Resting does not mean inactive
A resting neuron maintains an electrical potential difference across its membrane, with the inside negative relative to the outside. Unequal ion distributions, selective membrane permeability and trapped intracellular anions all contribute to this state.
The sodium–potassium pump uses ATP to move three sodium ions out for every two potassium ions moved in, maintaining the gradients over time. At rest, potassium leak channels allow more positive charge to leave than sodium leak permits to enter, helping make the interior negative.
| Component | Main role |
|---|---|
| Na⁺/K⁺ pump | Maintains sodium and potassium gradients using ATP |
| K⁺ leak channels | Permit resting potassium movement outward |
| Voltage-gated Na⁺ channels | Produce rapid depolarisation when opened |
| Voltage-gated K⁺ channels | Drive repolarisation and after-hyperpolarisation |
Threshold triggers an all-or-none event
A stimulus that depolarises the membrane to threshold opens many voltage-gated sodium channels. Sodium enters down its electrochemical gradient, causing further depolarisation and opening still more channels. This positive-feedback phase produces the rising action potential.
An action potential is all-or-none: once threshold is reached, its basic size is not graded according to stimulus strength. Stronger stimuli are represented mainly by a higher frequency of action potentials or recruitment of more sensory neurons.
Repolarisation resets electrical conditions
Voltage-gated sodium channels inactivate while voltage-gated potassium channels open more fully. Potassium leaves the cell and the membrane potential becomes negative again. Because potassium channels close slowly, the potential may briefly become more negative than the resting level.
During the absolute refractory period, inactivated sodium channels cannot reopen, preventing another action potential. During the relative refractory period, a stronger-than-usual stimulus may be required. These periods help enforce one-way propagation and limit firing frequency.
Ion gradients and leak permeability maintain negativity.
Sufficient depolarisation opens Na⁺ channels.
Rapid Na⁺ entry makes the inside positive.
Na⁺ channels inactivate and K⁺ leaves.
Channels reset before normal excitability returns.
Myelin makes propagation faster
Myelin electrically insulates sections of the axon. Voltage-gated channels are concentrated at gaps called nodes of Ranvier, so local current spreads rapidly beneath the myelin and action potentials are regenerated mainly at the nodes.
This saltatory conduction is faster and more energy-efficient than continuous regeneration along an unmyelinated membrane. Larger axon diameter also reduces internal resistance and can increase conduction speed.
A chemical synapse converts the signal twice
When an action potential reaches the presynaptic terminal, voltage-gated calcium channels open. Calcium entry triggers synaptic vesicles to fuse with the membrane and release neurotransmitter by exocytosis. The transmitter diffuses across the cleft and binds postsynaptic receptors.
Receptor activation changes ion permeability, producing an excitatory or inhibitory postsynaptic potential. The neurotransmitter is then removed by enzymatic breakdown, diffusion or reuptake. Transmission is one-way because vesicle-release machinery is presynaptic while receptors are concentrated postsynaptically.
The terminal membrane depolarises.
Voltage-gated calcium channels open.
Vesicles fuse by exocytosis.
Postsynaptic permeability changes.
Transmitter is removed from the cleft.
The ideas to carry forward
- Resting potential depends on ion gradients and selective permeability.
- Voltage-gated sodium entry causes depolarisation; potassium exit restores negativity.
- Refractory periods support one-way propagation.
- Calcium entry links an arriving impulse with neurotransmitter release.
Answer first. Then reveal the marking logic.
01Why does the sodium–potassium pump not directly create the rapid rising phase of an action potential?2 marks · show the biological link
Answer: The rising phase is produced by rapid sodium flow through voltage-gated channels. The pump works more slowly to maintain the gradients that make that flow possible.
02Why is transmission across a typical chemical synapse one-way?2 marks · show the biological link
Answer: Transmitter vesicles and release machinery are presynaptic, while the relevant receptors are on the postsynaptic membrane.
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