Final answer:
An excitatory neurotransmitter causes an excitatory postsynaptic potential (EPSP) that depolarizes the postsynaptic neuron, making it more likely to fire an action potential.
Step-by-step explanation:
An excitatory neurotransmitter will produce an excitatory postsynaptic potential (EPSP) and will depolarize the postsynaptic neuron. Therefore, the correct answer is EPSP; depolarize. Excitatory neurotransmitters bind to receptors on the postsynaptic neuron, causing ligand-gated ion channels to open. This typically results in positive ions, such as Na+, entering the neuron, leading to a decrease in the membrane potential difference (depolarization).
An EPSP brings the membrane potential closer to the threshold for firing an action potential. In contrast, inhibitory neurotransmitters such as GABA can lead to the opening of channels that allow negatively charged ions like Cl- to enter the neuron, resulting in hyperpolarization (increasing the membrane potential difference), which is indicated by an inhibitory postsynaptic potential (IPSP).