Synapses

Basic classification and localization

Based on the signal transmission method, there are two main types of synapses:

• Chemical — the most common type. The signal is transmitted through the release of neurotransmitter molecules that bind to specific receptors on the target cell.
• Electrical — excitation is passed directly to the neighboring neuron via specialized narrow gap junctions called connexons.

Synapses delivering information to a specific cell are called afferent. Depending on which part of the receiving neuron they are located on, they are divided into three groups:

1. Axodendritic — located on dendrites and make up the vast majority of afferent contacts. In many cells (e.g., cortical pyramidal neurons), dendrites end in spines that compartmentalize incoming signals. Inside them lies the spine apparatus, which stores calcium ions. Asymmetric excitatory synapses are most frequently located on spines.
2. Axosomatic — localized directly on the neuron cell body (perikaryon).
3. Axoaxonic — formed on the axon and its initial segment (axon hillock).

Organization of neural networks

Neurons constantly interact with each other, forming complex networks. This process relies on two fundamental principles:

• Convergence — a situation where a single neuron collects impulses from multiple other cells.
• Divergence — the spread of a signal from a single neuron to many others due to the branching of its axon (formation of countless collaterals).

Each nerve cell can be either in a resting state (returning to it under the influence of inhibitory impulses) or in an active state, where it continuously generates discharges under the action of excitatory signals.

Morphologically and functionally, neurons are divided into excitatory (usually principal cells with long axons transmitting signals over long distances) and inhibitory (most often interneurons with short processes).

Principles of neuronal inhibition

Inhibition in the nervous system is carried out via inhibitory interneurons. There are three key mechanisms:

• Recurrent inhibition. An axon collateral of the principal (excitatory) neuron activates an inhibitory cell. The axon of this inhibitory cell returns to the initial neuron and suppresses its activity. This is a classic example of negative feedback for autoregulation.
• Feedforward inhibition. An axon collateral of the principal neuron engages an inhibitory interneuron, which then suppresses the activity of other neighboring principal cells, limiting the spread of excitation.
• Disinhibition. One inhibitory neuron suppresses the activity of another inhibitory neuron. As a result, the inhibitory influence on the target cell drops, and the activity of postsynaptic principal neurons naturally increases.

Neurotransmitters and Receptors

Historically, neurons were classified by the shape and length of their processes (Golgi type I and II cells). Modern classification relies on the neurotransmitter phenotype—the type of substance released, which determines the final effect (excitation or inhibition).

Key neurotransmitters:

• Central nervous system (CNS): main excitatory is glutamate; main inhibitory are GABA (brain) and glycine (spinal cord).
• Autonomic nervous system (ANS): acetylcholine and norepinephrine are of key importance (they are also present in the CNS).
• Interneurons frequently use dopamine, serotonin, and various neuropeptides.

Neurotransmitters act on ligand-gated receptors—protein complexes spanning the membrane. Upon binding to a ligand (neurotransmitter), the receptor opens an ion channel for specific ions.

Function of glutamate receptors

Glutamate interacts with three main receptor types: kainate, AMPA, and NMDA. Excitatory signal transmission is staged:

1. Activation of AMPA receptors. Glutamate binds to the receptor, opening a channel for sodium ions. Sodium rushes into the cell, causing membrane depolarization.
2. Removal of the magnesium block. At rest, the NMDA receptor channel is blocked by a magnesium ion. This "plug" can only be dislodged by depolarization previously triggered by AMPA receptors.
3. Activation of NMDA receptors. As soon as the magnesium block is cleared, the NMDA channel opens, allowing not only sodium but also calcium ions into the cell.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Ultrastructural organization of connexons in electrical synapses
• Mechanisms of the spine apparatus and calcium accumulation
• Historical classification of neurons by Golgi (types I and II)
• The role of neuropeptides in interneuron function
• Features of signal transmission in the autonomic nervous system

Other topics in the section "DeMyer's The Neurologic Examination" / "Duus' Topical Diagnosis in Neurology"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.