Effect of aging on properties of motor unit action potentials in the rat medial gastrocnemius muscle

The purpose of this study was to investigate whether age-related changes in motor unit (MU) contractile properties are reflected in parameters of motor unit action potentials (MUAPs). MUs of the medial gastrocnemius muscle were functionally isolated in anaesthetized Wistar rats. A control group of young animals (5-10 mo) was compared to two groups of old rats (24-25 mo and 28-30 mo). The basic contractile properties of MUs as well as the amplitude, total duration, peak-to-peak time, and number of turns within MUAPs were measured. Effects of aging were mainly observed for fast fatigable MUs (a prolongation of MUAPs and increased number of turns). The MUAP amplitude did not change significantly with aging in either MU type, but it correlated to the twitch or tetanic forces, which tended to increase with age, especially for slow MUs. We concluded that the prolongation of MUAPs and the greater incidence of signal turns was probably a result of a decrease in muscle fiber conduction velocity and/or an increase in their dispersion, and enlargement of MU territories - presumably caused by axonal sprouting of surviving motoneurons. The latter might also be responsible for the observed age-related tendency for a increase in MUAP amplitudes in slow MUs.