I’ve said it before — Crash Course videos are awesome. This one is their introduction to motivation, which covers four key theories:
1. Evolutionary theory
2. Drive-reduction theory
3. Optimal arousal theory
4. Maslow’s hierarchy of needs
In typical Crash Course style the video is technical without being mind-boggling and in-depth without being overwhelming.
This is definitely focused on the psychology of motivation, rather than the physiology… you can find some more on that side of things (hormones, neurotransmitters) in this video.