What is the effect of an excitatory neurotransmitter?

The effect of an excitatory neurotransmitter primarily promotes neuronal firing. Excitatory neurotransmitters, such as glutamate, work by binding to receptors on the postsynaptic neuron and leading to depolarization. This process increases the likelihood that the postsynaptic neuron will reach the threshold needed to generate an action potential, thus contributing to the transmission of signals within the nervous system.

In contrast, the other options would imply different mechanisms that do not align with the functions of excitatory neurotransmitters. Inhibitory neurotransmitters would be responsible for inhibiting neuronal firing, which is not the role of excitatory neurotransmitters. Therefore, the correct understanding of excitatory neurotransmitter action aligns with the option that signifies their role in promoting neuronal activity.