Human brain the organ of the central nervous system
The brain is the organ of our body responsible for the perception and interpretation of the outside world. It is composed of a myriad of nerve cells, called neurons, which form an extremely efficient network of connections.
To give an order of magnitude, it is estimated that the brain contains a little more than 100 billion neurons, each of which can form up to 10,000 connections, all contained in a volume equivalent to that of a carton of milk. It is thanks to this formidable network of neurons that we can, among other things, enjoy a good movie, fall in love or solve a riddle.
Function of the brain
Located in the cranium, the brain is the seat of higher functions (cognitive functions, senses, nervous responses) and vegetative functions. It is thus an essential organ which ensures the regulation of all the vital functions. The brain receives information from the whole body via afferent nerves, integrates and analyzes it, and then responds by sending new signals to the parts of the body concerned by the efferent nerves.
The brain is thus responsible for the cardiac and respiratory rhythms, functions which for us are unconscious. But it is also involved in decision making, in the body’s motor skills, behavior, memory, consciousness…
The brain has been mapped and some areas seem to be associated with specific functions (speech area…), while more complex functions (memory, consciousness) seem to be much more distributed.
Structure of the brain
- The human brain contains about 100 billion neurons and has an average weight of 1,300 to 1,400 grams. It also contains glial cells and astrocytes.
- The brain is immersed in cerebrospinal fluid.
- The brain is composed of two hemispheres, right and left, connected by the corpus callosum, which is composed of white matter (axons covered with myelin). Divised into lobes, the components are the most advanced corridor in mammals.
- The brain is divided into many parts, depending on their embryonic origin:The telecephalon, the superior section, composed of the cerebral cortex and striatum ;
- The diencephalon, containing the thalamus and the hypothalamus (endocrine glands);
- The mesencephalon ;
- The cerebellum (motor coordination and the learning of routine movements);
- Varole’s bridge ;
- The medulla oblongata (control of reflex and vegetative movements: breathing, moderation of the cardiac rhythm, regulation of arterial smooth muscles) which communicates with the spinal cord.
The cerebral cortex is the external layer of the brain and contains the majority of neurons (gray matter). The meninges are protective envelopes (dura mater, arachnoid and pie mater) between the cortex and the skull bone. The cerebral cortex is subdivided into five cerebral lobes according to their position:
- The frontal lobe (motor skills, memory, reasoning) ;
- The parietal lobe (touch) ;
- The temporal lobe (hearing, smell) ;
- The occipital lobe (vision);
- The limbic lobe.
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The brain is the organ of the central nervous system that regulates all vital functions. The brain is a member of the central nervous system.
The pleasure centers of the human brain
The main centers of the reward circuit have been located along the MFB (“medial forebrain bundle”). The ventral tegmental area (VTA) and the nucleus accumbens constitute the major centers of this circuit, which includes several others such as the septum, the amygdala, the prefrontal cortex and certain regions of the thalamus. Each of these brain structures would participate in its own way in various aspects of the behavioral response.
Furthermore, all these centers are interconnected and innervate the hypothalamus (red arrows), informing it of the presence of a reward.
The hypothalamus then acts in return not only on the ventral tegmental area, but also on the vegetative and endocrine functions of the whole body via the pituitary gland.
Aversive stimuli that provoke flight or fight activate the punishment circuit – or “periventricular system” (PVS) – which allows us to cope with unpleasant situations. Discovered by De Molina and Hunsperger in 1962, this system involves different brain structures including the hypothalamus, the thalamus and the central gray matter surrounding the aqueduct of Sylvius. Secondary centers are also found in the amygdala and the hippocampus.
This circuit works in the brain through acetylcholine and stimulates ACTH (adrenal corticotrophic hormone), the hormone that stimulates the adrenal gland to release adrenaline to prepare the organs for flight or fight.
Interestingly, stimulation of the punishment circuit can inhibit the reward circuit, supporting the common observation that fear and punishment can drive out many pleasures.
The MFB and the PVS are thus two major motivational systems for the individual. They induce action in order to satisfy instinctive impulses and avoid painful experiences.
The same cannot be said for a third circuit, the Behavioral Inhibitory System “BIS”. The identification of this system was made by Henri Laborit in the early 1970s. It is associated with the septo-hippocampal system, the amygdala and the basal ganglia. It receives input from the prefrontal cortex and sends its output through the noradrenergic fibers of the locus coeruleus and through the serotonergic fibers of the median raphe. Some recognize a major role for serotonin in this system.
The AIS is activated when fight or flight appears impossible and the choice of behavior is reduced to passive submission. The pathological consequences of this inhibition of action have made it possible to understand how destructive chronic stress can be for human beings.
The brain has the size and aspect of a small cauliflower. But thanks to its 100 billion nerve cells (as many as there are stars in our galaxy!), we can think, plan, speak, imagine… and at the same time be aware of it all.
The brain has two cerebral hemispheres, each of which takes care of one side of the body. Except that the control is crossed: the right hemisphere takes care of the left side of the body and vice versa!
Each of these unfolded hemispheres would be as big as an extra-large pizza! That’s why it has to fold back on itself in many convolutions to fit in our skull.
THE NERVOUS SYSTEM
The brain is like the control tower of our body. It must be kept informed quickly of the body’s needs and the resources available in the environment to satisfy them.
It does this through a vast network of cables spread throughout the body: the nerves. Together with the brain and the spinal cord, they form the nervous system.