The brain functions as a structural and functional unit consisting primarily of two types of cells: neurons and glial cells. It is estimated that there are about 100 billion neurons in the entire human nervous system and about 1,000 trillion glial cells (there are 10 times more glial cells than neurons).
Neurons are highly specialized and their functions are to receive, process and transmit information through different circuits and systems. The process of transmitting the information is done through synapses, which can be electrical or chemical.
The Glial cells For their part, are responsible for regulating the internal environment of the brain and facilitate the process of neuronal communication. These cells are arranged throughout the nervous system forming structures and are involved in the processes of development and formation of the brain.
Formerly it was thought that glial cells only formed the structure of the nervous system, hence the famous myth that we only use 10% of Our brain . But today we know that it fulfills much more complex functions, for example, they are related to the regulation of the immune system and the processes of cellular plasticity after suffering an injury.
In addition, they are indispensable for neurons to function properly, as they facilitate neuronal communication and play an important role in the transport of nutrients to neurons.
As you can guess, the human brain is breathtakingly complex. It is estimated that an adult human brain contains between 100 and 500 trillion connections and our galaxy has about 100 billion stars, so it can be concluded that the human brain is much more complex than a galaxy (García, Núñez, Santín, Redolar, & Valero, 2014).
Communication between neurons: synapses
The cerebral functioning consists in the transmission of information between the neurons, this transmission is realized by means of a more or less complex procedure denominated synapsis.
The synapses can be electrical or chemical. The electrical synapses consist of the bidirectional transmission of electrical current between two neurons directly, whereas in the chemical synapses they need intermediaries denominated neurotransmitters.
In the background, when a neuron communicates with another it does to activate or inhibit it, the final effects observable in the behavior or in some physiological process are the result of excitation and inhibition of several neurons along a neural circuit.
Electrical synapses
Electrical synapses are much faster and simpler than chemical synapses. Simply stated, they consist of the transmission of depolarizing currents between two neurons that are quite close, almost glued together. This type of synapse does not usually produce long-term changes in postsynaptic neurons.
These synapses occur in neurons that have a tight junction, in which the membranes are almost touched, separated by scarce 2-4nm. The space between the Neurons Is so small because its neurons must be united by channels formed by proteins called connexins.
The channels formed by the connexins allow the interior of both neurons to be in communication. Through these pores small molecules can pass (less than 1kDa) so the chemical synapses are related to processes of metabolic communication, in addition to electrical communication, through the exchange of second messengers that occur in the synapse, such as inositoltrifosfato ( IP 3 ) Or cyclic adenosine monophosphate (cAMP).
Electrical synapses are usually performed between neurons of the same type; however, electrical synapses between neurons of different types or even between neurons and astrocytes (a type of glial cells) can also be observed.
The electrical synapses allow the neurons to communicate quickly and connect many neurons synchronously. Thanks to these properties we are able to perform complex processes that require a rapid transmission of information, such as sensory, motor and cognitive processes (attention, memory, learning...).
Chemical synapses
The chemical synapses occur between adjacent neurons in which a presynaptic element is connected, usually an axon terminal, which emits the signal, and another postsynaptic terminal, which is usually found in the soma or in the Dendrites , Which receives the signal.
These neurons are not glued, there is a space between them of a 20nm called synaptic cleft.
There are different types of chemical synapses depending on their morphological characteristics. According to Gray (1959), chemical synapses can be divided into two groups.
- Chemical synapses type I (Asymmetric). In these synapses the presynaptic component is formed by axonal terminals containing rounded vesicles and the postsynaptic is found in the dendrites and there is a high density of postsynaptic receptors.
- Chemical synapses type II (Symmetrical). In these synapses the presynaptic component is formed by axonal terminals containing oval vesicles and postsynaptic can be found in both soma and dendrites and there is a lower density of postsynaptic receptors than in type I synapses. Other of the differences of this Type of synapse compared to type I is that its synaptic cleft is narrower (about 12nm approximately).
The type of synapse depends on the neurotransmitters involved in it, so that type I synapses involve excitatory neurotransmitters, such as glutamate , While in type II, inhibitory neurotransmitters would act as GABA .
Although this does not occur throughout the nervous system , In some areas such as spinal cord , the Black substance , Basal ganglia and colliculi, there are GABA-synergic synapses with a type I structure.
Another way to classify the synapses is according to the presynaptic and postsynaptic components that form them. For example, if both the presynaptic component is an axon and the postsynaptic dendrite are called axodendritic synapses, this way we can find axonal, axosomatic, dendroaxonic, dendrodendritic synapses...
The most frequently occurring type of synapses in the central nervous system are the axespinous type I (asymmetric) synapses. It is estimated that between 75-95% of the synapses cerebral cortex Are type I, whereas only between 5 and 25% are type II synapses.
The chemical synapses can be summarized in a simple way as follows:
- An action potential reaches the axonal terminal, this opens the channels of calcium ions (Ca 2+ ) And a stream of ions is released into the synaptic cleft.
- The flow of ions triggers a process in which the vesicles, full of neurotransmitters, bind to the postsynaptic membrane and open a pore through which all its contents flow into the synaptic cleft.
- The released neurotransmitters bind with the postsynaptic receptor specific for that neurotransmitter.
- The binding of the neurotransmitter to the postsynaptic neuron regulates the functions of the postsynaptic neuron.
Neurotransmitters and neuromodulators
The concept of neurotransmitter includes all substances that are released at the chemical synapse and that allow neuronal communication. Neurotransmitters meet the following criteria:
- They are synthesized inside the neurons and are present in the axonal terminals.
- When a sufficient amount of the neurotransmitter is released this exerts its effects on the adjacent neurons.
- When they have finished their mission they are eliminated through mechanisms of degradation, inactivation or reuptake.
Neuromodulators are substances that complement the actions of neurotransmitters by increasing or decreasing their effect. This is done by binding to specific sites within the postsynaptic receptor.
There are numerous types of neurotransmitters, the most important being:
- Amino acids, which can be excitators, such as glutamate, or inhibitors, such as γ-aminobutyric acid, better known as GABA.
- Acetylcholine.
- Catecholamines, such as dopamine or Noradrenaline
- Indolamines, such as serotonin.
- Neuropeptides.
References
- García, R., Núñez, Santín, L., Redolar, D., & Valero, A. (2014). Neurons and neural communication. In D. Redolar, Cognitive Neuroscience (Pages 27-66). Madrid: Pan American Medical.
- Gary, E. (1959). Axo-somatic and axo-dendritic synapsis of the cerebral cortex: an electron microscope study. J.Anat, 93 , 420-433.
- Pasantes, H. (s.f.). How does the brain work? General principles. Recovered on July 1, 2016, from Science for all.