Estimates of persistent inward currents in human spinal motor neurons: non-linear and sex-dependent trajectories from young to very old age

The contribution of persistent inward currents (PICs) to spinal motor neurons discharge rate is lower in old compared to young adults. However, whether this difference further progresses at very old age (≥80 years) and differs between males and females remains unknown. Thirty young (26 ± 4 years, 15 females), thirty old (72 ± 5 years, 15 females) and twenty-nine very old adults (85 ± 3 years, 15 females) performed isometric dorsiflexion ramp contractions at 20% of maximal voluntary torque. Individual motor unit spike trains were identified from decomposed high-density surface electromyograms of the tibialis anterior. The paired motor unit technique was used to estimate the prolongation effect of PICs (ΔF and normalized ΔF), and the quasi-geometric approach to estimate neuromodulatory (brace height) and inhibitory pattern (attenuation slope) contributions. ΔF was lower in old and very old adults than in young adults, with no difference between old and very old adults regardless of sex. Compared with young adults, normalized ΔF was lower in both old and very old males, whereas in females it is only lower in the very old group. Brace height and attenuation slope did not differ across ages or sexes. These findings indicate that age-related changes in spinal motor neuron excitability are established by the seventh decade and do not progress further in healthy active very old adults, following a nonlinear trajectory that may differ between males and females. The absence of age-related differences in the neuromodulatory index does not support a lower monoaminergic drive in older adults.