Layman's guide to synapses

Different molecular components at synapses mediate their structure and function
Synapses are extremely complex cell junctions, and so far we have seen only a fraction of their typical components. This page provides an overview over most classes of components typically found at synapses and required for their formation, maintenance and function.

In the following pages we will see where such components come from, how their function and importance for the synapse can be studied, and how new synaptic components can be discovered.

Synaptic vesicles in the presynaptic terminal are filled with neurotransmitter.
Associated with the vesicles and the presynaptic membrane is a complex machinery of proteins mediating and regulating vesicle fusion with the presynaptic membrane; they ensure that the neurotransmitter content of vesicles can is released into the synaptic cleft in a controlled fashion.
A machinery of synaptic reuptake (endocytosis) mediates recycling of vesicles from the presynaptic membrane; vesicles are guided back to the release site through mechanisms regulating intracellular trafficking (not shown).
Certain neurotransmitters (red dots) are generated within the presynaptic terminal by catalytic enzymes (green ellipse).
Vesicular transporters load vesicles with neurotransmitter; they require an electrical and/or proton gradient across the vesicular membrane. These gradients are generated by ATP-dependent proton pumps which are likewise localised in the vesicular membrane (not shown).
Voltage dependent ion channels regulate the amplitude and timecourse of pre- and postsynaptic depolarisations. Densely clustered presynaptic calcium channels (green) cause calcium influx upon depolarisation which elicits presynaptic vesicle fusion.
Neurotransmitter receptors respond to binding of neurotransmitters; either they represent ion channels which permit ion flow across the membrane (ionotropic; red), or they trigger intracellular signalling events (metabotropic; not shown) leading to a great variety of possible structural or functional changes (depending on the type and condition of a synapse).
Non-vesicular transmitter transporters remove neurotransmitter from the synaptic cleft; in contrast to vesicular transporters, this activity depends on symport and/or antiport of inorganic ions using their gradients across the membrane. One specific neurotransmitter (acetyl choline) is enzymatically removed by enzymatic activity in the synaptic cleft (not shown).
Cytoskeletal components organise the shape of pre- and postsynaptic terminals and mediate precise localisation of functional synaptic components.
Synapses represent cell junctions that require adhesion across the synaptic cleft mediated by adhesion molecules.
The synaptic cleft is usually filled by diffuse extracellular material composed of structural proteins released from cells. At vertebrate neuromuscular junctions the extracellular material forms a prominent basement membrane which is used as an anchor for enzymes and signalling molecules.
Signalling molecules mediate information exchange between pre- and postsynaptic terminals. This allows co-ordination of their joint development but also of their functional or structural changes (plasticity) required to adapt synapse function to altered network requirements in the mature brain (e.g. changes underlying learning and memory).


Where do these components come from? >>>