Final answer:
Relaxation involves the parasympathetic nervous system that releases acetylcholine to decrease metabolic activity and stimulate the 'rest and digest' response in the body.
Step-by-step explanation:
Relaxation targets the parasympathetic nervous system, which releases acetylcholine, a neurotransmitter that decreases metabolic activity. The parasympathetic nervous system, associated with the 'rest and digest' state, includes preganglionic neurons with cell bodies in the brainstem and sacral spinal cord. These neurons release acetylcholine onto postganglionic neurons, which are generally located very near the target organs.
Most postganglionic neurons then release acetylcholine onto the target organs. This neurotransmitter binds to muscarinic receptors, inducing a response that leads to a slower heart rate, reduced blood pressure, and stimulated digestion.