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Nerve Tissue

Neuron_Structure_090420A
[Neuron Structure - NIH]


 
Although the nervous system is very complex, there are only two main types of cells in nerve tissue. The actual nerve cell is the neuron. It is the "conducting" cell that transmits impulses and the structural unit of the nervous system. The other type of cell is neuroglia, or glial, cell. The word "neuroglia" means "nerve glue." These cells are nonconductive and provide a support system for the neurons. They are a special type of "connective tissue" for the nervous system.
 

- Neurons

Your brain is made up of millions of cells called neurons. Neurons make connections with each other to create pathways that control all aspects of life, such as bodily functions, emotions, and movement. Each neuron is composed of three parts: the cell body, dendrites, and an axon.

Neurons, or nerve cells, carry out the functions of the nervous system by conducting nerve impulses. They are highly specialized and amitotic. This means that if a neuron is destroyed, it cannot be replaced because neurons do not go through mitosis. The image below illustrates the structure of a typical neuron.  

Each neuron has three basic parts: cell body (soma), one or more dendrites, and a single axon.


- Cell Body

The cell body, also called the soma, is the spherical part of the neuron that contains the nucleus. The cell body connects to the dendrites, which bring information to the neuron, and the axon, which sends information to other neurons. When information is received from another neuron, the dendrites pass the signal to the cell body. The cell body then may send the information to the axon, depending on the strength of the signal. 

In many ways, the cell body is similar to other types of cells. It has a nucleus with at least one nucleolus and contains many of the typical cytoplasmic organelles. It lacks centrioles, however. Because centrioles function in cell division, the fact that neurons lack these organelles is consistent with the amitotic nature of the cell.



- Dendrites

Dendrites and axons are cytoplasmic extensions, or processes, that project from the cell body. They are sometimes referred to as fibers. Dendrites are usually, but not always, short and branching, which increases their surface area to receive signals from other neurons. The number of dendrites on a neuron varies. They are called afferent processes because they transmit impulses to the neuron cell body. There is only one axon that projects from each cell body. It is usually elongated and because it carries impulses away from the cell body, it is called an efferent process.


- Axon

An axon may have infrequent branches called axon collaterals. Axons and axon collaterals terminate in many short branches or telodendria. The distal ends of the telodendria are slightly enlarged to form synaptic bulbs. Many axons are surrounded by a segmented, white, fatty substance called myelin or the myelin sheath. Myelinated fibers make up the white matter in the CNS, while cell bodies and unmyelinated fibers make the gray matter. The unmyelinated regions between the myelin segments are called the nodes of Ranvier.





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