Nature Research
Nature Education: Scitable: Essentials of Cell Biology
Explains how cells send and receive signals. Includes a quiz that can be completed after free registration.
National Cancer Institute at the National Institutes of Health
Seer Training Modules: Introduction to the Nervous System
Self-guided learning activity where students learn about the structure and function of the human nervous system. There is a short quiz at the end of the lesson to check for understanding.