Neurotransmitters are chemical messengers that transmit signals between neurons and target cells like other neurons, muscles, or glands. An imbalance in neurotransmitters can cause emotional, mental, or physical distress by impacting functions related to mood, thinking, and behaviors. Things like stress, addictions, medications, poor nutrition or sleep can cause this imbalance. The document discusses the types and functions of neurotransmitters in the brain.