Final answer:
Neurotransmitters bind to receptor proteins on the cell surface and can change ion channel permeability. Ionotropic receptors are directly opened by neurotransmitter binding, while metabotropic receptors initiate complex intracellular responses.
Step-by-step explanation:
Substances that bind to receptor proteins and change the permeability of ion channels are called neurotransmitters. When a neurotransmitter binds to a receptor on the cell surface, it can trigger a wide array of responses depending on the type of receptor. Ionotropic receptors are ligand-gated ion channels that open in response to neurotransmitter binding, while metabotropic receptors are part of a complex that leads to metabolic changes within the cell, involving a G protein and effector proteins which can result in the generation of a second messenger, affecting various cellular processes.