Brain cell (neuron) 
                      
                    The human brain contains more than 100 billion brain cells 
                      called nerve cells or neurons. All neurons have same basc 
                      structure. Many number of nerves cells together are called 
                      nerve. Nerve is a pale, tough and string like structure 
                      and acts as a living telephone wire. 
                      A neuron is a long cell that has a thick central area containing 
                      the nucleus; it also has one long structure called an axon 
                      and one or more short, bushy structures called dendrites. 
                      Dendrites receive impulses from other neurons. These impulses 
                      propagate electrically along the cell membrane to the end 
                      of the axon. At the tip of the axon the signal is chemically 
                      transmitted to an adjacent neuron or muscle cell. 
                      
                    When the electrical signal (action potential) reaches the 
                      tip of an axon, it stimulates small presynaptic vesicles 
                      in the cell.  
                      
                      
                    These vesicles contain chemicals called neurotransmitters, 
                      which are released into the microscopic space between neurons 
                      (the synaptic cleft). The neurotransmitters is attached 
                      to specialized receptors on the surface of the adjacent 
                      neuron. This stimulus causes the adjacent cell to depolarize 
                      and propagate an action potential of its own. The duration 
                      of a stimulus from a neurotransmitter is limited by the 
                      breakdown of the chemicals in the synaptic cleft and the 
                      re-uptake by the neuron that produced them. Formerly, each 
                      neuron was thought to make only one transmitter, but recent 
                      studies have shown that some cells make two or more. 
                       
                     
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