Synapses and excitatory postsynaptic potentials

Cells (Interaction and interdependence) · Neural signalling · note 5 of 7

Spec C2.2.5, C2.2.6, C2.2.7Download all Neural signalling notes (PDF)Download all notes (PDF)12 pages

Synapses and excitatory postsynaptic potentialsSpec C2.2.5, C2.2.6, C2.2.7

In short

A synapse is a junction between two neurons or between a neuron and an effector cell. Depolarization of the presynaptic membrane lets calcium ions in, which triggers release of neurotransmitter by exocytosis. Acetylcholine diffuses across the synaptic cleft, binds to transmembrane receptors and lets positive ions in, generating an excitatory postsynaptic potential.

Synapses are junctions between neurons and between neurons and effector cells, such as muscle fibres at neuromuscular junctions. At a chemical synapse (called simply a synapse here) the cells are separated by a narrow synaptic cleft.

A signal can only pass in one direction across a typical synapse. Only the presynaptic neuron has vesicles of neurotransmitter, and only the postsynaptic membrane has the receptors.

Release of neurotransmitter from the presynaptic membrane

An action potential arrives and depolarizes the presynaptic membrane.
Voltage-gated calcium channels open and Ca²⁺ diffuses into the presynaptic neuron (it is more concentrated outside).
Inside the neuron, calcium ions act as a signalling chemical: they cause vesicles of neurotransmitter to move to the presynaptic membrane and fuse with it.
The neurotransmitter is released into the synaptic cleft by exocytosis.

Excitatory postsynaptic potential (EPSP)

Acetylcholine diffuses across the synaptic cleft.
It binds to transmembrane receptors in the postsynaptic membrane.
The receptors open ion channels and positively charged ions, mainly Na⁺, diffuse into the postsynaptic cell.
The postsynaptic membrane is depolarized: an excitatory postsynaptic potential. If it reaches the threshold potential, an action potential starts in the postsynaptic cell.
The neurotransmitter is quickly removed from the synaptic cleft, so each signal is brief.

Beyond the syllabus: the guide does not name the enzyme that removes acetylcholine. It is acetylcholinesterase, which breaks acetylcholine into acetate and choline; the presynaptic neuron reabsorbs the choline to make more acetylcholine.

Acetylcholine is used at many types of synapse, including neuromuscular junctions, where it triggers contraction of skeletal muscle fibres.

Cholinergic synapse: an action potential arrives at the presynaptic neuron, voltage-gated Ca²⁺ channels let Ca²⁺ in, vesicles containing acetylcholine fuse with the presynaptic membrane (exocytosis), acetylcholine diffuses across the synaptic cleft and binds acetylcholine receptors (ion channels) in the postsynaptic membrane, Na⁺ enters and causes depolarization, an excitatory postsynaptic potential; the signal passes in one direction only. — At a cholinergic synapse, Ca²⁺ entry triggers exocytosis of acetylcholine, which causes an EPSP in the postsynaptic cell.

Exam tip:

Linking question: in what ways are biological systems regulated? Synapses are control points: whether a signal passes depends on how much neurotransmitter is released and how many receptors respond.

Written and checked against the IB Biology HL specification · Updated October 2026

Frequently asked questions

Why is the resting potential negative?

The resting potential is negative because sodium–potassium pumps move three Na⁺ out for every two K⁺ in, the membrane lets K⁺ leak out much faster than Na⁺ leaks in, and negatively charged proteins stay inside the cytoplasm. Together these leave the inside of the neuron about 70 mV more negative than the outside.

How does a nerve impulse cross the synapse?

The impulse depolarizes the presynaptic membrane, so calcium ions enter and cause vesicles to release neurotransmitter by exocytosis. The neurotransmitter, such as acetylcholine, diffuses across the synaptic cleft and binds to receptors on the postsynaptic membrane. Positive ions enter and depolarize it; if threshold is reached, a new action potential starts.

Why do myelinated neurons conduct impulses faster?

HL only Myelin insulates the axon, so action potentials can only occur at the nodes of Ranvier, where ion pumps and channels are clustered. Local currents spread under the myelin to the next node, so the impulse jumps from node to node. This saltatory conduction is much faster than depolarizing every part of the membrane in turn.

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