FoundationalConceptsNeuroscience

lipid bilayer membrane (Fig. 6.3). These are synaptic storage vesicles,
each filled with several thousand neurotransmitter molecules.
Specific proteins in the lipid membrane of vesicles function to attach
some of the vesicles to other specific proteins in the boundary mem-
brane of the axon terminal. Collectively, these attachment proteins are
called the SNARE complex, and they result in vesicles being poised to
fuse with the boundary membrane of the axon. (SNARE is an abbrevi-
ation for the rather unwieldly name soluble N-ethylmaleimide-sensi-
tive factor attachment protein receptor.)
What happens when a nerve cell passes a signal along to another
neuron at a chemical synapse? Let’s begin with an action potential
being initiated in the presynaptic neuron (we'll see how initiation
happens shortly). The action potential propagates along the length of
the axon until it reaches the axon terminal or, in most cases, multiple
axon terminals, because generally axons are branched at the ends.
When the action potential reaches the end of the axon, the membrane
no longer has any voltage-gated sodium and potassium channels.
Rather, it has another kind of voltage-gated ion channel, a calcium
channel. When the depolarization reaches the axon terminal, the
changing electrical forces on the voltage-gated calcium channel in-
duce the channel to open, and Ca** ions flow into the cell. The calcium
binds to proteins present in the SNARE complex, and a sequence of
molecular events results in fusion of the vesicular membrane with
the boundary membrane of the axon terminal. The contents of the
storage vesicle—the neurotransmitter molecules—then spill into the
synaptic cleft. Neurotransmitter release has occurred.