“Perception of events matters more than facts.”
Napoléon Bonaparte
From the very first organisms, an adequate perception of the surrounding environment is essential. It gradually led to the development of the human brain.
We will first analyze the evolution of the nervous system from invertebrates to humans.
We will then present the human brain in a few figures and briefly describe its architecture.
A study of the brain formation from the embryo to the
adulthood will follow.
We will conclude with an analysis of the tools developed by plants to explore their environment, to adapt and communicate.
1.1 Animals
A draft of the nervous system appeared very early in evolution, allowing organisms to react quickly to any changes in the environment.
Bacteria
Through a recognition and counting system, bacteria can
communicate and align their individual behaviour.
Quorum sensoring
This synchronization mechanism, called quorum sensoring* or quorum detection, is based on the exchange of a chemical messenger between individuals.
(words marked with an * are repeated at the end of the lexicon) The bacteria can thus carry out common actions, promoting 9
their survival, such as the production of a development support or toxin release:
- In Vibrio fischeri, this synchronized behaviour is
based on the synthesis of a specific molecule (AHL*)
and a protein sensor (LuxR*).
The disseminated molecule penetrates its neighbors
and the quorum reached, triggers a bioluminescence.
- In sufficient numbers in one host, Staphylococcus
aureus causes various infections up to septicemia*.
- Pseudomonas aeruginosa is responsible for
secondary infections in patients with cystic fibrosis.
The quorum sensoring regulates the formation in their
lungs of a biofilm stimulating their growth.
Invertebrates
Placozoans
Small marine organisms, placozoans are the simplest known animals. They are devoid of nerve or muscle cells, nervous system or internal organs.
However, they can synchronize their movements to perform more complex actions, such as climbing on marine rocks to feed on seaweed.
D.Fasshauer’s team highlighted intra-cellular communication, ensured by the transmission of small peptides* coordinating the different parts of their body.
Thus the origin of the nervous system goes back at least to that of placozoans, more than 600 million years ago.
Sponges
Sponges are a bag-shaped fixed marine animal, supple and supported by a limestone or silica structure.
Their cells communicate with each other through the transfer 10
of calcium ions, allowing them to harmonize their body movements to feed.
The proteins responsible for this ionic flow are encoded by the same genes as those involved in nerve transmission in humans.
Hydras
The hydra is a small animal living in fresh water, able to regenerate the amputated parts of its body or reproduce by budding.
Its rudimentary nervous system consists of a diffuse network of sensory nodes, it is decentralized and distributed uniformly throughout the body.
Earthworm
Earthworms are small terrestrial animals with ringed bodies, playing a major role in soil fertilization.
They have a primitive brain, formed of cerebral nodes,
extended by a ventral nerve cord from which come out at each segment small sensory endings.
In response to perceived stimuli, these transverse nerves allow a coordinated muscular contraction of their body.
Arthropods
The nervous system of crustaceans and insects consists of a brain and ganglia connected by a ventral nerve cord.
Arthropods equipped with sensory organs, benefit from vision and olfaction, essential to their social life.
Vertebrates
Thanks to the development of new areas and connections, the vertebrate brain is experiencing impressive growth.
The peripheral nervous system appears, allowing to connect the central nervous system to the whole body.
11
The brain* combines the signals of the sensory organs with those of the internal ones, improving the adaptation to the surrounding environment.
The developed cerebellum* allowing fish to control their movements and measure pressure variations.
Amphibians
The size of the forebrain largely increases, exceeding that of the cerebellum.
The amphibian brain is covered with a nerve tissue, analysing information from the olfactory bulb*.
Reptiles
Compared to fish, the base of the forebrain continues to grow, but the olfactory bulb retains a major role.
The reptilian brain or ancestral brain controls the innate behaviours and corresponds to the brainstem* and cerebellum of our brain*.
Birds
The brain of birds is close to that of reptiles with 3 differences:
- Its cerebellum is larger, controlling balance and flight,
- The existence of a single cerebral structure, the
caudolateral nidopallium*, considered to be the
analogue of the primates prefrontal cortex.
- The density of neurons* in the avian brain is much
higher than that of other vertebrates.
Mammals
The appearance of the cortex*, an external layer covering the cerebral hemispheres and interconnected to the limbic system*, allows a better processing of information.
12
The limbic system develops further, it is involved in olfaction but also memory and emotions regulation.
The cortex is particularly well developed in dolphins, elephants and humans.
Man
The human brain is distinguished primarily by the size and structure of its cortex.
The frontal lobe is large, hosting thought, consciousness and metacognition*.
Summary
Starting with the first organisms, an internal and external analysis system takes place, leading gradually to the brain.
Bacteria communicate by diffusion of a chemical messenger, allowing them to adjust their behaviour.
The first pluricellular animals synchronize their cellular movement through the exchange of small peptides or ions.
A rudimentary, decentralized nervous system appears in hydra consisting of a diffuse network of nerve nodes.
Worms, crustaceans and insects have a primitive brain, formed of cerebral nodes, prolonged by a nervous cord.
The evolution of the brain in vertebrates still accelerating:
- Fish associate signals from sensory organs with
them from internal ones.
- Amphibian telencephalon is covered with nerve
tissue.
- The reptilian brain controls innate behaviours.
- The mammal cortex, interconnected with limbic
system, improves information analysis.
1.2 Human brain
13
The human brain is the most sophisticated organ known, its functioning still unknown to a large extent.
It continuously coordinates the adaptation of our body to changes in the environment and plays a major role in the manifestation of consciousness and memory.
Throughout life, physical and mental activities change the structure of the brain.
Some figures
- Our brain alone accounts for 20% of our energetic
need, for only 2% of our body weight.
- 1 billion signals pass through it per second at a speed of 432.000 km/h.
- Like any living cell, neurons are regularly replaced,
their number decreasing by 5% over the course of life.
Anatomy
The encephalon* is composed of the brain stem, cerebellum and brain. The cortex covering the brain is divided into lobes separated by convolutions and grooves.
It consists of external lobes: frontal (cognition, motor skills, writing), temporal (hearing, smell, reading), occipital (sight) and parietal (taste, touch) and limbic and insular systems (olfaction, memory, emotion).
We distinguish the primary areas, sensory or motor, from the associative areas that integrate the information of the primary adjacent areas.
Brainwaves
5 types of brain waves are measured: gamma: intense mental activity, beta: active awakening, alpha: light relaxation, theta: hypnosis or meditation and delta: deep sleep or meditation.
Thus, in addition to the flow of biochemical messengers, the 14
brain is also crossed by electromagnetic waves.
1.3 Brain construct
The construction of the brain starts at the beginning of the gestation and continues during 25 years, it can be divided into five successive stages.
Before birth
About 20 days after conception, a neural plate appears on the back of the embryo. A week later, by folding it turns into a neural tube*.
Acting as a neural factory, it manufactures 3.000 cells every second and is at the origin of the spinal cord* and the brain.
After 7 weeks, the neural tube divides and the main brain structures are differentiated. At the 17th, the brain, the cerebellum and the brainstem are formed.
At birth, the brain is operational but its structure still has to evolve, its cortex already has its pleated appearance and it weighs 400 g.
Childhood
After birth, the brain continues to develop rapidly, creating more than a million synapses per second, connecting the
different neural zones together.
This important growth leads to an excess of brain connections, resulting in their pruning and only the most regularly solicited will be kept and reinforced.
At 2 years, the functional architecture is emerging, first by integrating the areas involved in the perception of oneself and the environment and then those of motor skills.
The visual cortex pruning is complete at 3 years, that relating to spatial orientation, the language one spreads until puberty.
In children under 4 years, hundreds of thousands of synapses 15
At 6 years the brain reaches 90% of its adult size, its structure and cortex continuing to evolve significantly.
Adolescence
From puberty to adulthood, the adolescent’s brain experiences a second period of intense restructuring. At 16 years only 50%
of the initial synapses have been preserved.
These changes mainly concern the epiphysis* regulating sleep, the limbic brain controlling emotions and after 20 years, the prefrontal brain involved in decision-making and planning.
At puberty, the production of testosterone amplifies the neuronal plasticity, which can lead to emotional instability, increased impulsivity or reckless risk-taking.
From 10 to 20 years, the deposition of myelin* around the axons* intensifies, optimizing the conduction of nerve
impulses while strengthening communication between the
cerebral hemispheres.
Maturity
The brain still evolves after its maturity, new synapses are formed, its architecture is modified according to circumstances and activities.
In adults, the neurogenesis* takes place exclusively in the hippocampus where a third of its neurons will be replaced over the course of life.
This structure plays an essential role in learning new skills and creating memories.
The prefrontal cortex is the last region to mature, around 25
years the myelination* of its nerve fibers ends, making it 100%
operational.
The maturation of the frontal cortex completed, the integration of information from other brain areas is optimal, leading to a 16
complete vision of events and better judgment.
Senescence
The brain experiences a slow natural cognitive decline. Its weight and volume decrease by 5 to 10% and anatomical
changes are observed.
The adoption of a healthy lifestyle and the regular practice of physical and mental activities slow down the process.
If the number of neurons decreases by less than 5%, their myelin sheath and their connections gradually slim down.
Most of the neural circuits being preserved, our brain retains its ability to adapt. Mental functions can also improve and new ones develop.
Such as the appearance of new connections between the motor and auditory zones by playing piano or the involvement of new ones compensating the deficits of those of the language.
Summary
Building the brain is a complex 25-year process. This is a permanent reconfiguration of the brain connections, renewed regularly.
With the exception of the hippocampus, the synthesis of
neurons stops in adulthood, yet its total number only decreases by 5%.
The brain gradually declines beyond the age of 60, but different mechanisms are being put in place to remedy this, a healthy lifestyle and various activities strengthening them.
1.4 Plants
Plants also have sensory, communication and defense systems, all the more important because they cannot move to feed, reproduce or flee.
Five senses and more
17
Despite the absence of complex sense organs, plants have our
«five senses» and even other detection systems for: magnetic fields, acidity, chemical gradient...
View
In addition to the chlorophyll needed for sugar photosynthesis, plants have other pigments analyzing their environment:
- Phototropins, capturing blue light, allow them to
orient themselves towards light.
- Cryptochromes, absorbing blue and ultraviolet light,
regulate the development of leaves, roots and flowers.
- Phytochromes, sensitive to red light, are involved in
germination and flowering.
Some plants perceive images, such as the wild vine copying the shape and color of its host or the thale cress distinguishing its related neighbors from other shoots.
F.Baluska and S.Mancuso propose the existence on the leaves of ocellae*, primitive eyes, similar to those of the starfish.
Hearing
Plants are able to perceive sound vibrations:
- Soft music promotes the growth of cultivated plants,
as opposed to hard rock.
- Pea roots ear the sound of groundwater that allows
them to head for it.
- Larvae feeding on thale cress leaves emit a typical
sound, its exposure to other non-infested ones triggers
their self-defense system.
Sense of smell
Plants emit and smell odorous molecules:
- The smell of cut grass is a red flag.
18
- The scent of flowers is used to attract pollinating
insects or repel predators.
Taste
Plants use taste to feed and defend themselves:
- Roots taste the soil for nutrients capable of detecting minute amounts of mineral salts.
- Jasmonic acid released from attacked leaves triggers
the defense system of neighbouring plants.
Touch
The sense of touch is quite elaborate in plants, able to distinguish the caress of the wind from the trampling of an insect:
- The sensitive folds its leaves when touched by an
insect and not by wind or rain.
- The fly trap has hairs on its mandibles, if two of them are affected in less than 20 seconds, the plant closes
them automatically.
- Coming into contact with a hard obstacle, the root
feels it and then bypasses it.
Root-brain
The movements of a root strangely recall those of a earthworm: groping, pauses, bypasses..., the meristem* at its tip acting as a mini brain.
The root continuously analyzes its environment, measuring many parameters: temperature, humidity, granularity,
brightness, electromagnetic fields...
In 2009, S.Mancuso and F.Baluska observed intense electrical activity in the root meristem, reflecting the ongoing exchange of information with the rest of the plant.
Neurotransmitters
19
In 2013, E.Farmer’s team demonstrated the spread of a weak electrical current from the attacked leaf to the others, followed by the production of jasmonic acid.
This team then identified the three genes involved in this defense process, they are identical to those responsible for the transmission of nerve impulses in humans.
In 2015, S.Mancuso and A.Viola detected in these plants the presence of 3 neurotransmitters: dopamine*, serotonin* and glutamate, circulating throughout the whole plant.
Summary
Morphologically different from animals, plants are equipped with analysis, communication and reaction tools with
impressive performances.
Able to see, hear, smell, taste, touch, analyze the surrounding environment and communicate internally and with each other.
The most striking is the similarity of the internal mode of plants communication with that of animals, based on the
exchange of identical messengers encoded by the same genes.
1.5 Conclusion
Since the very first organisms, the evolution of life has been joined by the parallel development of increasingly elaborate systems of analysis and communication.
Building the brain is a complex 25-year process. This is a permanent reconfiguration of the brain connections, renewed regularly.
Besides the flow of biochemical messengers, the brain is also crossed by electromagnetic waves.
Morphologically and physiologically different, plants have systems of analysis and communication as efficient as the animal nervous system.
20