Initiation of Breathing PDF

Initiation of breathing Dr Gwen Hughes Objectives • Describe, at a basic level, the neurogenic basis of breathing • Differentiate between the roles dorsal respiratory group, ventral respiratory group, the apneustic centre and the pneumotaxic centre, and explain consequences of injury to the latter...

Initiation of breathing Dr Gwen Hughes Objectives • Describe, at a basic level, the neurogenic basis of breathing • Differentiate between the roles dorsal respiratory group, ventral respiratory group, the apneustic centre and the pneumotaxic centre, and explain consequences of injury to the latter on the respiratory phases Neurogenic initiation - transection studies Rhythm defining CNS areas • • Thomas Lumsden (1923) • Anaesthetized cats • 3 discrete CNS respiratory centres initiate ventilation Lumsden T (1923) Observations on the respiratory centres in the cat. J Physiol. 57 (3-4): 153-160 3 The Respiratory Center • Bilateral groups of neurons send impulses to the respiratory muscles – Medullary rhythmicity – Inspiratory & expiratory groups (DRG and VRG) – Pneumotaxic centre – Feeds into DRG – Limits inspiration – Controls rate and depth of breathing – The stronger the signal, the shorter the inspiration and faster rate – Apneustic center DRG = dorsal respiratory group VRG = ventral respiratory group (DRG) – Modulates breathing cycle (e.g. no abrupt halt in • Dorsal Respiratory Group (DRG) – – – – Medull a Extends the length of the medulla to VRG The output is to the inspiratory muscles Drives movements and timing Active during inspiration - repetitive ‘ramped’ bursts of inspiratory neuronal action potentials for 2 secs on (allowing inspiration) then 3 sec off (allowing expiration) • Allows steady increase in lung volume rather than inspiratory gasps. – Neurones are located in the nucleus tractus solitarius (NTS) which is the sensory terminal for the vagus and glossopharyngeal nerves that transmit sensory signals from many different receptors to the respiratory centre e.g. proprioceptors, airways etc (VRG) Vagus or glossopharyngeal Sensory information as the Central pattern generation • Complex interaction of > 6 neurones with different firing patterns, concentrated near the pre-Bötzinger complex • 3 phases: – Inspiratory – Post inspiratory or expiratory phase I – Expiratory phase II • Excitatory and inhibitory neurotransmitters e.g. NMDA and GABA respectively in conjunction with neuromodulators Control of breathing Lumb, Andrew, MB BS FRCA, Nunn and Lumb's Applied Respiratory Physiology, 4, 42-58.e3 Copyright © 2021 Copyright © 2021, Elsevier Limited. All rights reserved. https://www.clinicalkey.com/student/content/book/3-s2.0B9780702079085000138 Efferent pathway from the respiratory centres: Bulbospinal inspiratory neurones • Nerves descend from the respiratory centre in the anterior part of the lateral column of the spinal cord and terminate in the anterior horn cells of the cervical and thoracic segments • Phrenic nerves give the motor control to the diaphragm – formed by rootlets exciting the cervical spine C3, C4 and C5 Pon s Pneumotaxic center (upper pons) is inhibitory Sends continual inhibitory impulses to inspiratory center of the medulla oblongata and finely tunes the respiratory rate and breathing pattern by signaling to the DRG. Its signals limit the activity of the phrenic nerve and inhibits the apneustic centre. It decreases the tidal volume which prevents the lungs becoming too full. Stimulated more? Breathe faster and shallower Stimulated less? E.g. upper pons damage - Depth of inspiration increases 9 Altered breathing rate and depth Normal breathing Upper pons damage Apneustic center (lower pons) – excitatory Pon s Smooths the breathing cycle and integrates inspiratory cutoff. Sends signals for inspiration for long deep breaths by stimulating the DRG and VRG to increase inspiration and tidal volume i.e. delays the ‘off signal’ Inhibited by stretch receptors at max inspiration or by the pneumotaxic centre Overstimulation? Apneusis (gasping) through exciting inspiratory neurones which prolong inspiration. Damage? The respiratory cycle is abrupt 11 Altered breathing rate Normal Upper pons damage Apneusis (gasping) Summary • • • • DRG – inspiratory VRG – inspiratory and expiratory Pneumotaxic inhibits the apneutic centre Many different neurotransmitters and neuromodulators • Co-ordinated activity is essential