有点看不懂，大家帮忙看看,xiexie

1.Local anesthetics with a pKa closest to physiological pH will have (at physiological pH) a greater fraction of nonionized base that more readily permeates the nerve cell membrane, generally facilitating a more rapid onset of action. A.drug with a higher pKa will exist predominantly in the unionized form at physiologic pH, making it easier to pass into cells and thus have a quicker time to onset. This is the logic behind adding bicarbonate to local anesthetics in order to speed their onset. It is the lipid-soluble form that more readily diffuses across the neural sheath (epineurium) and passes through the nerve membrane. Once inside the cell, the ionized molecule is what binds to the Na channel more readily. The pKa is not the only factor affecting the onset of LA action as 2-chloroprocaine, which has the fastest onset, has the greatest pKa of the commonly used LAs.麻烦帮我看下最后一句，最后一句2-氯普鲁卡因PKa最高，起效也最快，那就符合上面的具有较高pKa的药物将在生理pH下主要以非离子化形式存在，使其更容易进入细胞并因此具有更快的起效时间。但是为啥原文最后一句：说 pKa不是影响局麻药作用发生的唯一因素，因为具有最快起效的2-氯普鲁卡因具有最大的pKa的局麻药。似乎逻辑上2-氯普鲁卡因不符合上面说的规律，但是从内容上看来并没有转折啊？

2.The axillary artery is located close to, but distal from, the pectoralis muscle. 如何理解又是靠近又是远离？

3.Allergic reactions specifically caused by a local anesthetic are rare. If a reaction occurs after local anesthetic use, it is usually the result of sensitivity to methylparaben, a preservative, or sulfites, which prevent degradation of vasopressors such as epinephrine.

which指代什么？谢谢，我自己觉得是指代整个过敏反应，但是这个过敏反应是否会阻止血管升压药如肾上腺素降解，我就不知道了

methylparaben, a preservative, or sulfites这三者是什么关系？感觉a preservative是methylparaben的同位语，补充说明methylparaben，然后methylparaben, a preservative一起和sulfites并列？