Structure of the nervous system

The nervous system of animals in general and humans, in particular, performs two essential functions:

- Alone, he perceives the stimuli (plural of “stimulus ") of the environment, due to the senses of sight, hearing, taste, smell and touch in humans, and transmits this information to the centres that interpret them.

- Along with the much slower endocrine system, the nervous system sends information through the body very quickly, within a fraction of a second that controls muscle contraction or glandular secretions.

General organization of the nervous system

Anatomically, the nervous system is made up of two distinct but interconnected parts:

- The central nervous system (= CNS ), formed by the brain, enclosed in the skull and itself composed of the brain, brainstem and cerebellum, and its continuation in the spine, the spinal cord, is the centre where the information arrives, where the data is processed and where the control nerve impulses originate. It is, therefore, the party that perceives and decides. The CNS is surrounded by three membranes, the meninges, the outermost of which is the dura.

- The peripheral nervous system (= SNP) is made up of nerves that start from the CNS, branch out and connect it to sensory organs, muscles and glands. We distinguish:

- the cranial nerves which are connected to the brain;

- The spinal nerves which are connected to the spinal cord.

Functionally, the nervous system has two parts:

- The somatic nervous system, which, thanks to efferent nerves (which leave the CNS to the organs, unlike afferent nerves, which arrive at the CNS and bring sensory information there), voluntarily control the striated muscles. It is the nervous system controlled by the will.

- The autonomic or vegetative nervous system, which escapes the will and, thanks to efferent nerves, controls the smooth muscles (visceral muscles), the muscle of the heart and the glands. It is the involuntary nervous system. As the autonomic nervous system must ensure the homeostasis of the organism, that is to say, the stability of the internal conditions of our body in spite of the numerous disturbances, it is itself made up of two antagonistic control systems:

- the (ortho) sympathetic nervous system (whose nerves are connected by adrenergic synapses) which manages emergency reactions by preparing it for exertion, fight or flight (by accelerating the pulse, dilating the pupils, activating sweating, stimulating respiration and blood circulation, releasing fast sugars from the glycogen stores of the liver, etc.);

- The parasympathetic nervous system (whose nerves are linked by cholinergic synapses) which allows the conservation of energy and the normal accomplishment of physiological functions (by slowing the pulse after exertion, contacting the pupils, calming breathing and blood circulation. It also activates digestion and excretion by relaxing the sphincters and activating digestive secretions). The viscera receive nerves from these two systems, one stimulating the activity of the organ, the other inhibiting it.

Use of the cerebral cortex

The cortical surface or surface of the cortex of the brain contains the bodies of sensory and motor neurons that allow the perception of stimuli and the control of voluntary movements. The left side of the brain perceives and controls the right side of the body and vice versa.

But the surface area of ​​the cerebral cortex is proportional not to the size of the organ, but to the sensory sensitivity or motor dexterity of the innervated organ. Thus, the fraction of the brain occupied to perceive manual stimulation or control the movements of the hand is clearly greater than that devoted to the thorax, although the latter is larger than a hand.

To this end, we can redesign homunculus sensory or motor (or = Homunculus Homunculus) whose proportions betray the importance of consecrated surface to each organ. Careful observation of these representations is interesting.

The central nervous system

The nervous system is made up of two parts:

- The central nervous system, constituted by the encephalon comprising the brain, the brainstem, and the cerebellum located in the cranial box, and the spinal cord located in the spinal canal. Its role is to receive the record, interpret the signals arriving from the periphery, and organize the response to be sent. The peripheral nervous system made up of the cranial nerves and the spinal nerves, which are attached to the central nervous system. Its role is to convey information from peripheral receptors for sensitivity or pain to the central nervous system and to transmit motor commands emitted by the nervous centres.

1. The entire brain is located in the cranial box. It is made up of:

- The brain, situated entirely in the supratentorial space, and formed of two right and left hemispheres, incompletely separated from each other by the interhemispheric fissure marked by the scythe of the brain and united one to the 'other to their central part.

- The brainstem, which emerges from the underside of the brain, and has three parts from top to bottom: the right and left cerebral peduncles, the annular protuberance, and the medulla oblongata. From the brainstem emerge all the cranial nerves except the optic nerve and the olfactory nerve located entirely above the tent of the cerebellum.

- The cerebellum located like the brainstem in the posterior fossa and therefore separated from the brain by the tent of the cerebellum. It is formed by two right and left hemispheres, united by the vermis. They are connected to the brainstem on the right as well as on the left by the upper, middle, and lower cerebellar peduncles.

2. The spinal cord extends the brainstem and medulla oblongata. It begins immediately below the foramen magnum. It is located entirely in the spinal canal that it does not occupy over its entire height because the marrow ends approximately at the level of the first lumbar vertebra (L1). From the marrow and at each intervertebral space emerge, the spinal nerves made up of an anterior motor root and a posterior sensitive root.

Below L1 and up to the sacrum, the spinal canal is occupied by the roots of the spinal nerves from the lumbar cord; all of these roots form what is called by resemblance "the ponytail".

3. Gray matter and white matter

At whatever level, the central nervous system is made up of two different parts characterized by their color: the gray matter and the white matter. In the brain, we describe

- A layer of gray matter covering all the hemispheres whose grooves it follows: this is the cerebral cortex or cerebral cortex.

- A layer of the white matter immediately below the gray bark.

- A more complex central zone where one distinguishes a part of white matter which are the commissures uniting the two hemispheres, and a part of clusters of gray matter, the central gray nuclei.

In the brainstem, the gray matter predominates (reticulated substance) and occurs in clusters which are the original nuclei of the cranial nerves. In the cerebellum, gray matter occupies the cerebellar bark or cerebellar cortex, under which white matter and basal ganglia are found. At the level of the spinal cord, the gray matter forms the center, present throughout the height of the spinal cord, drawing an H or a butterfly shape on a cross-section.

Cellular Elements of the Nervous System

The gray matter is made up of cells; the white matter is made up of fibers. The cell and the fiber are just two parts of the same essential elements of the nervous system: the neuron. The neuron is not the only cell present in the nervous system.

Next to it, there is a supporting tissue and nourishing cells: the glial cells.

1. The neuron

The neuron is an anatomically and physiologically specialized cell in the reception, integration and transmission of information. This complex role is worth it to be an ordinary cell in the constitution of its membrane, its nucleus, its organelles, and a singular cell, excitable and secreting, adapted to the tasks of formation, maintenance and functioning of networks. Indeed, isolated neuron does not exist. Each of them is integrated into multiple, ordered and hierarchical networks responsible for receiving or transmitting a signal, or for coordinating a complex function.

Nerve transmission takes place through several neurons which are related to each other by their dendrites or by the articulation of an axon with the dendrites of one or more neighboring cells. The junction between the elements of two cells constitutes a synapse.

The axon does not reproduce, despite the importance of its enzymatic activity and its chromosomal stock. Its activity is entirely dedicated to its functions as receiver and transmitter. It consists of a cell body, dendrites, an axon, synapses and a cytoskeleton.

The cell body

The cell body of the neuron varies in shape and size. Rounded or oval, sometimes triangular or pyramidal, it can measure from 5 to 120 microns in diameter. It has a single, clear, central, well-defined nucleus and the cytoplasm which contains the elements common to all cells. The cytoplasm is rich in organelles: it contains many Nissl bodies, a characteristic basophilic substance that witnesses enzymatic activity, the Golgi apparatus, mitochondria and many elements of the cytoskeleton (microfilaments, neurofilaments, microtubules).

B. dendrites

These are short, branched, numerous extensions that stretch out like antennae from the cell body. This arborization thus offers a larger contact surface between the nerve cells.

Axon

The axon, usually single, is the longest extension of the neuron. It ends with numerous ramifications comparable to dendrites, the terminal buttons. The branching model of the axon and the dendrites is very diverse. The multiplicity of these dendritic endings means that an axon can receive up to 100,000 entries.

The axon is made up of an envelope, the axolemma and a cytoplasm called the axoplasm.

- Extension of the cell cytoplasm, axoplasm differs from it in the absence of Nissl body and Golgi apparatus. The organelles and elements of the cytoskeleton line up here longitudinally, parallel to the axis of the axon. Essential fact, the axoplasm cannot ensure the synthesis and the renewal of the macromolecules which constitute it. They come from the cell body and are provided through axonal transport.

- The axolemma is the extension of the cell membrane. It is covered with a sheath. There are two kinds of sheaths which make it possible to differentiate myelin fibres from amyelin fibers. In the myelin fibers, the axolemma is covered with coiling of myelin, and this sheath presents in place of the constrictions called nodes of Ranvier.

- This myelin sheath is itself surrounded by Schwann cells, the source of myelin. Myelin gives the fibers it covers a whitish appearance. Myelin fibers make up the white matter of brain tissue. Myelin fibers or bare fibers, obviously smaller in diameter, are covered directly by Schwann cells.

- Axonal transport occurs in both directions, from the cell body to the end of the axon and vice versa. It can be fast (transport of information) or slow (transport of substances). The axon's path varies in length, from a few millimeters (like the interneurons in the spinal cord which provide a short-distance interconnection) to over a meter (for those intended for the muscles of the lower limbs).

D. synapses

Synapses are specialized contact areas between neurons or between the neuron and its effector site (example: the neuromuscular junction). They ensure the transfer of signals between cells. We distinguish:

- Electrical synapses, in direct contact with each other, and which allow rapid propagation of electrical signals between two cells. They are rare in humans.

- Chemical synapses that use a chemical messenger (neurotransmitter) to transmit information. There is a wide space of 20 to 30 nanometers between the two cellular elements: this is the synaptic cleft. It is crossed by the neurotransmitter released by the presynaptic element which transmits the signal to the postsynaptic element.

- Mixed synapses, which combine a chemical synapse and an electrical synapse.

We speak of the synaptic complex to designate the presynaptic element, the synaptic cleft, and the postsynaptic element. Synaptic vesicles are neurotransmitter storage organelles that can be released into the synaptic cleft. Opposite, the postsynaptic membrane is composed of protein structures serving as an anchor point for the postsynaptic receptors. There is a great diversity of synapses between axons and dendrites, between axons, or between dendrites between them.

e. the cytoskeleton

The elements of the neuron's cytoskeleton (microfilaments, microtubules, and intermediate filaments) are more abundant than in other cells of the body. The neuronal cytoskeleton determines and maintains the morphology of the neuron, and plays a role in neurogenesis and synaptogenesis: it ensures the transfer of macromolecules between the cytoplasm and neuronal extensions (axons or dendrites), and, at the synaptic level, it participates in neurotransmitter release process because it allows the attachment of membrane receptors.

Glial cells Glial

Cells are the supporting tissue of the nervous system. They provide the link with the blood vessels and provide nutrients essential for the metabolic functioning of the nervous system. Unlike neuronal cells, glial cells can multiply or even proliferate and become cancerous. There are several types of glial cells: astrocytes, oligodendrocytes, microglia, and ependymal cells.

Astrocytes

These are the most numerous cells in the brain. True support tissue, they provide metabolic support and the synthesis of the main constituents of the nervous system. They have no direct role in the transmission of nerve impulses. They have a stellate, branched appearance around a large cell body. These extensions are of varied shape and short. They ensure intercellular contacts. Particularity, some of their extensions are in direct contact with the basement membrane of the vessels which they completely surround. The extracellular space between the astrocytes is important, except at the level of the foot of the astrocytes which are in contact with the blood vessels. Here the intercellular junctions are tight, not allowing the presence or circulation of extravascular extracellular fluid. Astrocytes here constitute an anatomical barrier opposing the penetration of intravascular fluids and substrates. They are the constituents of the blood-brain barrier.

b. Oligodendrocytes

These are cells that are smaller and fewer in number than astrocytes. Their main role is the development of the myelin which surrounds the axons. In peripheral nerves, Schwann sheath cells are analogous to oligodendrocytes. The microglia is formed of small cells with scanty cytoplasm. They have phagocytosis properties.

d. Ependymal cells

These are columnar or cubic cells with a bulky nucleus that cover and line the ventricular cavities of the brain and the central duct of the spinal cord. Their free edge looks like a brush. They play an important role in the exchange between cerebrospinal fluid and the cerebral parenchyma.

Author: Vicki Lezama