ionotropic and GPCR types. Chapter 6 discussed the actions of
glutamate, GABA, and acetylcholine at their associated ionotropic
receptors. Glycine also acts as a neurotransmitter at ionotropic recep-
tors. There is one type of serotonin receptor—the 5HT3 receptor—
that is ionotropic. There are receptors that respond to ATP, ADP, and
AMP as neurotransmitters that are ionotropic; these are called purine
receptors. Most other known neurotransmitter receptors are GPCRs.
There are GPCRs that respond to glutamate, GABA, and acetylcholine.
All the known serotonin receptors other than 5HT3 are GPCRs. There
are GPCR purine receptors. All the known dopamine, norepinephrine,
epinephrine, histamine, adenosine, cannabinoid, opioid, and other
neuropeptide receptors are GPCRs. Thus, there is enormous diversity
of neurotransmitter receptor types, both ionotropic and GPCR—hun-
dreds (minimally) of different receptor types.
The vast interconnectivity of the brain consists in large part of the
trillions of glutamatergic and GABAergic chemical synapses, medi-
ating an immense number of rapid excitatory and inhibitory effects.
In addition, there is extensive excitatory and inhibitory interconnec-
tivity via electrical synapses. And then there are trillions of chemical
synapses modulating neural excitation and inhibition via GPCR
effects. Allin all, such complex circuitry combining excitation and
inhibition provides opportunities for problems to occur if something
gets out of whack. Too much excitation and not enough inhibition
may, for example, set off a kind of explosive chain reaction of excita-
tion. One manifestation of such runaway excitation is the clinical con-
dition called a seizure (old French saisir = to take possession of).
A variety of different things may happen as a result of a seizure,
depending upon the region of the brain that is affected. There may be
sudden changes in sensory perception, such as visual disturbances