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C2.2.12 (HL)—Exogenous chemicals

Exogenous chemicals can alter synaptic transmission by blocking receptors, changing neurotransmitter release or preventing neurotransmitter reuptake, linking cellular mechanisms to rapid communication in nervous systems.

Syllabus
First assessment 2025
Objective
C2.2.12
Level
HL

Exam analysis

Chance of appearing17%of analysed past papers
Latest appearanceNovember 2025
Most common paperPaper3
Typical marks1–6

Common command terms

  • Explain
  • State
  • Suggest
  • Outline
  • Describe
  • Discuss
  • Label

Recent exam appearances

November 2025Paper2 ["HL"] · TZ33(c)[ 3 ]C2.2.12 (HL)—Exogenous chemicals
May 2025Paper1A ["HL"] · TZ318[ 1 ]C2.2.12 (HL)—Exogenous chemicals
May 2025Paper1A ["HL"] · TZ118[ 1 ]C2.2.12 (HL)—Exogenous chemicals
May 2025Paper1A ["HL"] · TZ217[ 1 ]C2.2.12 (HL)—Exogenous chemicals
May 2024Paper3 ["HL"] · TZ28[ 6 ]C2.2.12 (HL)—Exogenous chemicals
Practice this objective

Coverage 2013–2025 · Updated 15 Jul 2026

Exogenous Chemicals Can Alter Neural Signalling

HL only

Exogenous chemicals can alter synaptic transmission by acting on receptors or by changing how long neurotransmitter remains in the synaptic cleft.

Chemical Synaptic action Consequence
Neonicotinoid pesticide Binds to insect acetylcholine receptors and disrupts their normal activation Persistent abnormal stimulation followed by failure of normal synaptic transmission
Cocaine Blocks reuptake transporters for neurotransmitters such as dopamine Neurotransmitter remains in the cleft longer, prolonging postsynaptic signalling

If reuptake is blocked, a released neurotransmitter is cleared more slowly, so receptor stimulation continues after it would normally have ended.

The two examples act differently: neonicotinoids act at receptor sites, whereas cocaine blocks neurotransmitter reuptake. Do not infer a single behavioural outcome without considering dose, receptor distribution and organism.

Exogenous chemicals

HL only

Assessment in practice

1–3 marks
How it is assessed

This objective is assessed through essay response, commonly using Explain / State / Suggest.

Command terms

Explain / State / Suggest / Outline / Describe / Discuss / Label

What earns marks

Build the answer around this relationship: Cocaine blocks dopamine reuptake and prolongs postsynaptic stimulation.

Representative question

Question 1

[Maximum number: 6]

Discuss how drugs can affect the release and reception of chemical messengers at synapses.

Neural Control And Evidence

HL only
  • At threshold, voltage-gated Na+ channels depolarize the axon; K+ channels then repolarize it. Local currents propagate the impulse and the refractory period enforces direction.
  • Action potentials are all-or-nothing; stimulus intensity is encoded by frequency. Myelin enables fast saltatory conduction between nodes of Ranvier.
  • Synapses integrate excitatory and inhibitory inputs through spatial and temporal summation. Chemicals may mimic, block or prolong neurotransmitter action.
  • Pain receptors use TRP channels that respond to heat, acid, capsaicin or tissue damage.
  • EEG, MRI and fMRI link conscious processing to coordinated activity across brain regions, providing neural correlates rather than a single consciousness centre.

Concept essentials

  • Cocaine blocks dopamine reuptake and prolongs postsynaptic stimulation.
  • Neonicotinoids bind acetylcholine receptors in insect postsynaptic membranes.
  • Psychoactive drugs can increase or decrease synaptic transmission.
  • Exogenous chemicals may affect release, receptor binding, breakdown or reuptake of neurotransmitters.