Basic electrophysiological properties of spinal cord motoneurons during old age in the cat

Abstract

1. The electrophysiological properties of alpha-motoneurons in old cats (14-15 yr) were compared with those of adult cats (1-3 yr). These properties were measured utilizing intracellular recording and stimulating techniques. 2. Unaltered in the old cat motoneurons were the membrane potential, action potential amplitude, and slopes of the initial segment (IS) and soma dendritic (SD) spikes, as well as the duration and amplitude of the action potential's afterhyperpolarization. 3. In contrast, the following changes in the electrophysiological properties of lumbar motoneurons were found in the old cats: a decrease in axonal conduction velocity, a shortening of the IS-SD delay, an increase in input resistance, and a decrease in rheobase. 4. In spite of these considerable changes in motoneuron properties in the old cat, normal correlations between different electrophysiological properties were maintained. The following key relationships, among others, were the same in adult and old cat motoneurons: membrane potential polarization versus action potential amplitude, duration of the afterhyperpolarization versus motor axon conduction velocity, and rheobase versus input conductance. 5. A review of the existing literature reveals that neither chronic spinal cord section nor deafferentation (13, 21) in adult animals produce the changes observed in old cats. Thus we consider it unlikely that a loss of synaptic contacts was responsible for the modifications in electrophysiological properties observed in old cat motoneurons. 6. We conclude that during old age there are significant changes in the soma-dendritic portion of cat motoneurons, as indicated by the modifications found in input resistance, rheobase, and IS-SD delay, as well as significant changes in their axons, as indicated by a decrease in conduction velocity.

Age-related decline in rate of torque development is accompanied by lower maximal motor unit discharge frequency during fast contractions

“…Although a suboptimal cortical drive cannot be discounted, the progressive decline in the instantaneous motor unit discharge frequency during the successive discharges at the onset of a fast contraction is consistent with the hypothesis of an age-related alteration in the intrinsic properties of motoneurons (17,31). This early decline in motoneuron discharge rate when the cell is activated by constant-current injection, which is known as spike frequency adaptation (8,20), could be modified by aging as it seems to be the case with chronic changes in physical activity levels (18).…”

Discharge Characteristics of Biceps Brachii Motor Units at Recruitment When Older Adults Sustained an Isometric Contraction

“…For example, motor neurons from older animals exhibit greater input resistance (Chase et al 1985;Kalmar et al 2009;Morales et al 1987), reduced rheobase current (Kalmar et al 2009;Morales et al 1987), and either longer (Cameron et al 1991;Kalmar et al 2009) or unchanged (Engelhardt et al 1989Morales et al 1987) AHP duration. Such parameters can be estimated only with indirect approaches in humans, such as the interval death rate analysis of single motor unit ISIs (Matthews 1996) or evaluation of the relation between the mean and SD of ISIs within a train of single motor unit action potentials (Piotrkiewicz et al 2007).…”

“…Because recordings obtained from the same motor unit in a second set of data (n ϭ 12) were consistent with the larger set of data made from separate motor units (n ϭ 53), the difference in discharge characteristics for the two tasks was attributed to differences in the integration of synaptic input received by the motor neuron (Riley et al 2008). Given age-associated changes in both the synaptic input that is delivered to motor neurons (Boxer et al 1988;Eisen et al 1996;Oliviero et al 2006) and in the intrinsic properties of the motor neurons (Engelhardt et al 1989;Morales et al 1987;Piotrkiewicz et al 2007;Rossini et al 1992), it seems likely that the discharge characteristics of motor units at recruitment will differ for older adults when they perform such tasks. The purpose of the study was to compare the discharge characteristics of motor units recruited during an isometric contraction that was sustained with the elbow flexor muscles by older adults at target forces that were less than the recruitment threshold force of each isolated motor unit.…”

Disrupted myelin and axon loss in the anterior commissure of the aged rhesus monkey

“…In this study we have the first indication that in white matter myelin damage precedes axon loss, as the proportion of abnormal myelin profiles increases significantly in middle-aged animals compared to young animals. Disruption of myelin sheaths in the optic nerve is known to impair the conduction velocity of action potentials (Gutierrez et al, 1995), and an age-related reduction in axonal conduction velocity has been reported in several other systems (Aston-Jones et al, 1985;Morales et al, 1987;Xi et al, 1999). Reduction in conduction velocity might explain why the prevalence of myelin abnormalities in monkey prefrontal and visual cortex relates to the magnitude of age-related cognitive impairment Peters and Sethares, 2002), since the timing in neuronal circuits would thereby be altered with age.…”