MAIN FEATURES OF THE NERVOUS TISSUE
Neurons are the most important cells of the nervous tissue, as they release instructions to other cells of the body so that they works collectively in an organized way.
Neurons act in several ways, mainly through electrical signals transmitted along the cell membranes of their extensions. Chemical substances or electrical stimuli from other cells generate an instantaneous reversion of the electrical polarity an instantaneous reversion of the electrical polarity of the cell membrane of a dendrite or of neuron cell body. This reversion that may be transmitted along the cell mmbrane of an axon is called action potential (a “nerve impulse”). The action potential is transmitted up to a synapse, a place of very close proximity between the membranes of a neuron and of another cell (a dendrite, a muscular cell, a secretory cell). At the synape the action potential may be transmitted by way of chemical mediators or by ions that are caught by the next cell and result mostly in the transmission of another action potential by the next cell, the release of secretory products by a secretory cell or by the contraction of a muscle cell.
Main cell components of the nervous system
1- Neurons are formed by a cell body and its extensions. The nucleus and a large part of the cytoplasm surrounding it are located in the cell body. Neurons have two kinds of extensions: dentrites and axons. Dendrites are generally shorter and may show many branches at their first part than when they reach the neuron cell bodies. They as well as the cell body receive nerve impulses from other cells. Axons transmit nerve impulses from the cell body. They have a rather constant thickness and branch mainly at their end.
2- Neuroglial cells also called glial cells constitute a population of several cell types found in the central nervous tissue where they execute several important functions, such as supporting and nourishing neurons, isolating neurons, phagocytosis, repair of the nervous tissue.
Nerve Fibers
Axons are always surrounded by other nervous tissue cells. The association of an axon with its surrounding cells constiture a nerve fiber. As axons are generally quite long, many cells are needed to cover their entire length. Axons of the central nervous system are covered by a neuroglial cell called oligodendrocyte. In the peripheral nervous system, this function is performed by Schwann cells. Both types of cells may produce a material derived from their cell membrane called myelin, formed mainly by complex lipid molecules. Nerve fibers whose outer covering cells contain myelin are called myelinated nerve fibers, as opposed to unmyelinated or nonmyelinated nerve fibers that lack myelin.
Regarding its distribution in the body, the nervous tissue is divided into:
1 – Central nervous system, formed by two continuous portions, the brain and cerebellum are placed in the skull. The spinal cord is an extension of the brain placed inside the vertebral canal. The central nervous system holds neuron cell bodies and their extensions plus neuroglial cells and blood vessels. The brain neurons are concentrated in the cerebral cortex and in clusters of varying sizes called cerebral or brain nuclei. The areas heavily occupied by neurons and neuroglial cells constitute the gray matter whereas the white area is formed mainly by neural cell extensions together with neuroglial cells plus blood vessels.
2 – Peripheral nervous system, distributed throughout the body in the form of:
– nerves, composed of bundles of nerve fibers.
– nerve ganglia, clusters of neuronal cell bodies.
The organs of the central nervous system are covered by connective tissue membranes called meninges. There is very few connective tissue inside the central nervous system as opposed to the peripheral nervous system which contains a larger amount of this tissue.
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