4 Main mediators groups:

a) acetylcholine;

1. catecholamines:

• dopamine;

• noradrenaline;

• adrenaline;

• serotonine (monoamine);

2. aminoacids:

• glycine;

• GABA (gama-amino-butiric acid);

• L-glutamate;

• cysteine and so on;

d) peptides.

4. According to ending effect:

• Stimulating – both electrical and chemical ones.

• Inhibiting- only chemical ones.

SYNAPSE STRUCTURE:

1.Presynaptic terminal or part – usually is presynaptic axon ending.

2.Synaptic cleft – is a space dividing membranes of contacting cells.

3.Post-synaptic part – is cell locus to which presynaptic ending comes.

Fig.14

Synaptic binding is interneuronal interaction main mechanism. It provides all major expressions of nervous system activity. It is one of the most essential structural-functional brain elements.

Neuron from which axon arises is called presynaptic neuron and neuron on which axon ends is called postsynaptic neuron. Axon of presynaptic neuron divides into many small branches before forming the synapse. These branches are known as presynaptic axon terminals. The slightly expanded presynaptic terminal has a definite intact membrane known as presynaptic membrane. Presynaptic terminal has two important structures:

1) mitochondria, which help in the synthesis of chemical neurotransmitter substances;

2) synaptic vesicles, which store neurotransmitter substances.

Membrane of postsynaptic neuron is called postsynaptic membrane. It contains some receptor proteins. Small space in between presynaptic membrane and postsynaptic membrane is called synaptic cleft. Basal lamina of this cleft contains cholinesterase, which destroys acetylcholine.

Functions of synapse

Main function of synapse is to transmit impulses, i.e. action potential from one neuron to another. However, some of synapses inhibit these impulses and so impulses are not transmitted to the postsynaptic neuron. Thus, synapses are of two types.

1. Excitatory synapses, which transmit the impulses-excitatory function and

2. Inhibitory synapses, which inhibit transmission of impulses—inhibitory function.

Electrical Synapse or Ephapse

In electrical synapse, there is continuity between presynaptic and postsynaptic neurons. Continuity is provided by gap junction between two neurons. So, there is direct exchange of ions between two neurons. Because of this, action potential reaching presynaptic terminal produces potential change in postsynaptic neuron. Important feature of electrical synapse is that synaptic delay is minimum or even absent because of direct flow of current. If in ephapse fissure between pre- and postsynaptic elements is 2 nm, in chemical ones –50-300 nm. Thus, quicker impulse distribution is in ephapse. Moreover, unlike chemical synapse, impulse can be transmitted in either direction through the electrical synapse thus in 2 directions. They are considered to be only excitatory (due to depolarization development) comparatively to chemical synapses. Postsynaptic current source is presynaptic cell membrane. Chemical mediator is absent and all factors influencing on releasing and action or liquidating of enzymes decomposing mediator (extracellular calcium ions concentration decreasing) do not influence.

Ions transporting electrical currents can not pass through lipid membranes. That is why canal proteins are essential for ions transporting in “membrane contacts” between electrically conjugated cells. Such intercellular contacts are known as nexuses or “crack contacts”. There are connexones in every of two neighboring cellular membranes. They are distributed in definite gaps (spaces) and penetrate all membrane thickness. These connexones are located opposite each other in the contact locus and their lumens are located at one and the same level. So, there are 6 subunits. Everyone is 25000 Da. Big molecules (with molecular weight equal to 1000 D and 1,5 nm in diameter) come through them.

Nexuses form functional syncitium:

• in non-vertebrata;

• in vertebrata CNS;

• in vertebrata myocardium;

• in vertebrata smooth musculature.

Also all stages at embryonic stage and all epitheliocytes (particularly of lens) contain them.

Crack contacts are highly-regulated.

Closage is caused by: pH reducing, calcium ions level increasing.

Closage reasons:

• cells deep injury;

• metabolism disorder.

Closage role:

• injured places isolation and restriction (for example, at myocardial infarction);

• exchange with signal molecules (for example, with secondary messengers) which in turn is essential for regulation of:

• metabolism;

• mitosis;

• meiosis;

• cells migration during specific protection (immunity) as well as non-specific one (phagocytosis);

• regeneration;

• reparation;

• hemopoiesis;

• cells death.

Chemical synapses are dominant in CNS (comparatively to electrical ones) because:

• they are more specific (regulated by chemicals particularly medicines);

• provide bigger possibility to be regulated in intercellular communications.