What is the nervous system?

The nervous system, one of the most complex and important of our organism, is a set of organs and a network of nervous tissues whose basic unit are neurons. The neurons are arranged inside a framework with non-nervous cells, which together are called neuroglia.

The nervous system has three basic functions: the sensitive, the integrative and the motor.

The sensitive function allows it to react to stimuli coming from both inside the organism and from the outside environment.

Then, sensitive information is analyzed, some aspects of it are stored and decisions are made regarding the behavior to be followed; this is the integrating function.

Finally, it can respond to stimuli by initiating muscle contractions or glandular secretions; it is the motor function. To understand its functionality, the nervous system as a whole can be subdivided into two systems: the central nervous system (CNS) and the peripheral nervous system (SNP).

The CNS is connected to the sensory receptors, the muscles and the glands of the peripheral areas of the organism through the SNP.

The latter is formed by the cranial nerves, which are born in the brain and the spinal or medullary nerves, which are born in the spinal cord. A part of these nerves carries nerve impulses to the CNS, while other parts carry the impulses that leave the CNS.

Fuente

Neurons

The nervous system is a complex network of interconnected excitable cells. These cells, called neurons, are capable of transmitting an electrochemical signal when stimulated properly. Normally, neurons have a slightly negative electrical potential (relative to the exterior) inside them and are momentarily reserved during the transmission of the nerve impulse.

How does the nervous system work?

The nervous system works by sending electrical impulses through a specialized group of cells, called neurons. Neurons or nerve cells are the priority functioning units of the nervous system.

The nervous system of the human body comprises the brain, the spinal cord and the nerves connected to the brain and the spinal cord with other parts of the body. The work of the nervous system focuses on the property of the electrical excitation of neurons and on the interconnections with other neurons.

Source

The activation of a neuronal cell and the nervous impulse

The activation of a neuronal cell and the generation of a nerve impulse that moves away from the point where it occurs, is called an action potential. As I mentioned, the inside of the neuron is slightly electrically negative. In addition, it is deficient in sodium ions (positively charged) and, to some extent, rich in potassium ions (positively charged as well).

A nerve impulse is an action potential that propagates along the neuron, depending on the opening of the sodium and potassium channels.
Source

Normally, the membrane of the neuronal cell is not permeable to these sodium and potassium ions. However, there are some specialized channels that open up during the action potential, allowing the movement of these ions. When an action potential is initiated, sodium channels are first opened, which causes the entry of sodium into the neuronal cell, turning inside the positive neuron, relatively. Afterwards, the potassium channels are opened so that the potassium from inside the neuron comes out, turning its negative interior.

Synapse - The connection between neurons

The neurons are connected to each other by specialized unions called synapses. The electrical impulse that travels in one neuron is transferred to another by means of a synapse. The transfer of a signal usually involves the release of specialized chemicals, called neurotransmitters.

During the synapse, neurotransmitters that bind to the second neurons are released in order to propagate the information.
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When an electrical impulse reaches the end of a neuron, it causes the release of a neurotransmitter in the space between the two neurons. The released neurotransmitter binds to the receptors of the second neurons and initiates a cascade of reactions responsible for generating a new electrical impulse, which spreads even further.

Some synapses do not need a neurotransmitter. In these synapses, the nerve terminals of the two neurons are located very close to each other, so that the electrical impulse can simply "jump".

Nerves and muscles: the neuromuscular junction

Some nerves carry the signs of muscle contraction. The point of connection of the nerve and muscle is known as the neuromuscular junction. The transmission signal between the nerves and the muscles is similar to the synapse.

When the electrical signal that is traveling on the nerve fiber reaches the last portion of the nerve (near the muscle), it causes the release of a chemical neurotransmitter (acetylcholine). Acetylcholine binds to receptors in muscles and then initiates a chain of reactions that ultimately generates muscle contraction. The nerve endings are not physically connected to the muscles.

Peripheral nervous system

The brain and spinal cord constitute the central nervous system. The brain and spinal cord are connected to the other parts of the body (muscles, organs, etc.) by the nerves. These nerves, together with their neuronal bodies aggregated in the ganglia, located outside the spinal cord, form the peripheral nervous system. The peripheral nervous system is divided into two specialized systems, the somatic nervous system and the autonomic nervous system.

The somatic nervous system carries the voluntary nerve impulses, causing the contraction of the skeletal muscles. Therefore, this division of the nervous system controls the movements of the body. The nerve fibers of the somatic nervous system continue in the spinal nerves and cranial nerves.
The autonomic nervous system or visceral nervous system is the second major subdivision of the peripheral nervous system. It is also known as the involuntary nervous system because it is not under the conscious control of the brain. In some special cases (such as breathing), the functioning of the automatic nervous system can be altered slightly by conscious control.
The autonomic nervous system has control over a wide variety of functions such as heart rate, respiration, sexual activation, salivation, perspiration, the size of the pupils, etc. In addition, it is subdivided into two subsystems: the parasympathetic nervous system and the sympathetic nervous system.

Sensations - General and special senses

We all know the five senses of the human being. These are hearing, taste, smell, touch and vision. However, the human body has many more types of sensations, each with its specific receptors and nerves. Vision, hearing, taste and smell are considered special senses, and their receptors are much more complex when compared to other sensory modalities. Other important sensations that the human body possesses are:

Pain
Balance
Propioception (The sense of position - you know where your hands are without your eyes looking at them)
Muscle stretch receptors
Temperature

Apart from these, there is a great variety of sensory receptors in the human body that feel a wide variety of physiological parameters, for example, blood pressure (baroreceptors), carbon dioxide and oxygen in the blood (chemoreceptors), stretching pulmonary (pulmonary stretch receptors), etc.

The functioning of all sensory modalities is organized along the same general line. That is, a receiver capable of feeling a physiological parameter, is linked to a sensory nerve that carries information to the central nervous system. The information is processed in the central nervous system, generating an adequate response (if necessary) to the nerves.

Source:
http://www.profesorenlinea.cl/Ciencias/Sistema_nervioso/Sistema_nervioso.html
http://comofuncionaque.com/funciones-del-sistema-nervioso/