Organization and functions of the nervous system

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Organization and Functions of the Nervous System


Neuroanatomy is the study of the nervous system. The nervous system is the most complex, widely investigated and least understood system in the body. It along with endocrine system regulates the functions of all other systems of the body. Hence nervous system is also called master system of the body.


The functions of the nervous system include:




The nervous system consists of three basic functional types of neurons: sensory, motor and interneurons. The sensory neurons detect stimuli and motor neurons send commands to the effector organs. The interneurons confer on the nervous system its prodigious capacity to analyse, integrate and store information.


The mechanism of functioning of the nervous system is as follows: The sensory stimuli (afferent impulses) received from inside or outside the body are correlated within the nervous system and then coordinated motor response (motor impulses) is sent to the effector organs (muscles, glands, etc.) so that they work harmoniously for the well-being of the individual (Flowchart 2.1).




Divisions of Nervous System



Anatomical


Anatomically the nervous system is divided into two parts, the central nervous system and the peripheral nervous system (Fig. 2.1).






Functional


Functionally also the nervous system is divided into two parts, the afferent division and the efferent division (Fig. 2.2).





The efferent division has somatic and visceral components constituting somatic and autonomic nervous systems, res pectively.






Cellular Organization of the Nervous System


The highly specialized and complex nervous system consists of only two principal categories of cells, (a) neurons, and (b) neuroglia.





Neurons (Neuro, Nerve)


The neurons are the structural and functional units of the nervous system. They are specialized for reception, integration, interpretation and onward transmission of information. They conduct nerve impulses over long distances at great speeds. The nervous system consists of vast number (about 1012) of neurons.


Each neuron consists of a cell body (also called soma or perikaryon or nerve cell body) and its processes called neurites.


The typical neuron has a single long process called axon and many short processes called dendrites (Fig. 2.3).



The axon does not branch freely except at its termination; although it does give off side branches as collaterals by which it establishes interconnections with the other neurons. The axon conducts impulses away from the cell body.


The dendrites receive stimuli and conduct nerve impulses to the nerve cell body. The dendrites often branch profusely and form a major part of the receptive area of the neuron.





Classification of neurons (types of neurons)


The neurons exhibit considerable diversity in form and function. Therefore, they are classified structurally as well as functionally.



Anatomical (morphological) classification


According to polarity (Fig. 2.4)



• Pseudounipolar neurons. These neurons possess oval or rounded cell body. A single process emerges from the cell body and after a short convoluted course bifurcates at a T-junction into peripheral and central processes. They are called pseudounipolar neurons because it is thought that the two processes of the bipolar neurons, during the process of differentiation, are approximated and finally fused near the cell body to form a single process. Thus, it appears that the neurons possess a single process bifurcating in a T-shaped manner, a short distance from the cell body. Such neurons are found in dorsal root ganglia of spinal nerves and sensory ganglia of some cranial nerves.



• Bipolar neurons. They possess spindle-shaped cell body, from each end of which a single neurite (process) emerges. Thus, bipolar neurons have two processes, one dendrite and one axon, with the soma between them. Such neurons are found in olfactory epithelium of nasal cavity, retina of eyeball and sensory ganglia of cochlear and vestibular nerves.


• Multipolar neurons. Have multipolar cell body from which emerges several dendrites and a single axon. Most of the neurons in the body especially those in CNS belong to this category. For example, all the motor neurons that control skeletal muscles are multipolar neurons.


In fact multipolar neurons make up almost entire neuronal population of the CNS. Due to presence of several dendrites and their elaborate primary and secondary dendritic branches, these neurons enormously increase their synaptic surfaces.





According to relative lengths of axons and dendrites



They are found in large numbers in cerebral cortex, cere-bellar cortex and in the retina.


Table 2.1 summarizes the morphological (anatomical) classification of neurons.




Functional classification



They carry impulses from the receptor organs to the CNS.




Types of motor neurons

In the somatic nervous system they are divided into two types:


Jan 2, 2017 | Posted by in NEUROLOGY | Comments Off on Organization and functions of the nervous system
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