Project Summary/Abstract: The over-65 population is growing, and over 1/3 of aged people will experience a fall. Falls in the elderly often result from frailty caused by muscle weakness, balance deficits, and gait instability. Previously, these deficits were ascribed to declining muscle mass (sarcopenia), however, simply increasing muscle mass does not necessarily rescue age-induced functional deficits. The term dynapenia has been introduced to denote aging-induced loss of muscle strength, which includes deficits in motoneuron excitability, motor unit recruitment, transmitter release at the neuromuscular junction (NMJ), and muscle mass and function. As such, it is critical to fully understand the changes that occur within NMJs to identity new targets for therapy development. Here, we propose to define heterogeneous changes that occur within the transmitter release sites (active zones; AZs) within NMJs that underlying dynapenia in an aged mouse model. Specifically, we will (1) use a combination of electrophysiology and a new optical quantal analysis approach we developed to study changes in the control of transmitter release within single identified NMJs during aging, (2) use a new super-resolution imaging approach of the density and distribution of presynaptic voltage-gated calcium channels within single AZs and compare these results with our single AZ optical measurements of transmitter release in identified NMJs over the aging time course, and (3) use a realistic computer simulation to test the hypothesis that age-induced changes in AZ voltage-gated calcium channel density and distribution predict single AZ and whole synapse function at the aging NMJ. The results from these aims will elucidate the detailed structural and functional changes that occur within transmitter release sites (AZs) of aging NMJs over the time course of the dynamic alterations that occur as dynapenia develops. This information will guide future studies aimed at targeting these specific aging-induced changes with the goal of treating dynapenia.