Final answer:
Acetylcholine (ACh) is the neurotransmitter involved in muscle contractions, learning, memory, and REM sleep.
Step-by-step explanation:
The neurotransmitter in the brain, spinal cord, and peripheral nervous system involved in muscle contractions, learning and memory, and REM sleep is acetylcholine (ACh). Acetylcholine acts at the neuromuscular junction, triggering depolarization to activate muscles, which means that it can have significant effects on motor function. In the central nervous system, acetylcholine is used by motor neurons to communicate with muscles, and it is also active in the autonomic nervous system. Furthermore, acetylcholine plays a crucial role in various brain functions, contributing to arousal, attention, memory and motivation.