Control of Breathing Flashcards
What is the MRC and its components (& their functions?)
MRC is a system of neurons in the medulla oblongata and pons. Responsible for autonomic control of breathing.
The two most important groups of neurons are the DRG and VRG
DRG = inspiratory neurons. Outputs to diaphragm and external intercostals. Signal gradually ramps up until contraction occurs. Signals cease to allow passive expiration
VRG = contains both inspiratory and expiratory neurons. Has more to do with forced inspiration and expiration:
Outputs to abdominals and internal ic muscles for forced exiration
Outputs to scalines sternocleinomastoids and pectorals for forced inspiration
Note that during forced inspiration, both DRG and VRG are involved
Describe the Pontine Respiratory conrol centre and the functions of the centres there
Pontine Resp Centre fine-tumes breathing cycles
Pneumotaxic Centre = inhibits inspiratory neurons to shorten inspiration. Also smooths transition from in-expiration
Apneustic Centre = wants to prolong inspiration - usually kept in check by pneumotaxic centre
What factors alter ventilation (control centres, receptors, etc)
Cerebral cortex - voluntary override to a certain extend
MRC and Pontine RC (always)
Hypothalamus - emotional respone e.g. sharp intake of air upon shock
Peripheral and central chemoreceptors
Stretch receptors - Hering-Breuer Reflex
Proprioceptors in Joints and Muscles - ‘lear’ to increase ventilation upon increased movement
Irritant receptors - cough reflex
Receptors in airways & lungs
RAPR: Rapidly adapting pulmonary receptors - irritant receptors located between endothelial cells. mediate cough reflex / bronchiconstriction
C-Fibre endings - in pulmonary interstitial space. Respond to histamine, bradykinin, prostacyclin, etc. Play a role in asthma/allergic reaction. Also respond to smoke, capsaicin
Peripheral Sensory Receptors: located in upper airways mucosa. Respond to general sensory and chemosensory stimulation
Stretch-receptors - mediate Hering-Breuer reflex - mechanoreceptive, respond to increased stretch.
Proprioreceptors: in muscles and joints. Stimulate increased ventilation in response to incerase movement
Chemoreceptors = Central and Peripheral. Respond to O2, CO2, H+ etc. more on this later….
What is Hering-Hreuer Reflex?
Shortens inspiratory phase by increased pneumotaxic activity in response to increased stretch.
This only kicks in in adults at very large tidal volumes >1L etc.
Is the main control of tidal volume ventilation in infants, whose chemoreceptor control is not yet matured
Central Chemoreceptors and workings of
CCS located near surface of ventral medulla, away from MRC
Respond to changes in CO2 and alter ventilation accordingly.
Only CO2 can cross the BBB (not H+ and HCO3-).
But, CCs only have a limited response to CO2. Main effect is driven by H+. Thus, CO2 must be hydrolysed into HCO3- and H+ in the CSF. As CSF has low protein content (compared to plasma), it doesn’t buffer H+ well, thus the CCs respond strongly to the increased H+.
Peripheral chemoreceptors: workings and innervation*
Respond to changes in PO2. Usually a resting output to stimulate ventilation. This ceases at really high PO2, and sharply increases at ~60mmHg (which corresponds to the beginning of the steep slope of the Hb-O2 dissociation curve).
Located in or close to the walls of carotid arteries and aortic arch. Most concentrated in carotids.
Each carotid body is innervated by branches of the carotid sinus nerve, which is a branch of the glossopharylgeal nerve (CNIX). This projects to the medulla.
*also respond a small amount to CO2 and H+
Factors that decrease the ventilatory response to PCO2 changes:
Decrease response = don't respond until higher PCO2, and the steepness of the level of response is shallower Anaethetics Certain drugs: morphine, barbituaries SLeep Ageing Chronic hypercapnia**
Increase response = starts responding within tight range, and has a steeper incline of level of response as the PCO2 deviates further from ideal Hypoxia Metabolic acidosis Progesterone Neurochemicals
***Very high PCO2 can result in respiratory depression - CNS narcosis
