Department of Neurosurgery, School of Medicine, Johns Hopkins University, Baltimore, Maryland, United States of America.
PLoS One. 2010 Feb 5;5(2):e9076. doi: 10.1371/journal.pone.0009076.
Different classes of unmyelinated nerve fibers appear to exhibit distinct conductive properties. We sought a criterion based on conduction properties for distinguishing sympathetic efferents and unmyelinated, primary afferents in peripheral nerves.
METHODOLOGY/PRINCIPAL FINDINGS: In anesthetized monkey, centrifugal or centripetal recordings were made from single unmyelinated nerve fibers in the peroneal or sural nerve, and electrical stimuli were applied to either the sciatic nerve or the cutaneous nerve endings, respectively. In centrifugal recordings, electrical stimulation at the sympathetic chain and dorsal root was used to determine the fiber's origin. In centrifugal recordings, sympathetic fibers exhibited absolute speeding of conduction to a single pair of electrical stimuli separated by 50 ms; the second action potential was conducted faster (0.61 0.16%) than the first unconditioned action potential. This was never observed in primary afferents. Following 2 Hz stimulation (3 min), activity-dependent slowing of conduction in the sympathetics (8.6 0.5%) was greater than in one afferent group (6.7 0.5%) but substantially less than in a second afferent group (29.4 1.9%). In centripetal recordings, most mechanically-insensitive fibers also exhibited absolute speeding to twin pulse stimulation. The subset that did not show this absolute speeding was responsive to chemical stimuli (histamine, capsaicin) and likely consists of mechanically-insensitive afferents. During repetitive twin pulse stimulation, mechanosensitive afferents developed speeding, and speeding in sympathetic fibers increased.
CONCLUSIONS/SIGNIFICANCE: The presence of absolute speeding provides a criterion by which sympathetic efferents can be differentiated from primary afferents. The differences in conduction properties between sympathetics and afferents likely reflect differential expression of voltage-sensitive ion channels.
不同类别的无髓神经纤维似乎表现出不同的传导特性。我们寻求一种基于传导特性的标准，以区分外周神经中的交感传出纤维和无髓、初级传入纤维。
方法/主要发现：在麻醉猴中，从腓肠神经或腓浅神经中的单个无髓神经纤维进行离心或向心记录，并分别向坐骨神经或皮肤末梢施加电刺激。在离心记录中，使用交感神经链和背根的电刺激来确定纤维的起源。在离心记录中，交感纤维表现出对 50ms 间隔的单个电刺激对的绝对传导加速；第二个动作电位的传导速度比第一个未条件的动作电位快（0.61 0.16%）。这在初级传入纤维中从未观察到。在 2Hz 刺激（3 分钟）后，交感神经的活动依赖性传导减慢（8.6 0.5%）大于一个传入纤维组（6.7 0.5%），但远小于第二个传入纤维组（29.4 1.9%）。在向心记录中，大多数对机械刺激不敏感的纤维也表现出对双脉冲刺激的绝对加速。没有表现出这种绝对加速的子集对化学刺激（组胺、辣椒素）有反应，可能由对机械刺激不敏感的传入纤维组成。在重复双脉冲刺激期间，机械敏感传入纤维发生加速，而交感纤维的加速增加。
结论/意义：绝对加速的存在提供了一个标准，可以区分交感传出纤维和初级传入纤维。交感神经和传入纤维之间的传导特性差异可能反映了电压敏感离子通道的差异表达。
