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Respiratoryphysiologylecture5Dr.AmjedH.Abbas Nervous and Chemical Control of Respiration Nervous Control Inspiratory muscles, diaphragm and intercostal muscles, composed of skeletal muscle and must be stimulated to contract, two phrenic nerves responsible for rd th th diaphragm contraction originate at the 3 , 4 , and 5 cervical spinal nerves, 11 th pairs of intercostal nerves originate 1- 11 thoracic spinal nerves. Respiratory Areas in Brainstem These centers are responsible for automatic basic rhythm of respiration, located bilaterally in the reticular formation of the brain stem (which consists from medulla oblongata, pons and midbrain). The primary portions of the brainstem that control ventilation are the medulla oblongata and the pons. A. Medullary respiratory center: consists of dorsal groups which stimulate the diaphragm (inspiratory center) and ventral groups (expiratory center) which stimulate the intercostal and abdominal muscles. B. Pontine respiratory group It is involved with switching between inspiration and expiration, it consists of pneumotaxic and apneustic centers. 1 Respiratoryphysiologylecture5Dr.AmjedH.Abbas Medullary respiratory centers: The medulla oblongata is the primary respiratory control center. Its main function is to send signals to the muscles that control respiration to cause breathing to occur. There are two regions in the medulla that control respiration: The dorsal respiratory group stimulates inspiratory movements. The ventral respiratory group stimulates expiratory movements. 2 Respiratoryphysiologylecture5Dr.AmjedH.Abbas The medulla also controls the reflexes for non-respiratory air movements, such as coughing and sneezing reflexes. Inspiratory Center or Dorsal Respiratory Group (DRG) - Basic rhythmic breathing, it sends signal to the Phrenic nerve ----> Intercostal nerves ---> Diaphragm + external inter-costals it containing inspiratory neurons It sets the basic respiratory rate, stimulates the inspiratory muscles to contract (diaphragm). The signals it sends for inspiration start weakly and steadily increase for ~ 2 sec. This is called a ramp and produces a gradual inspiration. The ramp then stops abruptly for ~ 3 sec and the diaphragm relaxes. Ventral respiratory group (VRG): The neurons in the VRG remain almost inactive during normal quiet respiration. There is no evidence that VRG participates in the basic rhythmical oscillation that controls respiration. When the respiratory drive for increased pulmonary ventilation becomes more than normal as in exercise, respiratory signals spill over into VRG from the basic 3 Respiratoryphysiologylecture5Dr.AmjedH.Abbas oscillatory mechanisms of the DRG area. Then the VRG contribute to the respiratory drive. VRG area is very important in providing powerful expiratory signals to abdominal muscles during expiration. The VRG area operates as an overdrive mechanism when high levels of pulmonary ventilation are required. Pontine respiratory groups The pons is the other respiratory center and is located above the medulla. Its main function is to control the rate or speed of involuntary respiration. It has two main functional regions that perform this role: 1.Pneumotaxic center It is located in upper part of the pons, slightly inhibits medulla, it has inhibitory effect on inspiration causes shorter, shallower, quicker breaths. The pnuemotaxic center sends signals to inhibit inspiration that allows it to finely control the respiratory rate. Its signals limit the activity of the phrenic nerve and inhibits the signals of the apneustic center. It decreases tidal volume. when activity of inspiratory center stops, inhibitory impulses cease from pneumotaxic center and inspiratory impulses initiated. 2.Apneustic center It is located in lower portion of pons, stimulates the medulla, causes longer, deeper, slower breaths (prevent switch off), it has stimulatory effect on inspiratory center and inhibitory on expiratory center. Its activity is modulated on and off by pneumotaxic center. It is intermittently inhibited by vagal discharge arise from lung which appear during inflation of the lung and disappear during deflation of the lung. 4 Respiratoryphysiologylecture5Dr.AmjedH.Abbas The apneustic and pnuemotaxic centers work against each other together to control the respiratory rate. Mechanoreceptors: These receptors are placed in the walls of bronchi and bronchioles of the lung and the main function of these receptors is to prevent the over-inflation of the lungs. Inflation of the lungs activates these receptors and activation of the stretch receptors in turn inhibits the neurons in inspiratory center via vagus nerve. When the expiration starts activation of the stretch receptors gradually ceases allowing neurons in the inspiratory neurons become active again. This phenomenon is called Hering-Breuer Reflex. It is particularly important for infants. In adults it is functional only during exercise when the tidal volume is larger than normal. 5