Tejido Nervioso PDF

# TEJIDO NERVIOSO ## 7.3 GENERALIDADES The nervous system is a set of organs, ganglia, and nerve fibers that are disseminated throughout the body and have the function of regulating the functioning of the rest of the organism. This is achieved due to the fact that its anatomical and functional uni...

# TEJIDO NERVIOSO ## 7.3 GENERALIDADES The nervous system is a set of organs, ganglia, and nerve fibers that are disseminated throughout the body and have the function of regulating the functioning of the rest of the organism. This is achieved due to the fact that its anatomical and functional unit at the microscopic level is constituted by very specialized cells called neurons; grouped together they constitute the nervous tissue. ## 7.3.1 CÉLULA NERVIOSA O NEURONA Each nerve cell or neuron consists of a central portion or cell body, which contains the nucleus and pericarion, which is the cytoplasm that surrounds the nucleus. In the structure of the neuron, two types of prolongations are also identified: dendrites and axon. - The dendrites are fairly short extensions of the neuronal body and are involved in the reception of stimuli. - As a contrast, the axon is usually a single and elongated prolongation, very important in the transmission of impulses from the region of the neuronal body to other cells. ## 7.3.2 TIPOS DE NEURONAS There are three varieties of neurons: - **UNIPOLAR NEURONS:** They only have one prolongation. They are found in the ganglion of the posterior root of the spinal nerve. - **PSEUDOUNIPOLAR NEURONS:** They are bipolar, but as they approach the neuronal body they merge until they form only one prolongation. - **BIPOLAR NEURONS:** They emit a prolongation from each end of the neuronal body. They are found in bipolar cells of the retina and the cells of the cochlear and vestibular sensory ganglion. - **MULTIPOLAR NEURONS:** They have a large number of dendrites in addition to the axon. They are the pyramidal neurons of Betz of the cerebral cortex, the bottle-shaped neurons of Purkinje of the cerebellum, and the stellate neurons of the anterior horn of the medulla. ## 7.3.3 NEUROGLÍA O GLÍA As its name suggests (neuron: nerve, glia: glue or cement) it is a tissue that keeps the nervous tissue together, as it is made up of supporting cells, which outnumber neurons, and the ependyma. The supporting cells are differentiated into astrocytes, oligodendrocytes, and microglia. a) **ASTROCYTES:** They are star-shaped cells (gr. astron: star) and are differentiated into two types: - **PROTOPLASMIC ASTROCYTES:** They are found mainly in the gray matter, with very branched prolongations that present podocytes that are in contact with the vessels. - **FIBROUS ASTROCYTES:** They have long and thin prolongations and are found mainly in the white matter. b) **OLIGODENDROCYTES:** (gr. oligos: few) These glial cells are so named because they have shorter and fewer prolongations than astrocytes. They form and maintain the myelin sheaths of the fibers of the central nervous system. c) **MICROGLÍA:** They are small cells, with a reduced nucleus and thin prolongations with fine spines. They are phagocytic cells that are part of the defense of the nervous system against infection and injury. The **EPENDYMA** constitutes a cubic epithelium that lines the cavities of the brain and the central canal of the spinal cord. ## 7.3.4 REVESTIMIENTO DE LAS FIBRAS NERVIOSAS A nerve fiber consists of an axon with its corresponding nerve sheaths. All peripheral axons are surrounded by a Schwann cell sheath. In the case of larger peripheral axons, the Schwann cells also develop a myelin layer, the myelin sheath. In this way, myelinated and unmyelinated nerve fibers are distinguished. In myelinated nerve fibers, a small opening called the node of Ranvier is distinguished between two adjacent myelinated segments. In contrast, the unmyelinated fibers are wrapped only by Schwann cells, which constitute the Schwann sheath. ## 7.3.5 GANGLIOS A ganglion is defined as a cluster of nerve cell bodies outside the central nervous system. They are classified into: - **SENSORY GANGLIA:** They are fusiform thickenings located at the posterior root of each spinal nerve and in the course of the cranial nerves V-VII-VIII-IX-X. - **AUTONOMIC GANGLIA:** They are irregularly shaped and located along the efferent nerve fibers of the autonomic nervous system. They are found in the paravertebral sympathetic chains, around the roots of the large visceral arteries in the abdomen, and near or within the walls of various viscera.