health of the nervous system

Central nervous system

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 coming from the internal and external environment of the organism, then processes the most appropriate answers;
  • Peripheral Nervous System (SNP): captures the stimuli coming both from the external environment and from inside the body, then transmits them to the CNS; moreover, it transmits to the periphery the nervous stimuli (the responses) elaborated at central level.

In vertebrates, the central nervous system ( CNS ) is composed of the brain and the spinal cord .

Of soft and gelatinous consistency, both the enfalo and the spinal cord are immersed in a protective liquid, wrapped in membranes (the meninges ) and further protected by an external bone covering.

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 substance ; another part forms what is called white substance .

THE BONE-COATING OF THE SNC

The brain is stored inside the skull, which is a real protective bone box. The spinal cord, on the other hand, runs inside a spinal canal.

The vertebral column is so called because it is composed of vertebrae, 33 or 34, which are particular bony structures, formed by a body, an arch and separated by a gelatinous disk.

Skull and spine, as well as providing protection, perform support and containment functions.

LE MENINGI

The meninges are membranes located between the bone lining and the central nervous system. Therefore, the entire meningeal system wraps around both the brain and the spinal cord.

The meninges are three:

  • Pious mother . Very thin, is the membranous layer in direct contact with encephalon and spinal cord. In it, the arteries that supply the central nervous system are contained.
  • Arachnoid . It is the intermediate meningeal layer. Although it is connected to the pious mother, the link with it is slow, so a space is created, called the subarachnoid space, filled with liquid.
  • Hard mother . Very thick layer, which constitutes the outermost meninge of the three. It contains the venous vessels, which, through the venous sinuses, operate the drainage of the blood circulated 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 the spine.

THE PROTECTIVE LIQUID

Figure: an overview of the brain areas .

The protective fluid of the central nervous system cushions and absorbs 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 takes the name of cerebrospinal fluid or liquor .

The liquor, in addition to defending the central nervous system from trauma, contains salts that it exchanges with the interstitial liquid, and very few proteins; very important, 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 diseases are suspected (see rachicentesi).

NEURONS AND NERVES

Neurons are the cells of nervous tissue. Their function is to generate, exchange and conduct all those (nervous) signals that allow muscle movement, sensory perceptions, reflex responses and so on. In other words, neurons are information carriers. In an adult's nervous system, some tens (or even hundreds) of billions of neurons create an enormous network, which reaches and connects every part of the body.

In general, neurons consist of three parts:

  • the body or cellular soma
  • the dendrites
  • the axons .

The cell body contains the nucleus and all those organelles typical of every cell in the body.

Dendrites are extensions that allow reception of the nerve signal from other neurons.

Finally, axons are extensions, which spread and transmit the nervous signal to other neurons or organs.

The structure of a neuron may 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 contrary, do not.

A bundle of multiple neurons (or rather axons) constitutes 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, as they control the movement of the muscles; on the contrary, the afferent nerves are sensitive, as they signal to the central nervous system what they detected in the periphery.

In reality, next to the two aforementioned, in the CNS, there is a third category of nerves, that of mixed nerves . These possess bundles of sensory neurons and beams of motor neurons.

GRAY AND WHITE SUBSTANCE SUBSTANCE

The gray substance and the white substance 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 substance, unlike the white substance, contains neurons without myelin.

The figure shows how they appear and which areas occupy the white and gray matter in the brain and spinal cord.

Figure: the position of the gray substance and of the white substance inside the spinal cord (on the left) and of the encephalon (on the right). The gray substance, in the spinal cord, occupies the central area and has the shape of an H (or a butterfly); in the encephalon, instead, it takes place in the cortex and in some internal areas.

In the marrow, the white substance surrounds the gray one; vice versa, in the encephalon it is surrounded by the latter.

The brain

The brain is the structure of the most complex central nervous system, as it is formed by different areas or regions.

In adult humans, it weighs up to 1.4 kg (about 2% of total body weight) and can contain 100 billion neurons (one billion corresponds to 1012). Therefore, the connections it can establish are many and unimaginable.

The main regions of the brain are four. Each of them has a specific anatomy, with specialized compartments in different functions. In order not to complicate this text too much, it was preferred to provide a summary table of the main encephalic areas (ie the encephalon) and their relative functions.

The only information that will be limited to exposing is the following. Twelve pairs of cranial nerves depart from the encephalon, for which, for identification purposes, the Roman numeration I to XII is used. Except for the I and II pair of nerves, which originate respectively in the telencephalon and in the diencephalic, the remaining twelve pairs are born in the brainstem.

REGION

FUNCTION

1) Telencephalon or brain proper

Cerebral cortex

Perception; movement and coordination of voluntary musculature

Cagli of the baseMovement
Limbic systemEmotion; memory; learning
2) Diencephalon

Talamo

Transit station for motor and sensory information

Hypothalamus and pituitary gland

Instinctive behaviors; secretion of various hormones

Epitalamo and epiphysisMelatonin secretion
3) CerebellumCoordination of the movement
4) Brain stem

Mid-brain

Eye movement; coordination of auditory and visual reflexes

BridgeTransition station between brain and cerebellum; breath control
Elongated MedullaControl of visceral functions
Lattice formationState of consciousness; sleep-wake cycles; muscle tone; pain modulation

The spinal cord

The spinal cord is a cylindrical-shaped structure, on average 45 centimeters long and housed inside a vertebral column canal (this, in general, measures 70 centimeters).

Figure: the marrow contained in the vertebral column.

The sections of the spine:

  • Cervical: 7 vertebrae
  • Dorsal (or thoracic): 12 vertebrae
  • Lumbar: 5 vertebrae
  • Sacral: 5 vertebrae
  • Coccigee: 4/5 vertebrae

Above, it starts from the medulla oblongata (structure of the brainstem); inferiorly, it ends between the second and third lumbar vertebra and reaches, with the last extensions, the sacral region.

The nerve structure of the spinal cord is quite complicated. To facilitate understanding, the neurons of the gray substance will be analyzed first, then those of the white substance.

NB: clearly, the length of the medulla and of the vertebral column depend on the height of an individual. A person who is 160 centimeters tall will certainly not have a marrow as long as a basketball player's other 2 meters. Nevertheless, anatomy and functions do not change.

gray substance

As with the brain, nerve pairs (exactly 31 pairs), called spinal nerves, also arise from the spinal cord. Spinal nerves are mixed nerves, so they have both motor and sensory fibers.

Spinal nerves bind to the spinal cord through so-called roots : there are the roots of motor fibers (or ventral roots ) and the roots of sensory fibers (or dorsal roots ). The ventral and dorsal terms are used according to where the roots are inserted: the belly of the medulla looks towards the belly of the individual, the back of the medulla looks towards the back.

Each type of fiber belongs to the gray substance, contained in the central area of ​​the marrow: the motor originates from an area called ventral horn ; the sensitive one, instead, comes from a portion called dorsal horn .

The figure is of considerable help in understanding what has just been described.

Spinal nerves are:

  • 8 cervical
  • 12 thoracic
  • 5 lumbar
  • 5 sacral
  • 1 coccigeo

white substance

The neurons, or rather the axons, of the white substance of the spinal cord form real columns . These columns, called bundles or tracts, can run from top to bottom (ie from the CNS to the periphery) and vice versa (ie from the periphery to the CNS): if they run downwards, they are called descending bundles ; if they run upwards, ascending beams are defined.

The ascending beams carry sensitive information.

The descending beams carry motor type signals.

Figure: spinal cord anatomy. Besides the elements described in the text, it is also possible to recognize the ganglion of the dorsal root and its content, that is the body of one of the sensory neurons. The ganglion is, as can be seen, a bulge, 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 MIDOLLO, A SIGNAL INTEGRATION CENTER

The spinal cord must be considered in all respects a center of integration of nervous-type signals, as it has the extraordinary ability, when it receives sensitive signals, to formulate an autonomous motor response, without turning to the brain. This response, which is thus more rapidly constructed, is called the spinal reflex .

All this confirms, once again, the many potentials of our central nervous system.