IB Biology · Theme C: Interaction and interdependence · SL and HL
C2.2 Neural signalling
A one-page summary of C2.2 Neural signalling, the key terms to know, and sample exam questions with answers. For the full lesson, open the illustrated revision slides or practise in the app.
Guiding questions
How are electrical signals generated and moved within neurons?
How can neurons interact with other cells?
What C2.2 covers
Neuron structure and the resting potential
- C2.2.1One cell body, many fibres
- C2.2.2Charging the membrane at rest
Action potentials and impulse speed
- C2.2.3An impulse is an electrical event
- C2.2.4Bigger axons carry impulses faster
- C2.2.4Reading the strength of a relationship
Synapses and neurotransmission
- C2.2.5A junction with only one way through
- C2.2.6Calcium turns an impulse chemical
- C2.2.7One neurotransmitter, many synapses
The action potential mechanism · HL
- C2.2.8 · HLTwo channels, opposite directions
- C2.2.9 · HLHow an impulse travels without moving
- C2.2.10 · HLWatching a membrane's voltage change
- C2.2.11 · HLJumping from node to node
Synaptic transmission in depth · HL
- C2.2.12 · HLBlocking the receiving end
- C2.2.12 · HLBlocking the sending end
- C2.2.13 · HLPushing a membrane further from firing
- C2.2.14 · HLAdding up different inputs
- C2.2.14 · HLAdding up the same input, over time
Pain and consciousness · HL
- C2.2.15 · HLFree endings that sense danger
- C2.2.16 · HLA property of the whole network
C2.2 Neural signalling: summary
Neuron structure & resting potential
- Neuron: cell body (cytoplasm + nucleus) with one long axon and multiple shorter dendrites, all conducting impulses.
- Resting potential (~−70 mV, inside negative): maintained by ATP-driven pumping of Na⁺ out and K⁺ in, in opposite directions.
Action potentials & impulse speed
- Action potential: all-or-nothing electrical event, triggered only once threshold is reached; read on oscilloscope traces.
- Conduction velocity is negatively correlated with body size, positively correlated with axon diameter (R² quantifies each relationship's strength).
Synapses & neurotransmission
- Synapse: junction (neuron–neuron or neuron–effector) where a signal passes one direction only; depolarization → Ca²⁺ entry → neurotransmitter release.
- EPSP: neurotransmitter (e.g. acetylcholine) diffuses across the cleft, binds a receptor, depolarizes the postsynaptic membrane toward threshold.
HL · The action potential mechanism
- Depolarization = Na⁺ influx via voltage-gated Na⁺ channels; repolarization = K⁺ efflux via voltage-gated K⁺ channels.
- Local currents regenerate the impulse point-by-point (unmyelinated); saltatory conduction jumps node-to-node at nodes of Ranvier (myelinated) for speed.
HL · Synaptic transmission in depth
- Neonicotinoids block the postsynaptic acetylcholine receptor; cocaine blocks presynaptic reuptake of neurotransmitter (e.g. dopamine) — different sides, different mechanisms.
- IPSP hyperpolarizes the postsynaptic membrane; summation (spatial + temporal) combines many EPSPs/IPSPs into one all-or-nothing firing decision.
HL · Pain & consciousness
- Free nerve endings open positive-ion channels in response to heat, acid or capsaicin, sending impulses to the brain, where pain is perceived.
- Consciousness is an emergent property arising from the interaction of billions of individual neurons, not a property any single neuron has alone.
Key terms
- Resting potential
- The voltage difference across a neuron's membrane at rest, generated by active pumping of Na⁺ and K⁺ in opposite directions.
- Action potential
- An all-or-nothing electrical impulse triggered once a stimulus depolarizes the membrane to threshold.
- Synapse
- A chemical junction between two neurons, or between a neuron and an effector cell, across which a signal passes in one direction.
- Threshold potential
- The minimum depolarization needed to trigger an action potential.
- EPSP
- Excitatory postsynaptic potential: a depolarizing, graded shift in postsynaptic membrane potential toward threshold.
- Saltatory conduction HL
- The "jumping" of an action potential from node to node in a myelinated fibre, achieving faster impulses.
- Refractory period HL
- The brief period after an action potential during which a neuron cannot (absolute) or is less able (relative) to fire again.
- IPSP HL
- Inhibitory postsynaptic potential: a hyperpolarizing, graded shift in postsynaptic membrane potential away from threshold.
- Summation HL
- Combining multiple EPSPs and IPSPs at a postsynaptic neuron into one all-or-nothing firing decision.
Sample exam questions
Three of the 48 multiple-choice questions for C2.2. Try each one before opening the answer.
Question 1. Which combination correctly describes the parts of a neuron?
- Axon = multiple short fibres; dendrites = one long fibre
- Axon = one long fibre carrying impulses away from the cell body; dendrites = multiple shorter fibres
- Cell body = the myelin sheath; axon = the nucleus
- Dendrites = the region where ATP is generated; axon = the receptor region
Show the answer
Answer: B. A neuron's cell body (cytoplasm + nucleus) gives rise to elongated nerve fibres: a single long axon and multiple, shorter dendrites. Both conduct electrical impulses, but dendrites typically receive signals and the axon carries them onward.
Question 2. Why is the resting potential of a neuron negative rather than zero?
- Because active pumping maintains a higher concentration of positive ions outside the cell than inside, so the membrane is polarized
- Because the membrane is completely impermeable to all ions at rest
- Because the axon contains no ions when not stimulated
- Because ATP is only used during an action potential, never at rest
Show the answer
Answer: A. ATP-driven pumping maintains sodium and potassium concentration gradients across the membrane. Because the resulting distribution of charge leaves the inside of the cell more negative than the outside, the membrane is described as polarized, with a negative membrane potential at rest.
Question 3. Which statement correctly distinguishes the mechanisms of neonicotinoids and cocaine at a synapse?
- Both block the same postsynaptic receptor in the same way
- Cocaine acts at the postsynaptic receptor; neonicotinoids block presynaptic reuptake
- Neither chemical affects synaptic transmission
- Neonicotinoids act at the postsynaptic receptor; cocaine acts by blocking presynaptic reuptake of neurotransmitter
Show the answer
Answer: D. Neonicotinoids bind to postsynaptic acetylcholine receptors, disrupting normal transmission. Cocaine blocks reuptake of neurotransmitter by the presynaptic terminal, prolonging its effect. These are genuinely different mechanisms of exogenous chemicals affecting synaptic transmission.
Linking questions
Questions that connect C2.2 to other parts of the course, the kind that come up in Paper 2.
- The sodium-potassium pump (B2.1) maintains the Na⁺ and K⁺ gradients essential for action potentials. State how many Na⁺ and K⁺ ions the pump moves for each ATP used, and explain why this helps make the inside of the neuron negative. (see B2.1)
- The structure of proteins (B1.2) determines the specificity of neurotransmitter receptors. Explain how the tertiary structure of a ligand-gated ion channel enables it to open only in response to a specific neurotransmitter. (see B1.2)
- At the neuromuscular junction (B3.3), acetylcholine triggers muscle contraction. Compare the postsynaptic response at the NMJ with that at a neuron-neuron synapse in the central nervous system. (see B3.3)
- Many psychoactive drugs affect synaptic transmission. Using cocaine (dopamine reuptake inhibitor) as an example, explain how altering synaptic transmission can lead to addiction. (see C2.1)
Practise C2.2
Study notes, every question and full markschemes for C2.2 are in the app with Pro. Two lessons are completely free to try: A1.1 Water and B1.1 Carbohydrates and lipids.