Find out how a cell that is permeable to one ion can become charged (either positive or negative) if there is permeability and a concentration gradient in this video lecture. Also discussed is an equation to calculate the membrane...

Watch as the heart cells use energy to reset the concentration gradients for all of the ions after the action potential has gone through. Rishi is a pediatric infectious disease physician and works at Khan Academy. [14:58]

See how muscle cells in the heart contract by allowing Calcium to flow inside and bringing along some positive charge with it! Rishi is a pediatric infectious disease physician and works at Khan Academy. [12:11]

This video focuses on the painting "The Harrowing of Hell", c. 1440-70, carved painted and gilt alabaster, 58.3 x 31.5 cm (Victoria and Albert Museum). The speakers are Dr. Nancy Ross and Dr. Paul Binski. [5:18]