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Nervous System

The nervous system of an animal coordinates the activity of the muscles, monitors the organs, constructs and also stops input from the senses, and initiates actions. Prominent participants in a nervous system include neurons and nerves, which play roles in such coordination.


Nervous system consist of:
  1. Central Nervous System: It consist of the brain and spinal cord.
  2. Periphral Nervous System: It includes 12 pairs of cranial nerves and 31 pairs of spinal nerves. 
  3. Autonomous Nervous System: It consist of sympathetic and parasympathetic nerves. 
Nervous system is made up of nervous tissue which comprises of neurons and muscles cells. Neurons transmit nerve messages. Muscle cells are in direct contact with neurons and often surround them.
The neuron is the functional unit of the nervous system. Neurons have the amazing ability to gather and transmit electrochemical signals -- they are something like the gates and wires in a computer. Neurons share the same characteristics and have the same parts as other cells, but the electrical aspect lets them transmit signals over long distances and pass messages to each other.
Neurons have three basic parts:
Cell Body
This main part has all of the necessary components of the cell, such as the nucleus, endoplasmic reticulum, ribosomes and mitochondria. If the cell body dies, the neuron dies.
This long cable-like projection of the cell carries the electrochemical message along the length of the cell. Depending upon the type of neuron, axons can be covered with a thin layer of myelin, like an insulated electrical wire. Myelin is made of fat and it helps to speed transmission of a nerve impulse down a long axon. Myelinated neurons are typically found in the peripheral nerves, while non-myelinated neurons are found in the brain and spinal cord.
Dendrites or Nerve Endings
These small, branch-like projections of the cell make connections to other neuron. Dendrites can be located on one or both ends of the cell.



Many nerve fibres (axon fibres) bundled together make a nerve. Nerves may be sensory, motor and mixed.
Nerve fibres or neurons are sensory when the direction of impulse is from a receptor towards the brain or spinal cord. The cell body is not always present at one end. Nerves  from eyes, skin, ears etc. are sensory. 
When the direction is reverse, i.e. from the brain or spinal cord towards an effector which could be muscle or gland. Thus, nerve cells conduct impulses only in one direction. 
Nerves which contain both sensory and motor are called mixed nerves. Spinal nerves are mixed.

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