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All the functions of any organism are regulated and integrated by the nervous system.All the systems are coordinated with one another to perform normal functioning of the body. This co-ordination is by two systems. They are nervous and endocrine systems. Study of nervous and endocrine systems together called Cybernetics. The main and fast co-ordinating system is nervous system. For example the sensory organs first receive external stimuli. These are carried to the centres of brain through the sensory nerves. The centres of the brain take decessions. These decessions are carried to the muscles for contraction and to the glands for secretion. The entire nervous system is derived from embryonic ectoderm. The structural and functional units are neurons.
 
The nervous system is divided into three parts. These are i. Central ii. Peripheral and iii. Autonomous nervous system.
 
CENTRAL NERVOUS SYSTEM
 
It lies dorsally in the mid longitudinal axis of the body. The central nervous system contains two parts. They are brain and spinal cord.
 
PERIPHERAL NERVOUS SYSTEM
 
The peripheral nervous system includes cranial nerves and spinal nerves.
 
The nerves which arise from brain are cranial nerves and that of spinal cord are spinal nerves. The cranial nerves come out of cranial cavity through cranial foramina. Spinal nerves come out of the neural canal through the inter-vertebral foramina. The nerves of the peripheral nervous system innervate different parts of the body.
 
Based on the function, the nerves are of 3 types.
 
A. SENSORY NERVES
 
The sensory nerves are also known as afferent nerves. These nerves carry impulses from sense organs like nose, tongue, skin etc to the central nervous system.
 
B. MOTOR NERVES
 
These nerves are also known as efferent nerves. These nerves carry orders from central nervous system to the effectors ( muscles and glands).
 
C. MIXED NERVES
 
These nerves conduct impulses in both directions. They contain both afferent and efferent fibres.

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