MAIN CHARACTERISTICS OF THE NERVOUS TISSUE
The main characteristic of nervous tissue is the presence of the neurons. These are cells with extensions that have the ability to be stimulated by chemical substances or electrical stimuli, usually originating from other cells, and in turn, generate an action potential (“nerve impulse”). The action potential is transmitted along the plasma membrane that covers its extensions up to the synapses. At the synapses, the action potential can be transmitted to other cells.
Cells of the nervous system
1- Neurons. Neurons consist of a cell body, also called the perikaryon and extensions of the cell body. The nucleus and a portion of cytoplasm surrounding it are located in the cell body.
There are two types of extensions in neurons. Dendrites are generally short and highly branched at its ends. The axon, usually single, has a constant thickness and branches mainly at its end.
Synapses are points of close proximity between the neuron membrane and the surface of another cell. An action potential that reaches a synapse can trigger the release of chemical mediators and stimulate the other cell – another nerve cell or a cell of another tissue (such as a muscle cell or a secretory cell).
2- Neuroglial cells. In addition to neurons, nervous tissue has another population of cells, called glial cells or neuroglia. There are several types of neuroglial cells, which perform important functions in the nervous tissue, such as supporting and nourishing neurons, isolating neurons, phagocytosis, and repairing nervous tissue.
Nerve Fibers
Axons are always surrounded by other cells of nervous tissue. The combination of axon and their surrounding cels is called a nerve fiber. As axons are generally quite long, many cells are needed to cover their entire length.
In the central nervous system, the covering is made by a type of neuroglial cell called an oligodendrocyte. In the peripheral nervous system, this function is performed by cells called Schwann cells.
The cells surrounding the axons can produce a material called myelin, composed of complex lipid molecules and other types of molecules. Myelin is derived from the cell membranes of the surrounding cells. Nerve fibers whose outer covering cells contain myelin are called myelinated nerve fibers, as opposed to unmyelinated nerve fibers whose outer covering cells do not contain myelin.
Location of the nervous system
Regarding its location, nervous tissue can be classified as:
1 – Central nervous system, formed by two continuous portions, one located inside the cranial cavity, the brain, and the other constituting the spinal cord. In the central nervous system there are cell bodies and extensions of neurons in addition to many neuroglial cells. The neurons of the brain are concentrated in the cerebral cortex and in clusters of varying sizes called nuclei.
2 – Peripheral nervous system, is distributed throughout the body in the form of:
– nerves, which are bundles of nerve fibers.
– nerve ganglia, groups of neuronal cell bodies located outside the central nervous system.
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 conteins a greater amount of connective tissue.
The organs and structures of the .
Organization of nervous tissue in the central nervous system: white and gray matter
In the central nervous system, nervous tissue is organized in such a way that the cell bodies of neurons are concentrated in certain regions. These regions constitute the gray matter of the central nervous system.
The extensions of neurons (mainly their axons) are also organized and constitute regions called white matter. In the white matter, the axons form bundles of different calibers.