Biology Dept., Boston Univ, Boston, MA 02215, USA.
J Neurophysiol. 2012 May;107(10):2692-702. doi: 10.1152/jn.00857.2011. Epub 2012 Feb 15.
Current-clamp recordings were made from the primary (1°) and secondary (2°) branching points (BPs) of axons at the crayfish neuromuscular junction. Action potential (AP) firing initiated by current injected at the 2° BP showed strong adaptation or high-frequency firing at threshold current, whereas AP firing frequency at the 1° BP exhibited a gradual rise with increasing current amplitude. The voltage threshold for AP (V(TH)) was higher at the 2° BP than the 1° BP. 4-Aminopyridine (4-AP) at 200 μM increased AP amplitude and duration at both BPs but reduced threshold current at the 2° BP more than at the 1° BP. This blocker lowered V(TH) at both BPs, but the difference between the BPs remained. Firing patterns evoked at the 2° BP became similar to those evoked at the 1° BP in 4-AP. Thus 4-AP-sensitive channels may be more concentrated in the distal axon and control AP initiation and firing patterns there. Orthodromic APs between the two BPs were also compared. There was no difference in AP amplitude between the two BPs, but AP half-width recorded at the 2° BP was longer than that at the 1° BP. AP duration at both BPs increased gradually, by ∼17%, during a 100-Hz, 500-ms train (in-train rise). Normalized AP half-widths revealed a smaller fractional in-train rise at the 2° BP. Thus, although distal APs were broader, AP duration there was under more stringent control than that of the proximal axon. 4-AP increased AP amplitude and duration of the entire orthodromic train and reduced the magnitude of the in-train rise in AP half-width at both BPs. However, this blocker did not uncover a clear difference between the two BPs. Thus 4-AP-sensitive channels concentrated in distal axon may be essential in preventing unintended firing and modulating AP waveform without interfering with orthodromic AP propagation.
在螯虾的神经肌肉接点，我们从轴突的初级（1°）和次级（2°）分支点（BP）进行电流箝制记录。在 2°BP 处注入电流所引发的动作电位（AP），在阈电流时表现出强烈的适应或高频发放，而在 1°BP 的 AP 发放频率随电流幅度的增加而逐渐升高。在 2°BP 的 AP 电压阈值（V(TH)）高于 1°BP。200 μM 的 4-氨基吡啶（4-AP）增加了两个 BP 的 AP 幅度和时程，但在 2°BP 降低阈电流的程度大于 1°BP。这种阻断剂降低了两个 BP 的 V(TH)，但 BP 之间的差异仍然存在。在 4-AP 中，在 2°BP 诱发的放电模式变得类似于在 1°BP 中诱发的放电模式。因此，4-AP 敏感通道可能更集中在远端轴突中，并控制那里的 AP 起始和发放模式。在两个 BP 之间也比较了顺行性 AP。两个 BP 之间的 AP 幅度没有差异，但在 2°BP 记录的 AP 半宽度比在 1°BP 长。在 100-Hz、500-ms 串（串内上升）期间，两个 BP 的 AP 时程逐渐增加约 17%。归一化 AP 半宽度显示 2°BP 的串内上升幅度较小。因此，尽管远端 AP 较宽，但远端轴突的 AP 时程受到更严格的控制，而不是近端轴突的 AP 时程。4-AP 增加了整个顺行性串的 AP 幅度和时程，并减少了两个 BP 的 AP 半宽度的串内上升幅度。然而，这种阻断剂并没有揭示出两个 BP 之间的明显差异。因此，集中在远端轴突中的 4-AP 敏感通道可能对于防止意外放电和调节 AP 波形而不干扰顺行性 AP 传播是必不可少的。
