Biology HL Paper 1A (May 2025, TZ2) · Question 17
Mixed-topicMatching part: 17
Also tests: 7.2 Drugs, Pain and Consciousness
Practice this question →7.2 Synapses and Neural Integration
Trace cholinergic transmission and use excitatory, inhibitory, spatial and temporal summation to predict firing.
Estimated time: 105 minutes
IB syllabus: C2.2 · SL and HL
A Chemical Synapse Converts the Signal Twice
A synapse is a junction between neurons or between a neuron and a receptor or effector cell. At a chemical synapse, the presynaptic terminal and postsynaptic membrane are separated by a cleft about tens of nanometres wide. Arrival of an action potential depolarizes the terminal, opening voltage-gated Ca²⁺ channels. Ca²⁺ enters and triggers docked vesicles to fuse with the presynaptic membrane.
Neurotransmitter released by exocytosis diffuses across the cleft and binds receptors in the postsynaptic membrane. Ligand-gated channels change ion permeability, generating a graded postsynaptic potential. If depolarization at the axon hillock reaches threshold, the postsynaptic neuron produces its own action potential. The neurotransmitter does not itself become the action potential.
Acetylcholine Must Be Released and Removed
At a cholinergic synapse, acetylcholine binds cholinergic receptors; at many neuromuscular junctions this opens cation channels and depolarizes the muscle membrane. Acetylcholinesterase hydrolyses acetylcholine in the cleft. Choline is taken back into the terminal, used to resynthesize acetylcholine and repackaged into vesicles. Rapid clearance keeps separate presynaptic spikes from becoming one prolonged signal.
Chemical synapses are directional because vesicle release machinery is concentrated presynaptically and receptors are concentrated postsynaptically. They also introduce a short synaptic delay, but permit amplification, inhibition, plasticity and integration. An electrical synapse through gap junctions has different properties and should not be used as the model for acetylcholine transmission.
HL extensionC2.2 AHL
Excitation and Inhibition Describe Effects on Threshold
An excitatory postsynaptic potential, or EPSP, shifts membrane potential toward threshold, often through cation entry. An inhibitory postsynaptic potential, or IPSP, moves it away from threshold or stabilizes it, for example through Cl⁻ entry or K⁺ exit. Acetylcholine is excitatory at many synapses and GABA is the major inhibitory transmitter in much of the brain, but a transmitter's effect ultimately depends on its receptor and ion coupling.
One postsynaptic potential is normally too small to trigger a spike. Spatial summation combines potentials arriving from different presynaptic neurons at similar times. Temporal summation combines rapidly repeated potentials from one input before earlier effects decay. The axon hillock integrates both EPSPs and IPSPs. If the net voltage reaches threshold, an action potential starts; if not, the graded potentials fade.
Neural Computation Is Weighted, Not a Simple Vote
Counting excitatory and inhibitory synapses is not enough. Their strength, timing, duration and location matter. An inhibitory synapse near the axon hillock can strongly control firing, while a distant dendritic EPSP may decay as it spreads. Synaptic plasticity changes these weights with activity, contributing to learning and memory. Summation is therefore a dynamic calculation rather than a fixed subtraction of transmitter molecule counts.
Cholinergic synapse
Vary vesicle release and acetylcholinesterase activity, then block or restore postsynaptic receptors.
Detect · transduce · integrate · respond
Cell communication laboratory
Excitation–inhibition integrator
Combine graded excitatory and inhibitory inputs and test whether simultaneous arrival reaches threshold.
Detect · transduce · integrate · respond
Cell communication laboratory
Test Yourself
A model neuron rests at −70 mV and simultaneously receives four +2 mV EPSPs and two −1.5 mV IPSPs. Ignoring decay, what membrane potential results?
Test Yourself
Acetylcholinesterase is inhibited at a normally excitatory cholinergic synapse. Presynaptic firing then stops. Which prediction best follows?
Exam questions on this topic
Practice focused questions or see how IB combines this topic with ideas from elsewhere in the course.