CHAPTER 7Neuroanatomy and Excitability
Glutamate and GABA are the primary excitatory and inhibitory
neurotransmitters in the human brain, used as signaling molecules
by billions of neurons. Much of their action consists of evoking rapid
changes in cell excitability by interacting with ionotropic receptors.
In addition, these neurotransmitters also act at G-protein-coupled
receptors (GPCRs), producing varied and longer-term effects on cell
excitability, structure, and function.
Another neurotransmitter acting on both ionotropic receptors and
GPCRs is acetylcholine. Ionotropic acetylcholine receptors (AChRs)
are positive ion channels, primarily for Na. Thus, when acetylcholine
binds to an ionotropic AChR, Na flows from the outside of the cell
inward, depolarizing the membrane and increasing the excitability
of the postsynaptic cell. lonotropic AChRs mediate communication
between nerves and skeletal muscles in humans and other vertebrate
animals. This connection is called the neuromuscular junction. Acetyl-
choline was the first neurotransmitter to be identified, following Otto
Loewi’s discovery of chemical neurotransmission in his investigation
of the slowing of the heartbeat by the vagus nerve. The vagus nerve
is part of the brain’s connection with the autonomic nervous system,