Akaishi Tetsuya
Department of Neurology, Tohoku University Graduate School of Medicine.
Department of Neurology, Yonezawa National Hospital.
Tohoku J Exp Med. 2018 Feb;244(2):151-161. doi: 10.1620/tjem.244.151.
Saltatory conduction is an essential phenomenon to facilitate the fast conduction in myelinated nerves. The conventional conductive models assumed electric circuits with local current along the axonal membrane to explain the nerve conduction in unmyelinated nerves. However, whether such models with local current can be also applied to the saltatory conduction in myelinated nerves is unknown. In this report, I propose a new model of saltatory conduction by focusing on the behavior of electric charges in the axoplasm, not limited to the membrane. In myelinated nerves, because of the large internodal length and the low ion channel density in the internodal segment, the whole cross-section of the internodal axoplasm would contribute to the signal conduction. Because the conducted signals originate from the sodium ion influx through the voltage-gated sodium (NaV) channel at the Ranvier's nodes, an individual conducted signal can be described as a single electrostatic compressional wave of positive charges in the internodal axoplasm. Based on this model, the total number of NaV channels in one Ranvier's node would regulate the strength of the wave. Also, the internodal length would be important for the faster conduction in larger myelinated axons. Based on the linear relationships between axonal diameter, internodal length, and conduction velocity, the internodal length would be inversely proportional to the ratio of the transmitted overall wave strength at a Ranvier's node to the original strength at the proximal adjacent node. This new mathematical model may have wide applicability and usability for the conduction in myelinated nerves.
跳跃传导是促进有髓神经快速传导的一种重要现象。传统的传导模型假定沿轴突膜存在局部电流的电路，以解释无髓神经中的神经传导。然而，这种基于局部电流的模型是否也能应用于有髓神经的跳跃传导尚不清楚。在本报告中，我提出了一种新的跳跃传导模型，该模型关注轴浆中电荷的行为，而不仅限于膜。在有髓神经中，由于节间长度长且节间段离子通道密度低，节间轴浆的整个横截面都将参与信号传导。由于传导信号源于通过郎飞结处的电压门控钠通道（NaV）流入的钠离子，单个传导信号可被描述为节间轴浆中带正电荷的单个静电压缩波。基于该模型，一个郎飞结处的NaV通道总数将调节波的强度。此外，节间长度对于较大有髓轴突的更快传导也很重要。基于轴突直径、节间长度和传导速度之间的线性关系，节间长度将与郎飞结处传输的总波强度与近端相邻结处原始强度的比值成反比。这种新的数学模型可能对有髓神经的传导具有广泛的适用性和实用性。
