Final answer:
The effect of a neurotransmitter signal being inhibitory or excitatory is determined by the type of receptor protein on the postsynaptic membrane that it activates, with excitatory neurotransmitters causing depolarization and inhibitory neurotransmitters causing hyperpolarization.
Step-by-step explanation:
Whether a neurotransmitter signal is inhibitory or excitatory depends on the type of receptor protein it activates on the postsynaptic membrane. Excitatory neurotransmitters, like glutamate, generally bind to receptors that cause the postsynaptic cell to depolarize and become more likely to fire an action potential, known as an excitatory postsynaptic potential (EPSP). On the other hand, inhibitory neurotransmitters such as GABA (gamma-aminobutyric acid) bind to receptors that cause hyperpolarization of the postsynaptic membrane, leading to inhibitory postsynaptic potentials (IPSPs), which make the postsynaptic neuron less likely to fire an action potential. Specific neurotransmitters may have different effects on various target cells depending on the receptor type(s) present. For instance, acetylcholine causes depolarization when it binds to nicotinic receptors, which are ionotropic ligand-gated ion channels, but it might cause either depolarization or hyperpolarization when it binds to different variants of metabotropic muscarinic receptors.