A neurochemical is a chemical substance that influences or plays an important role in the functioning and communication within the nervous system. These substances include neurotransmitters, which are responsible for transmitting signals between neurons; hormones, which can affect moods and behaviors; and other molecules such as enzymes, receptors, and ion channels that help regulate neural activity. In summary, a neurochemical is any chemical substance that interacts with the nervous system to influence behavior or physiological processes.