Final answer:
When acetylcholine receptor sites are not occupied, the neurotransmitter does not have any effect on the postsynaptic cell and cannot elicit a response.
Step-by-step explanation:
When the acetylcholine receptor sites are not occupied, the neurotransmitter acetylcholine cannot bind to the receptors and open ion channels. This means that the postsynaptic cell does not receive the signal from the acetylcholine, and there is no depolarization or activation of the cell.
One example of this is the breakdown of acetylcholine by the enzyme acetylcholinesterase (AChE) in the synaptic cleft. This prevents acetylcholine from remaining bound to the receptors and causing extended muscle contraction.
Overall, when acetylcholine receptor sites are not occupied, the neurotransmitter does not have any effect on the postsynaptic cell and cannot elicit a response.