Anatomy And Physiology

Anatomy Physiology



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In this article, the types and functions of cells in the nervous system are discussed. There are in the human nervous system two types of cells that have completely different functions in the body. These are: Neurons and neuroglia.  Neurons are the functional cells of the nervous system.  They are present in both the central and the peripheral nervous systems. This is in contrast to neuroglia which have different types of cells in the central and the peripheral nervous systems.   


Neuroglia are supportive cells for neurons which have supplementary function to neurons.  Neurons are among the few cells in the body that do not divide nor regenerate.  This fact has both positive as well as negative consequences.  The positive thing about it is that these cells cannot undergo mutation or cannot become carcinogenic.  For this reason, cancer of the nervous system that arises in neurons is not possible.   


The bad thing about it is that due to the inability to reproduce these cells they cannot be compensated in the case of injury of the nervous system.  Therefore damage to the spinal cord for example can be irreversible and can lead to paralysis in the affected individual.  Neuroglia on the other hand can reproduce and divide.  Therefore they are the source of malignancy in tumours of the nervous system.   
Neurons differ to other cells by their excitable nature.


  In this respect they are similar to muscle cells which are also excitable.  Neurons are divided into three structures.  These are dendrites which receive information from other neuronal cells and cell bodies, in addition to axons which transmit information from one cell to another.    Neurons excitability is due to opening of ion channels in the cellular membrane mainly sodium ion channels.  The stimulus that triggers the excitation is usually due to a neurotransmitter or a chemical compound. 

Opening of the sodium ion channel leads to an increase in the electrical potential of the cellular membrane.  If this increase reaches a threshold, an action potential takes place in which a neurotransmitter is released into the space between the cells or the synaptic cleft. This released neurotransmitter can either inhibit or excite the neigbouring neurons.  This will depend on the type of ion channel that opens in the neigbouring cell.  

Opening of sodium or calcium ion channels leads to excitation of the neuron due to the inflow of sodium ions and calcium ions into the cell.  On the other hand, opening of chloride or potassium ion channels lead to inhibitory effect.    In this respect pharmacological drugs can be used to induce either excitation or inhibition of the formation of an action potential.  This again will depend on the channel that opens.

Neuroglia in the central nervous system differ to that in the peripheral nervous system.  Astrocytes are one type of neuroglia that are located in the central nervous system.  They have a supporting function to neurons due to the existence of microfilaments in them which give strength.  They help to form the blood-brain barrier by enclosing the blood vessels in the brain.   
The other type of cell in the central nervous system is oligodendrocytes.  These cells are responsible for the synthesis of the myelin sheath around the axons of neurons in the central nervous system. 


Myelin is a lipid compound that encloses the axons of neurons in both the central as well as the peripheral nervous system.     Other types of cells of the neuroglia group are the microglial cells.  These cells function as phagocytes or macrophages and perform an immune function in the nervous system.  They can phagocytose microbes during an infection of the nervous system.   
The last type of cells in the central nervous system are the ependymal cells. 


These cells produce the cerebrospinal fluid and are located in the ventricles of the brain.  Cells of the peripheral nervous system and which produce myelin are called schwann cells .  Another type of cell in the peripheral nervous system is called the satellite cell and perform also a supporting function of neurons in the peripheral nervous system.

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