Final answer:
Ionotropic receptors are directly connected to ion channels and open when a neurotransmitter binds to them. This response is fast-acting and does not involve second messengers or G proteins.
Step-by-step explanation:
Ionotropic receptors are directly connected to ion channels and open when a neurotransmitter binds to them. They are also known as ligand-gated ion channels. When the neurotransmitter binds to the ionotropic receptor, the ion channel opens, allowing ions to flow through and generating a response in the neuron. This response is fast-acting and does not involve second messengers or G proteins.