Biological intelligence

Image: Carlos Cruz-Diez


It's not you who controls your body, it's your brain

Integration and control systems

The human body has two control systems: the nervous system (SN) and the endocrine system.[1] The first is a system of rapid and fleeting action, which operates through electrical impulses, a type of signal that is conducted at very high speeds (eg, 100 ms-1). This provides agility, to the point where the system's reaction time, in certain cases, is practically zero.

The second is a system of slow and long-lasting action, which operates through hormones, chemical substances that are transmitted through the bloodstream.[2] The transmission speed is much slower, a difference that helps explain why the reaction time here is much slower. The signal, however, is persistent: As long as the hormone is circulating through the blood, cells with the appropriate receptors will continue to respond.

Another important difference concerns the size of the target and the accuracy of the control. The impulse transmitted by a chain of neurons is capable of reaching a small group of motor cells or even a single individual cell. Blood flow does not allow this. Strictly speaking, the hormones that are passing through the blood affect all cells that carry their respective receptors.

In what follows, we will look at some aspects of our nervous system.[3]

Chordates with skulls

The nervous system of craniates – read: chordates with a skull[4] – is formed by (i) the brain, a set of structures enclosed within the skull, with emphasis on the brain, a mass of cells with a gelatinous consistency and a globose appearance (especially in the case of birds and mammals); and by (ii) the spinal cord, a cylindrical tube that attaches to the back of the brain and runs through the interior of the spinal column.[5]

In the words of Hildebrand & Goslow (2008, p. 319 and 321): “The brain is the most complicated organ in the body and also the most wonderful organ for many people. […] The brain does not merely transmit, reject or store information in the 3 billion impulses that reach its 1010 cells every waking second.[6] It transforms information, adapts it, and chooses between alternative responses in ways that surpass our present understanding.”

Where does the brain come from?

Unlike the medulla, which has changed relatively little throughout the evolutionary history of vertebrates, the brain has undergone notable changes, whether in size or shape (see Fig. 1).

The mature brain develops from three embryonic regions: prosencephalon (or anterior brain), midbrain (midbrain) and hindbrain (hindbrain). Each region gives rise to organs or tissues with specific functions. The brain is one of these organs, the cerebellum is another.

The brain has increased greatly in size since the emergence of craniates. Both in absolute and relative terms. See, for example, how the (brain): (medulla) ratio varies between different lineages. Among the oldest (fish and amphibians), the proportion is around 1:1 – that is, the brain and spinal cord have more or less the same mass. Among mammals, however, the ratio is very unequal, reaching 50:1 in humans. The mass of our cerebral cortex, for example, is around 882 g (or 8 × 1010 cells), while that of the marrow does not exceed 18 g (2,1 × 109 cells).[7]

The size of the brain

Another type of relevant comparison involves the size of the brain (or brain) vs. the size of the body. There is a very significant positive correlation between one and the other.[8] Here is Bonner's comment (1983, p. 67-8): “There is a direct inverse correlation between the time of appearance of a group in the history of the Earth and the dimensions of the brain of that group. At one end of the spectrum, fish have small brains, and at the other end, mammals have larger ones. This suggests a tendency towards an increase in learning capacity, towards an increase in response flexibility. Note, however, that this brain expansion probably corresponds, in large part, to the expansion of new niches and not just the elimination of animals with smaller brains. […] [F]ish still exist, and they are abundant and successful as a group, despite the relative insignificance of their brains.”

But there are important deviations from this correlation. Among mammals, for example, primates stand out as having particularly large brains. Larger than would be expected if we only took body size into account. Among primates, humans stand out even more.

Here is Lewin's characterization (1999, p. 448-50): “[It] can be said that the brain size of australopithecines was almost 400 cm3, and which has increased only slightly throughout the history of this genre. A more marked expansion is observed with the origin of the genus Homo, specifically the homo habilis/rudolfensis, which lived between 2,5 and 1,8 million ago and had a brain size of 650 to 800 cm3. The size variation for the homo ergaster/erectus, dated from 1,8 million to 300.000 years ago, is 850 to just over 1.000 cm3. Equivalent measures for Homo sapiens archaic trees range between 1.100 and more than 1.400 cm3, that is, greater than in modern humans. Using the encephalization quotient (EQ), a measure of brain size relative to body size, we can discern this progression more objectively. Australopithecine species have EQs of around 2,5, compared to 2 for the common chimpanzee, 3,1 for the first homo ergaster/erectus, and 5,8 for modern humans.”

Relationships like these (I mean: brain vs. spinal cord or brain vs. body correlation) are converted into indices that can be used to compare the degree of intelligence of different groups of animals. As a general rule, the greater the relative size of the brain, the greater the degree of intelligence. A statement that is anchored in some biological assumptions. As Jerison (1985, p. 106) defined: “Biological intelligence in adults representative of a species is the behavioral consequence of the available neural information processing capacity, in addition to that necessary for the control of general body functions.”

Now, knowing that the brain is the control center for the other organs of the body, it is not surprising that larger animals have equally larger brains. After all, if an animal's body houses more cells, more neurons must be needed to control them.

The human nervous system

Our nervous system can be divided into (i) central nervous system (CNS); (ii) peripheral nervous system (PNS); and (iii) an autonomous division, which comprises the sympathetic and parasympathetic. This distinction is both morphological and functional, although the three portions are interdependent.

The central nervous system is a region for receiving and processing stimuli and issuing responses. Its constituent elements are housed inside the axial skeleton: the brain, inside the skull, and the spinal cord, inside the vertebral column. In what follows, we will only talk about the brain.

The development of the brain

The brain is formed from an embryonic structure called the neural tube, which itself comes from a previous structure called the neural plate.

The neural tube forms around the fourth week of gestation, when the embryo is 26-29 days old. The emergence of this structure marks the beginning of a developmental phase referred to as neurulation. A few more days and it will be possible to observe the presence of dilations in the anterior portion of the tube. These are the primary brain regions (forebrain, midbrain and hindbrain), already mentioned above.

The three regions will split into five: (i) Prosencephalon – This is the most anterior dilation. During development, the lateral portions expand, to the point of covering and hiding the central portion. Gives rise to the telencephalon and diencephalon; (ii) Midbrain – Does not subdivide. In the mature embryo, it continues to be recognized as a more or less narrow canal; and (iii) Hindbrain – This is the most posterior dilation. It passes through a longitudinal subdivision, giving rise to the metencephalon and the myelencephalon.

These five regions will then give rise to the structures that make up the brain (eg, cerebrum, cerebellum and medulla oblongata). Let's see.

The mature brain

First. Telencephalon and diencephalon give rise to the brain. The first gives rise to the two cerebral hemispheres. Separated by a deep fissure, the hemispheres are connected by a medial structure called the corpus callosum. There are smaller connections, but the corpus callosum is mainly responsible for the connection between the two hemispheres,[9]

The outer surface of the brain, as many of us have witnessed, exhibits a curious pattern of gyri or convolutions, which are separated by slits (or fissures) of varying depth. The exaggerated lateral growth of the telencephalon almost completely covers the diencephalon, which remains as a unique structure, in a median position.

The walls of the diencephalon give rise to the thalamus and the like (ie, metathalamus, hypothalamus, epithalamus and subthalamus).

Second. The midbrain changes relatively little and remains the same name.

Third. The metencephalon gives rise to the cerebellum and pons, while the myelencephalon gives rise to the medulla oblongata. The surface of the cerebellum is covered with grooves (fissures) of varying depth. These fissures divide the organ into lobes; these, however, do not exhibit the topographic specialization that is observed in the cerebral hemispheres.[10]

The mature brain, therefore, houses three sets of structures (i) the brain (cerebral hemispheres, thalamus and the like); (ii) the brain stem (midbrain, pons and medulla oblongata), which joins the hemispheres through the so-called cerebral peduncles; and (iii) the cerebellum.

In addition to the central nervous system, the human body is equipped with a peripheral nervous system (PNS) and an autonomic nervous system (ANS).

Peripheral nervous system.

The peripheral nervous system includes nerve endings, ganglia and nerves. Nerve endings are associated with sensory and motor fibers, being found both in motor plates and in the form of free nerve endings. Accumulations of cell bodies outside the central nervous system generally take the form of small dilations, referred to as ganglia. Nerves are formed by nerve fibers associated with connective tissue. They appear as whitish cords, whose function is to conduct (take and bring) impulses to the central nervous system. They are divided into two large groups: the cranial nerves (12 pairs connected to the brain) and the n. spinal cords (31 pairs connected to the spinal cord).[11]

Autonomic nervous system

In functional terms, the nervous system can be divided into somatic and visceral. The first is responsible for intermediation between the central nervous system and stimuli coming from outside (via the sense organs). The second is responsible for intermediation between the central nervous system and other organs of the body. Controlling respiratory rate and heartbeat, for example, is the task of the visceral system.

Both somatic and visceral have two components: an afferent or outward component (carries impulses from the viscera to specific areas of the central nervous system) and an efferent or return component (brings impulses from specific areas of the central nervous system to the viscera, generally ending in a gland or muscle). The efferent (or motor) component of the visceral nervous system is commonly referred to as the autonomic nervous system. This, in turn, is divided into the sympathetic nervous system and the parasympathetic nervous system, which are distinguished by both morphological and physiological criteria. For the purposes of this article, it is enough to point out that the sympathetic and parasympathetic in general exert antagonistic effects on the organs they innervate (when the sympathetic stimulates, the parasympathetic inhibits).


To conclude, let's look at a very familiar example of how the nervous system controls our body. Consider what happens in so-called (involuntary) spinal reflexes.[12]

An action or reaction is said to be voluntary when we have a significant degree of conscious control over it. It turns out that many of our reactions, especially in dangerous situations, are involuntary. In an involuntary reaction, we are initially unaware of what is being processed, which means that we do not deliberately choose this or that response. This is what happens, for example, in the so-called withdrawal reflexes. Stop and think: What happens when you inadvertently stick your finger on a needle or bang your foot on the corner of the bed? I assume your answer isn't much different from mine: We react immediately, without thinking.

Briefly, what happens is more or less the following: The signal coming from outside is transmitted to the brain via an input route from the nervous system, passing through the spinal cord. The immediate reaction (removing the finger from the needle or the foot from the obstacle) is determined by nervous circuits that act at the level of the spinal cord itself, from where a response signal is transmitted via an output pathway to an appropriate muscle group.

In cases like this, we only begin to be aware of what happened (the accident, the wound, etc.) – including our own reaction (the muscular movements that resulted in the hand or foot moving away from the source of pain) – a few seconds after the end of the episode. As the reaction was not decided on the level of consciousness, it is said to be an involuntary reaction.

Much of what happens in our body is involuntary. In the end, therefore, do not deceive yourself: Whoever is controlling your body (I mean: the internal physiology and part of the external behavior) is not you (I mean: it is not your self-conscious self), but rather your brain ( I mean: your nervous system).

*Felipe APL Costa is a biologist and writer. Author, among other books by What is darwinism.


Bonner, J.T. 1983 [1980]. The evolution of culture in animals. RJ, Zahar.

Cingolani HE & Houssay, AB, eds. 2004 [2000]. Houssay's human physiology, P Alegre, Artmed.

Dangelo, JG & Fattini, CA. 2007. Human anatomy, 3rd ed. SP, Athenaeus.

Guyton, AC & Hall, JE. 2006. Textbook of medical physiology, 11th ed. SP, Elsevier.

Hatton, IA & more 5. 2023. The human cell count and size distribution. Proceedings of the National Academy of Sciences 120: e2303077120.

Herculano-Houzel, S & Lent, R. 2005. Isotropic fractionator: a simple, rapid method for the quantification of total cell and neuron numbers in the brain. Journal of Neuroscience 25: 2518-21.

Hickman, CP, Jr & more 2. 2004 [2001]. Integrated principles of zoology, 11th ed. RJ, G Koogan.

Hildebrand, M & Goslow, G. 2006 [2004]. Vertebrate structure analysis, 2rd ed. SP, Athenaeus.

Jerison, H.J. 1985. Issues of brain evolution. In: R Dawkins & M Ridley, eds. Oxford Surveys in Evolutionary Biology, v. 2. Oxford, OUP.

Lent, R, ed. 2008. Neuroscience: Of mind and behavior. RJ, G Koogan.

Lewin, R. 1999 [1998]. Human evolution. SP, Athenaeus.

Schmidt-Nielsen, K. 2002 [1997]. Animal physiology, 5th ed. SP, Santos.

Shultz, S & Dunbar, R. 2010. Encephalization is not a universal macroevolutionary phenomenon in mammals but is associated with sociality. Proceedings of the National Academy of Sciences 107: 21582-6.

Standring, S, ed. 2010 [2008]. Gray's, anatomy, 40th ed. RJ, Elsevier.

Tortora, GJ & more 2. 2005 [2004]. Microbiology, 8th ed. P Alegre, Artmed.

Voet, D & Voet, JG. 2006 [2004]. Biochemistry, 3th ed. P Alegre, Artmed.


[1] For a detailed discussion, see Schmidt-Nielsen (2002). This article corresponds to the third and final part of a trilogy entitled Nerves, brain and behavior (to consult Parts I and II, see here e here).

[2] In addition to the so-called endocrine hormones (whose molecules will act on distant cells), two other types are recognized, paracrine hormones (act only in the vicinity of the cell that releases them) and h. autocrine (act on the cell that secreted them). In strictly chemical terms, animal hormones are generally peptide molecules (eg, glucagon and insulin) or steroids (eg, testosterone and estrogen) – for details and examples, see Schmidt-Nielsen (2002) and Voet & Voet (2006).

[3] For a detailed examination of the structure and functioning of the nervous system, Brazilian readers have some manuals at their disposal. For example: (i) human anatomy – Standring (2010) or, for a simpler introduction, Dangelo & Fattini (2007); (ii) comparative anatomy – Hickman et al. (2004) and Hildebrand & Goslow (2006); and (iii) neurophysiology – Cingolani & Houssay (2004), Guyton & Hall (2006) and Lent (2008).

[4] The phylum Chordata is commonly divided (eg, Hickman et al. 2004; Hildebrand & Goslow 2006) into three subphyla: Urochordata (Gr., oura, tail + L., chorda, string + ata, characterized by), houses the ascidians (tunicates); Cephalochordata (Gr., Kephale, head + L., chorda, rope), houses the amphioxus; and Vertebrata (L. vertebratus, with vertebrae), brings together a wide variety of 'fish', as well as amphibians, reptiles, birds and mammals. Ascidians and amphioxus are said to be acraniate (read: lacking a skull), while the other chordates are said to be craniate. The skull is a bony or cartilaginous box that houses the brain. It is customary to divide the skull into two portions, the neurocranium and the viscerocranium. The first, posterior and superior, houses the brain; the second, anterior and inferior, is related to two major systems, the respiratory and the digestive. In the case of human beings, specifically, here is the comment by Dangelo & Fattini (2007, p. 399): “The viscerocranium is commonly known as the face. At birth, the neurocranium is much larger than the viscerocranium, as the former is related to the growth of the brain, eyes and hearing and balance organs and these are already well developed at the time of birth. However, the development of the viscerocranium is linked to the appearance of teeth and maxillary sinuses. So, until this happens, the height of the face is small. Even in adults, the disproportion between neurocranium and viscerocranium continues, but it is smaller than that which occurs at birth and in childhood.”

[5] Extending through the interior of the central spinal canal, from the base of the skull to approximately the 2nd lumbar rib, the spinal cord is surrounded by three layers of membranes. These are called spinal meninges. They are (from outside to inside): dura-, arachnoid- and pia mater (Dangelo & Fattini 2007). Infections of the meninges, collectively referred to as meningitis, can be caused by different pathogens (eg, viruses and bacteria). Although relatively episodic and localized in occurrence, meningitis can have very serious consequences, especially in the case of those of bacterial origin (eg, meningococcal and pneumococcal meningitis) – for details, see Tortora et al. (2005).

[6] Brazilian researchers (Herculano-Houzel & Lent 2005) revealed that the number of cells that make up the human brain is lower than the 100 billion that was previously mentioned. For a census of the different cell types found in our bodies, see Hatton et al. (2023).

[7] In the words of Dangelo & Fattini (2007, p. 88): “The cerebral cortex is the layer of gray matter that covers the cerebral hemispheres […]; corresponds to 40% of the weight of the brain.” Hatton et al. (2023) was the source of the cited numbers.

[8] Among mammals, specifically, there is an even more significant correlation between brain size and the degree of socialization – see, eg, Shultz & Dunbar (2010); in port., Bonner (1983).

[9] Agenesis of the corpus callosum (CCA) is a congenital brain disorder characterized by the total or partial absence of the corpus callosum. It is a relatively rare condition, which helps explain why not every pediatrician is able to offer a correct diagnosis. In cases like this, the family needs to consult a pediatric neurologist.

[10] Topographic specialization alludes to the fact that different parts of the brain are responsible for specific functions. For a map of the cerebral cortex, see, eg, Standring (2010); for functional details, Guyton & Hall (2006).

[11] Of the 12 cranial nerves (I-XII), two are perhaps most familiar to the reader: the trigeminal (V) and the vagus (X). The trigeminal remains sensitive to most of the face and oral mucosa; the vagus innervates all the thoracic and almost all abdominal viscera – regarding the cranial nerves, see Dangelo & Fattini (2007); for more details, Standring (2010).

[12] For details, see, e.g., Guyton & Hall (2006).

the earth is round exists thanks to our readers and supporters.
Help us keep this idea going.

See this link for all articles




Random list of 160 from over 1.900 authors.
Erico Andrade Ricardo Antunes Matheus Silveira de Souza Eleonora Albano Antonio Martins Luiz Roberto Alves Ronaldo Tadeu de Souza Paulo Fernandes Silveira Rodrigo de Faria Marcus Ianoni Fernando Nogueira da Costa Francisco Pereira de Farias Lincoln Secco Leonardo Avritzer José Geraldo Couto Slavoj Žižek Luiz Eduardo Soares Denis de Moraes André Singer Bruno Machado Daniel Costa Chico Whitaker Marcos Aurélio da Silva Andre Marcio Neves Soares Jose Luis Fiori Paulo Capel Narvai Vladimir Safari Marcelo Modolo Celso Frederick Thomas Piketty berenice bento Luiz Marques michael roberts Luis Felipe Miguel Andrew Korybko Milton Pinheiro Eliziário Andrade Salem Nasser Igor Felipe Santos Alexandre de Freitas Barbosa Priscila Figueiredo Marilena Chauí Carla Teixeira Paulo Nogueira Batista Jr. Katia Gerab Baggio Fernão Pessoa Ramos Octavian Helene Flavio R. Kothe Michel Goulart da Silva Joao Carlos Salles Lucas Fiaschetti Estevez Joao Paulo Ayub Fonseca Bernardo Ricupero Lorenzo Stained Glass Jean-Pierre Chauvin Luiz Renato Martins Marjorie C. Marona Francisco de Oliveira Barros Junior João Feres Junior Airton Paschoa Luiz Carlos Bresser-Pereira daniel afonso da silva Maria Rita Kehl Alysson Leandro Mascaro Gabriel Cohn Sandra Bitencourt Fabio Konder Comparato Edward Borges Leonardo Sacramento Plínio de Arruda Sampaio Jr. Antonino Infranca Sergio Amadeu da Silveira Chico Alencar Joao Carlos Loebens Luiz Bernardo Pericas Leda Maria Paulani Alexandre de Lima Castro Tranjan José Micaelson Lacerda Morais Remy Jose Fontana Ricardo Abramovay Eugenio Trivinho Tales Ab'Saber Paulo Martins Marilia Pacheco Fiorillo Jorge Luiz Souto Maior Ronald Rocha Ladislau Dowbor Marcelo Guimaraes Lima Ricardo Musse Daniel Brazil Samuel Kilsztajn Luiz Werneck Vianna Everaldo de Oliveira Andrade Annateresa Fabris Osvaldo Coggiola Eleutério FS Prado Mario Maestri Joao Lanari Bo Paulo Sergio Pinheiro Yuri Martins-Fontes Celso Favaretto Afrânio Catani Jose Raimundo Trinidad Luis Fernando Vitagliano Valerio Arcary Carlos Tautz Tadeu Valadares John Adolfo Hansen Heraldo Campos Gilberto Lopes Benicio Viero Schmidt Atilio A. Borón Denilson Cordeiro Gerson Almeida Tarsus-in-law Rubens Pinto Lyra Flavio Aguiar Andres del Rio Claudius Katz Renato Dagnino Mark Silva Joao Sette Whitaker Ferreira Gilberto Maringoni Bento Prado Jr. pressure gauge Mariarosaria Fabris Henri Acselrad Valerio Arcary Julian Rodrigues Armando Boito Leonardo Boff Ronald Leon Núñez Rafael R. Ioris Jorge Branco Henry Burnett Eugenio Bucci Boaventura de Sousa Santos Francisco Fernandes Ladeira Vinicio Carrilho Martinez Alexandre de Oliveira Torres Carrasco Juarez Guimaraes José Machado Moita Neto Liszt scallop Alexandre Aragão de Albuquerque Jean Marc Von Der Weid Ari Marcelo Solon Dennis Oliveira Walnice Nogueira Galvão Bruno Fabricio Alcebino da Silva Anselm Jappe Luciano Nascimento Vanderlei Tenorio Caio Bugiato Elias jabbour Michael Lowy Manuel Domingos Neto Jose Costa Junior Dirceu Ricardo Fabbrini Antonio Sales Rios Neto