Brain

[Nivedita Vaidya]

 Nervous system

 An organism can not live in isolation. It is subjected all the time to several influences that come from surrounding or external environment. One of the salient characteristics of living beings is their response to environment. Since protoplasm possesses the property of irritability , all  living forms are able to respond to certain factors in the environment called stimuli. Even a single organism such as paramecium, if touched with a needle , reverses its motion.  In higher organisms, the neurons that constitute the units of nervous tissue, are specially  designed to receive stimuli from the environment and to show appropriate responses to these stimuli. In fact, a vast multitude of such neurons constitute the nervous system in the mammals. These neurons comprise the central nervous system including the brain and spinal cord. and peripheral nervous system consisting of nerve cells that lie outside of them.