Psychological aspects of pain perception can influence the release of the neurotransmitters called ________, the body's natural version of morphine.

A) endorphins
B) substance P
C) serotonin
D) acetyl choline

The neurotransmitters called endorphins are the body's natural version of morphine and are released in response to pain. Psychological factors such as stress and anxiety can influence the release of endorphins. Substance P is a neurotransmitter that transmits pain signals to the brain. Serotonin and acetylcholine are neurotransmitters that have various functions in the body but are not directly involved in pain perception.