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Lecture 4

Lecture 4. Control of V E Ventilatory response to CO 2 Ventilatory response to O 2 Ventilatory response to pH Ventilatory response to exercise. Control of VE. The three basic elements of the respiratory control system are; SENSORS, CENTRAL CONTROLLER and EFFECTORS.

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Lecture 4

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  1. Lecture 4 • Control of VE • Ventilatory response to CO2 • Ventilatory response to O2 • Ventilatory response to pH • Ventilatory response to exercise

  2. Control of VE • The three basic elements of the respiratory control system are; SENSORS, CENTRAL CONTROLLER and EFFECTORS. 1- SENSOR; which gather information and feed it to the  2- CENTRAL CONTROLLER (in the brain), which coordinates the information and, in turn, sends impulses to the  3- EFFECTORS (respiratory muscles), which cause VE.

  3. Central controller • Central control of breathing is achieved at the brainstem, specially the pons and midbrain (responsible for involuntary breathing) and the cerebral cortex (responsible for voluntary breathing). • The respiratory centre is divided into 4 groups of neurones spread throughout the entire length of the medulla and pons; • (1) DRG: - It is located in the entire length of the dorsal aspect of the medulla. - It lies in close relation to the NTS where visceral afferents from cranial nerves IX and X terminate. - It comprises inspiratory neurons. Thus, they are almost entirely responsible for inspiration.

  4. (2) VRG: • It is located in each side of the medulla, about 5 milliliters anterior and lateral to the DRG. • They are inactive during quiet breathing, but become activated during ↑ pulmonary VE, as in exercise. • They are mainly expiratory neurons with some inspiratory neurons, both of which are activated when expiration becomes an active process. • They are comprises 4 nuclei; a) the nucleus retroambigualis (NR); which is predominantly expiratory with upper motor neurons passing to the expiratory muscles of the other side. b) the nucleus ambiguous (NA); which controls the dilator function of larynx, pharynx and tongue. c) the nucleus para-ambigualis (NP); which is mainly inspiratory and control the force of contraction of the inspiratory muscles of the opposite side. d) the Botzinger complex (BC); within the nucleus rterofacialis) has widspread expiratory functions.

  5. (3) AC ???: • It is located in the lower pons. • They sends excitatory impulses to the DRG of neurons and potentiates the inspiratory drive. • It receives inhibiting impulses from the sensory vagal fibers of the Hering-Breuer inflation reflex and inhibiting fibers from the pneumotaxic centre in the upper pons. • (4) PC; • It is located dorsally in the upper pons. • It transmits inhibitory impulses to the AC and to the inspiratory areas to switch off inspiration. • The function of this centre is primarily to limit inspiration. This has a secondary effect of increasing the rate of breathing. • Some investigators believed that the role of this centre is “fine tuning” of respiratory rhythm because a normal rhythm can exist in the absence of this centre.

  6. 2) Effectors • They are the muscles of respiration, including the diaphragm, intercostal muscles, abdominal muscles and accessory muscles as sternocleidomastoid. • It is crucially important that these various muscle groups work in a coordinated manner, and this is the responsibility of the central controller. • There is some evidence that some newborn children, particularly those who are premature, have uncoordinated respiratory muscle activity, especially during sleep. For example, the thoracic muscle may try to inspire while the abdominal muscle expire. This may be a factor in the “sudden infant death syndrome” (SIDS).

  7. 3) Sensors • The sensors that contribute to the control of breathing include lung stretch receptors in the smooth muscle of the airway, irritant receptors located between airway epithelial cells, joint and muscle receptors that stimulate breathing in response to limb movement, and juxtacapillary (or J) receptors located in alveolar walls which sense engorgement of the pulmonary capillaries and cause rapid shallow breathing.  • The most important sensors are central chemoreceptors in the medulla as well as peripheral chemoreceptors in the carotid and aortic bodies.  

  8. Central chemoreceptors (CC) • They are most probably located on the ventrolateral surfaces of the medulla oblangato, which bathed CSF. • The CCs in the medulla respond to changes in the pH of the CSF.  ↓ in CSF pH produce ↑ in breathing (hyperVE) whereas ↑ in pH result in hypoVE. • They are highly sensitive to [H+] of the CSF evoked by PaCO2, since CO2 can freely cross the blood-brain barrier into the CSF while the barrier is relatively impermeable to H+ and H2CO3. • Stimulation of these receptors ↑ both the rate of rise and the intensity of the inspiratory signals, thereby ↑ the frequency of the respiratory rhythm.

  9. Peripheral chemoreceptors (PC) • They are located in the carotid bodies at the bifurcation of the common carotid arteries and in the aortic bodies above and below the aortic arch. • They cause an ↑ in VE in response to ↓ in PaO2, ↑ in PaCO2 and ↑ in arterial hydrogen concentrations (↓ in pH). • The carotid bodies are most important in humans. They contain glomus cells of two or more types which show an intense fluorescence staining because of their large content of dopamine. • The mechanism of chemoreception is not yet understood. A popular view has been that glomus cells themselves are chemoreceptors. • They are highly sensitive to changes in PaO2 and to a lesser extent to PaCO2 and pH. They are also sensitive to temperature of the blood and blood flow. • The response of the PCs to PaCO2 is much less important than that of the CCs.

  10. Lung and airway receptors • Receptors in the lung and airways are innervated by myelinated and unmyelinated vagal fibers. • The unmyelinated fibers are C fibers. • The myelinated fibers are commonly divided into SARs and RARs on the basis of whether sustained stimulation leads to prolonged or transient discharge in their afferent fibers. • SARs are also known as pulmonary stretch receptors.They are thought to participate in ventilatory control by prolonged inspiration in conditions that reduce lung inflation. • RARs are stimulated by chemicals such as histamine, dust, cigarette smoke. Therefore, they have been called irritant receptors. • Activation of RARs in the lung may produce hyperpnea. • J receptors are stimulated by hyperinflation of the lung. They play a role in the dyspnea associated with left heart failure, interstitial lung disease, pneumonia and microembolism.

  11. The Hering-Breuer reflexes thought to play a major role in VE by determining the rate and depth of breathing. This can be done by using the information from the SARs to modulate the “switching off” mechanism in the medulla. • The Hering-Breuer inflation reflex is an ↑ in the duration of expiration produced by steady lung inflation, and the Hering-Breuer deflation reflex is a ↓in the duration of expiration produced by marked deflation of the lung. • In human beings, the Hering-Breuer reflex probably is not activated until VT increases to more than three times normal (i.e. < 1.5 l/breath). Therefore, this reflex appears to be mainly a protective mechanism for preventing excess lung inflation rather than an important ingredient in normal control of VE.

  12. Ventilatory response to CO2 • The most important factor in the control of VE is PaCO2. • The ventilatory response to CO2 is normally measured by having the subject inhale CO2 mixture or rebreathe from a bag so that the inspired PCO2 gradually ↑. • With a normal PO2 the VE ↑ by about 2-3 l/min for each 1 mmHg rise in PCO2. Lowering the PO2 produces 2 effects; • 1) there is a higher VE for a given PCO2 • 2) the slope of the line becomes steeper. • The ventilatory response to CO2 is reduced by sleep, increasing age, and genetic, racial and personality factors. It can also be reduced by if the work of breathing is ↑.

  13. Ventilatory response to O2 • The way in which a reduction of PaO2 stimulates VE can be studied by having the subject breathe hypoxic gas mixture. • When the PCO2 is ↑ a reduction in PO2 below 100 mmHg causes some stimulation of VE. • Hypoxemia reflexly stimulates VE by its action on the carotid and aortic body chemoreceptors.

  14. Ventilatory response to pH • A reduction in arterial blood pH stimulates VE. • The chief site of action of a reduced arterial pH is the PCs, especially the carotid bodies in humans. It is also possible that the CCs itself is affected by a change in blood pH if it is large enough.

  15. Ventilatory response to exercise • On exercise, VE ↑ promptly and, during strenuous exercise, it may reach very high levels. • The ↑ in VE closely matches the ↑ in VO2 and VCO2. • The PaCO2 does not ↑ during most form of exercise, however, during sever exercise it falls slightly. • The PaO2 ↑ slightly, and it may fall at very high work levels. • The arterial pH remains nearly constant for moderate exercise, and falls during heavy exercise. • Factors which play a role in the ↑ in VE during exercise includes; - ↑ body temperature - ↑ plasma epinephrine conc - ↑ plasma potassium conc - ↑ CO2 load to the lung - Passive movement of the limbs

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