (about -55 mV) opens voltage-gated sodium channels: sodium rushes in and the membrane depolarises. Sodium channels then close and voltage-gated potassium channels open: potassium leaves and the membrane repolarises, briefly overshooting into hyperpolarisation. This action potential travels along the axon. At the synapse, calcium ions enter the axon terminal, vesicles release neurotransmitter into the synaptic cleft, and it binds to receptors on the next cell, passing the signal on.
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What this dot point is asking
Nerve impulses are part of criterion 6 (Section C of the TASC exam). The external assessment specifications list the parts of a neuron, the stages and ion channels of the action potential, and the events at a synapse. Types of neuron, glial cells, divisions of the nervous system and reflex arcs are excluded.
Neuron structure
- Dendrites: receive signals from other neurons or receptors.
- Cell body: contains the nucleus; integrates incoming signals.
- Axon: carries the action potential away from the cell body.
- Myelin sheath: a fatty insulating layer around the axon that speeds up conduction.
- Schwann cells: the cells that form the myelin sheath around axons in the peripheral nervous system.
- Axon terminals: the ends of the axon, where neurotransmitter is released at synapses.
The resting potential
At rest, the inside of the axon is about -70 mV compared with the outside.
- The sodium-potassium pump uses ATP to move 3 sodium ions out for every 2 potassium ions in.
- Leak channels let ions diffuse through the membrane; there are more potassium leak channels, so potassium leaks out more easily than sodium leaks in.
- The result is more positive charge outside than inside.
The action potential
| Stage | What happens to the ion channels | Membrane potential |
|---|---|---|
| Stimulus and threshold | A stimulus makes the inside less negative; if it reaches threshold, the next stage is triggered | Rises to about -55 mV |
| Depolarisation | Voltage-gated sodium channels open; sodium ions flow in | Rises to about +30 to +40 mV |
| Repolarisation | Sodium channels close; voltage-gated potassium channels open; potassium ions flow out | Falls back towards negative |
| Hyperpolarisation | Potassium channels close slowly, so extra potassium leaves | Briefly below -70 mV |
| Return to resting | The sodium-potassium pump and leak channels restore the ion distribution | Back to about -70 mV |
An action potential is all or nothing: if threshold is reached it fires fully; if not, it does not fire. The action potential in one section of axon depolarises the next section, so the impulse travels along the axon.
Leak channels are always open and help set the resting potential. Voltage-gated channels open only when the membrane potential changes, producing the action potential.
Synaptic transmission
A synapse is the junction between the axon terminal of one neuron and the next cell (another neuron, a muscle or a gland), separated by a tiny gap, the synaptic cleft.
- The action potential reaches the axon terminal and opens voltage-gated calcium channels; calcium ions enter.
- Calcium causes vesicles of neurotransmitter to fuse with the membrane and release neurotransmitter into the cleft.
- The neurotransmitter diffuses across the cleft.
- It binds to specific postsynaptic receptors, changing the permeability of the next cell's membrane (signal transduction), which can trigger a new action potential.
- The neurotransmitter is broken down or taken back up, so the signal stops.
Examples of neurotransmitters include acetylcholine, noradrenaline, dopamine, serotonin, glutamate and GABA. You need to know names, not their specific functions.
Membrane potential against time
A graph starts flat at -70 mV, rises slowly to -55 mV, then shoots up to +35 mV, falls steeply to -80 mV and returns to -70 mV.
- -70 mV: resting potential.
- -55 mV: threshold; voltage-gated sodium channels open.
- Rise to +35 mV: depolarisation (sodium in).
- Fall: repolarisation (potassium out).
- -80 mV: hyperpolarisation.
- Back to -70 mV: resting potential restored by the pump and leak channels.
Marker's note: label each section with the ion movement, not just the name of the stage.
- Saying potassium moves in during depolarisation
- Sodium moves in during depolarisation; potassium moves out during repolarisation.
- Saying the pump creates the action potential
- Voltage-gated channels create the action potential; the pump maintains the resting potential.
- Saying the impulse "jumps" across the synapse electrically
- Transmission across the cleft is chemical, by neurotransmitter.
Exam-style questions
Questions in the style of TASC exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
Original6 marksDescribe the changes in ion movement that produce the depolarisation, repolarisation and hyperpolarisation phases of an action potential.Show worked answer →
Two marks for each phase.
- Depolarisation
- When a stimulus raises the membrane potential to threshold (about -55 mV), voltage-gated sodium channels open. Sodium ions rush into the axon, making the inside positive (about +30 to +40 mV).
- Repolarisation
- Voltage-gated sodium channels close and voltage-gated potassium channels open. Potassium ions move out of the axon, making the inside negative again.
- Hyperpolarisation
- The potassium channels close slowly, so extra potassium ions leave and the membrane potential briefly becomes more negative than the resting potential. The sodium-potassium pump and leak channels then restore the resting potential of about -70 mV.
Original5 marksExplain how a nerve impulse is transmitted from one neuron to the next across a synapse.Show worked answer →
One mark for each step in order.
- The action potential arrives at the axon terminal and opens voltage-gated calcium channels, so calcium ions enter.
- Calcium ions cause vesicles containing neurotransmitter to fuse with the membrane and release the neurotransmitter into the synaptic cleft.
- The neurotransmitter diffuses across the cleft.
- It binds to specific receptors on the postsynaptic membrane, changing its permeability to ions (signal transduction).
- If enough neurotransmitter binds, the postsynaptic neuron reaches threshold and a new action potential is generated; the neurotransmitter is then removed from the cleft.