Department of Medicine University of Louisville, Louisville, KY, 40292, USA; Robley Rex VA Medical Center, Louisville, KY, 40206, USA.
Respir Physiol Neurobiol. 2021 May;287:103595. doi: 10.1016/j.resp.2020.103595. Epub 2020 Dec 9.
Bronchopulmonary mechanosensors play an important role in the regulation of breathing and airway defense. Regarding the mechanosensory unit, investigators have conventionally adhered to 2 doctrines: one-sensor theory (one afferent fiber connects to a single sensor) and line-labeled theory. Accordingly, lung inflation activates 2 types of mechanosensors: slowly adapting receptors (SARs) and rapidly adapting receptors (RARs) that also respond to lung deflation to produce Hering-Breuer deflation reflex. RARs send signals to a particular brain region to stimulate breathing (labeled as excitatory line) and SARs to a different region to inhibit breathing (inhibitory line). Conventionally, RARs are believed to be mechanosensors, but are also stimulated by a variety of chemicals and mediators. They are activated during different disease conditions and evoke various respiratory responses. In the literature, RARs are the most debatable sensors in the airway. Recent physiological and morphological studies demonstrate that a mechanosensory unit consists of numerous sensors with 4 types, i.e., an afferent fiber connects to multiple homogeneous or heterogeneous sensors (multiple-sensor theory). In addition to SARs and RARs, there are deflation-activated receptors (DARs), which can adapt slowly or rapidly. Each type senses a specific force and generates a unique response. For example, RAR (or SAR) units may respond to deflation if they house DARs responsible for the Hering-Breuer deflation reflex. Multiple-sensor theory requires a conceptual shift because 4 different types of information from numerous sensors carried in an afferent pathway violates conventional theories. Data generated over last eight decades under one-sensor theory require re-interpretation. Mechanosensors and their reflex functions need re-definition. This detailed review of the RARs represents our understanding of RARs under the conventional doctrines, thus it provides a very useful background for interpretation of RAR properties and reflex function against the new proposed multiple-sensor theory.
气道机械感受器在呼吸调节和气道防御中发挥着重要作用。关于机械感受器单元，研究人员传统上坚持两种学说：单感受器理论（一条传入纤维连接到一个单一的感受器）和线标记理论。相应地，肺充气激活两种类型的机械感受器：慢适应感受器（SARs）和快适应感受器（RARs），它们也对肺萎陷作出反应，产生赫令-布勒反射。RAR 向特定脑区发送信号以刺激呼吸（标记为兴奋性线），SAR 向不同脑区发送信号以抑制呼吸（抑制性线）。传统上，RAR 被认为是机械感受器，但也受到各种化学物质和介质的刺激。它们在不同的疾病状态下被激活，并引起各种呼吸反应。在文献中，RAR 是气道中最具争议的传感器。最近的生理和形态学研究表明，一个机械感受器单元由多个感受器组成，即一条传入纤维连接到多个同型或异型感受器（多感受器理论）。除了 SAR 和 RAR 之外，还有萎陷激活受体（DARs），它们可以缓慢或快速适应。每种类型都感知特定的力并产生独特的反应。例如，如果 RAR（或 SAR）单元包含负责赫令-布勒反射的 DAR，则它们可能对萎陷作出反应。多感受器理论需要概念上的转变，因为传入通路中来自大量感受器的四种不同类型的信息违反了传统理论。在单感受器理论下产生的过去八十年的数据需要重新解释。机械感受器及其反射功能需要重新定义。对 RAR 的详细综述代表了我们在传统学说下对 RAR 的理解，因此为解释 RAR 特性和反射功能提供了非常有用的背景，以对抗新提出的多感受器理论。
