Synapses are sites of functional contact between two neurons, that is, between two nerve cells. Also called synaptic junctions, these junctions allow the transmission of information in the form of electrical signals. Depending on the structures involved, these impulses can be transmitted from one neuron to another (interneuronic synapses), from a sensory receptor to a nerve end (cyto-neural synapses) or from a neuron to a peripheral effector cell, for example to a fiber or to a glandular cell (peripheral synapses). Specifically, the neuron-muscle fiber synapse is called a motor plate or neuromuscular junction. Regardless of the cellular elements that come into contact, the cell that transmits the information is called presynaptic, while the one that receives it is called postspinpathic.
Synapses between neurons (interneuronic synapses)
- asso-dendritic synapses (the most numerous;
- axosomatic synapses;
- axonal synapses.
As can be seen, the presynaptic neuron always uses the terminal branches of its own axon, which represents the extension through which it communicates with other nerve cells.
Near the synapses the axonic branches lose their myelin sheath and swell in the so-called terminal buttons or synaptic buttons.
Despite the figure, it is important to note that the number of synapses in a single neuron can be quite numerous, up to several thousand. Some of these are of the excitatory type, others of the inhibitory type.
Chemical synapses and electrical synapses
From a functional point of view - in relation to the type of signal that is transmitted from the presynaptic to the postsynaptic cell - there are two different types of synapses: electrical synapses and chemical synapses.
In electrical synapses the conduction of the nerve impulse is particularly fast and virtually instantaneous, thanks to the direct passage of current from one cell to another. This is thanks to the extreme proximity or even to the cytoplasmic continuity between the presynaptic cell and the postsynaptic one, and to specialized structures, the gap junctions or communicating junctions, which allow themselves to be crossed by the wave of depolarization of the action potential, opposing a very low resistance. communication is entrusted to ionic currents and is generally bidirectional, which allows to synchronize the neuronal population responses and obtain a massive and very rapid activation.
In chemical synapses, which are far more frequent in our body, the transmission of signals is entrusted to a chemical mediator, called a neurotransmitter. Compared to the previous ones, there is a point of structural discontinuity between the presynaptic cell and the postsynaptic cell; in this way the membranes of the two cells always remain distinct and separated by a space (20-40 millionths of a millimeter) called the synaptic cleft. Examining them under a microscope, we realize that chemical synapses comprise three different structures: the presynaptic membrane, the synaptic cleft (or synaptic wall) and the postsynaptic membrane. Unlike the previous ones, the chemical synapses are unidirectional and have a certain delay in the transmission of the electrical signal (from 0.3 ms to a few ms). When the nerve impulse arrives at the synaptic button, the vesicles it contains, rich in chemical messengers (neurotransmitters), merge with the cell membrane, releasing their contents in the synaptic cleft.The neurotransmitters are then picked up by specific receptors placed on the postsynaptic membrane, modifying their permeability to the passage of ions. Thus, a depolarizing post-synaptic potential (opening of the ion channels, with resulting excitation) or hyperpolarizing (closing of the ion channels, with resulting inhibition) is generated.
Once the signal is transmitted, the neurotransmitter is then reabsorbed by the presynaptic termination or degraded by specific enzymes present in the synapse gap; a small amount can also diffuse out of the fissure and enter, for example, the bloodstream. Both neurotransmitters and protein enzymes necessary for metabolism must be synthesized by the soma, as the axonal terminal participating in the synapse does not contain the organelles necessary for protein synthesis.
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