Final answer:
The type of neurotransmitter binding to the receptor is the most important factor in determining the nature of a receptor-operated ion channel, whether it is inhibitory or excitatory.
Step-by-step explanation:
The most important factor in determining whether a receptor-operated ion channel is inhibitory or excitatory is the type of neurotransmitter binding to the receptor. When neurotransmitters such as acetylcholine bind to their receptors at the neuromuscular junction, they cause channels to open that allow positive ions like sodium (Na+) to enter the postsynaptic cell, leading to depolarization and an excitatory effect. Alternatively, neurotransmitters like GABA can bind to receptors and cause the opening of channels for negative ions such as chloride (Cl-), resulting in hyperpolarization and an inhibitory effect. These ligand-gated ion channels are thus influenced by the specific neurotransmitter involved and the consequent flow of ions, which affects the membrane potential and the excitability of the neuron.