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In-Depth Information
Dendrite
Spine
Receptors
mGlu
AMPA
NMDA
Postsynaptic
Neuro−
transmitter
(glutamate)
Na+
Cleft
Ca++
Presynaptic
Ca++
Vesicles
Terminal
Button
Figure 2.4: Electron-microscope image of a synapse. The
arrows show the location of two synaptic release sites, with
the terminal button extending below (the orientation is the
same as in figure 2.5). The small circles are the synaptic vesi-
cles containing neurotransmitter. The large dark structures in
the terminal button are cellular organelles. Reproduced from
Kandel et al. (1991).
Axon
Microtubule
Figure 2.5: Diagram of the synapse. The action potential
causes Ca ++ ions to be mobilized in the button, causing vesi-
cles to bind with the presynaptic membrane and release neu-
rotransmitter (NT) into the cleft. NT (e.g., glutamate) then
binds with the postsynaptic receptors, which are either associ-
ated with channels and thus allow ions to flow (e.g., Na + ions
for AMPA channels or Ca ++ ions for NMDA channels), or
cause postsynaptic chemical processes to take place (e.g., the
metabotropic glutamate receptor, mGlu). NT is produced in
the soma and transported to the terminal via microtubules.
transmitter at the synapse. As we have said, the synapse
is the junction between the sending neuron's axon and
the receiving neuron's dendrite (figures 2.5 and 2.4).
The end of the axon that enters into the synapse is called
the axon terminal or button (pronounced “boo-tohn”).
In some types of synapses, there is a special process
called a spine on the dendrite where the synapse is
formed. In other cases, the synapse is formed directly
with the dendritic membrane.
When the electrical pulse reaches the terminal button,
it mobilizes calcium ions (Ca ++ ) by opening voltage-
sensitive channels that allow Ca ++ ions to flow into the
terminal, and also possibly by releasing internal stores
of these ions. These calcium ions then promote the
binding of little sacks or vesicles of neurotransmitter
( NT ) to the membrane of the terminal (this process is
not yet fully understood at a detailed level). Upon bind-
ing with the membrane, the vesicles then release the NT
into the synaptic cleft (the very narrow gap between the
axon and dendrite).
After diffusing across the cleft, the released NT then
binds to the postsynaptic receptors on the receiving neu-
ron.
with the receptor to open, allowing ions to flow (for
ionotropic receptors), or results in the initiation of var-
ious chemical processes in the postsynaptic neuron (for
metabotropic receptors that are not associated with
channels). There are different types of NT chemicals
that are released by different types of sending neurons,
and these different chemicals bind to different recep-
tors, causing different kinds of effects in the receiving
neuron. We will discuss later some of the different types
of NTs in the context of the different dendritic recep-
tors.
To maintain the continued release of NT over many
spikes, the vesicle material that gets bound to the mem-
brane is later recycled to make new vesicles, and new
NT and other important molecules are sent in from the
cell body via microtubules . Once the NT is released
into the cleft, it must also be broken down or taken back
This binding either causes a channel associated
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