Department of Clinical and Movement Neurosciences, UCL Queen Square Institute of Neurology, London, WC1N 3BG, UK.
Institute of Neuroscience, Newcastle University, Newcastle upon Tyne, NE2 4HH, UK.
Cereb Cortex. 2020 May 14;30(5):3403-3418. doi: 10.1093/cercor/bhz318.
Anatomical studies report a large proportion of fine myelinated fibers in the primate pyramidal tract (PT), while very few PT neurons (PTNs) with slow conduction velocities (CV) (<~10 m/s) are reported electrophysiologically. This discrepancy might reflect recording bias toward fast PTNs or prevention of antidromic invasion by recurrent inhibition (RI) of slow PTNs from faster axons. We investigated these factors in recordings made with a polyprobe (32 closely-spaced contacts) from motor cortex of anesthetized rats (n = 2) and macaques (n = 3), concentrating our search on PTNs with long antidromic latencies (ADLs). We identified 21 rat PTNs with ADLs >2.6 ms and estimated CV 3-8 m/s, and 67 macaque PTNs (>3.9 ms, CV 6-12 m/s). Spikes of most slow PTNs were small and present on only some recording contacts, while spikes from simultaneously recorded fast-conducting PTNs were large and appeared on all contacts. Antidromic thresholds were similar for fast and slow PTNS, while spike duration was considerably longer in slow PTNs. Most slow PTNs showed no signs of failure to respond antidromically. A number of tests, including intracortical microinjection of bicuculline (GABAA antagonist), failed to provide any evidence that RI prevented antidromic invasion of slow PTNs. Our results suggest that recording bias is the main reason why previous studies were dominated by fast PTNs.
解剖学研究报告称，灵长类动物的皮质脊髓束 (PT) 中有很大比例的细有髓纤维，而电生理学研究报告的具有较慢传导速度 (CV) (<~10 m/s) 的 PT 神经元 (PTNs) 却很少。这种差异可能反映了记录偏向于快速 PTNs，或者是由于更快的轴突的回返抑制 (RI) 阻止了缓慢 PTNs 的逆行入侵。我们在麻醉大鼠 (n=2) 和猕猴 (n=3) 的运动皮层上使用多探针 (32 个紧密间隔的接触点) 进行记录，研究了这些因素，重点关注具有长逆行潜伏期 (ADL) 的 PTNs。我们在大鼠中鉴定出 21 个 ADL >2.6 ms、CV 为 3-8 m/s 的 PTN，在猕猴中鉴定出 67 个 ADL >3.9 ms、CV 为 6-12 m/s 的 PTN。大多数慢 PTNs 的尖峰幅度较小，只出现在一些记录接触点上，而同时记录的快传导 PTNs 的尖峰幅度较大，出现在所有接触点上。快速和慢速 PTNs 的逆行阈值相似，而慢速 PTNs 的尖峰持续时间明显较长。大多数慢速 PTNs 没有出现逆行反应失败的迹象。多项测试，包括皮质内注射荷包牡丹碱 (GABAA 拮抗剂)，都未能提供任何证据表明 RI 阻止了慢速 PTNs 的逆行入侵。我们的结果表明，记录偏向是以前的研究主要集中于快速 PTNs 的主要原因。
