Final answer:
The control of the slight decrease in pacing rate due to expiration-activated inhibition of the SA node is mainly regulated by the parasympathetic nervous system through the release of acetylcholine (ACh)
Step-by-step explanation:
The control of the slight decrease in pacing rate due to expiration-activated inhibition of the SA node is primarily controlled by the parasympathetic nervous system.
Parasympathetic stimulation, via the vagus nerve, releases acetylcholine (ACh) which slows down the heart rate by opening potassium ion channels and extending repolarization. This inhibitory effect allows for a longer time between heartbeats, contributing to the decrease in pacing rate.