How can acetylcholine act both in milliseconds and over seconds? · Grey Matter

It has two receptor families.

How can acetylcholine act both in milliseconds and over seconds?

It has two receptor families. Nicotinic receptors are ion channels, so acetylcholine opens them within a millisecond; at the neuromuscular junction this fires every muscle fibre the motor neuron reaches. Muscarinic receptors are coupled to G proteins, so their effects start more slowly and last seconds; in the cortex they mostly close potassium channels and make neurons more excitable. Acetylcholinesterase in the cleft ends the fast signal within about a millisecond, which keeps neuromuscular transmission sharp, while in the brain the slower muscarinic actions shape attention and arousal.