Factors governing speed of action potential conduction and neuromuscular transmission in aged rats

Impaired motor coordination is a widespread attribute of senescence. As this might be due to slower action potential propagation in peripheral nerves or to prolonged delays of neuromuscular transmission, axon conduction velocity, synaptic delays, and associated morphologic features governing speed of conduction were studied in the phrenic-diaphragm preparations of rats aged 10 (mature adult) and 28 (aged) months. Conduction velocity and synaptic delay of the motoneurons were measured with electrophysiologic recording techniques. Speed of conduction did not change with age, but the average synaptic delay increased from 0.28 in the younger rats to 0.40 ms in the 28-month-old animals. Nerve composition was examined using the electron microscope. There were no age-related changes in the number of myelinated or nonmyelinated axons, capillaries, or myelin sheath invaginations. Moreover, fiber diameter and myelin sheath width did not vary with age. The total extracellular space was about 32% larger in the aged rats: the synaptic cleft widths were 42.7 and 50.6 nm in the 10- and the 28-month animals, respectively. We concluded that in the rat age-related changes in the transmission properties of peripheral nerve are minor. Furthermore, temporal motor deficits during aging must be associated primarily with changes in central pathways or at the muscular apparatus.