Several physical phenomena accompany the firing of electrical impulses by axons. Some of these, such as the microscopic swelling of axons, alter the transmission of light through axons. This produces what are called "intrinsic optical signals" because optical methods can be used to see axons fire without adding voltage-sensitive dyes or using electronic amplifiers. These physical changes allow neurons tocommunicate through nonsynaptic signals to adjacent cells, such as other neurons or glia. Two of the three videos in this Teaching Resource show the optical manifestations of the microscopic swelling of axons that accompanies the firing of action potentials in cultured neurons, and one shows the nonsynaptic release of ATP that occurs through membrane channels that are stimulated by neuronal swelling.