The brain is a complex structure, consisting of approximately 86 billion neurons (according to calculations by scientist Suzane Herculano-Houzel) linked together by connections called synapses. In a general distinction, we can divide synapses into chemical and electrical. In humans, we can find predominantly chemical synapses, whereas there are far fewer electrical synapses. The latter can be found, for example, in the retina and certain parts of the brain, as well as in the heart and muscles.

What is the difference between these two types of synapse and what is their function? The main difference, which determines their different physiology, is the presence or absence of a synaptic gap - the space that exists between the presynaptic and postsynaptic neuron (Fig.1). In chemical synapses, there is a synaptic gap, whereas in electrical synapses there is none, and the cells are located very close to each other and are connected by gap junctions. Gap junctions are special types of channels that allow ions to flow freely between two cells. As a result, cells are able to communicate with each other at lightning speed. In this case, molecules are not placed in synaptic vesicles and do not connect to receptors on postsynaptic neurons, as is the case with neurons communicating with each other via chemical synapses.

Why then are chemical synapses predominant in our brain when it is the electrical ones that provide a faster flow of information? The answer is simple, it is the chemical synapses that are more specialised. Their action is more precise, profiled to activate or inhibit specific neurons. It is thanks to chemical synapses that we can think, remember, feel emotions, experience pleasure, hunger, fatigue. This does not mean that electrical synapses do not participate in these processes, but they play a supporting role by enabling, for example, the synchronous activation of neurons in a particular brain structure.

The flow of information between chemical synapses is mediated by neurotransmitters, which the presynaptic neuron packs into the aforementioned synaptic vesicles (Fig.1). Which molecules can be neurotransmitters? They could be, for example, serotonin (5-HT), norepinephrine (NA), acetylcholine (Ach), dopamine (DA), glutamic acid (Glu), gamma-aminobutyric acid (GABA) or glycine (Gly). In order for neurotransmitters to activate subsequent neurons, they must bind to a receptor specific only to themselves, which is located on the surface of the postsynaptic neuron. This is followed by the opening of channels for positive ions or negative ions, which enter the neuron and initiate the depolarisation or hyperpolarisation of the neuron in turn. This is how the signal is transmitted to the subsequent nerve cells and the brain is able to carry out its specialised functions.


Author: Suri Stawicka

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