HOW DOES THE HUMAN BRAIN WORK? - NEUROPSYCHOLOGY - 2024

The brain functions as a structural and functional unit made up mainly of two types of cells: neurons and glial cells. It is estimated that there are about 100 trillion 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 information is carried out through synapses, which can be electrical or chemical.

Glial cells, on the other hand, are responsible for regulating the internal environment of the brain and facilitating the process of neuronal communication. These cells are found throughout the nervous system forming its structure and are involved in the development and formation processes of the brain.

In the past 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 performs 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 essential for neurons to function properly, since they facilitate neuronal communication and play an important role in the transport of nutrients to neurons.

As you can guess, the human brain is impressively 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.

How is information transmitted in the brain?

Brain function consists of the transmission of information between neurons, this transmission is carried out through a more or less complex procedure called synapse.

Synapses can be electrical or chemical. Electrical synapses consist of the bidirectional transmission of electrical current between two neurons directly, while chemical synapses require intermediaries called neurotransmitters.

Ultimately, when one neuron communicates with another, it does so to activate or inhibit it, the final observable effects on behavior or on some physiological process are the result of excitation and inhibition of several neurons throughout a neuronal circuit.

Electrical synapses

Electrical synapses are considerably faster and easier than chemical ones. Explained in a simple way, they consist of the transmission of depolarizing currents between two neurons that are quite close, almost stuck together. This type of synapse usually does not produce long-term changes in postsynaptic neurons.

These synapses occur in neurons that have a tight junction, in which the membranes almost touch, separated by a scant 2-4nm. The space between neurons is so small because their neurons must join together through channels made of proteins called connexins.

The channels formed by the connexins allow the interior of both neurons to communicate. Small molecules (less than 1kDa) can pass through these pores, so chemical synapses are related to metabolic communication processes, in addition to electrical communication, through the exchange of second messengers that are produced in the synapse, such as inositol triphosphate (IP ₃) or cyclic adenosine monophosphate (cAMP).

Electrical synapses are usually made between neurons of the same type, however, electrical synapses can also be observed between neurons of different types or even between neurons and astrocytes (a type of glial cells).

Electrical synapses allow neurons to communicate quickly and many neurons to connect synchronously. Thanks to these properties we are able to carry out complex processes that require a rapid transmission of information, such as sensory, motor and cognitive processes (attention, memory, learning…).

Chemical synapses

This image shows the axon from where the neurotransmitters are released towards the dendrite receptors

Chemical synapses occur between adjacent neurons in which a presynaptic element connects, usually an axonal terminal, which emits the signal, and another postsynaptic, which is usually located in the soma or in the dendrites, which receives the signal. signal.

These neurons are not attached, there is a space between them of one 20nm called the 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 can be simply summarized as follows:

1. An action potential reaches the axon terminal, this opens the calcium ion channels (Ca ²⁺) and a flow of ions is released to the synaptic cleft.
2. 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 their contents exit towards the synaptic cleft.
3. The released neurotransmitters bind to the postsynaptic receptor specific for that neurotransmitter.
4. The binding of the neurotransmitter to the postsynaptic neuron regulates the functions of the postsynaptic neuron.

Types of chemical synapses

Type I chemical synapses (asymmetric)

In these synapses, the presynaptic component is made up of axonal terminals that contain rounded vesicles and the postsynaptic component is found in the dendrites and there is a high density of postsynaptic receptors.

The type of synapse depends on the neurotransmitters involved, so excitatory neurotransmitters, such as glutamate, are involved in type I synapses, while inhibitory neurotransmitters, such as GABA, act in type II synapses.

Although this does not occur throughout the nervous system, in some areas such as the spinal cord, substantia nigra, basal ganglia, and colliculi, there are GABA-ergic synapses with a type I structure.

Type II chemical synapses (symmetric)

In these synapses, the presynaptic component is formed by axonal terminals containing oval vesicles and the postsynaptic component can be found both in the soma and in the dendrites and there is a lower density of postsynaptic receptors than in type I synapses.

Other differences of this type of synapse with respect to type I is that its synaptic cleft is narrower (approximately 12nm approximately).

Another way to classify synapses is according to the presynaptic and postsynaptic components that form them. For example, if the presynaptic component is an axon and the postsynaptic component is a dendrite, they are called axodendritic synapses. In this way we can find axoaxonic, axosomatic, dendroaxonic, dendrodendritic synapses…

The type of synapse that occurs most frequently in the central nervous system is axospinous type I (asymmetric) synapses. It is estimated that between 75-95% of the synapses in the cerebral cortex are type I, while only between 5 and 25% are type II synapses.

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 within neurons and are present at axonal terminals.
• When a sufficient amount of the neurotransmitter is released, it exerts its effects on adjacent neurons.
• When they have finished their task, they are eliminated through degradation, inactivation or reuptake mechanisms.

Neuromodulators are substances that complement the actions of neurotransmitters by increasing or decreasing their effect. They do this by binding to specific sites within the postsynaptic receptor.

There are numerous types of neurotransmitters, the most important of which are:

• Amino acids, which can be excitatory, such as glutamate, or inhibitors, such as γ-aminobutyric acid, better known as GABA.
• Acetylcholine.
• Catechollamides, such as dopamine or norepinephrine
• Indolamines, such as serotonin.
• Neuropeptides.

References

1. García, R., Núñez, Santín, L., Redolar, D., & Valero, A. (2014). Neurons and neural communication. In D. Redolar, Cognitive Neuroscience (pp. 27-66). Madrid: Panamerican Medical.
2. Gary, E. (1959). Axo-somatic and axo-dendritic synapsis of the cerebral cortex: an electron microscope study. J. Anat, 93, 420-433.
3. Pasantes, H. (nd). How does the brain work? General principles. Retrieved on July 1, 2016, from Science for all.