Sensory Inputs to the Medulla

Abstract

INTRODUCTION +1298Respiratory neurons in the medulla receive afferent information from many different sensory systems. Principal among these are the central and pe ripheral chemoreceptors, mechanoreceptors in the lung, and proprioceptors in the chest and abdominal wall. These sensory systems have been investi gated with respect to both the information they provide to the medulla and the nature of their transduction mechanisms. This short review focuses on the pulmonary vagal receptors, and on literature that has appeared since the two most recent extensive reviews of this topic (32, 68); both the transduc tion mechanisms and refl ex role of the receptors are considered. Other sensory systems-peripheral and central chemoreceptors-have been the subject of recent symposia (1, 51, 69). PULMONARY VAGAL RECEPTORSDuring the 1960s different names were employed by different investigators for essentially the same pulmonary vagal receptor. Further ANNUAL REVIEWS PACK PULMONARY STRETCH RECEPTORSFrom a functional point of view the main interest is in the stretch receptors (PSR). Evidence indicates that their afferent activity is involved in control of inspiratory and expiratory durations, and the instantaneous magnitude of neural output to a number of the main respiratory muscles. Distribution of Stretch ReceptorsAnatomical studies on the stretch receptors remain scarce although anatomical information on their distribution within the bronchial tree has been obtained by indirect physiological means (5,57,58). The receptors are thought to be situated in the bronchial smooth muscle. In the trachea, when the receptor is silenced by local dissection, the last piece of tissue to be removed is smooth muscle (5). The majority of the receptors (55.6%) are in the extrapulmonary airways (57). The total surface area of the extrapul monary airways is orders of magnitude less than that of the intrapulmonary airways (98); this implies large differences in the density of these receptors (per unit surface area of bronchial wall) between the extra and intrapulmo nary airways. This density of receptors in the extrapulmonary airways may however not refl ect their functional significance (49, 50).Within the intrapulmonary airways the receptors are also denser proxi mally (58). Such a distribution may have advantages, relating the distorting force on the bronchial wall more to the overall degree of lung infl ation and being less infl uenced by local mechanical derangements. A significant number of stretch receptors (of the order of 17% of the total) are located in the extrathoracic trachea (5). The mechanical forces on the extrathoracic trachea during ventilation are completely different from those on the intrathoracic airways. Transmural pressure, which distorts the wall, in the extrathoracic trachea is dependent on intratracheal pressure alone, whereas in the intrathoracic airways it refl ects transpulmonary pres sure. Since airfl ow is related to intratracheal pressure, the presence of receptors in this site may allow the control system to moni...

Effects of Vocal Task and Respiratory Phase on Prephonatory Chest Wall Movements

“…Sensory information is potentially available from a variety of afferent systems, including central and peripheral chemoreceptors, as well as mechanoreceptors in the lungs, airways, muscles, joints, and skin of the respiratory system (Pack, 1981;Paintal, 1973;Porter, 1963;Remmers & Bartlett, 1977;Rossi & Brodal, 1956;Sessle, Greenwood, Lund, & Lucier, 1978;Shannon, 1980). For metabolic breathing, these respiratory afferents are involved in the adaptive control of the basic breathing pattern for changes in ventilatory drive and disturbances in respiratory mechanics (Newsom Davis & Stagg, 1975;Sears, 1971;Shannon, 1986;Euler, 1973Euler, ,1981Widdicombe, 1986).…”

Projection of pulmonary rapidly adapting receptors to the medulla of the cat: an antidromic mapping study.

“…Thus, our results suggest the possibility of a direct excitatory action by some r.a.r.s on inspiratory cells of the dorsal respiratory group and, in turn, on phrenic motoneurones. Such a connexion could be responsible for the excitatory, paradoxical response in the phrenic nerve to lung hyperinflation and for the facilitation of the phrenic output during normal hyperpnoeic states (see Paintal, 1973;Pack, 1981;Widdicombe, 1982 for references). However, branching of r.a.r.…”

“…They are also stimulated during hyperpnoea, pneumothorax and anaphylaxis, and by a variety of noxious agents (see Pack, 1981;Sant'Ambrogio, 1982; Widdicombe, 1982 for reviews). Rapidly adapting receptors may be involved in the reflex bronchoconstriction associated with many pathological states, the triggering of spontaneous sighs, the normal control of expiratory duration and airflow, and a facilitation of the phrenic output during normal hyperpnoeic states (see Paintal, 1973;Pack, 1981 ; Widdicombe, 1982, for reviews). The pathways through which these reflex effects could be produced are unknown.…”

Morphology of pulmonary rapidly adapting receptor relay neurons in the rat

“…RARs respond to inflation and deflation of the lungs with an irregular discharge of action potentials that adapts rapidly when the volume stimulus is maintained. They are also known as lung irritant receptors because they are stimulated by a variety of respiratory irritants such as ammonia, histamine, dust, and smoke (Pack, 1981; Sant'Ambrogio, 1982; Widdicombe, 1986; Lai and Kou, 1998).…”