General structure of the synapse
A chemical synapse has three mandatory structural elements:
- Presynaptic membrane is the membrane of the axon terminal of the transmitting cell. Inside this terminal are vesicles containing a specific chemical substance—the neurotransmitter. The delivery of the vesicles themselves and the necessary neurotransmitter components to the axon terminal is provided by a specialized transport mechanism—axoplasmic flow.
- Synaptic cleft is the microscopic space that physically separates the contacting neurons or a neuron and a target organ.
- Postsynaptic membrane is the region of the membrane of the receiving cell. It features specialized receptors ready to recognize and bind the neurotransmitter released into the cleft.
Electrical synapses (ephapses)
Electrical synapses, also called ephapses or gap junctions, are gap junctions. The width of the synaptic cleft here is 10 times smaller than in chemical synapses.
Excitation transmission in them occurs without the release of a neurotransmitter—via an electrical mechanism through local ionic currents between the excited presynaptic and unexcited postsynaptic membranes. Fundamentally, this mechanism differs little from the conduction of excitation between Nodes of Ranvier in a myelinated fiber.
These contacts are localized in the myocardium, smooth muscle, and partially in the central nervous system. Their main functional feature is bidirectional conduction, meaning excitation can freely pass in both directions.
Chemical synapses
In chemical contacts, excitation conduction is carried out exclusively by means of neurotransmitters. This is the main mode of signal transmission between CNS neurons, in the ganglia of the nervous system, as well as in neuroglandular and neuromuscular junctions.
A special type of transmission is volume transmission (synapses at a distance). In these, a neurotransmitter, released from a single cell, excites the postsynaptic membranes of multiple targets simultaneously via diffusion. This pattern is characteristic of sympathetic nervous system ganglia (where a single preganglionic fiber can excite about 20 ganglionic neurons) and certain neuroeffector synapses of the autonomic nervous system.
Morphological and pharmacological classification
According to structure (morphology), synapses are divided into two large groups:
- Neuronal (central and ganglionic): axosomatic, axodendritic, axoaxonic, and dendrodendritic.
- Neuroeffector: neuromuscular and neuroglandular.
According to the type of released neurotransmitter (pharmacological classification), they are distinguished into:
- Adrenergic;
- GABAergic;
- Serotonergic;
- Dopaminergic;
- Glutamatergic;
- Cholinergic (conduct excitation using acetylcholine). They are additionally subdivided into nicotinic (N-cholinergic) and muscarinic (M-cholinergic).
Mechanisms of excitation and inhibition
Physiological classification divides synapses into excitatory and inhibitory, depending on the ion channels that open on the postsynaptic membrane:
- Excitatory synapse: The neurotransmitter binds to receptors and opens channels for sodium ions (Na⁺). The influx of positively charged sodium into the cell causes local depolarization. As a result, an EPSP—Excitatory PostSynaptic Potential—is formed.
- Inhibitory synapse: The neurotransmitter binds to receptors and opens channels for chloride ions (Cl⁻). The influx of negatively charged chloride causes hyperpolarization of the cell membrane. An IPSP—Inhibitory PostSynaptic Potential—is formed, which leads to the suppression of excitation.
Briefly: answers to topical questions
By what mechanisms is the neurotransmitter removed from the synaptic cleft?
The removal of the neurotransmitter from the synaptic cleft occurs via two main pathways.
- Molecule destruction — enzymatic cleavage of the neurotransmitter. Acetylcholine is cleaved by cholinesterase/acetylcholinesterase, norepinephrine by COMT and MAO. Degradation products can be absorbed into the blood or undergo reuptake for the synthesis of new portions of the neurotransmitter.
- Reuptake (reuptake) — the neurotransmitter is transported from the synaptic cleft back into the presynaptic terminal and is used for the refilling of vesicles.
Which specific proteins ensure the fusion of the synaptic vesicle with the presynaptic membrane?
The fusion of the synaptic vesicle with the presynaptic membrane is ensured by secretion machinery proteins forming the SNARE complex, as well as calcium sensors.
- SNAP-25 — a presynaptic membrane protein.
- Syntaxin — a presynaptic membrane protein.
- Synaptobrevin — a vesicle membrane protein.
- Synaptotagmin — a calcium sensor on the vesicle that, upon binding to calcium ions, induces a conformational change in secretion proteins and the initiation of exocytosis.
To avoid confusing the ions: Na — Nucleates excitation (forms EPSP), while Cl — Causes inhibition (IPSP).
What else to study on this topic
- Mechanisms of exocytosis and neurotransmitter release into the synaptic cleft
- Fate of the neurotransmitter: enzymatic degradation and reuptake
- Spatial and temporal summation of EPSPs and IPSPs on the neuronal membrane
- Properties of synapses: synaptic delay, unidirectional conduction, and fatigue
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