Nerve impulse generation and transmission — A/L Biology

What must I remember about Nerve impulse generation and transmission?

• Resting Membrane Potential (-70 mV): Maintained by the active Na+/K+ ATPase pump (pumping 3 Na+ out for every 2 K+ in) and differential membrane permeability (high K+ leak channel permeability, low Na+ permeability).
• Depolarization & Action Potential: An adequate stimulus above threshold (-55 mV) opens voltage-gated Na+ channels; rapid influx of Na+ reverses polarity up to +30 mV in an all-or-none fashion.
• Repolarization & Hyperpolarization: Voltage-gated Na+ channels inactivate, and voltage-gated K+ channels open; rapid efflux of K+ restores negative potential inside, often briefly undershooting to -80 mV before resting potential is restored.
• Saltatory Conduction: In myelinated neurons, action potentials jump rapidly between Nodes of Ranvier where voltage-gated channels are concentrated, consuming less ATP and propagating far faster than in unmyelinated fibers.
• Chemical Synaptic Transmission: Action potential arrives at axon terminal -> Voltage-gated Ca2+ channels open -> Ca2+ influx triggers exocytosis of neurotransmitters (e.g., Acetylcholine) into synaptic cleft -> binding to ligand-gated ion channels on postsynaptic membrane generates EPSP or IPSP -> enzymatic degradation (Acetylcholinesterase) terminates signal.

Test yourself (1 minute)

1. All-or-None Law

   If the depolarizing stimulus does not reach threshold (-55 mV), no action potential fires; once reached, an action potential of maximal, constant amplitude is generated.

2. Refractory Periods

   Absolute refractory period (voltage-gated Na+ channels inactivated; impossible to fire another action potential) ensures unidirectional impulse propagation; Relative refractory period requires a stronger-than-normal stimulus.

3. Excitatory vs Inhibitory Synapses

   EPSPs (e.g., Acetylcholine at neuromuscular junction) depolarize postsynaptic membrane towards threshold; IPSPs (e.g., GABA) open Cl- or K+ channels, causing hyperpolarization away from threshold.

Practise Nerve impulse generation and transmission questions today

Source: NIE G.C.E. (A/L) Biology Syllabus & Resource Book, Unit 5

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Year 2 · Term 1