Which statement best describes excitatory neurotransmitters?

• A. They decrease the rate of firing
• B. They cause hyperpolarization
• C. They block the synapse
• D. They increase the likelihood of the postsynaptic neuron firing

Answer: (D)

Excitatory neurotransmitters increase the likelihood that the postsynaptic neuron will fire an action potential. They do this by causing depolarization of the postsynaptic membrane, an excitatory postsynaptic potential (EPSP), usually through opening channels that let positive ions like Na+ into the neuron. When enough EPSPs occur—either at the same time or in quick succession via spatial or temporal summation—the membrane potential moves toward the threshold, making an action potential more likely.

In contrast, inhibitory neurotransmitters typically cause hyperpolarization (an inhibitory postsynaptic potential, or IPSP), moving the membrane potential away from threshold and reducing firing probability. They don’t block the synapse.

So the statement that best describes excitatory neurotransmitters is that they increase the likelihood of the postsynaptic neuron firing.