channel. The channel or hole is generally closed and opens when a
specific neurotransmitter molecule binds to a particular site on the
surface of the receptor protein. This binding induces a shift in the
shape of the protein, which opens the channel. Depending on the size,
shape, and electric charge properties of the channel, different recep-
tors allow different types of ions to cross the membrane. Na‘* or Ca*,
which would both flow into a neuron, produce EPSPs. Cl flowing in or
K flowing out both produce IPSPs.
In addition to the ionotropic type, there is a second major category
of neurotransmitter receptor protein, called metabotropic (Greek
metabole = change, tropos = turn toward). Metabotropic receptors
affect chemistry inside the cell (intracellular chemistry). Binding of a
neurotransmitter to a metabotropic receptor does not directly open
an ion channel (as ionotropic receptors do) but can cause a variety of
different things to happen: ion channels may open or close, enzymes
may be activated or inactivated, gene transcription may be turned on
or off, and so forth.
After the initial observations of the existence of this kind of
receptor, many years of experimental study worked out that intricate
sequences of events take place after the binding of neurotransmitter.
Here is one summary; consider reading through it as you might a
poem on first reading—don’t get caught up in the details...
The binding of a neurotransmitter molecule to the appropriate
location on the extracellular side of the metabotropic receptor pro-
tein shifts the shape of the receptor to make its intracellular surface
available to bind another protein—called a G-protein—present
inside the cell. When the G-protein binds, it changes shape and be-
comes activated in the following way: a molecule of GDP (guanosine
diphosphate) attached to the G-protein comes off and is replaced