Synaptic Transmission and Neurotransmitter Function
Synaptic transmission is the process by which one neuron communicates with another neuron or an effector cell across a synapse. At the dominant chemical synapse, an arriving action potential triggers calcium-dependent release of neurotransmitter, which diffuses across the synaptic cleft and binds postsynaptic receptors to change the target cell's membrane potential. This area covers the cellular and molecular machinery of that signalling chain and the neurotransmitter systems that carry it.
Definition
Synaptic transmission is the conduction of a signal from a presynaptic neuron to a postsynaptic cell, achieved at chemical synapses by the regulated, calcium-triggered release of a neurotransmitter that binds receptors on the target cell and alters its excitability.
Scope
The area orients the reader to chemical synaptic signalling in the nervous system: how transmitter is synthesised and packaged, how its release is coupled to calcium entry, how postsynaptic receptors transduce the chemical signal, how excitatory and inhibitory inputs are integrated, and how synaptic strength is modified by use. It is organised as a physiology reference and does not provide clinical management guidance.
Sub-topics
Core questions
- How is presynaptic depolarisation converted into neurotransmitter release?
- How do postsynaptic receptors translate transmitter binding into electrical or biochemical change?
- How are many converging excitatory and inhibitory inputs integrated by a single neuron?
- How is synaptic strength altered over short and long timescales?
Key concepts
- Chemical synapse and synaptic cleft
- Presynaptic terminal and active zone
- Calcium-triggered vesicle exocytosis
- Neurotransmitter receptors (ionotropic and metabotropic)
- Excitatory and inhibitory postsynaptic potentials
- Synaptic integration
- Synaptic plasticity
Key theories
- Quantal hypothesis of transmitter release
- Neurotransmitter is released in discrete, roughly uniform packets (quanta) corresponding to the contents of single synaptic vesicles, so that the postsynaptic response is built from integer multiples of a unit miniature response.
- Chemical theory of synaptic transmission
- Most central and peripheral synapses transmit by releasing a diffusible chemical messenger rather than by direct electrical continuity, with release tightly coupled to presynaptic calcium influx.
Mechanisms
An action potential invading the presynaptic terminal opens voltage-gated calcium channels; the resulting calcium influx triggers fusion of neurotransmitter-filled vesicles with the plasma membrane at the active zone, releasing transmitter into the cleft. Transmitter binds postsynaptic receptors, which either gate ion channels directly (ionotropic) or act through second messengers (metabotropic), producing depolarising (excitatory) or hyperpolarising (inhibitory) potentials. The postsynaptic neuron sums these inputs in space and time, and repeated activity can strengthen or weaken the connection, the basis of synaptic plasticity.
Clinical relevance
Synaptic transmission is the level at which many neurological and psychiatric disorders, and the drugs used to treat them, exert their effects, since transmitter synthesis, release, receptor binding, and reuptake are common pharmacological targets. This area describes the normal physiology that such interventions act upon and is intended as background understanding rather than as diagnostic or treatment guidance.
History
The chemical nature of synaptic transmission was established in the early twentieth century and refined by Bernard Katz and colleagues, whose work at the neuromuscular junction in the 1950s revealed that transmitter is released in quanta. Subsequent molecular work, recognised by Nobel Prizes to Katz and later to Südhof and others, identified the vesicle-fusion machinery and the calcium sensor that couple electrical activity to release.
Key figures
- Bernard Katz
- Thomas Südhof
- Eric Kandel
Related topics
Seminal works
- fatt-katz-1952
- sudhof-2013
- kandel-2001
Frequently asked questions
- What is the difference between a chemical and an electrical synapse?
- A chemical synapse signals by releasing a neurotransmitter that crosses a cleft and binds receptors on the target cell, whereas an electrical synapse passes current directly through gap-junction channels; this area concerns chemical transmission, the predominant mode in the mammalian nervous system.
- Why is calcium central to synaptic transmission?
- Calcium entering the presynaptic terminal through voltage-gated channels is the trigger that couples electrical activity to vesicle fusion, so that transmitter release is steeply dependent on the calcium signal.