Respiration Lesson 7 Objectives • Identify where in the brain the basic rhythm of breathing is generated. • Specify the key sources of sensory information to the medullary respiratory center that modify its output to the respiratory muscles. • Provide examples of modification arising from pontine structures lung mechanoreceptors. central & peripheral chemoreceptors • Describe the sequence of events by which changes in arterial levels PCO2, PO2 & pH stimulate ventilation. 5) Explain why increases in arterial hydrogen ion concentration do not stimulate the central chemoreceptors. Respiration Lesson 7 • What is it that the respiratory control system controls? • What is the primary drive to breathe in man at rest? • What is the primary stimulus to increase ventilation during exercise? 1 Control of Breathing Components of the neural control system What is controlled? respiratory center : medulla chemoreceptor spinal cord respiratory muscles movement of chest wall & lungs ventilation alveolar-capillary membrane gas exchange arterial blood PCO2 , PO2 , pH Control of Breathing Respiratory Centres in the brainstem establish a rhythmic breathing pattern Whereas the heart can generate its own rhythm due to intrinsic pacemaker activity, the respiratory muscles are skeletal and are innervated by spinal nerves which require neural stimulation. The rhythmic neural activity that establishes the normal breathing pattern arises in the PreBötzinger medulla complex 2 Control of Breathing Breathing Is Initiated in the Medulla The Medullary Respiratory Centres consist of the: PreBötzinger Complex Most researchers currently believe that this is the source of the respiratory rhythm PreBötzinger complex Control of Breathing Breathing Is Initiated in the Medulla The Medullary Respiratory Centres consist of the: Dorsal Respiratory Group These cells reside in the nucleus tractus solitarius. They receive sensory information from lung mechanoreceptors and peripheral arterial chemoreceptors which they relay to the PreBötzinger Complex They receive motor output from PreBötzinger the PreBötzinger Complex which complex they relay to the phrenic motor nucleus (which resides in the 3rd, 4th and 5th cervical segments of the spinal cord 3 Control of Breathing PreBötzinger complex The Diaphragram Is Innervated by Phrenic Nerves Control of Breathing Breathing Is Initiated in the Medulla The Medullary Respiratory Centres consist of the: Ventral Respiratory Group Receives motor output from the PreBötzinger Complex which it relays to the motor nuclei of the intercostal and abdominal muscles in the thoracic and lumbar segments of the spinal PreBötzinger cord complex 4 Control of Breathing PreBötzinger complex The Intercostal Muscles are Innervated by Thoracic Spinal Nerves Control of Breathing Hering-Breuer Reflex respiratory center : medulla spinal cord mechanoreceptors respiratory muscles movement of chest wall & lungs ventilation Mechanoreceptors in the lungs (pulmonary stretch receptors = PSR) sense the rate and volume of inflation and act to prevent the lungs from over inflating. They act to terminate inspiration and promote expiration. 5 Control of Breathing Breathing Is Initiated in the Medulla The Medullary Respiratory Centres consist of the: Pneumotaxic Centre These neurons provide a tonic excitatory input to the PreBötzinger Complex They also act like the PSR and serve to help terminate inspiration and promote expiration PreBötzinger complex Control of Breathing Breathing Is Initiated in the Medulla The Medullary Respiratory Centres consist of the: Apneustic Centre There is no apneustic centre However, if the pneumotaxic centre is removed along with input from the PSR, a pattern of prolonged inspiratory gasps occurs called apneusis PreBötzinger If the pons is then removed, complex breathing returns towards a normal pattern 6 Control of Breathing Pons respiratory center : medulla chemoreceptor spinal cord mechanoreceptors respiratory muscles movement of chest wall & lungs ventilation alveolar-capillary membrane gas exchange arterial blood PCO2 , PO2 , pH Control of Breathing CHEMORECEPTORS • peripheral chemoreceptors • central chemoreceptors 7 Control of Breathing Peripheral Chemoreceptors Carotid Bodies & Aortic Bodies* Control of Breathing Note: These receptors do not begin to respond strongly until PO2 falls below 60 mmHg (i.e. they are not strongly stimulated during resting breathing at sea level) Remember - this corresponds to the plateau region of the oxygen equilibrium curve 8 Control of Breathing Thus these receptors do not begin to stimulate breathing strongly until Hb begins to desaturate significantly Large decreases in O2 act on other nervous tissue to reduce activity – which could set up a vicious cycle of respiratory depression if not for the action of the carotid bodies Control of Breathing Peripheral Chemoreceptors 9 Control of Breathing CO2/pH Chemoreceptors PreBötzinger complex CO2/pH provides the primary stimulus to breathe at rest acting via both peripheral and central chemoreceptors Control of Breathing Central Chemoreceptors The central chemoreceptors are located a few millimeters below the ventral surface of the medulla stimulated by: small changes (few mm Hg) in arterial PCO2 via the associated changes in [H+] in the brain ECF PreBötzinger complex 10 Control of Breathing Peripheral Chemoreceptors Peripherally, CO2/pH acts on the same chemoreceptor cells in the carotid body as O2. The peripheral receptors provide a rapid initial response to changes in arterial CO2/pH until the central receptors take over and provide a sustained response. Control of Breathing CO2 Chemoreceptors 2 3 sustained response 1 Rapid, initial response 11 Control of Breathing CO2 Chemoreceptors Despite the fact both peripheral and central chemoreceptors respond to change in pH – the response to changes in arterial pH are due exclusively to stimulation of the peripheral chemoreceptors Control of Breathing 12 Control of Breathing Examples of Non-co2 (Metabolic) Acidosis That Stimulate Ventilation Through Peripheral Chemoreceptors • In health: lactic acidosis of intense exercise • In disease states: ketoacidosis in diabetes mellitus Control of Breathing Chemoreceptors & Negative Feedback Control of Ventilation 13 Control of Breathing Exercise alters ventilation through a variety of different means 1) Blood gases usually do not change 2) Feedforward from locomotor control centres 3) Reflexes originating from joints and muscle receptors (neural and humoral) 4) Increases in body temperature 5) Release of epinephrine Control of Breathing Ventilation can be influenced by factors other than the need for gas exchange higher brain regions Pons respiratory center : MEDULLA chemoreceptor mechanoreceptors spinal cord respiratory muscles movement of chest wall & lungs ventilation alveolar-capillary membrane gas exchange arterial blood PCO2 , PO2 , pH 14.