Control of Breathing Flashcards
What system establishes control of breathing?
CNS
Where is breathing initiated, and what is it modified by?
initiated in medulla
modified by:
- higher structures in CNS
- sensory input from central and peripheral chemoreceptors, mechanoreceptors in lungs, chest wall, & other peripheral mechanical signals
What is automatic control of breathing in CNS?
central pattern generator
- initial phase of breathing in and out is automatically controlled
- info travels to make chest wall move and result in minute ventilation of breathing
- feedback from periphery (chemoreceptors, mechanoreceptors in airway/chest wall/lungs) to automatic control
What is voluntary control of breathing in CNS?
cortical input feeding forward to automatic control
What is integrated control of breathing in CNS?
- comes to chest wall to initiate breathing
- can create all kinds of sensory sensations that feedback to medulla or brainstem, or cortex
Compare control of rhythmic activity of heart and generation of cardiac output, to control of rhythmic activity of medullary respiratory neurons and generation of ventilation using respiratory muscles.
heart: rhythmic activity comes from heart tissue
chest wall muscles for breathing: rhythmic activity is controlled by medulla or brainstem
Peripheral Sensory Input and Voluntary Control
What feeds forward to the medulla, where breathing is initiated?
from upper areas of brain
- cerebral cortex that voluntary controls
from other receptors
- stretch receptors in lung
- irritant receptors
- receptors in muscle and joints when exercising
- receptors in skeletal muscles of respiratory system
- changes in arterial blood in terms of O2 and CO2 and H+
Peripheral Sensory Input and Voluntary Control
What other areas of brain are under behavioural control?
temperature and pain
all of these cortices feed forward into system
What are the two medullary respiratory groups?
dorsal respiratory group (DRG)
ventral respiratory group (VRG)
What is the dorsal respiratory group (DRG)?
mainly inspiratory neurons driving inspiratory muscles to contract during active inspiration
receives input from peripheral chemoreceptors and mechanoreceptors
What is the ventral respiratory group (VRG)?
mainly expiratory neurons driving expiratory muscles to contract during active/forced expiration
normal breathing is passive elastic recoil of lungs
silent during quiet breathing during inspiration
What can the control of breathing be significantly modified by?
- pontine regions and cortical regions in CNS
- complex feedforward systems
- feedback mechanisms from periphery outside
History About Trying to Figure Out How Breathing is Controlled
areas in pons that refine rhythm generated by medulla by DRG and VRG
- rhythmicity would be altered by destroying pontine regions
pneumotaxic centre:
- stops inspiration
- allows for expiration (inspiratory off switch)
- when destroyed, leads to apneusis (prolonged deep, sustained inspiration)
apneustic centre: leads to apneusis
What are the 3 types of nerves that send information from medulla to muscles of respiration?
- phrenic nerve
- intercostal nerve
- cranial nerve
What do phrenic nerves supply? What are they formed by?
supply motor output to diaphragms
formed by joining rootlets exiting cervical spine C3-C5
one nerve each on left and right sides of neck (two nerves total – one nerve for each half/side of diaphragm)
What do intercostal nerves supply? Where are they located?
supply motor output to intercostal and abdominal muscles
exit from thoracic and lumbar spine
bulbospinal neurons impinge on neuron in spinal cord at lower region, then go to their muscles for breathing
What do cranial nerves supply? What do they do when breathing?
supply motor output to upper airway dilator muscles
when breathing, info from bulbospinal region and cranial nerves keep upper airway open during inspiration (so it doesn’t collapse), and impinges on respiratory muscles to allow for breathing
Feedback and Feed Forward Input to Medulla
see notes
Feedback and Feed Forward Input to Medulla
What are the feedback loops? (2)
movement of chest wall / lungs (mechanoreceptors)
arterial PCO2, PO2, pH (chemoreceptors)
Feedback and Feed Forward Input to Medulla
What are the feedforwards? (3)
voluntary/behavioural control from cerebral motor cortex
pons – areas that refine rhythm
hypothalamus – ie. temperature regulation
What is the Hering Breuer reflex?
(mechanoreceptor feedback to medulla)
reflex triggered to prevent over inflation of lungs
- mediated by vagus nerve (CN X)
- stretch receptors in smooth muscle of airways respond to stretching of lung during inflation to allow expiration
- provide feedback to medulla saying there’s plenty of air, so you can stop breathing and expire
What does the Hering Breuer reflex play a major role in establishing?
rate + depth (rhythm) of breathing in most mammals, but not adult humans at rest
When does the Hering Breuer reflex operate in humans?
only operates when we’re breathing fairly large tidal volume
When does the Hering Breuer reflex play a role in modifying respiratory rhythm in newborn humans?
at rest
When does the Hering Breuer reflex play a role in modifying respiratory rhythm in adults?
at tidal volumes > 1 L (exercise)
Key Mechanical Reflex Mechanisms of Respiratory Control
see notes
What are the two types of chemoreceptors that feedback to medulla?
central chemoreceptors – slow, feedback 80%
peripheral chemoreceptors – fast, feedback 20%
What happens when peripheral and central chemoreceptors mechanisms are not acting?
ncreased levels of CO2 on brain tissue (including medulla) directly is inhibitory
kills neurons
Central Chemoreceptors
Where are they located?
in CNS, brain
Central Chemoreceptors
What do they sense?
CO2, and ultimate changes in H+
DO NOT sense O2
What is PaCO2?
key regulator of breathing
tight homeostatic control with normal range: 35-45 mmHg
Central Chemoreceptors Mechanism
see notes
Central Chemoreceptors
What would happen to PaCO2 if you held your breath?
arterial CO2 increases
blood acidifies
increase ventilation
Central Chemoreceptors
What would happen to PaCO2 if you hyperventilated?
arterial CO2 decreases less H+ inhibits medulla decrease ventilation CO2 increase (negative feedback loop)
Peripheral Chemoreceptors
What are they?
carotid and aortic bodies
- minuscule structures ‘tasting’ blood – high blood supply
- carotid body at carotid sinus
- aortic body at aorta
(separate from baroreceptors – stretch receptors)
Peripheral Chemoreceptors
What is sensory info carried by?
CB sensory info carried by glossopharyngeal nerve
AB sensory info carried by vagus nerve
Peripheral Chemoreceptors
What do they sense?
mainly PaO2
but also sense PaCO2 and arterial pH
What is hyperventilation?
breathing out more CO2 (results in low CO2)
What is hypoventilation?
breathing out less CO2 (results in high CO2)
What do metabolic acids do?
stimulate peripheral chemoreceptors, increasing ventilation
What are some metabolic acids? (2)
- lactic acid: produced in skeletal muscle during intense exercise
- diabetic ketoacidosis (Kussmaul breathing)
Peripheral Chemoreceptor – Mechanism
see notes
Sensing Arterial Plasma pH – Mechanism
see notes
What is congenital central hypoventilation syndrome (Ondine’s curse)?
rare disorder in children (1200 cases known world wide)
breathing is adequate when awake – conscious/voluntary breathing is working
breathing is inadequate/absent during sleep – automatic breathing is not working
- alveolar hypoventilation automatic control inadequate – PHOX2B mutations common but not in all cases nor the only gene mutations known
some patients with CCHS have low/absent ventilatory response to elevated CO2 (hypercapnia), low O2 (hypoxia), and metabolic acidosis
treatment: mechanical ventilation / diaphragm pacing
central mechanism unclear
