This proposal takes advantage of advances in laser technology that make it possible to detect electrical activity in unstained living tissue in a noninvasive manner. The first aim is to build a laser microscope that can detect SH signals generated by focused laser radiation at wavelengths that will minimize the possibility of tissue damage. The optics and data acquisition will be engineered for optimal use in a noninvasive manner and to maximize user-friendliness. The second aim is to evaluate and test the sensitivity of the microscope including its spatial and temporal resolution in detecting action potentials in a classical neurophysiological preparation. The third aim is to use the SH microscope for pilot tests on the functional development of synapses at the developing neuromuscular junction of vertebrates. These studies could provide information about the mechanisms of synapse formation that are not currently possible with other techniques. In addition, the ability to measure activity at the neuromuscular junction in a noninvasive way would have value in a number of clinical applications. Diseases like Myasthenia Gravis, the Muscular Dystrophies, ALS, and Guillain-Barr syndrome affect the activity of nerve and/or muscle cells. Optical microscopy could lead to new diagnostic methods and a better understanding of the mechanisms of disease progression.