Synapses

Main types by transmission mechanism

Based on the signal transmission mechanism, all synapses can be divided into two large groups:

1. Chemical synapses. This is the dominant type of contact in humans. The signal is transmitted using a specific chemical agent—a neurotransmitter. A key feature of this contact is strictly unidirectional conduction. Molecules diffuse across the intercellular space and bind to target receptors, making backward transmission physically impossible.
2. Electrical synapses. These are relatively rare in the nervous system (e.g., in the retina). The cells are physically connected by gap junctions—nexuses. Ion channels pierce the membranes directly, uniting the cytoplasm of neighboring cells. This allows the depolarization wave to be transmitted directly. Similar structures exist in the myocardium and smooth muscles, but they connect muscle cells to each other, so they do not always fit the strict definition of a synapse.

Structural triad of a chemical synapse

A classical chemical synapse always consists of three mandatory components forming a single functional complex:

• Presynaptic terminal. As a rule, this is an expanded axon terminal. Inside it, vesicles filled with neurotransmitter are anchored to the cytoskeleton elements. Voltage-gated calcium channels, which are tightly closed at rest, are embedded in the plasmalemma of this terminal.
• Synaptic cleft. A microscopic space 20–30 nm wide. In this zone, special filaments are located that mechanically bridge and bind both membranes together, preventing the contact from falling apart.
• Postsynaptic membrane. This is a specialized region of the receiving cell's plasmalemma. It features a complex protein apparatus: specific neurotransmitter receptors, effector proteins (which execute the response), and enzymes whose task is to rapidly break down the spent neurotransmitter to terminate signal transmission.

Mechanism of neurotransmitter release

Signal transmission is triggered when an electrical impulse reaches the axon terminal. The process occurs in several stages:

1. The excitation wave reaches the presynaptic membrane.
2. Voltage-gated $Ca^{2+}$ channels open, and calcium ions rush into the terminal.
3. A sharp surge in $Ca^{2+}$ concentration causes synaptic vesicles to detach from the cytoskeleton that holds them.
4. The vesicle membrane fuses with the presynaptic plasmalemma (exocytosis occurs).
5. A portion of neurotransmitter is released into the synaptic cleft and begins to diffuse toward the target cell.

Ionotropic and metabotropic receptors

When the neurotransmitter reaches the postsynaptic membrane, the cell's response can proceed via one of two pathways, depending on the receptor type:

• Ionotropic mechanism (fast). Here, the receptor molecule and the ion channel form a single protein complex. Upon neurotransmitter binding, the channel opens instantly. If it is a cation channel (as in nicotinic cholinergic synapses), $Na^+$ enters, causing depolarization and excitation. If it is an anion channel (GABAergic or glycinergic synapses), $Cl^-$ enters, causing hyperpolarization and inhibition.
• Metabotropic mechanism (slow). The receptor is not a channel. Neurotransmitter binding activates intracellular messengers that alter enzyme activity (usually protein kinases). This triggers the phosphorylation of targets in intracellular metabolism. In neurons, this ultimately leads to the opening or closing of channels, while in muscle cells, it can directly alter the activity of contractile proteins without changing the plasmalemma potential. This is how adrenergic synapses (norepinephrine) and muscarinic acetylcholine receptors operate.

Network architectonics and modulation

Neurons form highly complex networks thanks to divergence (the spreading of a signal via axon collaterals to numerous cells) and convergence (the convergence of hundreds of contacts onto the body of a single neuron). The cell sums up all incoming impulses and forms an integrative response.

In addition to classical synapses, non-synaptic contacts exist. In this case, axon terminals have varicosities from which neurotransmitters (often neuropeptides or norepinephrine) are released directly into the wide intercellular space. This process is slow, but it exerts a generalized modulatory effect on a group of neighboring cells simultaneously, fine-tuning their activity.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Features of target protein phosphorylation in metabotropic transmission
• Differences in norepinephrine effects on alpha- and beta-adrenergic receptors
• Ultrastructure of gap junctions in electrical synapses
• Morphology of varicosities in non-synaptic transmission
• The role of calcium ions in detaching synaptic vesicles from the cytoskeleton

Other topics in "Nervous tissue: nerve endings and synapses"

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