Elaborate and complete summary of the third task of the course Body and Behavior (PSY1023 / IPN1023). Summary contains a lot of figures. Resources used: parts of Carlson (2017), Breedlove (2017) or Pinel (2017). All tasks available as bundle!
Synaptic transmission requires a sequence of events;
1) The action potential traveling down the axon arrives at the axon terminal.
2) This depolarization opens voltage-gated calcium channels in the membrane of the
axon terminal, allowing calcium ions (Ca2+) to enter the terminal.
3) The Ca2+ causes synaptic vesicles filled with neurotransmitter to fuse with the
presynaptic membrane and rupture, releasing the transmitter.
4) Transmitter molecules bind to postsynaptic receptors and cause ion channels to open.
5) Ion flow creates a local EPSP or IPSP.
6) Synaptic transmitter is either inactivated by enzymes or removed from the synaptic
cleft by transporters.
7) Synaptic transmission may also activate presynaptic autoreceptors, regulating future
transmitter release.
When an action potential reaches a presynaptic terminal, it opens voltage-gated calcium
channels that allow influx of calcium into the axon terminal. This activates enzymes that
cause vesicles near the presynaptic membrane to fuse with the membrane. This process is
called exocytosis.
, PSY/IPN1023 Body and Behavior
Synapse and axonal terminal button
Two prominent structures are
located in the cytoplasm of the
terminal button: mitochondria and
synaptic vesicles. We also see
microtubules, which are responsible
for transporting material between
the soma and terminal button. The
presence of mitochondria implies
that the terminal button needs
energy to perform its functions.
Synaptic vesicles are small, rounded
objects in the shape of spheres or
ovoids.
Exocytosis
How does an action potential cause synaptic vesicles to release neurotransmitters? The
process begins when a population of synaptic vesicles becomes docked against the
presynaptic membrane, ready to release their neurotransmitter into the synaptic cleft. Docking
is accomplished when clusters of protein molecules attach to other protein molecules located
in the presynaptic membrane. Thus, when the voltage-dependent calcium channels open,
Ca2+ flows into the cell, propelled by electrostatic pressure and the force of diffusion. The
entry of Ca2+ is an essential step; if neurons are placed in a solution that contains no calcium
ions, an action potential no longer causes the release of a neurotransmitter.
Several proteins mediate exocytosis. One family, SNAREs, serve as tethers; those attached to
vesicles are called v-SNAREs, while those attached to the presynaptic membrane are called t-
SNARES (t for target). When the v-SNAREs on the vesicle attach to the t-SNAREs, the
vesicle is said to be docked, ready to be released (figure B on next page).
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