What is a quantum of neurotransmitter?

It is the amount of transmitter contained in a single synaptic vesicle; release happens in whole-number multiples of this packet rather than as a continuous stream.

How do electrical and chemical synapses differ?

Electrical synapses pass current directly between cells through gap junctions for very fast signalling, whereas chemical synapses use neurotransmitter release, allowing amplification, sign changes, and modulation but adding a brief delay.

Synaptic Transmission (Comparative)

How signals pass from one excitable cell to the next, whether through the rapid electrical coupling of gap junctions or the chemical relay of neurotransmitter release across a synaptic cleft.

Definition

Synaptic transmission is the process by which a signal is conveyed from a presynaptic cell to a postsynaptic cell, either electrically through gap junctions or chemically by the regulated release of neurotransmitter that binds postsynaptic receptors and changes the target cell's membrane conductance.

Scope

This topic covers the physiology of the synapse across animals: the structure of chemical synapses, calcium-triggered quantal release of neurotransmitter, postsynaptic receptor and conductance changes, and the contrast with electrical synapses that couple cells directly. It addresses excitatory and inhibitory postsynaptic potentials, temporal and spatial summation, and short-term plasticity, drawing on classic invertebrate and neuromuscular preparations. The treatment is comparative and mechanistic rather than pharmacological prescription.

Core questions

How does an action potential at the nerve terminal trigger the release of neurotransmitter?
Why is neurotransmitter released in discrete quantal packets, and what is the role of calcium?
How do postsynaptic receptors convert a chemical signal into excitation or inhibition?
When are electrical synapses used instead of chemical ones, and what are their trade-offs?

Key theories

Quantal hypothesis of transmitter release: Neurotransmitter is released in discrete multimolecular packets (quanta) corresponding to synaptic vesicles, so the postsynaptic response is built from integer multiples of a unit miniature potential.
Calcium hypothesis of release: Depolarisation of the presynaptic terminal opens voltage-gated Ca2+ channels, and the resulting brief Ca2+ entry is the immediate trigger for vesicle fusion and transmitter release.

Mechanisms

When an action potential invades a chemical synaptic terminal, voltage-gated Ca2+ channels open and local Ca2+ entry triggers fusion of neurotransmitter-filled vesicles with the presynaptic membrane. Released transmitter diffuses across the cleft and binds postsynaptic receptors: ionotropic receptors gate ion channels directly to produce fast excitatory or inhibitory postsynaptic potentials, while metabotropic receptors act through second messengers for slower, modulatory effects. Transmitter action is terminated by reuptake or enzymatic breakdown. Many small postsynaptic potentials sum in time and space to determine whether the postsynaptic cell reaches threshold. Electrical synapses bypass this chemistry, passing current directly through gap-junction channels for very fast, often bidirectional, signalling.

Clinical relevance

Classic studies of the neuromuscular junction established principles that explain the action of nerve agents, toxins, neuromuscular blockers, and many drugs that target receptors and transporters; the same framework underlies the study of synaptic plasticity. This entry is educational comparative physiology, not treatment guidance.

History

Otto Loewi's demonstration of chemical neurotransmission and Eccles's intracellular recordings of postsynaptic potentials established the chemical synapse, while Katz and colleagues used the frog neuromuscular junction to reveal quantal release and the calcium trigger. Comparative work later mapped the wide use of electrical synapses and modulatory transmission across invertebrate and vertebrate nervous systems.

Key figures

Bernard Katz
Ricardo Miledi
John Eccles
Otto Loewi

Seminal works

katzmiledi1967
randall2002
kandel2021

Frequently asked questions

What is a quantum of neurotransmitter?: It is the amount of transmitter contained in a single synaptic vesicle; release happens in whole-number multiples of this packet rather than as a continuous stream.
How do electrical and chemical synapses differ?: Electrical synapses pass current directly between cells through gap junctions for very fast signalling, whereas chemical synapses use neurotransmitter release, allowing amplification, sign changes, and modulation but adding a brief delay.