Synapses: what are they, types and functions

April 25, 2024

The nervous system is one of the most important elements for our existence and survival, since it allows the management, organization and operation of the rest of the corporal systems. This system works by sending electrochemical impulses with different information and orders for the different structures that are part of our organism.

Formerly it was believed that the nervous system was a continuous network and no separation between elements, until Ramón y Cajal, through dyes such as Golgi, allowed to identify that it is actually made up of a set of cells separated from each other: neurons . These are separated by small spaces, but they do not stop communicating with each other. The existing connection between them is what is known as synapses .

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What is a synapse?

The concept of synapses, described for the first time by Ramón y Cajal and baptized by Sherrington, refers to the existence of a connection between two neurons, characterized by the presence of a small space that serves as a way for the transmission of information .

The main function of this connection is to allow the transmission of information between the different neurons. It is therefore a fundamental element in the functioning of the organism, making possible the realization and coordination of all the processes that allow to perform the different vital functions, as well as the physical and mental capacities, both basic and superior.

This connection is also very useful not only to transmit information but also to regulate it: the presence of the synaptic space makes that the presynaptic neuron can recapture the neurotransmitters if an excessive amount has been released . Likewise, it is very useful in the sense that it allows the waste generated by neuronal functioning to be eliminated by each cell, preventing its wear due to the concentration of said residues.

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Main components

The synapse between two neurons, the connection and link between them that allows information to be transmitted, is not an isolated element, but is composed of three main components among which we find part of both neurons in interrelation: presynaptic neuron, synaptic space and postsynaptic neuron.

1. Presynaptic neuron

This part refers to the neuron that sends the information to another. This action is usually carried out through the emission of neurotransmitters by the synaptic vesicles of the terminal buttons of the end of the axon, which in turn will be received by the membrane of the postsynaptic neuron.

2. Synaptic space

The synaptic space or synaptic cleft is the space between two neurons , generally between twenty and forty nanometers. This is the space in which the transmission of information between neurons takes place.

3. Postsynaptic neuron

It is the receptor part in the relationship between neurons. More than the neuron itself, reference would be made to the part of it that receives the information from the presynaptic neuron. Usually it's about the dendrites , although depending on the type of connection can also be the soma or the axon.

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Types of synapses

There is not only one type of synapse, but different classifications and typologies can be found depending on different parameters, such as the place where they generate the connection with another neuron or the type of elements that circulate between them. Thus, we can find among others the following types.

Types according to what is transmitted

According to the type of element that is transmitted between neurons, we can find the following ones. Despite its distinction, it must be taken into account that It is common for the same neuron to have a chemical and electrical connection at the same time , as well as the fact that the information that the system goes through is generally bioelectric (that is, although chemical elements are transmitted between neurons, what these generate are alterations of an electrical type).

Chemical synapses

Its about type of major synapse in our body . In these synapses the information is transmitted chemically, through the sending by the presynaptic neuron of different neurotransmitters that the postsynaptic neuron captures through different receptors, whose action generates an alteration in the form of excitatory or inhibitory postsynaptic potential that can end or not with the generation of an action potential by the postsynaptic neuron. They are versatile synapses, since some neurons can inhibit the action of others depending on what is activated. There is no physical contact between both neurons.

Electric synapses

In this type of synapse, the information is transmitted directly at the electrical level as the ions directly flow between the pre- and postsynaptic component. They do not have versatility, since its performance does not allow one neuron to inhibit the action of another . In this type of synapse there is actually a contact between pre and postsynaptic neuron, through gap junctions or channels formed by proteins.

They are typical of the optic nerve and its connection with cones and rods in the eye . Also of invertebrate animals.

Types according to the effect

The interaction between neurons can have mainly two effects, which correspond to the following types of synapses.

Excitatory synapses

Type of synapse in which the transmission of information has excitatory effects, facilitating that the postsynaptic neuron realizes an action potential and the message transmission continues to generate the depolarization of its membrane.

Inhibitory synapsis

In this case, the actuation or activation of this type of synapse hinders the appearance of an action potential when hyperpolarizing the postsynaptic cell. It becomes more difficult for information to be transmitted through the postsynaptic neuron to others connected to it.

According to connection place

Depending on the place where they connect with each other, we can find the following types of synapses.

Axodendritic synapses

The most frequent and prototypical type of connection. The synaptic connection occurs between the axon of the presynaptic neuron and the dendrites of the postsynaptic neuron . Generally it has excitatory effects.

Axosomatic synapses

In this type of synapse, the axon of the presynaptic neuron connects with the soma or postsynaptic nucleus . Generally it has inhibitory effects in the second.

Axo-axonal synapses

This type of connection usually occurs in such a way that modulating effects are exerted when a neuron releases certain amounts of neurotransmitter to another. There is a connection between the axon of the presynaptic neuron and the postsynaptic neuron, altering the possibility that it releases certain amounts of neurotransmitters to a third with which it is connected by another route.