Brain

The brain (from the Latin cerebrum, with its Indo-European root ker, head, on top of the head and brum , 'to carry'; having the archaic meaning of "what the head carries") is an organ that centralizes the activity of the nervous system and exists in most animals.

In animals, such as bilateral invertebrates, Brain is understood as a series of ganglia around the esophagus in the most anterior part of the body, (protostomes) comprised by the protocerebrum, deutocerebrum and tritocerebrum in arthropods, cerebral, pleural and pedal ganglia in gastropod mollusks and supraesophageal and subesophageal masses in mollusks cephalopods. They also have very archaic or simple bilateral brains such as flatworms, nematodes or hemichordates. However, there are bilaterals that show very few distinctive features of cephalization such as bivalves or bryozoans. In some species of invertebrates there is no brain because they completely lack a nervous system, such as porifera, placozoans, mesozoans, and others, although they have a nervous system, lack defined centralization or cephalization features by showing non-bilateral symmetries such as cnidarians, ctenophores or echinoderms.
In vertebrates the brain is located in the head; closely related to the sense organs such as vision, hearing, balance, taste and smell.
The mammalian brain is the most complex organ in the body. The telencephalon reaches its maximum development and is formed by the cerebral hemispheres. The human brain contains, in the cerebral cortex, an estimated number of 20,000,000,000 (20 billion, 2 × 10¹⁰) of neurons.

The function of the brain as an organ, from an evolutionary and biological point of view, is to exert centralized control over the other organs of the body. The brain acts on the rest of the organism by generating patterns of muscle activity or by the production and secretion of chemical substances called hormones. This centralized control allows rapid and coordinated responses to changes that occur in the environment. Some basic types of response such as reflexes may be mediated by the spinal cord or peripheral ganglia, but control of behavior that is based on sensory information requires the ability to integrate information from a centralized brain.

From an idealistic philosophical perspective, what makes the brain special compared to other organs is that it forms the physical structure where the material correlate of the different activities of the mind occurs. However, multiple contemporary philosophical currents discard the idea that there is a non-material entity that exerts causality on the physical (although on the other hand it has been shown that psychotherapy and obviously the environment influence our behavior, cognition, etc. it seems that there is no possibility of proving the existence of such an entity).

During the early stages of psychology, it was believed that the mind should be separated from the brain. However, later, scientists carried out experiments that came to determine that the mind was a component in brain functioning, by the expression of certain behaviors based on its external environment and the development of its organism. The mechanisms by which the brain activity gives rise to consciousness and thought are very difficult to understand: despite many rapid scientific advances, much about how the brain works remains a mystery. Today, the operations of individual brain cells are more fully understood, but how they cooperate among collections of millions has been very difficult to decipher. Also, the most promising approaches treat the brain as a "biological computer", totally different in mechanism from electronic computers, but similar in the sense that they take in information from the surrounding world, store it, and process it in multiple ways.

This article compares the properties of the brains of the entire range of animal species. There is a specific article for the human brain.

General characteristics

The brain is the largest organ of the central nervous system and is part of the control center for the entire body. In humans, it is also responsible for thought, memory, emotions, speech, and language.

In vertebrates, the brain is located in the head, protected by the skull and close to the primary sensory apparatus of vision, hearing, smell, taste and the sense of balance.

In vertebrates, the brain is divided into three parts: cerebrum, cerebellum, and brainstem. The term "brain" is sometimes mistakenly used as a synonym for encephalon. Actually, the cerebrum is the anterior and most voluminous part of the entire brain.

Our brain represents only 2% of our body weight and consumes 20% of energy, it is the fattiest organ in the body. ^{[citation required]}

Brains are extremely complex. The complexity of this organ emerges from the nature of the unit that nourishes its operation: the neuron. These communicate with each other by means of long protoplasmic fibers called axons, which transmit trains of signal pulses called action potentials to distant parts of the brain or body, depositing them in specific receptor cells.

Brains control behavior by causing muscles to contract, or by stimulating the secretion of chemicals such as some hormones.

Sponges that do not have a central nervous system are able to coordinate the contractions of their bodies and even their locomotion.

Grey matter and white matter

Cutting a human brain in which areas formed by gray substance and white substance can be appreciated.

If a section of the brain is observed with the naked eye, two different-looking areas can be seen. One of them, darker in color, is called the gray matter and is made up of the neuronal bodies, while the other, lighter one, is called the white matter and is made up of the myelin-covered axons that depart from the neurons to transmit the nerve impulse. The white matter is made up of the pathways through which information is transmitted at a distance within the nervous system, while the gray matter is made up of the bodies of the neurons, which is where impulses are generated.

On the surface of the brain of vertebrates is the cerebral cortex, which is made up of gray matter, below is a central mass of white matter that surrounds a set of nuclei of gray matter located in the center of the brain, between which includes the thalamus and the so-called basal ganglia or basal ganglia.

Neuron

Neurons of the cerebral cortex.

The neuron is the basic unit on which the brain is built. According to recent estimates, an average human brain contains about 86 × 10⁹ or 86,000,000,000 (86 billion) of neurons. A neuron is a cell that has specialized in the transmission of nerve impulses that consists of a cell body or soma, a large number of small processes called dendrites and a main process that can be very long and is called of axon, which in turn can branch into many branches at the end of its journey. The axon forms in a thickening of the cell body and extends for variable distances ranging from a few micrometers (μm) to more than a meter (m) in some neurons in certain locations. The connections established between two neurons are called synapses. According to the principle of specific connectivity established by Ramón y Cajal, neurons do not connect with each other randomly, but establish specific connections in certain places with other nerve cells, so the apparent tangle of branches that is observed when looking through Under the microscope, a sample of brain tissue is not a set of random connections, but a perfectly organized network of contacts between cells that makes possible the functioning of the nervous system and all brain activities.

Each neuron continuously integrates numerous electrical impulses that it receives through its dendrites and emits a unique response through its axon. There are sensory neurons that capture information from the different senses and motor neurons that emit impulses that generate voluntary muscle movements, but most of those that exist in the brain are interneurons that are part of very precise anatomical circuits.

Neurotransmitters

Dopaminergic paths in the brain.

A neurotransmitter is a chemical produced by neurons that is released into the synaptic gap of a chemical synapse by the action of a nerve impulse or action potential. It interacts with a specific receptor in the postsynaptic neuron where it produces a certain response that can be excitatory or inhibitory. Neurotransmitters are a fundamental aspect of brain function.

There are different substances that act as neurotransmitters, some of the most important are the following: GABA, an acronym for g-aminobutyric acid, serotonin, acetylcholine, dopamine, norepinephrine, and endorphin. Dopaminergic pathways, for example, are pathways of neurons that function with dopamine as a neurotransmitter. These pathways are of great interest in brain function and it has been proven that their alteration can cause different diseases, including Parkinson's disease.

Anatomy

The cerebrum is part of the brain. The human brain is divided into three parts: cerebrum, cerebellum, and brainstem. Of these, the brain is the one with the greatest weight and volume of neurons.

Morphology

Central Surco of the brain or scent of Rolando

Side Surc of the Brain or Silvio Scent

The vertebrate brain is divided into two hemispheres, connected by the corpus callosum. The surface is called the cerebral cortex and is made up of the gray matter.
In an adult human it is estimated that the cerebral neocortex contains 20,000,000,000 (sixteen billion, 2 × 10¹⁰) neurons.
Each cerebral hemisphere has several fissures that divide the cerebral cortex into lobes.
Underlying the cortex is the white matter.

Cerebral Ventricles

The human brain has 4 interconnected cerebral ventricles inside that are filled with a clear fluid called cerebrospinal fluid.

Lobules

Each hemisphere has several fissures that subdivide, more or less clearly, the cerebral cortex into lobes:

• The frontal lobe is limited by Silvio's scent and Rolando's scent.
• The parietal lobe is delimited in front by the scent of Rolando, below by the scent of Silvio and behind by the occipital scent.
• The occipital lobe is limited by the external and internal perpendicular cissures, ahead; there is no limit on the inner face of it. It is located at the back of the brain.
• The temporal lobe is defined by Silvio's scent and is located in a lateral position.
  
• The lobe of the Insula, which is not visible from the outside, is found in the inner part of the brain; it is observed opening the scent of Silvio.

Thalamus

The thalamus is located above the brainstem, almost in the center of the brain. It is about 3 cm long and is made up of gray matter, that is, the soma of neuronal cells. It fulfills the function of a relay station for nerve signals and an integration center where sensory impulses are processed before continuing their journey to the cerebral cortex. It also receives signals that follow the opposite direction and reach the thalamus from the cerebral cortex.

Hypothalamus

The hypothalamus is a small region of the brain made up of gray matter. It is located immediately below the thalamus. It is about the size of an almond and performs important functions, including linking the nervous system with the endocrine system through the pituitary gland.

Basal ganglia

The basal ganglia should actually be called basal ganglia as they are not true ganglia. They are a set of brain structures made up of gray matter that are located below the cortex and perform important functions, one of the main ones being the control of voluntary movements, but they are also involved in the processing of sensory information and in aspects related to memory. memory and emotions. They are connected to the cerebral cortex and function with a high degree of integration. The following can be differentiated:

• Striated body, formed by the locked core, whoremen and the accumbens core.
• Pale balloon.
• Black substance
• Underground core.

Hippocampus

The hippocampus is a brain structure that plays important roles in memory and spatial orientation. It is made up of gray matter and comes from the temporal lobe, although it is located below the cerebral cortex. It owes its name to the fact that its shape is somewhat reminiscent of that of a seahorse. The hippocampus is part of the limbic system and is one of the few regions of the brain in which the phenomenon of neurogenesis (formation of new neurons) occurs.

Corpo callosum

Average vision of the brain. The red arrow indicates the situation of the silent body (corpus callosum) which serves as communication between the right and left cerebral hemisphere.

The corpus callosum is an important brain structure that is made up of fibers that act as a communication channel between the right and left cerebral hemispheres, so that both work together and complement each other.

Inner capsule

The internal capsule is a thick set of both ascending and descending nerve fibers that communicate the cortex with the lower regions of the central nervous system, the fibers are of diverse origin, but many of them carry motor or sensory information. On their way they pass near the region of the thalamus and the basal ganglia. The internal capsule is a very sensitive region, any injury to this area damages numerous nerve fibers and consequently causes severe neurological deficits.

Animated images

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  Hipocampo

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  Nucleus enclosed

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  Quiet body

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  Diencephalus (thalamus and hypothalamus)

Functions

The brain processes sensory information, both visual and tactile, auditory and olfactory. The motor areas control and coordinate movement, while the association areas are responsible for complex functions such as memory and reasoning. The basal ganglia act in the coordination of movement, while the limbic system is responsible for emotional responses. Although certain areas of the brain are in charge of certain functions, it is a system with a high degree of integration that is also related to other parts of the brain, such as the cerebellum, which is responsible for coordinating complex sequences of movements initiated by the motor areas and the brainstem..

Motor function

Vertical cut of the brain in which the path of the pyramidal path can be observed.

Motor function in the brain is carried out primarily through the pyramidal or corticospinal pathway, a group of nerve fibers that arise from neurons located in the primary motor cortex located in the posterior part of the frontal lobe and terminate in the horn. of the spinal cord, where they connect with a second neuron from which axons originate that converge in the different motor nerves that make voluntary control of the musculature of the entire body possible. The pyramidal pathway crosses at the base of the brain stem, in the so-called decussation of the pyramids, in such a way that the fibers coming from the right cerebral hemisphere control the muscles of the left half and those of the left cerebral hemisphere the right half. This route is of great importance because it is the one that allows the necessary movements to be carried out for most of the vital functions, including moving, speaking, chewing, etc. If the pyramidal pathway is injured, paralysis of the corresponding muscles occurs.

Pain perception

Pain is defined as an unpleasant sensory and emotional experience associated with actual or potential tissue damage. It has the function of warning or warning to inform of a danger that must be avoided, thus preventing more serious injuries.

The sensation of pain is initiated in certain receptors located in the tissues that are called nociceptors and are sensitive to tissue damage. Nerve impulses generated by these receptors travel via sensory nerves to the dorsal horn of the spinal cord, from where they travel up a bundle of nerve fibers called the spinal thalamus to reach the brain. They first reach the region of the thalamus, from where they reach the sensory cortex of the temporal lobe, which is where the signal is processed and the sensation of pain becomes conscious.

There are different diseases of congenital origin in which the affected people are unable to perceive pain. This group of disorders is known generically as congenital insensitivity to pain, it is usually accompanied by a lack of sensitivity to temperature and causes significant health problems, including bone or skin lesions that go unnoticed because the person does not feel any pain after severe trauma. and continues his usual activity without realizing that he has suffered a bone fracture or an injury.

Cognitive abilities

The emotional system and the value system develop in the parietal lobes. The emotional system —although it involves the entire brain, and in feedback, the entire body of the individual— is located mainly in the quite archaic area called the limbic system, within the limbic system the two cerebral tonsils, the basic emotions are focused (fear, aggression, pleasure) that we have and that we give when something or someone interferes with the activity that is being done abroad.
On the other hand, there is the value system, this is the relationship that exists between the prefrontal lobes (which, as its name indicates, is behind the forehead) and the cerebral tonsils, that "physical relationship" is called the hippocampus.^{[citation required]}

Brain and language

Broca area.

Wernicke area.

The vast majority of language-enabled processes take place in different association areas. There are two well-identified areas, which are considered vital for human communication: Wernicke's area and Broca's area. These areas are located in the dominant hemisphere (which is the left in 97% of people) and are considered the most important in terms of language processing. This is the reason why language is considered a lateralized function. However, the non-dominant hemisphere also participates in language, although there are questions about the level of participation of the areas located in said hemisphere.

Wernicke's area is named after the neurologist who first described it. It is especially developed in the dominant hemisphere for language, which is generally the left side. The development of this area allows reaching high levels of comprehension and processing most of the intellectual functions of the brain. It is in charge of decoding what has been heard and preparing possible responses. It is important for the comprehension of words and in meaningful speech.

It then gives way to Broca's area, also known as the motor area of words, which is connected to Wernicke's area by the superior longitudinal fasciculus. It is located in the prefrontal cortex, in the anterior part of the inferior portion of the primary motor cortex, close to the lateral fissure (FL). In most cases, it is dominant on the left side of the brain. Its function is to allow the realization of motor patterns for the expression of words, articulating the spoken and written language. It is responsible for the formation of words in which the actuation of the speech muscles is activated, that is, the laryngeal, respiratory and mouth muscles, to ensure the production of articulated sounds, which takes place in the primary motor area, of where the orders to the phonatory muscles depart. It is also connected to the supplementary motor area, which is related to the initiation of speech.

Functional differences between hemispheres

Even though both human hemispheres are opposite, they are not the inverted geometric image of each other. From a purely morphological point of view they are asymmetrical. This asymmetry depends on a pattern of gene expression that is also asymmetric during the embryonic development of the individual and it has been proven that it is not exclusive to the human species, since it is present, although to a lesser degree, in close relatives in the phylogeny of humans, such as the chimpanzee.

The study of cranial impressions of ancestors of the genus Homo has among its objectives to determine the presence or not of asymmetry in the telencephalon, since it is a trait of increased specialization, of a cognitive capacity more complex.

Functional differences between hemispheres are minimal and differences in function have only been found in a few areas, with exceptions in people where no differences were observed. The currently best-known specialized areas for language are Broca's and Wernicke's, although in doing linguistic processing it is likely that the whole brain is involved—almost certainly memory areas are involved in language processing. Broca's and Wernicke's areas are found, in most individuals, in the left hemisphere. For their part, the areas most involved in logic and intellectual activities are located mainly in the prefrontal cortex, perhaps having the left temporal areas of great importance for analysis and synthesis processes such as those that allow (mathematical) calculations; These areas provide the individual with a greater capacity to adapt to the environment, but with much longer learning processes, and as such are more dependent on their parents during the breeding stage.

Neurogenesis

Neurogenesis is the name given to the production, differentiation, and migration of new neurons in the nervous system. Until the 60s of the 20th century, it was believed that it was impossible for this to occur in adult life, and it was considered that the same neurons that existed at the time of birth would last until death without incorporating new units. In the second half of the 20th century, several studies were published that contradicted this ancient dogma of biology. It is currently proven that in the human and mammalian brain there are two areas of adult neurogenesis, the hippocampus and the area located below the lateral ventricles of the brain. It has been observed that certain learning processes dependent on the hippocampus, such as spatial learning in a maze, act as stimulants of the neurogenesis process.

Stem cells are the ones that give rise to new neurons, however, the regenerative capacity of the brain is very low compared to other tissues in the body.

Neuroplasticity

Neuroplasticity is the process of modification of the neuronal organization of the brain as a result of experience. The concept is based on the ability to modify the activity of neurons, and as such was described by the Polish neuroscientist Jerzy Konorski. The ability to modify the number of synapses, neuron-neuron connections, or even the number of cells, gives rise to neuroplasticity. Historically, neuroscience conceived during the 20th century a static schema of the oldest structures of the brain as well as the neocortex. However, today it is known that encephalic connections vary throughout adult life, as well as the generation of new neurons in areas related to memory management (hippocampus, dentate gyrus). This dynamism in some areas of the adult brain it responds to external stimuli, and even reaches other parts of the brain such as the cerebellum.

Comparative anatomy

Three groups of animals, with a few exceptions, have remarkably complex brains: the arthropods (for example, insects and crustaceans), the cephalopods (octopuses, squids, and similar mollusks), and the craniates (primarily vertebrates). The brain of arthropods and cephalopods arises from a pair of parallel nerves that run the length of the animal's body. The arthropod brain has large optic lobes behind each eye for visual processing and a central brain with three divisions.

In insects, the brain can be divided into four parts: the optic lobes, which, located behind the eyes, process visual stimuli; the protobrain, which responds to smell; the deutocerebrum, which receives information from tactile receptors in the head and antennae; and the tritocerebro.

In cephalopods, the brain is divided into two regions separated by the animal's esophagus and connected by a pair of lobes. They are called supraesophageal mass and subesophageal mass.

The cranian brain develops from the anterior section of a single dorsal nerve tube, which later becomes the spinal cord, then the spinal cord (always evolutionarily and phylogenetically) would have been bound (using Piaget's terminology or evolved by becoming more complex and successively transforming into the pons and the brainstem, already in fish and, mainly, in primitive tetrapods (amphibians, reptiles) the "limbic brain" (limbic system) would have emerged. by the bones of the neurocranium Vertebrates are characterized by increasing complexity of the cerebral cortex as one moves up the phylogenetic and evolutionary trees The large number of gyri appearing in the mammalian brain is characteristic of animals with advanced brains These convolutions arose from evolution to provide more surface area (with more gray matter) to the brain: the volume remains constant as the number of neurons increases. Therefore, it is the surface, and not the volume (absolute or relative), which determines the level of intelligence of a species. This is a very common mistake that must be taken into account. However, if we were to compare two brains of the same species, we could approximate that there are more possibilities that the larger brain of the two has a greater surface area, although this is not defining cognitive intellective quality either, but is considered a key factor for greater intellective and cognitive capacities to the architecture of the brain: for example, Homo neanderthalensis could have brains as large or larger than those of today's Homo sapiens, but the cortical architecture of their brains was more dedicated to controlling their strong muscles, while in Homo sapiens the most developed cortical areas are located in the areas dedicated to symbolic language, and the prefrontal and frontal —especially of the left hemisphere— where the syntheses that result in elaborate processes of reflection, cognition and intellection are carried out.