Synapse

Author: Christina Servou • Reviewer: Alexandra Pliakopanou
Last reviewed: November 13, 2024
Reading time: 7 minutes

Our nervous system consists of billions of nerve cells that exchange signals between them and also transmit information to effector cells, so that the human body can function effectively. A synapse is defined as the junction between two cells, serving as the primary means of communication between a presynaptic neuron initiating the signal and a postsynaptic cell receiving the signal. This type of connection facilitates communication not only between two neurons, but also between a neuron and an effector cell, i.e. a muscle cell or gland cell.

Key facts about synapses
Definition	The site of interaction between a neuron (presynaptic cell) and another cell; neuron, muscle cell or gland cell (postsynaptic cell)
Function	Signal transmission between two cells
Classification	Based on the way they function: - Chemical, communicating through chemical messengers - Electrical, communicating through direct ion flow between cells Based on the location of interaction: - Axodendritic - Axosomatic - Axoaxonic - Dendrodendritic - Dendrosomatic - Neuromuscular - Neuroglandular
Anatomy	Chemical synapses - Synaptic knob (or presynaptic terminal or axon terminal) - Neurotransmitters, serving as chemical messengers - Synaptic cleft, a narrow gap between the presynaptic and postsynaptic membrane - Postsynaptic membrane, containing receptors that bind to neurotransmitters Electrical synapse - Gap junctions, providing direct communication between adjacent neurons - Connexons, hexameric protein complexes forming channels for the passage of ions and small molecules

Contents

Classification of synapses
Anatomy of the synapse
1. Chemical Synapses
2. Electrical synapses
Sources

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Classification of synapses

Synaptic transmission can be either chemical or electrical, and thus, synapses can be classified into two types based on the way they function: chemical synapses, the most common ones in the human nervous system, and electrical synapses, mainly found in non-mammals.

Chemical and electrical synapses differ in several aspects, including their structure, the mode of communication between the connected cells, the speed of transmission and the directionality of the signal.

Depending on the location of interaction between the two cells involved, synapses can alternatively be categorized as:

Axodendritic; an interaction between the axon terminal of the presynaptic neuron and the dendrite of the postsynaptic neuron.
Axosomatic; an interaction between the axon terminal of the presynaptic neuron and the cell body (soma) of the postsynaptic neuron.
Axoaxonic: an interaction between the axon terminal of one neuron and the axon of another neuron.
Dendrodendritic; an interaction between the dendrites of two neurons.
Dendrosomatic; an interaction between the dendrites of one neuron and the the cell body (soma) of another neuron.
Somatic; an interaction directly between the cell bodies (somata) of two neurons.

A neuromuscular junction is the synapse between the axon terminal of a motor neuron and a muscle fiber and a neuroglandular junction that between an axon terminal of a neuron and glandular cell.

Anatomy of the synapse

Chemical Synapses

A chemical synapse consists of several key components:

Terminal bouton (or presynaptic terminal/knob or axon terminal), located at the end of the presynaptic neuron’s axon that houses synaptic vesicles (i.e., membrane-bound spheres which store neurotransmitters) and contain voltage-gated Ca2+ channels.
Neurotransmitters: chemical molecules serving as messengers for the transmission of information between the two cells involved in the chemical synapse. They can be characterized as excitatory or inhibitory, based on their effect on the postsynaptic neuron; excitatory neurotransmittors, such as glutamate, make it more likely for the postsynaptic neuron to generate an action potential, and inhibitory neurotransmittors, such as γ-aminobutyric acid (GABA), less likely.
Synaptic cleft: a narrow gap between the presynaptic and postsynaptic membranes across which neurotransmitters diffuse to reach the postsynaptic neuron.
Postsynaptic membrane, containing protein receptors that bind to neurotransmitters released by the presynaptic neuron. Receptors are usually ligand-gated channels (ionotropic receptors), or G-protein-coupled receptors activating ion channels through a signaling pathway (metabotropic receptors).

When an action potential reaches the axon terminal, it activates voltage-gated Ca2+ ion channels, allowing the influx of Ca²⁺ into the cell. The neurotransmitter-containing vesicles fuse with the presynaptic membrane and neurotransmitters are released into the synaptic cleft via exocytosis. Neurotransmitters bind to receptor proteins on the postsynaptic (or presynaptic) membrane leading to the opening or closure of ion channels. Depending on the neurotransmitter and ions involved, the cell interaction can result in an excitatory postsynaptic potential (EPSP) which is depolarizing (i.e., makes the inside of the cell more positive), or an inhibitory postsynaptic potential (IPSP) which is hyperpolarizing (i.e., makes the inside of the cell more negative). For the synapse to function properly, there must be a process for signal termination so that the postsynaptic cell returns to its resting state, ready for the arrival of new signals. For this purpose, the synaptic cleft is cleared from neurotransmitters through enzymatic deactivation, reuptake by the presynaptic neuron via endocytosis or transporters, or simply through diffusion.

Electrical synapses

Electrical synapses provide a direct route for electrical communication between neurons. Electrical transmission is achieved through:

Gap junctions: specialized protein channels that physically connect membranes of adjacent neurons. As a result, voltage changes in the presynaptic cell can directly induce voltage changes in the postsynaptic cell.
Connexons: hexameric protein complexes consisting of connexins and forming channels within gap junctions for the passage of ions and small molecules.

Since they do not require the release of a neurotransmitter, electrical synapses transfer signals very fast, allowing for synchronized activity among neurons. Moreover, transmission can take place in both directions, meaning that ions can flow either way, depending on the location the action potential was first generated, thus bending the definition of presynaptic and postsynaptic. Apart from ions, other small molecules can also pass through the pore of a gap junction such as ATP and second messengers. This showcases the role of electrical synapses in neuron signaling and metabolism.

Sources

All content published on Kenhub is reviewed by medical and anatomy experts. The information we provide is grounded on academic literature and peer-reviewed research. Kenhub does not provide medical advice. You can learn more about our content creation and review standards by reading our content quality guidelines.

Barrett, K. E., Barman, S. M., Boitano, S., & Brooks, H. L. (2012). Ganong’s Review of
Medical Physiology 24th Edition. New York: McGraw Hill Professional.

Caire, M. J., & Varacallo, M. (2018, November 13). Physiology, Synapse. Retrieved from Nih.gov website: https://www.ncbi.nlm.nih.gov/b...

Hyman, S. E. (2005). Neurotransmitters. Current Biology, 15(5), R154–R158.
https://doi.org/10.1016/j.cub....

Purves, D., Augustine, G. J., Fitzpatrick, D., Katz, L. C., Anthony-Samuel LaMantia,
McNamara, J. O., & S Mark Williams. (2012). Electrical Synapses. Retrieved from Nih.gov website: https://www.ncbi.nlm.nih.gov/b...

Purves, D., Augustine, G. J., Fitzpatrick, D., Katz, L. C., Anthony-Samuel LaMantia,
McNamara, J. O., & S Mark Williams. (2016). Excitatory and Inhibitory Postsynaptic
Potentials. Retrieved from Nih.gov website: https://www.ncbi.nlm.nih.gov/b...

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Synapse

Classification of synapses

Anatomy of the synapse

Chemical Synapses

Electrical synapses

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