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Nerve impulses and synapsesEdexcel A-Level Biology B: Revision notes

Section 1

Resting potential

At rest the inside of an axon is about −70 mV relative to the outside. This resting potential is set up by:

  • The sodium-potassium pump, which uses ATP to actively transport 3 Na⁺ out and 2 K⁺ in
  • K⁺ leak channels, so K⁺ diffuses out down its gradient, while the membrane is much less permeable to Na⁺

Net positive charge leaves, so the inside is negative. The membrane is polarised.

Key termsresting potentialsodium-potassium pumppolarised

Section 2

The action potential

A stimulus opens some voltage-gated Na⁺ channels. If the membrane reaches the threshold (about −55 mV), more open in a positive feedback loop.

  1. Depolarisation: Na⁺ rushes in and the potential rises to about +40 mV
  2. Repolarisation: Na⁺ channels close and voltage-gated K⁺ channels open, so K⁺ diffuses out
  3. Hyperpolarisation: K⁺ channels close slowly, so the potential overshoots below −70 mV
  4. Resting potential is restored by the pump and K⁺ leak

The all-or-nothing law applies: a stimulus below threshold gives no action potential, and above threshold the size is always the same. Intensity is coded by frequency of impulses.

Key termsaction potentialthresholddepolarisationrepolarisation
Common mistake

Do not say that the pump causes the rise or fall of an action potential. The pump is too slow; it restores the ion gradients afterwards.

Section 3

Propagation and myelin

Depolarisation at one point creates local circuits of current that depolarise the next region of membrane. The previous region is in its refractory period, when voltage-gated Na⁺ channels cannot open, so the impulse travels in one direction and impulses stay discrete.

In myelinated axons the myelin sheath is an electrical insulator. Action potentials occur only at the nodes of Ranvier, and the impulse jumps from node to node. This is saltatory conduction, and it is much faster than continuous conduction in a non-myelinated axon.

Key termsrefractory periodmyelinnode of Ranviersaltatory conduction
Exam tip

In a calculation question, time = distance ÷ speed, and keep units consistent (m and m s⁻¹, or mm and ms).

Section 4

Structure and function of a synapse

A synapse is a junction between neurones. The presynaptic knob contains mitochondria and vesicles of transmitter. The synaptic cleft separates it from the postsynaptic membrane, which has receptors.

Transmission at a cholinergic synapse:

  1. The action potential opens voltage-gated Ca²⁺ channels and Ca²⁺ enters
  2. Vesicles fuse with the membrane and release acetylcholine by exocytosis
  3. Acetylcholine diffuses across the cleft and binds to receptors
  4. Na⁺ channels open and the postsynaptic membrane depolarises
  5. Acetylcholinesterase breaks down acetylcholine, which is recycled

Noradrenaline is a second transmitter, released at some synapses of the sympathetic system.

Key termssynapseacetylcholinenoradrenalineacetylcholinesterase

Section 5

EPSPs, IPSPs and summation

At an excitatory synapse, Na⁺ entering depolarises the postsynaptic membrane, giving an excitatory postsynaptic potential (EPSP). A single EPSP is usually too small to reach threshold.

  • Temporal summation: repeated impulses at one synapse add together
  • Spatial summation: impulses at several synapses add together

At an inhibitory synapse, ions such as Cl⁻ entering or K⁺ leaving make the membrane more negative: an inhibitory postsynaptic potential (IPSP), or hyperpolarisation. The postsynaptic neurone fires only if the net effect of EPSPs and IPSPs reaches threshold, so it integrates many inputs.

Key termsEPSPIPSPtemporal summationspatial summation

Must know

  • Resting potential −70 mV from the pump (3 Na⁺ out, 2 K⁺ in) and K⁺ leak
  • Action potential: Na⁺ in, then K⁺ out; threshold −55 mV; all or nothing
  • Refractory period gives one direction
  • Myelin and nodes: saltatory conduction
  • Synapse: Ca²⁺ in, exocytosis of acetylcholine, Na⁺ in
  • EPSP, IPSP and summation

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Nerve impulses and synapses

  1. A researcher records the potential difference across the membrane of a resting mammalian axon using a microelectrode. The inside of the axon is 70 mV negative relative to the outside. The membrane contains sodium-potassium pumps and potassium ion leak channels, but very few sodium ion leak channels.
    Predict what would happen to the resting potential over several hours if the axon was supplied with a poison that stopped ATP production. Explain your answer.2 marks
  2. A neurone is stimulated and the membrane potential is recorded. The potential rises slowly from −70 mV to about −55 mV, then rises rapidly to +40 mV. It then falls rapidly, overshoots to about −80 mV, and finally returns to −70 mV. The whole event lasts about 3 ms.
    Explain the fall in potential from +40 mV to below −70 mV.2 marks
  3. A myelinated mammalian motor axon conducts impulses at 80 m s⁻¹, whereas a non-myelinated axon of similar diameter conducts impulses at 1 m s⁻¹. In the myelinated axon, Schwann cells wrap the axon in layers of myelin, leaving small gaps, the nodes of Ranvier, about 1.5 mm apart. A motor axon carrying impulses from the spinal cord to a foot muscle is 1.2 m long.
    Explain why the myelinated axon conducts impulses faster than the non-myelinated axon.3 marks
See the full worksheet

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).