Endogenous

The term "endogenous" originates from the Greek words "endon" meaning "inside" and "gignomai" meaning "to produce." In the context of biology and neuroscience, endogenous refers to processes, substances, or phenomena that are produced or originate from within an organism, tissue, or cell.

For instance, when we talk about endogenous substances, we are referring to materials such as hormones or neurotransmitters that are produced inside the body. Endogenous opioids, for example, are naturally produced in the body and function to relieve pain and induce pleasure. Similarly, endogenous rhythms, like the circadian rhythm, are internally generated and help regulate sleep-wake cycles, among other biological processes.

The concept of endogenous is the counterpart to exogenous, which refers to processes or substances that originate from outside of an organism. For example, an exogenous hormone would be a hormone introduced to the body from an external source, like medication.

The distinction between endogenous and exogenous is crucial in understanding many aspects of biology, physiology, and neuroscience, from the origin of biochemical compounds to the source of different influences on behaviour and cognition.