Institute for Physical Activity and Nutrition (IPAN), School of Exercise and Nutrition Sciences, Faculty of Health, Deakin University, Geelong, VIC, Australia.
School of Medical and Health Sciences, Centre for Human Performance, Edith Cowan University, Joondalup, WA, Australia.
J Physiol. 2023 Sep;601(17):3705-3716. doi: 10.1113/JP284603. Epub 2023 Jul 24.
Nervous system deterioration is a primary driver of age-related motor impairment. The motor neurones, which act as the interface between the central nervous system and the muscles, play a crucial role in amplifying excitatory synaptic input to produce the desired motor neuronal firing output. For this, they utilise their ability to generate persistent (long-lasting) depolarising currents that increase cell excitability, and both amplify and prolong the output activity of motor neurones for a given synaptic input. Modulation of these persistent inward currents (PICs) contributes to the motor neurones' capacities to attain the required firing frequencies and rapidly modulate them to competently complete most tasks. Thus, PICs are crucial for adequate movement generation. Impairments in intrinsic motor neurone properties can impact motor unit firing capacity, with convincing evidence indicating that the PIC contribution to motor neurone firing is reduced in older adults. Indeed, this could be an important mechanism underpinning the age-related reductions in strength and physical function. Furthermore, resistance training has emerged as a promising intervention to counteract age-associated PIC impairments, with changes in PICs being correlated with improvements in muscular strength and physical function after training. In this review, we present the current knowledge of the PIC magnitude decline during ageing and discuss whether reduced serotonergic and noradrenergic input onto the motor neurones, voltage-gated calcium channel dysfunction or inhibitory input impairments are candidates that: (i) explain age-related reductions in the PIC contribution to motor neurone firing and (ii) underpin the enhanced PIC contribution to motor neurone firing following resistance training in older adults.
神经系统恶化是与年龄相关的运动损伤的主要驱动因素。运动神经元作为中枢神经系统和肌肉之间的接口，在放大兴奋性突触输入以产生所需的运动神经元放电输出方面起着至关重要的作用。为此，它们利用产生持久（持久）去极化电流的能力来增加细胞兴奋性，并放大和延长运动神经元的输出活动，以产生给定的突触输入。这些持久内向电流（PIC）的调节有助于运动神经元达到所需的放电频率，并迅速调节它们以胜任地完成大多数任务。因此，PIC 对于适当的运动生成至关重要。内在运动神经元特性的损伤会影响运动单位的放电能力，有令人信服的证据表明，老年人运动神经元放电中 PIC 的贡献减少。事实上，这可能是导致与年龄相关的力量和身体功能下降的一个重要机制。此外，阻力训练已成为对抗与年龄相关的 PIC 损伤的一种有前途的干预措施，PIC 的变化与训练后肌肉力量和身体功能的改善相关。在这篇综述中，我们介绍了 PIC 在衰老过程中幅度下降的现有知识，并讨论了减少的血清素能和去甲肾上腺素能输入到运动神经元、电压门控钙通道功能障碍或抑制性输入损伤是否是：（i）解释与年龄相关的运动神经元放电中 PIC 贡献减少的候选因素，以及（ii）支持老年人进行阻力训练后运动神经元放电中 PIC 贡献增加的候选因素。
