Jankelowitz S K, McNulty P A, Burke David
Institute of Clinical Neurosciences, University of Sydney and Royal Prince Alfred Hospital, Sydney, Australia.
Clin Neurophysiol. 2007 Jun;118(6):1397-404. doi: 10.1016/j.clinph.2007.02.025. Epub 2007 Apr 23.
Threshold tracking is a novel technique that permits examination of the excitability of human axons in vivo. Protocols have been validated for sensory and motor axons, but there are limited data on the changes in the excitability of motor axons with age. This study aimed to determine such changes from the third to the eighth decades.
Sixty healthy subjects aged 22-79, 10 per decade, were studied using the TRONDXM4 protocol of the QTRAC threshold-tracking program to assess motor axon function. The median nerve was stimulated at the wrist and the compound muscle action potential was recorded from the thenar muscles.
There was an increase in threshold in elderly subjects, associated with a decrease in slope of the stimulus-response curves. Strength-duration time constant and threshold electrotonus to depolarising and hyperpolarising currents of up to 40% did not change significantly with aging. The current-threshold relationship was similar across all decades for subthreshold depolarising currents, but the slope of the current-threshold relationship was significantly steeper the older the subjects for hyperpolarising currents, particularly those greater than 40% of threshold. There was also a significant decrease in supernormality in the recovery cycle with increasing age.
The threshold of axons increases with age and the extent of supernormality decreases. There may also be greater inward rectification in motor axons, perhaps due to greater activity of I(H), the hyperpolarisation-activated conductance, though this is only significant with hyperpolarising currents greater than 40% of the threshold current.
Many indices of axonal excitability, such as strength-duration time constant, the relative refractory period, late subnormality, threshold electrotonus and the depolarising side of the current-threshold relationship, do not change significantly with age. For other indices, age-related changes may be due to a combination of non-neural factors that alter current access to the node of Ranvier, changes in the axon and its myelination and, possibly, changes in channel activity and/or changes in extracellular K(+).
阈下刺激追踪是一种可在体内检测人类轴突兴奋性的新技术。该方法已在感觉和运动轴突方面得到验证，但关于运动轴突兴奋性随年龄变化的数据有限。本研究旨在确定从第三个十年到第八个十年间运动轴突兴奋性的变化情况。
采用QTRAC阈下刺激追踪程序的TRONDXM4方案，对60名年龄在22 - 79岁之间、每十年10名的健康受试者进行研究，以评估运动轴突功能。在腕部刺激正中神经，并记录鱼际肌的复合肌肉动作电位。
老年受试者的阈值升高，同时刺激 - 反应曲线的斜率降低。强度 - 时间常数以及高达40%的去极化和超极化电流的阈下电紧张在衰老过程中无显著变化。对于阈下去极化电流，所有十年间的电流 - 阈值关系相似，但对于超极化电流，受试者年龄越大，电流 - 阈值关系的斜率越陡，尤其是那些大于阈值40%的超极化电流。随着年龄增长，恢复周期中的超常期也显著降低。
轴突阈值随年龄增加而升高，超常期程度降低。运动轴突可能也存在更大的内向整流，这可能是由于超极化激活电导I(H)的活性增加所致，不过这仅在超极化电流大于阈值电流的40%时才显著。
许多轴突兴奋性指标，如强度 - 时间常数、相对不应期、晚期亚正常期、阈下电紧张以及电流 - 阈值关系的去极化部分，随年龄无显著变化。对于其他指标，与年龄相关的变化可能是由多种因素共同作用引起的，包括改变电流进入郎飞结的非神经因素、轴突及其髓鞘的变化，以及可能的通道活性变化和/或细胞外K(+)的变化。
