The nervous system of humans produces phenomena as wonderful as memory, perception, thought and language. It is also he who controls the movements, or who deciphers the messages picked up by the sense organs. It is made up of around 100 billion nerve cells, also called neurons. All brain activities result from the interactions of these neurons with each other. And it is the synapse, that is, the area between two nerve cells, which ensures the transmission of information between these two nerve cells. And so, the synapse is therefore the histological structure by which the axon of one neuron articulates with the dendrites of another neuron. Thanks to the intervention of a mediator such as acetylcholine or noradrelanin, the synapse is the seat of the transmission of nerve impulses. Various works carried out in the course of the XXth century demonstrated that the two types of synapses, electrical and chemical, exist, and the electron microscopy, thereafter, made it possible to prove that these two types are very different.

Synpase is the contact region between two neurons (also called nerve cells). This zone, located between two neurons, thus ensures the transmission of information from one to the other. Etymologically formed from the Greek word sunapsis, which means union, the synapse is therefore an essential link in the central nervous system of humans, as well as that of all mammals. This central nervous system in humans consists of the brain, divided into two hemispheres, the right hemisphere and the left hemisphere separated from each other by a median groove, narrow but very deep. The central nervous system is based on well-defined synaptic circuits. These circuits are put in place during the development of the brain: they are based on billions of connections established between neurons and synapses.

The neuron is made up of a cell body called soma, from which radiate several expansions, called dendrite or axon, depending on the direction of propagation of the nerve impulse. Thus, in a dendrite, the nerve impulse propagates towards the soma, while in an axon, it leaves. In fact, the axon carries neuronal information either to other neurons in the brain or to muscle fibers located outside the brain. Finally, the synapse is the area where the end of an axon comes into contact with another neuron or with a muscle fiber. On a neuron are attached many synaptic buttons, not only to the cell body of the neuron, but also to the dendritic branches.

A presynaptic neuron is located upstream of the synapse while a postsynaptic neuron is located downstream. It is the presynaptic neuron which will act on the postsynaptic by secreting through the synapse a substance called neurotransmitter, such as acetylcholine or norepinephrine. The postsynaptic neuron integrates the messages received by its synapses, synthesizes them at each moment and deduces the message that it sends itself.

There are chemical synapses, which are communicated through molecules called neurotransmitters, as well as electrical synapses.

Chemical synapses

Synapses that use a neurotransmitter, also called a neurotransmitter, are chemical synapses (or most synapses). This chemical molecule is actually a messenger. It accumulates in vesicles located in the end of the axon of the presynaptic neuron (called the presynaptic termination). When the nerve impulse arrives (called the action potential), the neurotransmitter is released into the synaptic space: “bundles” of molecules are then excreted in the synapse.

This phenomenon is linked to a massive influx of calcium, which will induce a succession of chemical reactions ultimately resulting in the release of a given quantity of neurotransmitter. These neurotransmitters will thus cross the synapse (or synaptic space) and they attach to specific receptors located either on the postsynaptic neuron or on the effector cell. This binding of the neurotransmitter molecule to its receptor will induce hyper or depolarization of the postsynaptic cell. In a chemical synapse, this transmission of information is necessarily directed from the presynaptic element to the postsynaptic element.

Nearly a hundred chemical molecules are currently known for their role as neurotransmitters. The smallest are monoamines, such as dopamine, adrenaline or even serotonin. The largest are proteins that can contain several dozen amino acids. Not all of these neurotransmitters are present within the same synapse. On the other hand, the fact that several neurotransmitters can coexist within the same termination probably explains a very great finesse of nuances in the processes of cellular communication.

Electrical synapses

The so-called electrical synapses are frequently gathered in zones which make it possible to coordinate rhythmically the activity of a group of neurons. In fact, a tight junction connects the cytoplasm of two neurons. And in this case, in principle, communication can occur in both directions.

Excitatory and inhibitory synapses

The modes of action of synapses differ: and thus, there are so-called excitatory synapses given that they excite the postsynaptic neuron, as well as so-called inhibitory synapses because of the inhibitory effect they induce.

The nervous system faces a major problem: it must constantly adapt to a changing environment, while maintaining a level of stability allowing it to maintain its integrity and guarantee consistency at the behavioral level. This balance between adaptation and stability changes profoundly during the life of an organism. The main anomalies or pathologies of the nervous system, which in fact often constitute a disruption of this balance, are as follows:

• Parkinson disease ;
• epilepsy;
• Alzheimer’s disease ;
• psychiatric disorders;
• autism.

Even if the complex molecular mechanisms which are involved are not all deciphered, for each of its disorders a dysfunction of the synapses can come into play, since the synapses constitute indeed a key element of the functioning of this central nervous system.

Parkinson disease

Parkinson’s disease is a neurodegenerative disease of the nerve centers controlling in particular automatic movements. This disease is characterized by a progressive degeneration of dopaminergic neurons in the area of ​​the brain controlling motor skills: the lack of dopamine, a neurotransmitter regulator of movements, also leads to cholinergic hyperactivity.

Epilepsy

Epilepsy is a chronic neurological disease characterized by the sudden, simultaneous and excessive activation of a large number of brain neurons. This results in a disorderly electrical discharge of neurons. These convulsive phenomena turn out to be the consequence of the disruption of the balance between stimulating neuro-mediators, such as glutamate, and inhibitory neurotransmitters such as GABA, aminobutyric acid. The crisis is caused by the decrease in the efficiency of the Gabaergic mechanisms.

Alzheimer’s disease

This neurodegenerative disease of the central nervous system is characterized by a progressive and lasting deterioration of cognitive functions. The clinical picture belongs to dementia syndromes. In terms of physiopathology, we find intracellular filaments of the tau protein. There is tau hyperphosphorylation with aggregation, disruption of axonal flow. This leads to neuronal death with a decrease in neurotransmitters.

Psychiatric diseases and autism

A European research project (called CELLSYNCIRCUITS) looked at synapses, and it appears from this work, scientists say, that synapse dysfunction could be a root cause of certain mental disorders such as schizophrenia and autism. . Doctor Joao Peça, from the University of Coimbra in Portugal, is notably carrying out investigations in this area. He reveals : “In this regard, studying the role of synapses has helped us understand how information is transmitted in the brain and how synaptic dysfunctions could lead to behavioral disorders such as autism or schizophrenia.. ” And so, the Cordis site of the European Commission affirms: “A closer look at how the neural circuits in our brains control certain behaviors, and what happens when they malfunction, could improve our knowledge of mental illnesses and lead to better treatments and health policies.«

Parkinson disease

Parkinson’s disease is treated with specific drugs such as L-Dopa, dopamine agonists, dopamine metabolism inhibitors, anticholinergics. In addition, there are physiotherapy and speech therapy sessions.

Epilepsy

Treatment of epilepsy consists of eliminating contributing factors and taking anti-epileptic drugs.

Alzheimer’s disease

Therapeutic management is that of associated disorders: depression, anxiety, delusions and hallucinations, agitation or aggression. In addition, the management of the disease is essentially psychosocial, with rehabilitation as well as environmental planning.

Psychiatric diseases and autism

Schizophrenia is managed both by taking antipsychotic drugs, to which are sometimes added anxiolytics, associated with psychotherapeutic work. The chances of disease stabilization and patient rehabilitation are increasingly favorable, and early detection and early treatment improve long-term functioning.

The management of austime is often plural and complex. Thus, the notion of care in the management of autism can be considered in the perspective defined in one of her works by Marie-Dominique Amy, clinical psychologist and psychoanalyst, as “a totalizing and encompassing consideration of all the child’s needs“. For Claude Burzstejn, professor of child and adolescent psychiatry, “asking the question of care for people with autism in terms of need is problematic from the outset since there is currently no consensus or specific studies on the respective indications of the various therapeutic techniques currently applied“. And this psychiatrist thus to consider the place of this care within the framework of a multidimensional care. Indeed, the educational dimension must necessarily be associated and articulated with actions with a more specifically therapeutic aim: the physical health needs of the child must not be neglected.

Parkinson disease

The diagnosis of Parkinson’s disease can be made by physical examination and the observation of marked improvement after administration of dopamine. In addition, medical imaging examinations are necessary to rule out so-called parkinsonian syndromes such as brain tumor or Wilson’s disease.

Epilepsy

The diagnosis of epilepsy is made in view of the symptoms, associated with the electroencephalogram (EEG).

Alzheimer’s disease

The diagnosis with certainty can only take place post mortem. There is as yet no reliable biomarker or routine imaging technique. The current diagnosis is clinical, excluding other causes of dementia, but the clinical signs are very variable, and often associated with a depressive syndrome present at the onset of the disease, which hinders questioning. A consultation is recommended for the evaluation of cognitive functions MMS (Mini Mental Status). These are the current recommendations in France which have a test value.

Psychiatric illnesses: schizophrenia

There is no definitive diagnostic test for schizophrenia. The diagnosis is based on a global evaluation of the history of the disease, the symptomatology. The clinical criteria are described in DSM V (Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition).

Autism

The earliest possible diagnosis is recommended but remains complex to establish, especially during early childhood, and early warning signs can be detected by parents and childcare professionals. A consultation by specialized teams, in the event of a confirmed suspicion of autism, will be indicated.

The area that makes the junction between two neurons was first described by Santiago Ramon y Cajal (1852 – 1934), and it is at the end of the XIX^e century that the British physiologist Charles Scott Sherrington (1857 – 1952) gave to this specialized zone ensuring the junction between two neurons the name of synapse.

Otto Loewi (1873 – 1961) demonstrated in the mid-1920s the role of acetylcholine: it is, therefore, responsible for the transfer of information between the termination of the vagus nerve and the heart. A heated debate then ensued: was the transmission of information between two neurons, as well as between a neuron and its peripheral target, electrical or chemical?

Several works in the course of the XX^e century concluded that both types of synapses, electrical and chemical, exist. The electron microscopy images subsequently made it possible to show that these two types of synapses turn out to be very different.