I – Organization of the nervous system:
It is divided into two main parts: The central nervous system (CNS), comprising the brain and spinal cord and peripheral nervous system (PNS), comprising parts nervous outside the CNS.

• The CNS is the central regulator and integrator of sensory information, from which it develops appropriate motor responses.
• The SNP consists mainly of the nerves, whose main function is to enable the transmission of information and the return of the response prepared by the CNS. Functionally, there are two types of channels: the sensory (or afferent), composed of nerve fibers carrying nerve impulses, and motor or efferent pathways, carrying the response of the CNS. The motor pathways are also divided into the somatic nervous system, also called the voluntary nervous system, because it allows the use of skeletal muscles consciously, and autonomic nervous system, which regulates unconscious (hence independent) activity smooth muscle (heart, digestive system ….). This system also faces two subdivisions, the sympathetic nervous system and parasympathetic, each opposing the other.

II – Histology:
The nervous tissue is very rich in cells, and contains little extracellular space. It is composed of two types of cells, neurons and glial cells.


The glial cells are not excitable cells surrounding neurons and protecting them. They form the backbone of the nervous system and give a good portion of firmness. Distinguish between glial cells, whose function is to support, nutrition and isolation of neurons. There are five types. Unlike neurons, glial cells retain their mitotic capacity, this explaining that the majority of brain tumors are gliomas.
Schwann cells: they form the myelin sheaths surrounding the axons of neurons in the peripheral nervous system
Oligodendrocytes: like Schwann cells, they form sheaths of myelinated, but along the thick axons of neurons in the CNS.
Astrocytes: star-shaped, hence their name, represent more than half the volume of the SN. They attach with their extensions to neurons and capillaries, and thereby promote their nutrition. They also govern the chemical environment, buffered potassium and reuptake and recycling certain neurotransmitters.


The microglial cells: macrophages are individuals who contribute to the protection of the CNS in phagocytosis of pathogens and dead cells
The ependymal cells: they line the cavities of the brain and spinal cord, forming a barrier between the CSF and interstitial fluid.
The neurons are the functional units of the nervous system. They have extreme longevity (> 100 years) are amitotiques, which means that a neuron destruction will never be replaced, and offer an exceptional rate of metabolism, which explains why they can not live more than a few minutes without oxygen and glucose. The cells are complex, with three functional areas, an area receiving a conducting zone and a zone secretory.
A – Structure of the neuron:
The cell body or soma, is composed of a large nucleus and granular cytoplasm. It contains all the usual organelles except centrioles, related to its structure amitotique. Its rough endoplasmic reticulum or Nissl bodies and Golgi apparatus are well developed, indicating a very intense activity.
There follow numerous cytoplasmic extensions neural begin in the perikarya. There are two types, dendrites and axons.
The dendrites are relatively short extensions to the many ramifications, so many. They form the structure receiving, receiving very large numbers of signals through the area they cover. Their points of contact, the synapses, are sites of chemical transmission (neurotransmitters) and appliances. They then carry this information to the cell body by short-range signals, potential graduates.
The axon is a unique cytoplasmic extension, from a conical region of the cell body, the axon hillock. Their size can be very short, as extremely long, like the axons of motor neurons of the toes, more one meter, making the axon of the cell the longest in the human body. The axon may form some ramifications, called collaterals. It ends with a bunch of very many short branches, the terminal arborization, including the bulbous tip is called the terminal button. The axons are conducting structure of the neuron, generating nerve impulses and propagated to the target, the axon terminal. In these buttons, the nerve impulse causes the release of neurotransmitters, chemical substances stored in vesicles of endings. These neurotransmitters are released into the extracellular space, and excite or inhibit neurons with which they come into contact.
The myelin sheath: the long axons and / or large diameter, are coated with a substance, lipid and segmented, the myelin sheath. This sheath electrically insulates axons from each other, but also increases the speed of electric transport. The sheath is composed of a large number of cells, called Schwann cells, not touching. These intervals are called nodes of Ranvier.

B – Neuronal Classification:
It is based on the number of extensions of the neuron. The neuron is thus unipolar, bipolar or multipolar. The multipole are most common in humans, particularly in the CNS. The Bipolar are quite rare, rather they are found, indeed, in some sense organs, such as eye or olfactory mucosa. The unipolar neurons when their part, are found primarily in the lymph.
It also classifies the neuron according to its function:

• sensory neurons, or related, which are mostly unipolar stimuli related to the CNS
• Motor neurons, or efferent, carrying impulses from the CNS to the PNS. They are mostly multipolar and form synapses with target cells.
• Finally, also speak of interneurons, located between the sensory and motor

III – Physiology:
The human body is a holistic perspective, electrically neutral. From a microscopic point of view, including the neural level, there are energy flows. This energy is expressed in volts or millivolts. This measure represents the potential difference between two different point of loads. Basically, to distinguish the voltage of the amperage, you can make the connection with a waterfall. The height of the waterfall, shown between the drop point and end point represents voltage, while its speed is the amperage. Thus, we can have a very low voltage with a large amperage or conversely, a large voltage with low amperage. The product of two represents a power expressed in watts.

I – The resting membrane potential:
At rest, if measure the potential difference between the neuronal cytoplasm and membrane, then records a potential difference of-70mV. The membrane is thus negatively charged relative to the cytoplasm. It is the resting potential (Vm). The membrane is called “biased”. This potential is created by differences in ion concentrations: the cytoplasm, for example, is rich in potassium and low in sodium, in contrast to the extracellular fluid. This difference is maintained through ion channels, passive or active. This causes a concentration gradient, whose translation is the electric potential of resting membrane.
II – Action potential:
Neurons communicate with each other through action potentials along their axons: it is a reversal of the membrane potential of about 100mV (the potential goes thus-70mV to +30 mV). These potentials are identical (their intensity does not decrease) along the axon. Its production is based on several changes occurring in its area “trigger” of the neuron:
there is first an increase in permeability to Na ions, due to the opening of sodium channels, voltage-dependent response to stimuli. The extracellular Na diffuses into the cell. This influx depolarizes the axonal membrane to a critical threshold, the excitation threshold, approximately-55mV. At this threshold, the process s’entretien itself, below, it turns off. Other voltage-dependent sodium channels open, so too, until a depolarization reaching 30 mV
Then, a phase of decreased permeability to Na settled: the intracellular load causes a natural resistance at the threshold of 0 mV increased (repulsion of electric charges of same sign), the channels close after a few milliseconds of depolarization. The sodium diffusion decreases, then stops.
Meanwhile, voltage dependent potassium channels open, causing a release of K into the extracellular medium. It re polarization. Sometimes there is a small hyperpolarization due to higher input of K, which then exceed the resting potential of -70 mV.
These phenomena cause disparities ion over the initial balance. Indeed, there are therefore in excess of K and Na deficit in the cell. This equilibrium is soon restored by the actuation of pumps sodium and potassium correcting these changes.
It is important to understand that this will spread not only from obtaining the threshold. Thus, all stimuli will not create any potential actions, the law of all or nothing, and it is our only similarity with the laws of binary computing. Stimuli below causes a so-called subthreshold depolarization.

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