A, ANC
LittNeurotransmitter molecules are contained in the storage vesicles of
an axon terminal and are released into the synaptic cleft as a result of
an action potential in the presynaptic neuron. After bouncing around
in the synaptic cleft, they interact with neurotransmitter receptor
proteins found in the membrane of the postsynaptic cell. One major
type of neurotransmitter receptor is called an ionotropic receptor.
These proteins are channel proteins similar in function to voltage-
gated ion channels: when the channel is open, ions of a specific kind
flow across the cell membrane. But an ionotropic receptor channel is
opened not by changes in membrane voltage but, rather, by the bind-
ing of a specific neurotransmitter molecule to a particular location on
the receptor protein. This causes the receptor protein to shift shape,
opening the channel. Another name for an ionotropic receptor is a
ligand-gated channel receptor, where the term ligand means a small
molecule (in this case, the neurotransmitter molecule) that binds to
a larger molecule (in this case, the receptor protein) and causes some-
thing to happen (in this case, the opening of an ion channel). When an
ion channel opens and ions flow across the membrane, either a depo-
larization or a hyperpolarization results, depending upon which kind
of ion the channel allows through.
The most abundant neurotransmitter molecule in the human brain
is glutamic acid, or glutamate. Glutamate is also one of the twenty
building-block amino acids used to construct proteins for all of life
on Earth. As an amino acid component of proteins, glutamate was
discovered in the mid-nineteenth century as a breakdown product of