Estimates of persistent inward currents are reduced in upper limb motor units of older adults

Abstract

Aging is a natural process that causes alterations in the neuromuscular system, which contribute to weakness and reduced quality of life. Reduced firing rates of individual motor units (MUs) likely contribute to weakness, but the mechanisms underlying reduced firing rates are not clear. Persistent inward currents (PICs) are crucial for the initiation, gain control, and maintenance of motoneuron firing, and are directly proportional to the level of monoaminergic input. Since the concentration of monoamines (i.e. serotonin and norepinephrine) are reduced with age, we sought to determine if estimates of PICs are reduced in older (>60 years old) compared to younger adults (<35 years old). We decomposed MU spike trains from high-density surface electromyography over the biceps brachii and triceps brachii during isometric ramp contractions to 20% of maximum. Estimates of PICs (i.e. {Delta}F) were computed using the paired MU analysis technique. Regardless of the muscle, peak firing rates of older adults were reduced by ~1.6 pulses per second (pps) (P = 0.0292), and {Delta}F was reduced by ~1.9 pps (P < 0.0001), compared to younger adults. We further found that age predicted {Delta}F in older adults (P = 0.0261), resulting in a reduction of ~1pps per decade, but there was no relationship in younger adults (P = 0.9637). These findings suggest that PICs are reduced in older adults, and, further, age is a significant predictor of estimates of PICs in older adults. Reduced PIC magnitude represents one plausible mechanism for reduced firing rates and weakness in older individuals. KEY POINTSO_LIPersistent inward currents play an important role in the neural control of human movement and are influenced by neuromodulation via monoamines originating in the brainstem. C_LIO_LIDuring aging, motor unit firing rates are reduced, and there is deterioration of brainstem nuclei, which may reduce persistent inward currents in alpha motoneurons. C_LIO_LIHere we show that estimates of persistent inward currents ({Delta}F) of both elbow flexor and extensor motor units are reduced in older adults. C_LIO_LIEstimates of persistent inward currents have a negative relationship with age in the older adults, but not young. C_LIO_LIThis novel mechanism may play a role in alteration motor firing rates that occurs with aging, which may have consequences for motor control. C_LI