Nervous tissue (neural tissue)

Nervous tissue allow the propagation of electrochemical signals in the form of nerve impulses that communicate between different regions of the body. Nervous tissue is formed by neurons (excitable cells) and non-excitable cells, which together constitute the neuroglia (see: glial cells) with functions of support, nutrition, facilitation in the conduction of the nerve impulse. Neurons are cells with very variable shape and size. They present a cell body and extensions of different types. The cell body or pyrenophore or soma or pericarion, is generally large, the nucleus in large neurons is euchromatic, with nucleolus evident. The prolongations are divided into orthophytes (dendrites and axons) and paraphytes (the latter present only in special cases).

Generally the axon (or neurite) is single and departs from the pyrenophore with a cone of emergence (a small cone that narrows abruptly) and then continues without varying in caliber; the axon can also be very long and generally branches only at the end; any collateral branches detach at right angles at the nodes of Ranvier.

The dendrites are generally multiple and emerge from various points of the pyrenophore, are shorter than the axon and branch repeatedly thinning progressively; their surface has the characteristic dendritic spines (synapses). In large neurons, with a very long axon and without dendrites (e.g. spinal ganglion cells of vertebrates of large body size) there is another type of extension, the paraphytes, which serve to increase the surface of the neuron by promoting the nutritional exchange.

Nervous tissue is found in the brain, spinal cord, and nerves. It is responsible for coordinating and controlling many body activities. It stimulates muscle contraction, creates an awareness of the environment, and plays a major role in emotions, memory, and reasoning. To do all these things, cells in nervous tissue need to be able to communicate with each other by way of electrical nerve impulses. The cells in nervous tissue that generate and conduct impulses are called neurons or nerve cells. These cells have three principal parts: the dendrites, the cell body, and one axon. The main part of the cell, the part that carries on the general functions, is the cell body. Dendrites are extensions, or processes, of the cytoplasm that carry impulses to the cell body. An extension or process called an axon carries impulses away from the cell body.

Nervous tissue also includes cells that do not transmit impulses, but instead support the activities of the neurons. These are the glial cells (neuroglial cells), together termed the neuroglia. Supporting, or glia, cells bind neurons together and insulate the neurons. Some are phagocytic and protect against bacterial invasion, while others provide nutrients by binding blood vessels to the neurons.

Disorders of the nervous tissue

Several diseases can result from the demyelination of axons. The causes of these diseases are not the same; some have genetic causes, some are caused by pathogens, and others are the result of autoimmune disorders. Though the causes are varied, the results are largely similar. The myelin insulation of axons is compromised, making electrical signaling slower.

Multiple sclerosis (MS) is one such disease. It is an example of an autoimmune disease. The antibodies produced by lymphocytes (a type of white blood cell) mark myelin as something that should not be in the body. This causes inflammation and the destruction of the myelin in the central nervous system. As the insulation around the axons is destroyed by the disease, scarring becomes obvious. This is where the name of the disease comes from; sclerosis means hardening of tissue, which is what a scar is. Multiple scars are found in the white matter of the brain and spinal cord. The symptoms of MS include both somatic and autonomic deficits. Control of the musculature is compromised, as is control of organs such as the bladder.

Guillain-Barré syndrome is an example of a demyelinating disease of the peripheral nervous system. It is also the result of an autoimmune reaction, but the inflammation is in peripheral nerves. Sensory symptoms or motor deficits are common, and autonomic failures can lead to changes in the heart rhythm or a drop in blood pressure, especially when standing, which causes dizziness.

References

  1. Nervous Tissue. National Cancer Institute.

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