Nervous system

Anatomically we have:

Encephalon - that is the part of the nervous system that is inside of the cranial box and that divides in:

a) Brain, constituted by the cerebral cortex that comprises the gray substance, and the area subcortical, the diencephalon, of which makes part the hypothalamus that has a lot to do with our emotional reactions.

Hypothalamus - Area of the diencephalon, under the thalamus, that forms the floor and part of the lateral wall of the third ventricle. He covers the optical quiasma, the corpuscles mamilares, the tubercinereum, the infundibulum and the neuro-hypophysis. The nuclei of this area exercise control over the visceral activities, the balance hydric, the temperature, the sleep, the regulation of the metabolism of the carbohydrates and fats, as well as they govern the mechanisms of certain primitive reactions of defense or attack in the animals and they influence on the human emotional states - fear, angers etc.

b) Trunk encephalic, that is between the brain and the marrow, and the base of the trunk encephalic that makes the connection with the marrow receives the name of bulb.

c) Cerebellum, that is behind the bulb.

In the zoological scale, the more developed the animal; more compound is his nervous system. Unicellular organisms don't have nervous system. Alive waters, earthworms, insects, fish, amphibians, reptiles and mammals go on having, progressively, more complex nervous system until arriving to the man. Popularly, the word brain is used to identify the man's rational capacity, but actually, it is of the whole nervous system that depend the activities considered human.

The whole nervous system works starting from each one of the cells that constitute him, the neurons. Very specialized cells are capable to capture external incentives, as heat, cold, pain or irritability and of driving those incentives through the organism, under the form of nervous pulse. They are attributions accomplished by numerous neurons. As any other cell, possesses a nucleus and a cytoplasm. In this cytoplasm are the neurofibril and the corpuscles of Nissl  and the prolongations - on a side the dendrites and on the other side the axons that can have several meters in length and that establish contact with other nervous cell, muscle or gland. It is the axon, with their membranes, that will constitute a nervous fiber.

Starting from the nervous cells of the central axis, they are diffused by the whole body bunches of nervous fibers, that come to constitutes in the nerves. Some take pulses that leave from the central axis to the muscles, glands or viscera and that are the motor nerves or efferent. Others take pulses captured by the periphery, skin, muscles, viscera, until the central axis and they are the sensitive nerves or afferents.

To the long of those bunches there exist groupings of cells, the nervous ganglions, that work as stations relay of the pulses. Only the motor commands sent to the muscles don't pass for that stage, because they are going from the central axis to the muscles without passing by the ganglions.

In the structure of the nervous system, the neurons are clamped each other, as wagons of a train, forming chains neuronal. The clamp among a neuron and other, called synapse is done between the ending of an axon of a cell and the dendrite and cellular body of other. The direction of the impulse is from the cellular body to the axon. The synapse is responsible for the switch on or off a cell from another. The excessive operation causes wear of the synapses; it is what happens with the physical fatigue.

The high specialization of the brain permits the rational control and the training-oriented movements and actions that are driven through the marrow. The act of taking something, for example, is primitive reflex, but the man can train this reflex in order to guide the motor behavior for several uses - drawing, sculpture, machines control, games etc. Such use is controlled by the brain but the Centers for relaying orders of movement to the members are located in the marrow. Besides acting as relays to the orders issued by the brain and to the external stimuli to the centers of the encephalon, the marrow has also an autonomous function. In simpler pulses don’t have to pass through the supervision of the encephalon. For example, when the doctor uses the hammer to check the nervous reflex, the individual cannot hold the movement, because it is independent of the order of the brain.

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The woman of Galvani used to hang the legs of the frogs in a copper thread and Galvani was able, then, to observe that, when there were rays, the legs moved. Galvani noticed that the movement was provoked by the energy that traveled the nerve.

From Galeno, in 1.100, it was current the idea that the movement of the muscles was provoked for a type of spirit to which they gave the name of it "anima."

Today, it is known that the voluntary and involuntary corporal movements are executed as motor mechanisms stimulated by an electric power that runs through the nerves. The science knows that the senses emit electric potentials along the nerves until the brain, to where go all the information. This made to disappear the mysterious idea of the "anima."

In what consists the cerebral action? In the displacement of electric potentials along circuits formed by bunches of nervous fibers inside of the brain. Those fibers are ramifications of the gray mass, known as cortex. It is believed that there are 12 billion cells in the human brain. Each one of them is capable to produce a small amount of electric power that can intensify, to block or to alter the currents that pass and pass again.

 

(**) Franz Nissl (1860 - 1919) - German doctor, discoverer of the corpuscle of Nissl, substance chromophilia of the cellular body of the neuron.