Final answer:
Endorphins are the peptide neurotransmitters released in the brain responsible for the 'runner's high,' leading to feelings of euphoria and pain reduction after vigorous exercise like running.
Step-by-step explanation:
The feeling of happiness and relaxation after a five-mile run is partly due to the brain's release of peptide neurotransmitters known as endorphins. These neurotransmitters are responsible for inhibiting pain signals and can lead to a sensation commonly referred to as the "runner's high". The effect of endorphins can be compared to that of a natural opiate, influencing feelings of pleasure, euphoria, and a reduction in pain perception.
It's important to note that vigorous aerobic exercise like running causes the pituitary gland and hypothalamus to produce these opiate-like neurotransmitters. In addition to endorphins, a runner's high may involve the release of other neurochemicals like endocannabinoids, which also contribute to the sense of well-being. Nonetheless, endorphins are widely recognized for their role in the positive feelings associated with prolonged exercise.