Respiratory Control and Stimulation

The respiratory muscles p. 108) are innervated by nerve fibers extending from the cervical and thoracic medulla (C4 -C8 and T1 -T7). The most important control centers are located in the medulla oblongata and cervical medulla (C1-C2), where interactive inspiratory and expiratory neurons on different levels (^ A1, red and green areas). The network of these spatially separate neuron groups form a rhythm generator (respiratory "center") where respiratory rhythm originates (^ A1). The neuron groups are triggered alternately, resulting in rhythmic inspiration and expiration. They are activated in a tonic (non-rhythm-dependent) manner by the formatio g reticularis, which receives signals from respira-'■C tory stimulants in the periphery and higher .- centers of the brain.

Respiratory sensors or receptors are in-^ volved in respiratory control circuits (^ p. 4). 1/1 Central and peripheral chemosensors on the medulla oblongata and in the arterial circulation continuously register gas partial pressures in cerebrospinal fluid (CSF) and blood, respectively, and mechanosensors in the chest wall respond to stretch of intercostal muscles to modulate the depth of breathing (^ A2). Pulmonary stretch sensors in the tracheal and bronchial walls respond to marked increases in lung volume, thereby limiting the depth of respiration in humans (Hering-Breuer reflex). Muscle spindles (^ p. 318) in the respiratory muscles also respond to changes in airway resistance in the lung and chest wall.

Chemical respiratory stimulants. The extent of involuntary ventilation is mainly determined by the partial pressures of O2 and CO2 and the pH of blood and CSF. Chemosensors respond to any changes in these variables. Peripheral chemosensors in the glomera aortica and carotica (^ A3) register changes in the arterial PO2. If it falls, they stimulate an increase in ventilation via the vagus (X) and glossopharyngeal nerves (IX) until the arterial PO2 rises again. This occurs, for example, at high altitudes (^ p. 136). The impulse frequency of the sensors increases sharply when the Po2 drops below 13 kPa or 97mmHg (peripheral 132 ventilatory drive). These changes are even stronger when PCO2 and/or the H+ concentration in blood also increase.

Central chemosensors in the medulla react to CO2 and H+ increases (= pH decrease) in the CSF (^ A4 and p. 126). Ventilation is then increased until Pco2 and the H+ concentration in blood and CSF decrease to normal values. This mostly central respiratory drive is very effective in responding to acute changes. An increase in arterial PCO2 from, say, 5 to 9 kPa increases the total ventilation Ve by a factor of ten, as shown in the CO2 response curve (^ A6).

When a chronic rise in Pco2 occurs, the previously increased central respiratory drive decreases (^ p. 126). If O2 supplied by artificial respiration tricks the peripheral chemosen-sors into believing that there is adequate ventilation, the residual peripheral respiratory drive will also be in jeopardy.

During physical work (^ A5), the total ventilation increases due to (a) co-innervation of the respiratory centers (by collaterals of cortical efferent motor fibers) and (b) through impulses transmitted by proprioceptive fibers from the muscles.

Non-feedback sensors and stimulants also play an important role in modulating the basic rhythm of respiration. They include

! Irritant sensors in the bronchial mucosa, which quickly respond to lung volume decreases by increasing the respiratory rate (deflation reflex or Head's reflex), and to dust particles or irritating gases by triggering the cough reflex.

! J sensors of free C fiber endings on alveolar and bronchial walls; these are stimulated in pulmonary edema, triggering symptoms such as apnea and lowering the blood pressure.

! Higher central nervous centers such as the cortex, limbic system, hypothalamus or pons. They are involved in the expression of emotions like fear, pain and joy; in reflexes such as sneezing, coughing, yawning and swallowing; and in voluntary control of respiration while speaking, singing, etc. ! Pressosensors (^ p. 214), which are responsible for increasing respiration when the blood pressure decreases.

! Heat and cold sensors in the skin and thermoregu-latorycenter. Increases (fever) and decreases in body temperature lead to increased respiration. ! Certain hormones also help to regulate respiration. Progesterone, for example, increases respiration in the second half of the menstrual cycle and during pregnancy.

A. Respiratory control and stimulation

Voluntary action

Respiratory neurons (respiratory "center"): expiratory inspiratory

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