Generality
The nervous system receives the different stimuli coming from the inside and from the outside of the body, analyzes them, processes them and generates appropriate responses to favor the survival of the organism itself.
The vertebrate nervous system comprises two components:
- Central Nervous System (CNS): receives and analyzes information arriving from the internal and external environment of the organism, then elaborates the most appropriate responses;
- Peripheral Nervous System (PNS): it captures the stimuli coming from both the external environment and from the inside of the organism, then transmits them to the CNS; moreover, it transmits the nerve stimuli (responses) processed centrally to the periphery.
In vertebrates, the central nervous system (CNS) is made up of the brain and spinal cord.
The tissues that make up the central nervous system are made up of different nerve cells (called neurons): a part of them forms the so-called gray matter; another part forms what is called white matter.
THE BONE COATING OF THE CNS
The brain is placed inside the skull, which is a real protective bone box. The spinal cord, on the other hand, runs inside a canal in the spinal column.
The vertebral column is so called because it is composed of vertebrae, 33 or 34, which are particular bone structures, formed by a body, by an arch and separated by a gelatinous disc.
The skull and spine, in addition to providing protection, perform support and containment functions.
THE MENINGES
The meninges are membranes located between the bone lining and the central nervous system. Therefore, the entire meningeal apparatus envelops both the brain and the spinal cord.
There are three meninges:
- Pious mother. Very thin, it is the membranous layer in direct contact with the brain and spinal cord. It contains the arteries that supply the central nervous system.
- Arachnoid. It is the middle meningeal layer. Although connected to the pia mater, the link with it is loose, so that a space is created, called the subarachnoid space, filled with liquid.
- Tough mother. Very thick layer, which constitutes the outermost menynx of the three. It contains the venous vessels, which, through the venous sinuses, drain the blood circulating in the CNS.
The function of the meninges is to protect the delicate nervous tissue from all those traumas that could affect the skull and spine.
THE PROTECTIVE LIQUID
Figure: an overview of the brain areas.
The protective liquid of the central nervous system softens and absorbs the shocks that can affect the brain or spinal cord. This liquid is contained in different locations: between the cells, where it takes the name of interstitial fluid, and in the subarachnoid space, where it assumes the name of cerebrospinal fluid or CSF.
The liquor, in addition to defending the central nervous system from trauma, contains salts that it exchanges with the interstitial fluid, and very few proteins; very importantly, it also represents a way to remove waste products.
The cerebrospinal fluid is a source of considerable information, so much so that it is taken when infections or neurological pathologies are suspected (see rachicentesis).
NEURONS AND NERVES
Neurons are the cells of the nervous tissue. Their function is to generate, exchange and conduct all those (nerve) signals that allow muscle movement, sensory perceptions, reflex responses, etc. In other words, neurons are carriers of information. In an adult's nervous system, a few tens (or even hundreds) of billions of neurons create a huge network, reaching and connecting every part of the body.
- the body or cellular soma
- the dendrites
- the axons.
The cell body contains the nucleus and all those organelles typical of each cell of the organism.
The dendrites are the extensions that allow the reception of the nerve signal coming from other neurons.
Finally, the axons are the extensions that spread and transmit the nerve signal to other neurons or organs.
The structure of a neuron can vary slightly depending on the area in which it resides and the task it performs. For example, there are neurons with axons covered with myelin (an insulator made of lipids and proteins) and neurons, which, on the other hand, are devoid of it.
A bundle of several neurons (or rather axons) makes up a nerve.A nerve, depending on the neurons it contains, can carry information and signals in two directions: from the central nervous system to peripheral organs / tissues (efferent nerves) or vice versa, that is, from the periphery to the CNS (afferent nerves).
The efferent nerves are of the motor type, since they control the movement of the muscles; on the contrary, the afferent nerves are of the sensory type, in that they signal to the central nervous system what they have detected in the periphery.
In reality, alongside the two aforementioned, in the CNS, there is a third category of nerves, that of mixed nerves. These possess bundles of sensory neurons and bundles of motor neurons.
GRAY SUBSTANCE AND WHITE SUBSTANCE
Gray matter and white matter are the two tissues that make up the central nervous system.
The difference, which distinguishes these two substances, lies in the cellular composition: the gray matter, unlike the white matter, contains neurons devoid of myelin.
The figure shows how they appear and which areas the white and gray matter occupy in the brain and spinal cord.
Figure: the location of the gray and white matter within the spinal cord (left) and the brain (right). The gray matter, in the spinal cord, occupies the central area and has the shape of an H (or a butterfly); in the brain, on the other hand, it takes place in the cortex and in some internal areas.
In the medulla, white matter surrounds the gray matter; vice versa, in the brain it is surrounded by the latter.
The brain
The brain is the most complex structure of the central nervous system, as it is made up of different areas or regions.
In the adult man, it weighs up to 1.4 kg (about 2% of the total body weight) and can contain 100 billion neurons (one billion corresponds to 1012). Therefore, the connections that it can establish are many and unimaginable.
There are four main regions of the brain. Each of them has a specific anatomy, with compartments specialized in different functions. In order not to complicate this text too much, it was preferred to report a summary table of the main brain areas (ie of the brain) and their relative functions.
The only information that we will limit ourselves to expounding is the following. Twelve pairs of cranial nerves branch off from the brain, for which, for identification purposes, the Roman numbering from I to XII is used. Except for the I and II pair of nerves, which originate in the telencephalon and diencephalon respectively, the remaining twelve pairs arise in the brain stem.
REGION
FUNCTION
Cerebral cortex
Perception; movement and coordination of the voluntary muscles
Thalamus
Passage station for motor and sensory information
Instinctive behaviors; secretion of various hormones
Mid-brain
Eye movement; coordination of auditory and visual reflexes
The spinal cord
The spinal cord is a cylindrical-shaped structure, on average 45 centimeters long and housed inside a canal in the spinal column (this usually measures 70 centimeters).
Figure: the medulla contained in the spinal column.
The sections of the spine:
- Cervical: 7 vertebrae
- Dorsal (or thoracic): 12 vertebrae
- Lumbar: 5 vertebrae
- Sacral: 5 vertebrae
- Coccygee: 4/5 vertebrae
Superiorly, it starts from the medulla oblongata (structure of the brain stem); inferiorly, it ends between the second and third lumbar vertebrae and reaches, with the last extensions, the sacral region.
The nerve scaffolding of the spinal cord is somewhat complicated. To facilitate understanding, we will first analyze the neurons of the gray matter, then those of the white matter.
N.B: clearly, the length of the medulla and the spine depend on the height of an individual. A person who is 160 centimeters tall will certainly not have a medulla as long as that of a basketball player with another 2 meters. Nevertheless, anatomy and functions do not change.
gray matter
As with the brain, pairs of nerves (exactly 31 pairs) are also born from the spinal cord, called spinal nerves. Spinal nerves are mixed nerves, therefore they have both motor and sensory fibers.
The spinal nerves bind to the spinal cord via the so-called roots: there are the roots of the motor fibers (or ventral roots) and the roots of the sensory fibers (or dorsal roots). The terms ventral and dorsal are used according to where the roots are inserted: the belly of the medulla looks towards the abdomen of the individual, the back of the medulla looks towards the back.
Each type of fiber belongs to the gray matter, contained in the central area of the medulla: the motor one originates from an "area called the ventral horn; the sensitive one, on the other hand, originates from a portion called the dorsal horn.
The figure is of considerable help to understand what has just been described.
The spinal nerves are:
- 8 cervical
- 12 thoracic
- 5 lumbar
- 5 sacral
- 1 coccygeal
white matter
The neurons, or rather the axons, of the white matter of the spinal cord form real columns. These columns, called bundles or tracts, can run from the top to the bottom (ie from the CNS to the periphery) and vice versa (i.e. from the periphery to the CNS): if they run downwards, they are called descending bundles; if they run upwards , are defined ascending beams.
The ascending beams carry sensitive information.
The descending beams conduct motor-type signals.
Figure: spinal cord anatomy. Alongside the elements described in the text, it is also possible to recognize the dorsal root ganglion and its contents, i.e. the body of one of the sensory neurons. The ganglion is, as can be seen, a swelling, which acts as a container for the bodies of all the sensory neurons of a spinal nerve (in the figure, for simplicity, there is only one body
THE MARROW, A SIGNAL INTEGRATION CENTER
The spinal cord must be considered to all intents and purposes a center of integration of signals of the nervous type, as it has the extraordinary ability, when it receives sensory signals, to formulate an autonomous motor response, without addressing the brain. built is even faster, it is called the spinal reflex.
All this confirms, once again, the numerous potentialities of our central nervous system.
