Spectrum of myelinated pulmonary afferents (III) cracking intermediate adapting receptors

Conventional one-sensor theory (one afferent fiber connects to a single sensor) categorizes the bronchopulmonary mechanosensors into the rapidly adapting receptors (RARs), slowly adapting receptors (SARs), or intermediate adapting receptors (IARs). RARs and SARs are known to sense the rate and magnitude of mechanical change, respectively; however, there is no agreement on what IARs sense. Some investigators believe that the three types of sensors are actually one group with similar but different properties and IARs operate within that group. Other investigators (majority) believe IARs overlap with the RARs and SARs and can be classified within them according to their characteristics. Clearly, there is no consensus on IARs function. Recently, a multiple-sensor theory has been advanced in which a sensory unit may contain many heterogeneous sensors, such as both RARs and SARs. There are no IARs. Intermediate adapting unit behavior results from coexistence of RARs and SARs. Therefore, the unit can sense both rate and magnitude of changes. The purpose of this review is to provide evidence that the multiple-sensor theory better explains sensory unit behavior.

Deflation-activated receptors, not classical inflation-activated receptors, mediate the Hering-Breuer deflation reflex

Many airway sensory units respond to both lung inflation and deflation. Whether those responses to opposite stimuli come from one sensor (one-sensor theory) or more than one sensor (multiple-sensor theory) is debatable. One-sensor theory is commonly presumed in the literature. This article proposes a multiple-sensor theory in which a sensory unit contains different sensors for sensing different forces. Two major types of mechanical sensors operate in the lung: inflation- and deflation-activated receptors (DARs). Inflation-activated sensors can be further divided into slowly adapting receptors (SARs) and rapidly adapting receptors (RARs). Many SAR and RAR units also respond to lung deflation because they contain DARs. Pure DARs, which respond to lung deflation only, are rare in large animals but are easily identified in small animals. Lung deflation-induced reflex effects previously attributed to RARs should be assigned to DARs (including pure DARs and DARs associated with SARs and RARs) if the multiple-sensor theory is accepted. Thus, based on the information, it is proposed that activation of DARs can attenuate lung deflation, shorten expiratory time, increase respiratory rate, evoke inspiration, and cause airway secretion and dyspnea.

Spectrum of myelinated pulmonary afferents (II)

Recently, it has been recognized that a single airway sensory unit may contain multiple receptive fields and that each field houses at least one encoder. Since some units respond to both lung inflation and deflation, we hypothesized that these units contain heterogeneous encoders for sensing inflation and deflation, respectively. Single unit activities were recorded from the cervical vagus nerve in anesthetized, open chest, and mechanically ventilated rabbits. Fifty-two airway sensory units with multiple receptive fields that responded to both lung inflation and deflation were identified. Among them, 13 units had separate receptive fields for inflation and deflation, where one of the fields could be blocked by local injection of 2% lidocaine (10 μl). In 8 of the 13 units, the deflation response was blocked without affecting the unit's response to inflation, whereas in the remaining five units, the inflation response was blocked without affecting the deflation response. Our results support the hypothesis that a single mechanosensory unit may contain heterogeneous encoders that can respond to either inflation or deflation.

Structure and development of larval antennae in embryos of Lytta viridana LeConte (Coleoptera: Meloidae)

At time of hatch (252–264 h at 25 ± 0.5 °C), each antenna in Lytta viridana has three flagellomeres, three extrinsic muscles, and 25 sensilla of five different types, including a large composite sensillum of 19 sensory units on flagellomere II. Each antenna evaginates from epidermis on either side the stomodaeum beginning at 16% of embryogenesis. At 21%, a cell near its apex divides into two pioneer neurons that move into its lumen and project their axons to the brain by 29%. Sensillar stem cells begin to emerge at 23%, those of the appendix within a large embryonic placode and, from 26 to 48%, divide asymmetrically to generate the neurons and accessory cells of each sensillum. Sensillar axonogenesis begins at 34%, the first axons contact the brain at 35%, and antennal glomeruli begin to form within the deutocerebra at 57%. At 35%, the trichogen cell of each sensillum begins to grow out and larval cuticle is deposited about these, beginning at 57%. Upon withdrawal of trichogen cytoplasm from within the appendix at 81%, the dendrites of each sensory unit grow into it and begin to branch. Functional aspects are addressed and the observations compared with the limited information available on embryos of other insects.