Abstract
The squid giant axon has had a critical role in the introduction of optical methods for monitoring membrane potential and for monitoring intracellular free calcium using metallochromic indicator dyes. The chapter discusses different kinds of optical signals that have been examined on squid axons. First, changes in the intrinsic optical properties of axons have been measured in an attempt to study the alterations in axon structure that generate or accompany the action potential. Second, absorption and fluorescence changes from axons stained with a variety of dyes, referred to as “extrinsic signals,” allow the optical measurements of membrane potential when the electrode measurements of potential are difficult or impossible. Third, both intrinsic and extrinsic signals have been found whose time course far outlasts the time course of the action potential. These include light scattering, birefringence, and absorption of stained axons. The chapter describes the biological use of metallochromic indicator dyes to measure changes in the concentration of intracellular free calcium.