Respiratory Physiology LEC 6 PDF

Respiratory Physiology.LEC.6 Dr.Zainab Ali Altufailie \2024 Regulation of Respiration At the end of the lecture, you will be able to explain the : Regulation of respiration 1. Nervous regulatory mechanism 2. Chemical regulatory mechanism...

Respiratory Physiology.LEC.6 Dr.Zainab Ali Altufailie \2024 Regulation of Respiration At the end of the lecture, you will be able to explain the : Regulation of respiration 1. Nervous regulatory mechanism 2. Chemical regulatory mechanism 3. Reflex control Structure and function of the respiratory center pulmonary ventilation is adjusted according to metabolic demands of the body. Unlike all other organ system, respiration demonstrates: 1. automaticity 2. self modulation Rate, depth and rhythm of respiration is controlled by group of neurons situated reticular formation of brainstem. The collection of these neurons are called respiratory center. The rhythmic discharge from the brain that produces spontaneous respiration is regulated by two mechanisms: a. Nervous regulatory mechanism b. Chemical regulatory mechanism c. Reflex control The three basic elements of the respiratory control system are: 1. Sensors that gather information 2. Central controller in the brain, which coordinates the information 3. Effectors (respiratory muscles), which cause ventilation Page | 1  Respiratory Physiology.LEC.6 Dr.Zainab Ali Altufailie \2024 Respiratory Centers The rhythmic discharge is initiated by a group of pacemaker cells in the brainstem referred to as Pre-Botzinger complex of neurons (PBZ). They are situated in either sides of medulla. These neurons discharge spontaneously and rhythmically. They produce rhythmic discharges in phrenic motor neurons. All the neurons regulating respiration project into PBZ. The respiratory centers are situated in the reticular formation of the brainstem and depending upon the situation in the brainstem, they are classified into medullary and pontine center. A. Medullary centers include two group of neurons: 1. the dorsal respiratory group (DRG) 2. ventral respiratory group (VRG) which generate the basic respiratory rhythm B. The pontine centers include : 1. apneustic center (APN) 2. pneumotaxic center (PNC) Both of which modify the activity of medullary respiratory centers. Ponto-medullary respiratory center neurons are of two types: 1. I neurons are active during inspiration 2. E neurons are active only during forceful expiration. Page | 2 Respiratory Physiology.LEC.6 Dr.Zainab Ali Altufailie \2024 Page | 3  Respiratory Physiology.LEC.6 Dr.Zainab Ali Altufailie \2024 I. Dorsal Respiratory Group Neurons Most of the neurons are located within the nucleus tractus solitaries’ (NTS) and some in the adjacent reticular substance. The basic rhythm of respiration is generated mainly in the (DRG). They contain mainly I neurons. DRG send fibers to phrenic motor neurons which innervate diaphragm. NTS is the sensory termination of both vagal and glossopharyngeal nerve which transmit sensory signals into respiratory center from peripheral baroreceptors, chemoreceptors and pulmonary receptors. Page | 4  Respiratory Physiology.LEC.6 Dr.Zainab Ali Altufailie \2024 II. Ventral Respiratory Group Neurons Ventral group is a long column of neurons that extends through nucleus ambiguous and nucleus retroambiguus in the ventrolateral medulla. They contain both I and E neurons. Normally, this center is inactive during quiet breathing and it becomes active during forced breathing or when the inspiratory center is inhibited. The rhythmic discharge of the neurons in the medullary respiratory center is spontaneous, but is modified by: (i) neurons in the pons (ii) by afferents in the vagus nerves from receptors in the airways and lung 2.Pontine respiratory centres: 1.Pneumotaxic Center Situated in dorsal part of upper pons in nucleus parabrachialis and Kolliker-fuse nuclei. It contains both I and E neurons. Main function of PNC is to switch off inspiration so that duration of inspiration is controlled. Indirectly PNC increases respiratory rate by limiting duration of inspiration, i.e. when the duration for inspiration is reduced, naturally the expiration time is also reduced so that respiratory rate increases. 2.Apneustic Center Situated in the reticular formation of lower pons. It is made up of diffusely located neurons in the region of nucleus pontocaudalis and rostral part of nucleus gigantocellularis. This center is always excitatory to medullary inspiratory center and it increases the depth of inspiration. Cortex Breathing is under voluntary control to a considerable extent, and the cortex can override the function of the brainstem within limits. Other parts of the brain, such as the limbic system and hypothalamus, can alter the pattern of breathing, for example, in emotional states such as rage and fear. Page | 5 Respiratory Physiology.LEC.6 Dr.Zainab Ali Altufailie \2024 Inspiratory ramp signal The nervous signal that is transmitted to the inspiratory muscles, mainly diaphragm is not an instantaneous burst of action potentials. It begins weakly and increases steadily in a ramp manner for about 2 sec to cause contraction of diaphragm (inspiration). Then the excitatory signals ceases abruptly for the next 3 sec During this period expiration occurs to allow relaxation of diaphragm. At the end of 3 sec, the inspiratory ramp signals reappear in the same pattern and the cycle is repeated. The obvious advantage of the ramp is that it causes a steady increase in the volume of the lungs during inspiration, rather than inspiratory gasps. Page | 6 Respiratory Physiology.LEC.6 Dr.Zainab Ali Altufailie \2024 2. Chemical Regulation The chemical mechanism of regulation of respiration is operated through the chemoreceptors. These receptors are stimulated by any change in the concentration of O2, CO2, H+ and other drugs, chemical hormones. Chemoreceptors are of 2 types: i. Central chemoreceptors ii. Peripheral chemoreceptors. 1.Central Chemoreceptors Central chemoreceptors are situated in the ventral part of medulla. They are distinct and separate from pontomedullary respiratory center. Stimulus is increase in pCO2. A chemoreceptor is a receptor that responds to a change in the chemical composition of the blood or other fluid around it. The most important receptors involved in the minute-by-minute control of ventilation are those situated near the ventral surface of the medulla in the vicinity of the exit of the 9th and 10th nerves The central chemoreceptors are surrounded by brain extracellular fluid and respond to changes in its H+ concentration. An increase in H+ concentration stimulates ventilation, whereas a decrease inhibits it. The composition of the extracellular fluid around the receptors is governed by : 1. the cerebrospinal fluid (CSF) 2. local blood flow, 3. local metabolism. CSF is apparently the most important. It is separated from the blood by the blood- brain barrier, which is relatively impermeable to H+ and HCO3- ions, although molecular CO2 diffuses across it easily. Page | 7  Respiratory Physiology.LEC.6 Dr.Zainab Ali Altufailie \2024 When the blood Pco2 rises, CO2 diffuses into the CSF from the cerebral blood vessels, liberating H+ ions that stimulate the chemoreceptors. Thus, the CO2 level in blood regulates ventilation chiefly by its effect on the pH of the CSF. The resulting hyperventilation reduces the Pco2 in the blood and therefore in the CSF. CO2 Stimulates the Chemo sensitive Area Although CO2 has little direct effect in stimulating the neurons in the chemosensitive area, it does have a potent indirect effect. It has this effect by reacting with the water of the tissues to form carbonic acid, which dissociates into hydrogen and bicarbonate ions; the hydrogen ions then have a potent direct stimulatory effect on respiration. Why does blood CO2 have a more potent effect in stimulating the chemosensitive neurons than do blood hydrogen ions? The cerebral vasodilation that accompanies an increased arterial Pco2 enhances diffusion of CO2 into the CSF and the brain extracellular fluid. The normal pH of the CSF is 7.32, and because the CSF contains much less protein than blood, it has a much lower buffering capacity. As a result, the change in CSF pH for a given change in Pco2 is greater than in blood. If the CSF pH is displaced over a prolonged period, a compensatory change in HCO3-occurs as a result of transport across the blood-brain barrier. However, the CSF pH does not usually return all the way to 7.32. The change in CSF pH occurs more promptly than the change of the pH of arterial blood by renal compensation a process that takes 2 to 3 days. Page | 8 Respiratory Physiology.LEC.6 Dr.Zainab Ali Altufailie \2024 2.Peripheral Chemoreceptors They are carotid body and aortic body. There are two carotid bodies, one on either side of midline near bifurcation of common carotid artery. These contain chemoreceptors which are sensors of arterial O 2 tension, CO2 tension and blood pH.. From carotid body impulses are transmitted via Hering nerve The carotid bodies have a very high blood flow for their size. Usually two or more aortic bodies are present near the arch of aorta and impulses are transmitted through vagus N. The peripheral chemoreceptors are responsible for all the increase of ventilation that occurs in humans in response to arterial hypoxemia. the chemoreceptors are exposed at all times to arterial blood, not venous blood, and their PO2 values are arterial PO2 values. Decreased Arterial Oxygen Stimulates the Chemoreceptors. When the oxygen concentration in the arterial blood falls below normal, the chemoreceptors become strongly stimulated. Page | 9  Respiratory Physiology.LEC.6 Dr.Zainab Ali Altufailie \2024 Homework…….. Q:Define hypoxia and Enumerate different types of it? ---------------------------------------------------------------------------------------------------- ---------------------------------------------------------------------------------------------------- ---------------------------------------------------------------------------------------------------- ---------------------------------------------------------------------------------------------------- ---------------------------------------------------------------------------------------------------- Q:define COPD? ---------------------------------------------------------------------------------------------------- ---------------------------------------------------------------------------------------------------- ---------------------------------------------------------------------------------------------------- ---------------------------------------------------------------------------------------------------- --------------------------------------------------------------------------------------------------- Good luck Page | 10