Control of respiration 529870 223726372 2008-07-05T13:45:30Z 121.131.141.105 more detail on pneumotaxic and apneustic control of respiration - sorry don't know how to make it look good in HTML '''Control of ventilation''' (''control of respiration'') refers to the [[physiology|physiological]] mechanisms involved in the control of [[ventilation (physiology)|physiologic ventilation]]. Gas exchange primarily controls the rate of respiration. The most important function of [[breath]]ing is gas exchange (of [[oxygen]] and [[carbon dioxide]]). Thus the control of respiration is centered primarily on how well this is achieved by the [[lungs]]. == Control Unit == The control unit of ventilation consists of a processor (the breathing centre in the [[brain]]) which integrates inputs (emotional, chemical and physical stimuli) and controls an effector (the [[lungs]]) via [[motor nerves]] arising from the [[spinal cord]]. In humans, quiet breathing occurs by the cyclical contraction of the inspiratory muscles, particularly the [[Diaphragm (anatomy)|diaphragm]]. [[Inhalation]] is normally an active process, and [[exhalation]] is passive. However, when ventilation is increased (over 40 litres per minute), such as during heavy [[exercise]], muscle activity becomes involved in exhalation. Under these circumstances, the work of breathing over time can exceed the metabolic rate of the rest of the body. == Ventilatory Pattern == Ventilation is normally autonomic, with only limited voluntary override, but an exception to this is [[Ondine's curse]], where autonomic control is lost. The pattern neuronal firing when breathing can be divided into inspiratory and expiratory phases. Inspiration shows a sudden ramp increase in motor discharge to the inspiratory muscles (including pharyngeal dilator muscles). Before the end of inspiration, there is a decline in motor discharge. Exhalation is usually silent, except at high [[minute ventilation]] rates. The mechanism of generation of the ventilatory pattern is not completely understood, but involves the integration of neural signals by respiratory control centres in the [[medulla]] and [[pons]]. The nuclei known to be involved are divided into regions known as the following: * medulla ([[reticular formation]]) ** [[ventral respiratory group]] ([[nucleus retroambigualis]], [[nucleus ambiguus]], [[nucleus parambigualis]] and the [[pre-Botzinger complex]]). The ventral respiratory group controls voluntary forced exhalation and acts to increase the force of inspiration. ** [[dorsal respiratory group]] ([[nucleus tractus solitarius]]). The dorsal respiratory group controls mostly inspiratory movements and their timing. * pons ** [[pneumotaxic centre]]. Coordinates transition between inhalation and exhalation Sends inhibitory impulses to the inspiratory areaThe pneumotaxic centre is involved in fine tuning of respiration rate. ** [[apneustic centre]] • Coordinates transition between inhalation and exhalation • sends stimulatory impulses to the inspriatroy area – activates and prolongs inhalation long deep inhalation • pnemotaxic area overides apneustic area There is further integration in the [[anterior horn]] cells of the [[spinal cord]]. == Determinants of Ventilatory Rate == Ventilatory rate (minute volume) is tightly controlled and determined primarily by blood levels of [[carbon dioxide]] as determined by [[metabolic rate]]. Blood levels of [[oxygen]] become important in [[Hypoxia (medical)|hypoxia]]. These levels are sensed by [[chemoreceptor]]s in the [[medulla oblongata]] for pH, and the [[carotid body|carotid]] and [[aortic body|aortic]] bodies for oxygen and carbon dioxide. Afferent neurons from the carotid bodies and aortic bodies are via the [[glossopharyngeal nerve]] (CN IX) and the [[vagus nerve]] (CN X), respectively. Levels of CO<sub>2</sub> rise in the blood when the metabolic use of O<sub>2</sub> is increased beyond the capacity of the lungs to expel CO<sub>2</sub>. CO<sub>2</sub> is stored largly in the blood as bicarbonate (HCO<sub>3</sub><sup>-</sup>) ions, by conversion first to carbonic acid (H<sub>2</sub>CO<sub>3</sub>), by the enzyme carbonic anhydrase, and then by disassociation of this acid to H<sup>+</sup> and HCO<sub>3</sub><sup>-</sup>. Build-up of CO<sub>2</sub> therefore causes an equivalent build-up of the disassociated hydrogen ion, which, by definition, decreases the pH of the blood. During moderate [[exercise]], ventilation increases in proportion to [[metabolism|metabolic]] production of carbon dioxide. During strenuous exercise, ventilation increases more than needed to compensate for carbon dioxide production. [[Lactic acid]] produced during [[anaerobic metabolism]] lowers [[pH]] and thus increases breathing. In [[aerobic metabolism]], one molecule of acid (CO<sub>2</sub>) is produced in order to produce 6 molecules of the energy carrier [[Adenosine triphosphate|ATP]], whereas in anaerobic metabolism, 6 molecules of lactic acid are produced to provide the same amount of energy. Mechanical stimulation of the lungs can trigger certain reflexes as discovered in animal studies. In humans, these seem to be more important in neonates and ventilated patients, but of little relevance in health. The tone of respiratory muscle is believed to be modulated by [[muscle spindle]]s via a reflex arc involving the spinal cord. Drugs can greatly influence the control of respiration. [[Opioids]] and anaesthetic drugs tend to depress ventilation, especially with regards to [[Carbon Dioxide]] response. Stimulants such as [[Amphetamines]] can cause [[hyperventilation]]. [[Pregnancy]] tends to increase ventilation (lowering plasma carbon dioxide tension below normal values). This is due to increased [[progesterone]] levels and results in enhanced gas exchange in the [[placenta]]. Ventilation is temporarily modified by voluntary acts and complex reflexes such as sneezing, coughing and vomiting. == Feedback control == [[sensory receptor|Receptor]]s play important roles in the regulation of respiration; central and peripheral [[chemoreceptor]]s, and [[mechanoreceptor]]s. * '''[[Central chemoreceptors]]''' of the central nervous system, located on the ventrolateral medullary surface, are sensitive to the [[pH]] of their environment. * '''[[Peripheral chemoreceptors]]''' act most importantly to detect variation of the [[oxygen]] in the [[arterial blood]], in addition to detecting arterial carbon dioxide and pH. * '''[[Mechanoreceptors]]''' are located in the [[airway]]s and [[parenchyma]], and are responsible for a variety of reflex responses. These include: ** The [[Hering-Breuer reflex]] that terminates inspiration to prevent over inflation of the lungs, and the reflex responses of [[coughing]], [[airway constriction]], and [[hyperventilation]]. ** The upper airway receptors are responsible for reflex responses such as, [[sneezing]], [[bradycardia]], coughing, closure of [[glottis]], and [[hiccups]]. ** The [[spinal cord]] reflex responses include the activation of additional respiratory muscles as compensation, gasping response, hypoventilation, and an increase in breathing frequency and volume. In addition to involuntary control of respiration by the respiratory center, respiration can be affected by conditions such as emotional state, via input from the [[limbic system]], or [[temperature]], via the [[hypothalamus]]. Voluntary control of respiration is provided via the [[cerebral cortex]], although chemoreceptor reflex is capable of overriding conscious control. == External links== * Paul, Anthony D., et al., Neuronal Connections of a Ventral Brainstem Respiratory Chemosensitive Area: “Ventral Brainstem Mechanisms and Control of Respiration and Blood Pressure” [http://www.oakwood.edu/ocgoldmine/adoc/faculty/apaul/] * Rabbany, Sina Y., “Breathing Coordination”, Hofstra University [http://people.hofstra.edu/faculty/sina_y_rabbany/engg81/breathingcoordination.html] * Webber, Charles L., Jr., Ph.D, Pulmonary Curriculum Function:“Neural Control of Breathing”, Stritch School of Medicine, Loyola University-Chicago [http://www.meddean.luc.edu/lumen/MedEd/medicine/pulmonar/physio/pf11.htm] {{Respiratory physiology}} [[Category:Respiration]]