It was once thought that the brain was a single network that transmitted electrical pulses to the nerves. The Spanish neuroscientist Remon y Cajal showed that the cells of the brain and nervous system actually had tiny gaps too small to be seen by the microscopes of the day. Communication between neural cells is made with molecules called neurotransmitters which are ejected from the neuron -- brain cell -- when it "fires."
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Vesicles are small containers inside of living cells that are made out of the same two layer walls that make up cell walls. Some vesicles contain garbage that the cell wants to expel. These vesicles are pushed into the cell wall and the wall of the vesicle merges with the cell wall and the interior of the vesicle is suddenly on the outside of the cell. Vesicles containing neurotransmitters manufactured in a brain cell are expelled in the same way. An electrical potential pushes a vesicle full of neurotransmitters into the cell wall at the synapse and the neurotransmitters are expelled from one brain cell toward the next brain cell.
The tiny gap between brain cells is called a synapse. Neurotransmitters are expelled from the presynaptic neuron and cross the synaptic cleft toward the post synaptic neuron. During this journey, the neurotransmitters are subject to reaction with whatever chemicals are in the interstitial fluid -- the fluid between the cells. Some of the chemicals in the interstitial fluid -- like hormones -- exist just to interfere with neurotransmitters. This is the chemistry behind how out emotions interfere with our thought processes.
Some neurotransmitters are small molecules and some are large, complex molecules. The smaller molecules may actually enter the post synaptic neuron, but the larger neurotransmitters attach to special docking stations -- called receptacles -- on the surface of the post synaptic neuron. Attaching to the receptacle can imitate all kinds of activities inside the post synaptic neuron. Some of these activities increase the likelihood that the neuron will "fire" and push its own vesicles of neurotransmitters into the next synapse. Some neurotransmitters cause the opposite effect when they dock into receptacles -- they reduce the likelihood that the neuron will fire. Neurons in different parts of the brain have different receptacles, and this causes different behaviours. Most neurons have a variety of different kinds of receptacles.
Neurotransmitters -- especially the large, complex ones -- must be separated from their receptacle and returned to the neurons they came from. This process is accomplished via a variety of mechanisms that are collectively known as reuptake. Some neurotransmitters are dislodged from receptacle by the action of other molecules in the interstitial fluid, and some are dislodged because of the action of the receptacle over time. There are locations -- usually right behind the synapses -- where the loose neurotransmitters are attracted and then recaptured by being engulfed by a section of cell wall to create a new vesicle.
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