FUNCTIONAL UNIT of life. They are the smallest units of life and hence are also known as the "building blocks of life".All living organisms are either composed of single cells i.e. unicellular or are multicellular organisms composed of many cells working together.Neurons also called nerve cells are the longest cell in humans.These are the fundamental units of the brain and nervous system.Neurons communicate with each other as well as with other cells through electric signals which in turn allows EFFECTOR organs to respond to the STIMULI. Neurons have three essential parts i.e. a cell BODY, an axon, and dendrites.