General Structure of a Synapse
A chemical synapse consists of three essential structural components:
- Presynaptic membrane — 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 these vesicles and necessary transmitter components to the axon terminal is maintained by a specialized transport mechanism called axoplasmic flow.
- Synaptic cleft — a microscopic space that physically separates the contacting neurons or a neuron and its target organ.
- Postsynaptic membrane — a specialized region of the receiving cell's membrane. It contains 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 specialized intercellular channels. The width of the synaptic cleft here is 10 times smaller than in chemical synapses.
Excitation transmission occurs without the release of a neurotransmitter—via electrical currents through local ionic flows between the excited presynaptic and unexcited postsynaptic membranes. Mechanistically, this is very similar to the propagation of excitation between nodes of Ranvier in myelinated fibers.
These junctions are located in the myocardium, smooth muscle, and partly in the central nervous system. Their primary functional feature is bidirectional conduction, meaning excitation can pass freely in both directions.
Chemical Synapses
In chemical junctions, excitation transmission is carried out exclusively by neurotransmitters. This is the primary method of signal transmission between CNS neurons, in autonomic ganglia, and at neuroeffector and neuromuscular junctions.
A special mode of transmission is volume transmission (synapses at a distance). In this case, a neurotransmitter released from a single cell diffuses to simultaneously excite the postsynaptic membranes of several target cells. This pattern is characteristic of sympathetic nervous system ganglia (where a single preganglionic fiber can excite about 20 ganglionic neurons) and certain neuroglandular synapses of the autonomic nervous system.
Morphological and Pharmacological Classification
Based on structure (morphology), synapses are divided into two major groups:
- Neuronal (central and ganglionic): axosomatic, axodendritic, axoaxonic, and dendrodendritic.
- Neuroeffector: neuromuscular and neuroglandular.
Based on the type of released neurotransmitter (pharmacological classification), they include:
- Adrenergic;
- GABAergic;
- Serotoninergic;
- Dopaminergic;
- Glutamatergic;
- Cholinergic (transmit excitation using acetylcholine). They are further subdivided into nicotinic (N-cholinergic) and muscarinic (M-cholinergic).
Mechanisms of Excitation and Inhibition
The 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 sodium (Na⁺) channels. The influx of positively charged sodium into the cell causes local depolarization, generating an EPSP (Excitatory PostSynaptic Potential).
- Inhibitory synapse: The neurotransmitter binds to receptors and opens chloride (Cl⁻) channels. The influx of negatively charged chloride causes cellular membrane hyperpolarization, generating an IPSP (Inhibitory PostSynaptic Potential), which suppresses excitation.