How does the human brain work? Anatomy, structure, functions, and potential of the main organ of the central nervous system
The nervous system receives stimuli from both the external and internal environment and "analyzes" them in order to produce appropriate and coordinated responses in the various organs. In the Central Nervous System (CNS) of Man, we distinguish the brain and spinal cord (located in the vertebral canal), which are connected to the sensory receptors and the muscles through long axons that form the peripheral nerves.
The spinal cord is the seat of elementary reflexes such as that of knee extension and retraction of a limb by a caloric or puncture stimulus, but also of more complex reflexes.
The encephalus includes all the formations that are contained in the skull and plays a fundamental role in the acquisition of information, in motor control and in maintaining the homeostasis of our body. The brain can be divided into four regions:
1. The brain stem: It contains nerve centers that control, for example, breathing and blood pressure. Through the brainstem, information from the spinal cord reaches the rest of the brain and vice versa.
2. The cerebellum: controls posture, balance, and coordination of movements and are located at the base of the skull.
3. The diencephalon: It is located above the brain stem in the center of the skull, it is made up of three structures:
- thalamus (receives and sorts information from the sense organs)
- Hypothalamus (controls body temperature, water balance, metabolism and daily biorhythms such as sleep and hunger. It is home to emotions, the center of pleasure and habituation)
- Pituitary gland (endocrine gland).
The telencephalon: it is the most voluminous part of the whole brain and is commonly called the brain.
Brain and spinal cord are covered with three layers of tissue (meninges) of different thickness and structure:
- The very thick dura mater adheres to the bone;
- The pia mater, very thin and vascularized, adheres to the nervous tissue
- The arachnoid, with a net structure, located between the other two meninges.
Between the arachnoid and the nervous tissue, there is the subarachnoid space that contains the liquor that fills all the cavities of the central nervous system.
The brain is home to many functions of our organism. The brain is still considered the most complex and mysterious organ of our body, capable of controlling multiple functions that are very different from each other and at the basis of all our lives, such as memory, the language, the movements of the arms and legs and the functioning in general of all the organs present in the body with consequent regulation, for example, of breathing and heart rate.
Much of the development of the human brain occurs in the womb. During intrauterine life, the size of the brain increases greatly due to the rapid increase both in the number of specialized cells (neurons) that compose it and in the support structures and connections.
The adult brain is made up of billions of neurons, and the connections they develop between them are tens of thousands. Later in the text, we will try to understand the functioning of the brain by describing the structure and mechanisms that characterize it at the level of individual cells.
Anatomically, the brain is made up of two specular cerebral hemispheres that are divided by an interhemispherical sulcus. At the base, the two hemispheres are connected by a network of nerve fibers that make up the corpus callosum.
- The superficial part of the brain ( cerebral cortex ) is made up of gray matter (containing nerve cells and synaptic connections)
- The deepest part is made up of white matter (nerve fibers)
The surface of the brain hemispheres has folds that separate the brain convolutions. Deeper introflexion allows dividing the hemispheres into brain lobes.
The main part of the brain consists of the cerebral hemispheres in which four lobes are identified (frontal, parietal, temporal, and occipital). The hemispheres are connected to the underlying structures through the brain stem.
Each of the lobes is specialized for a certain function:
- Frontal lobe >> movement programming and execution
- Parietal lobe >> perception of somatic sensations
- Occipital lobe >> view
- Temporal lobe >> hearing, learning, and memory
Each part of the brain corresponds to a function, for example, "you read" with the occipital lobe while learning languages with the temporal one.
In man, the cortex is the most developed cerebral area, and we have already mentioned how the functions of many cortical areas are known. Many of these functions are performed by both hemispheres, while some by only one. Some areas involved in some higher functions have been identified, such as language, which is lateralized to the left in most people.
The pathways that go from the sensory receptors to the cortex and those go from the cortex to the muscles cross. For this, the movements of the right side of the body are controlled by the left part of the cortex, and vice versa, as well as the left part of the body, sends sensory signals to the right hemisphere. Nonetheless, the two halves of the brain do not work separately, as the right and left cerebral cortex is connected by the corpus callosum.
The brain is also composed of cells. At this point, to better understand how the brain is able to process the sound, visual, or tactile signals it receives through the sense organs, let us go into the structure and functioning of the single cell.
The nervous system is made up of a kind of network of specialized cells known as neurons (excitable cells) and support cells known as glial cells (non-excitable). The excitability of neurons is determined by the presence of voltage-dependent ion channels in the cell membrane, and the nerve impulse is generated precisely by the change in the difference in membrane potential.
In practice, if the neuron is stimulated, the membrane potential can increase from about –70 mV to about –50 mV (threshold potential). Once this threshold value is reached, many sodium channels open, and a large number of Na + ions pass from outside to inside the cell.
As the concentration of positive charges inside increases, the potential reverses sharply and reaches a value of +35 mV, which is called the action potential.
This sequence of events, called membrane depolarization, follows immediately after the restoring conditions are restored (membrane repolarization).
Neurons are made up of dendrites and axons. All neurons have the same basic structure. We can distinguish a large cell body, which contains the nucleus and the cytoplasm, from which two processes start, the dendrites and an axon.
- The dendrites are much-branched and form synapses, or specialized communication sites, with the adjacent neurons, and for this reason, they can be defined as structures in charge of the entry of information.
- The axon (or nerve fiber) is a portion of the cell body that extends and ends either on other neurons or on organs and has the function of transmitting action potentials up to the synapse, the area where the axon comes into contact with the dendrites of other neurons.
The synapse is composed of the presynaptic nerve termination separated by a thin space from the postsynaptic component. The transmission of the signal in this space takes place by means of the neurotransmitters which are contained in vesicles (synaptic vesicles) thickened at the ends of the axon, which contracts synaptic relationship with other neurons.
The moment the neuron is reached by a stimulus, the synaptic vesicles merge with the presynaptic membrane, pouring their content into the synaptic space and bind to receptors or located on the postsynaptic membrane. The interaction between the neurotransmitter and the receptor triggers a response in the postsynaptic neuron.
Specifically, the axon termination that conducts the nerve impulse releases the neurotransmitter, and, in this way, the electrical impulse is translated into a chemical signal.
The postsynaptic termination that receives the nerve signal has specific receptors on its membrane that are capable of binding neurotransmitters. This bond causes electrical changes in the postsynaptic neuron membrane. If the threshold value is reached, all this translates into a nerve impulse that travels rapidly and spreads to other neurons.
We have, therefore, seen how real electrical signals are generated in the brain that travels through the neuronal network. Cerebral activity is detectable through the electroencephalogram, able to highlight the generation, from the cerebral cortex, of different electric waves depending on the state of the person (alert, sleep, dream, etc.).
The control of movements starts from the cerebral cortex. In primates and in humans, the cerebral cortex plays a predominant role in the control of movements. Here are the motor areas (especially at the level of the parietal and frontal regions of the cerebral hemispheres) and these are connected to the subcortical formations with which they interact allowing the coordination of attitudinal movements (reflexes) with those much more precise and fine (typical of the voluntary motility).
The brainstem control of motility is implemented through pathways that carry signals to the spinal motor neurons and those of the cranial nerves (the head and neck muscles).
Neurodegenerative diseases cause disturbances in movement and mental functioning. The neurodegenerative diseases are debilitating diseases that cause progressive degeneration and/or death of nerve cells, causing disturbances in the movement (ataxia) or mental function (dementia). Some examples of neurodegenerative diseases are:
- Alzheimer's
- Parkinson
- Motor neuron diseases
- Huntington's disease
- Spinocerebellar ataxia
- Spinal muscular atrophy
Alzheimer's disease now affects about 5% of people over 60 years.
In patients, a loss of nerve cells is observed in the brain areas vital for memory and other cognitive functions, and, in addition, there is a low level of neurotransmitters such as acetylcholine.
This disease, therefore, affects memory, the ability to speak and think, and can also cause states of confusion and Spatio-temporal disorientation.
After Alzheimer's disease, Parkinson's is the most common neurodegenerative disease.
This disease is characterized by the neuronal degeneration of the black substance and, from a biochemical point of view, by a reduction in the amount of dopamine at the level of SNpc. It manifests itself with a typical tremor at rest, rigidity, slowness of automatic movements, and postural instability.
The causes of Parkinson's are not yet fully known. The causes of Parkinson's disease are not fully known to date, but the hypothesis that both environmental and genetic factors determine it is accepted.
Stem cells can repair damaged neurons. The potential use of stem cells to repair damaged neurons in the adult brain represents a very important, current, and promising line of research.
The discovery of the "nervous growth factor" Neurologist and life senator, Rita Levi Montalcini, was awarded the Nobel Prize for medicine in 1986. About the NGF, she said: "The discovery of the NGF in the early 1950s is an example fascinating about how an acute observer can extract valid hypotheses from apparent chaos. Previously
, neurobiologists had no idea what processes intervened in the correct innervation of the organs and tissues of the organism ".
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