What is the synaptic space?

Two neurons connect in synapses, so that they transmit information to each other. These synapses do not involve direct contact between both neurons, but rather occur in a synaptic space or cleft, which is the place where the exchange occurs. What happens in the synaptic space and how does it work? Let’s try to answer this question.

During chemical synapsis, the neuron that passes the information (presynaptic) releases a substance, in this case a neurotransmitter, through the synaptic button, being released into the synaptic space, also called the synaptic cleft. Subsequently, the postsynaptic neuron, which has specific receptors for each neurotransmitter, is responsible for receiving the information through the dendrites.

It was the electron microscope that allowed us to discover that the communication that occurred between neurons did not imply contact between them, but that there is a space where release neurotransmitters. Each of these neurotransmitters has different effects that affect the functioning of the nervous system.

chemical synapses

There are mainly two types of synapses: electrical and chemical.. The space between presynaptic and postsynaptic neurons is substantially larger in chemical synapses than in electrical synapses, receiving the name synaptic space. The key characteristic of these is the presence of membrane-bound organelles, called synaptic vesicles, inside the presynaptic terminal.

Chemical synapses occur as a result of the release of chemicals (neurotransmitters) in the synaptic cleft, which act on the psotsynaptic membrane, producing depolarizations or hyperpolarizations. In front of the electrical synapse, the chemical can modify its signals in response to events.

Neurotransmitters are stored in the vesicles of the terminal button. When an action potential reaches the terminal button, Depolarization causes the opening of Ca++ channels, that penetrates the cytoplasm and causes chemical reactions that cause the vesicles to expel neurotransmitters.

The vesicles are filled with neurotransmitters that act as messengers between communicating neurons. One of the The most important neurotransmitter in the nervous system is acetylcholine.which regulates the functioning of the heart or acts on different postsynaptic targets of the central and peripheral nervous system.

Properties of neurotransmitters

It was previously thought that each neuron was capable of synthesizing or releasing only one specific neurotransmitter, but today it is known that each neuron can release two or more. For a substance to be considered a neurotransmitter, it must meet the following requirements:

The substance must be present within the pre-synaptic neuron, in the terminal boutons, contained in vesicles. The pre-synaptic cell contains enzymes suitable for synthesizing the substance. The neurotransmitter must be released when certain nerve impulses reach the terminals. It is necessary that high affinity receptors are present in the postsynaptic membrane. The application of the substance produces changes in the postsynaptic potentials. There must be mechanisms of inactivation of neurotransmitters in or around the synapse. The neurotransmitter must fulfill the principle of synaptic mimicry. The action of a supposed neurotransmitter should be reproducible by the exogenous application of a substance.

Neurotransmitters affect their targets by interacting with receptors. A substance that binds to a receptor is called a ligand and can have 3 effects:

agonist: Starts the normal effects of the receptor.Antagonist: is a ligand that binds to a receptor and does not activate it, thus preventing other ligands from activating it.inverse agonist: binds to the receptor and initiates an effect that is the opposite of its normal function.

What types of neurotransmitters exist?

In the brain, most synaptic communication is carried out by 2 transmitter substances. Glutamate with excitatory effects and GABA with inhibitory effects, the rest of the transmitters, in general, serve as modulators. That is, its release activates or inhibits circuits involved in specific brain functions.

Each neurotransmitter, released into the synaptic space, has its own function, it may even have several. It binds to a specific receptor, and they can also influence each other, inhibiting or enhancing the effect of another neurotransmitter. More than 100 different types of neurotransmitters have been detected and the following are some of the best known:

Acetylcholine: It is involved in learning and controlling the stage of sleep in which dreams occur (REM).Serotonin: It is related to sleep, moods, emotions, control of intake and pain.Dopamine: involved in movement, attention and learning about emotions. It also regulates motor control.Epinephrine or adrenaline: It is a hormone when it is produced by the adrenal gland.Norepinephrine or norepinephrine: Its release produces an increase in attention, vigilance. In the brain it influences emotional responses.

Synapse pharmacology

In addition to the neurotransmitters that are released in the synaptic space, affecting the receiving neuron, there are exogenous chemicals that can cause the same or similar response. When we talk about exogenous substances, we are talking about substances coming from outside the body, such as drugs. These can produce agonist or antagonist effects and can also affect different levels of the chemical synapse:

Some substances have effects on the synthesis of transmitter substances. The synthesis of the substance is the first stage, it is possible that the production rate can be increased by administering a precursor. One of them is L-dopa, a dopamine agonist. Others act on the storage and release of these. For example, reserpine prevents the storage of monoamines in synaptic vesicles and therefore acts as a monoamine antagonist. They may have an effect on the receptors. Some substances can bind to receptors and activate or block them.On the reuptake or degradation of the transmitting substance. Some exogenous substances can prolong the presence of the transmitter substance in the synaptic space, such as cocaine, which delays the reuptake of norepinephrine.

Repeated treatments with a certain drug can reduce its effectiveness, which is called tolerance. Tolerance, in the case of drugs, can lead to an increase in consumption, increasing the risk of overdose. In the case of drugs, they can cause a decrease in the desired effects, which can lead to discontinuation of the drug.

As has been observed, in the synaptic space, exchanges occur between pre- and post-synaptic cells through the synthesis and release of neurotransmitters with various effects on our body. This complex mechanism can also be modulated or altered through multiple drugs.

Bibliographic references

Carlson, N. (1996). Behavioral physiology. Barcelona: Ariel.

Haines, DE. (2003). Principles of Neuroscience. Madrid: Elsevier Science.

Kandel, E.R., Schwartz, Jh, & Jesell, T.M. (19996). Neuroscience and behavior. Madrid: Prentice Hall.

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