What happens when neurotransmitters bind to receptors on the postsynaptic neuron?

• A. They have an excitatory or inhibitory effect on the postsynaptic neuron
• B. They always excite the postsynaptic neuron
• C. They create new neurons
• D. They are destroyed at the synapse

Answer: (A)

When neurotransmitters bind to receptors on the postsynaptic neuron, they influence whether that neuron will fire, not by creating new cells or being destroyed, but by changing the membrane potential. If the binding opens channels that let positive ions (like Na+ or Ca2+) flow in, the membrane depolarizes, producing an excitatory postsynaptic potential that brings the neuron closer to the firing threshold. If the binding opens channels that allow negative ions (like Cl−) in or positive ions (like K+) out, the membrane hyperpolarizes, producing an inhibitory postsynaptic potential that moves the neuron away from threshold. Receptors can be fast-acting ion channels (ionotropic) or slower, signal-cascade receptors (metabotropic), but the key idea is that the effect can be either excitatory or inhibitory depending on the receptor type and the ions involved. Whether the neuron actually fires depends on the sum of all excitatory and inhibitory inputs it receives.