Final answer:
The nature of an action potential as excitatory or inhibitory is primarily determined by the type of receptors on the postsynaptic cell that interact with specific neurotransmitters, resulting in either depolarization or hyperpolarization.
Step-by-step explanation:
What determines whether an action potential is inhibitory or excitatory? The primary factor that determines the nature of an action potential is (a) The type of receptors present on the postsynaptic cell. When neurotransmitters are released into the synapse, their effect on the postsynaptic neuron is dictated by whether the neurotransmitter binds to excitatory or inhibitory receptors. For example, the neurotransmitter acetylcholine at a neuromuscular junction causes an excitatory postsynaptic potential (EPSP) by opening sodium channels, leading to depolarization and increasing the likelihood of an action potential. Conversely, GABA results in an inhibitory postsynaptic potential (IPSP) by opening chloride channels, causing hyperpolarization and making the neuron less likely to fire an action potential.