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Making Connections - The Synapse
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 is a small
gap separating neurons. The synapse consists of:
1. a presynaptic ending that contains
neurotransmitters, mitochondria and other cell
organelles,
2. a postsynaptic ending that contains receptor sites
for neurotransmitters and,
3. a synaptic cleft or space between the presynaptic
and postsynaptic endings.
Axon
Dendrite Myelin Neuron
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See some synapses "Up Close and Personal".
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Electrical Trigger for Neurotransmission
Synapse Vesicle
For communication between neurons to
occur, an electrical impulse must travel down
an axon to the synaptic terminal.
Neurotransmitter Mobilization and Release
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.
Diffusion of Neurotransmitters Across the Synaptic Cleft
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.
Types of Synapses
Axodendritic Synapse
Axosomatic Synapse
Axoaxonic Synapse
For another explanation of the synapse, the Society for Neuroscience has written a short
summary called How do nerve cells communicate?
Play the Lost Synapse Game from the Nobel e-Museum.
Happy 106th Birthday to the word "SYNAPSE". In 2003, the word "synapse"
turned 106 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" comes from
Greek: "syn" meaning "together" and "haptein" meaning "to clasp."
If this site is still available, it can be found at
http://faculty.washington.edu/chudler/synapse.html