Nervous tissue

The nervous tissue is a specialized tissue whose functional unit is the neuron. Neurons have specialized receptors to perceive different types of stimuli, whether mechanical, chemical, thermal, etc., and translate them into nerve impulses that will lead them to the nerve centers. These impulses are successively propagated to other neurons for processing and transmission to the higher centers and to perceive sensations or initiate motor reactions.

In addition to neurons, nervous tissue contains many other cells that are collectively called glial cells, which neither receive nor transmit impulses, their mission is to support the main cell: the neuron.

Nervous tissue provides the integrated functioning of animal organisms. The cells responsible for this activity are neurons (which make up 20% of the system) and glia (80%), specialized in close communication with other cells of the same type. It is, then, a tissue that has cultivated Sensory Perception from Receptors, integrates them and causes an adapted reaction from the organism transmitting the information to the effectors (the main ones are the muscles and glands). The complexity of the nervous tissue of some organisms gives rise to extremely complex and varied reactions.

Nervous tissue differs from the hormonal system in that communication between cells is brief and very short-range, while hormones act on the entire organism.

Nervous tissue is distributed throughout the body and is organized into a system, the nervous system. Through a network of specialized cells, neurons, the nervous system perceives information from the internal and external environment through sensory receptors and decodes it to give an adequate response from the organism.

In vertebrates, the nervous system is epineural. It is subdivided into two parts: the central nervous system (CNS) and the peripheral nervous system (PNS). Microscopically made up of gray and white matter, the central nervous system includes the cerebrum (brain, brainstem, cerebellum) housed in the skull, and the spinal cord, housed in the spinal canal of the vertebral column. The peripheral nervous system includes the nerves that connect the different organs with the central nervous system, as well as the nerve relays, called nerve ganglia. The nervous system of the most primitive chordates is similar, except that the brain is absent, since the tissue structure of the head is indistinguishable from the rest of the neural tube.

In hyponeurians, the nervous system consists of a double chain of ventrally located ganglia from which motor and sensory nerves depart. Frequently, the anterior ganglia are fused into a brain that performs the more advanced functions of the nervous system. In some groups, such as molluscs, this structure is poorly visible due to evolution.

The third type of nervous tissue observed, present in the most primitive animals, is the formation of a diffuse tissue without identifiable structures within the other tissues. It is found in diploblastic animals (jellyfish, corals, etc.) and in primitive worms. In some animals, the neurons can be grouped in ganglia, but these do not seem to have any additional organization as in more evolved animals.

Cells of the nervous system

The cells of the nervous system are divided into two broad categories: neurons and supporting cells, also called glial cells or simply glia.

• Neuron: They have a diameter ranging from 5μm to 150μm are therefore one of the largest and smaller cells at the same time. The vast majority of neurons are made up of three parts: a single cellular body, multiple dendrites and a single axon. The cellular body, also called pericarion or soma, is the central portion of the cell in which the nucleus and the perinuclear cytoplasm are found. From the cellular body the dendrites are projected, specialized extensions to receive stimuli from the Zaccagnini apparatus, located near the rachid bulb.

It was previously believed that these were the only cells that did not reproduce and that when they died they could not be replenished; however, recently^{[citation needed]} it was shown that its regenerative capacity is extremely slow, but not null. Three types of neurons are recognized:

• Sensitive neurons: they receive the impulse originated in the receptor cells.
• Motor neurons: transmit the momentum received to the effecting organ.
• Connective or association neurons: link the activity of sensitive neurons and motors.

• Glyal cells: Non-nervous cells that protect and lead nutrients to cells. Glia means glue, it is a tissue that forms the substance of nerve centers. It is composed of a fine network that includes highly branched special cells. It is divided into:

• Central Glia: It is in the CNS (encephalo and marrow):

• Astrocytes.
• Oligodendrocitos.
• Microgy.
• Ependy cells.

• Glia Periférica: It is in the SNP (nervous geglios, nerves and nerve endings):

• Schwann cells.
• Capsular cells.
• Cells of Müller.

Neuroglia

One of the purposes of these cells was to hold neurons together and in place according to Virchow. It is now known that it is one of several functions. Microglia are small cells with an elongated nucleus and short, irregular processes that have phagocytic capacity. They originate in bone marrow precursors and reach the nervous system through the blood; They represent the mononuclear phagocytic system in the central nervous system.

They contain lysosomes and residual bodies. It is generally classified as a neuroglia cell. They present the common leukocyte antigen and the class II histocompatibility antigen, typical of antigen-presenting cells.

Nerve tissue regeneration

The regenerative capacity of nerve tissue is very limited compared to other tissues, mainly because the nerve cells are no longer capable of dividing.

In early embryonic development, the anlage of the nervous system is, for some time, the region with the highest rate of cell division, and fetally, at the height of the human being, several thousand young neurons are produced per second. But these neurons are no longer capable of cell division afterwards, they are postmitotic. And not all of them live as long as the organ of the organism in whose tissue they seek their place (see selective apoptosis).

In the adult brain, undifferentiated neuronal precursor cells remain in only a few regions, which can continue to divide and are capable of forming neuroblasts and young neurons (see adult neurogenesis). In humans, for example, young neurons can also form alongside glial cells, for example in regions of the hippocampus or in the subventricular zone to replace neurons in the olfactory bulb and olfactory mucosa. To do this, these young neurons have to migrate to that region of the brain and find a place (with chemotaxis or haptotaxis). Haptotaxis), extending extensions (axogenesis), forming transmission sites (Synaptogenesis), establishing contacts in the found network of other neurons, receiving signals and sending signals, finally also those with which the state of excitation of some other cells can be changed individual (Excitation or Inhibition).

On the way and in the process on the way, a neuron differentiates - to take a position in a cellular environment with specific connections. If it doesn't, the neuron doesn't survive long. If it succeeds, the neuron occupies a special place in the neural network, and can only be replaced in this place by young neurons that follow a similar differentiation process. But these cannot be formed from mature neurons by cell division. For this, the neurons would have to be rounded, the processes would regress, they would lose their contacts and, therefore, they would stop being functional. Therefore, the replacement of differentiated and functional neurons within a neural network is limited by the complexity of the neural connections.

In the Peripheral Nervous System, on the other hand, after damaging a nerve fiber, the extension of a neuron can regrow in the spinal sheath canal as an axon - if still present - at about the same rate that grows hair