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Neurons have specialized projections called dendrites and axons. Dendrites bring information to the cell body and axons take information away from the cell body.
Information from one neuron flows to another neuron across a synapse. The synapse contains a small gap separating neurons. The synapse consists of:
For communication between neurons to occur, an electrical impulse must travel down an axon to the synaptic terminal.
At the synaptic terminal (the presynaptic ending), an electrical impulse will trigger the migration of vesicles (the red dots in the figure to the left) containing neurotransmitters toward the presynaptic membrane. The vesicle membrane will fuse with the presynaptic membrane releasing the neurotransmitters into the synaptic cleft. Until recently, it was thought that a neuron produced and released only one type of neurotransmitter. This was called "Dale's Law." However, there is now evidence that neurons can contain and release more than one kind of neurotransmitter.
The neurotransmitter molecules then diffuse across the synaptic cleft where they can bind with receptor sites on the postsynaptic ending to influence the electrical response in the postsynaptic neuron. In the figure on the right, the postsynaptic ending is a dendrite (axodendritic synapse), but synapses can occur on axons (axoaxonic synapse) and cell bodies (axosomatic synapse).
When a neurotransmitter binds to a receptor on the postsynaptic side of the synapse, it changes the postsynaptic cell's excitability: it makes the postsynaptic cell either more or less likely to fire an action potential. If the number of excitatory postsynaptic events is large enough, they will add to cause an action potential in the postsynaptic cell and a continuation of the "message."
Many psychoactive drugs and neurotoxins can change the properties of neurotransmitter release, neurotransmitter reuptake and the availability of receptor binding sites.
Happy 119th Birthday to the word "SYNAPSE." In 2016, the word "synapse" turned 119 years old. The word synapse was first used in a book called A Textbook of Physiology, part three: The Central Nervous System, by Michael Foster and assisted by Charles S. Sherrington, in 1897. It was probably Charles S. Sherrington who coined the term synapse. The word "synapse" is derived from the Greek words "syn" and "haptein" that mean "together" and "to clasp," respectively.
"You are your synapses. They
are who you are."
--- Joseph LeDoux, 2002 (in Synaptic Self)
See some synapses "Up Close and Personal".
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