neural control of breathing

Question 1

why do we need to modulate the rate of ventilation ?

Answer 1

• The rate of ventilation is constantly adjusted to meet the body’s demand for O2 and production of CO2.
  *Adequate absorption of O2 and expulsion of CO2 to/from the body is achieved by maintaining pressure gradients between alveoli & blood.

Question 2

what physiological processes initiate breathing ?

Answer 2

• respiratory muscles provide the movement required for ventilation
  *as respiratory muscles consist of skeletal muscle they require neural stimulation to contract
• innervation from motor neurons synapsing from descending spinal tracts provide the contractile signal.

Question 3

which muscles are utilized in quiet/forced inspiration/expiration ?

Answer 3

Question 4

what is the basic pattern of ventilation determined by ?

Answer 4

• determined by a complex system of neurons within the brainstem called the central pattern generator (CPG) AKA respiratory pattern generator (RPG)
• behavior of this system is controlled via inputs from receptors and sensors within the body that provide feedback for the required level of ventilation needed to maintain healthy CO2 , O2 and pH levels.
• inputs from higher somatic and emotional centers also feed into the GPG so breathing can be voluntarily controlled and affected by extreme emotional states.
• it is impossible to voluntarily hold your breath until you die as either the urge to breath caused by excess CO2 will be overpowering, or acute hypoxemia will result in loss of consciousness (at which point involuntary breathing will begin).

Question 5

what are central respiratory chemoreceptors (CRC) ?

Answer 5

• located in the medulla and indirectly monitor changes in paCO2 by responding to changes in the pH of the cerebrospinal fluid (CSF).
• an increase in paCO2 will decrease blood pH however H ions present in arterial blood cannot pass the blood brain barrier as they are charged so CRC don’t respond to blood pH directly
• arterial CO2 can pass through the blood brain barrier into the CSF and react to produce carbonic acid which results in activating the CRCs.
• CRC respond to paCO2 and provides the signal in initiating the urge to breathe.

Question 6

what are peripheral chemoreceptors ?

Answer 6

• present within carotid and aortic bodies.
• detect levels of 02 , CO2 and pH within arterial blood.
• activated by low O2 , high CO2 and low pH and signal to medullar centers to increase ventilation

Question 7

what is the relationship between PCO2 and ventilation due to chemoreceptors ?

Answer 7

• due to the dominant role of CRCs ventilation is generally proportional to paCO2.
• pH is closely linked to CO2 levels and hypoxic drive only occurs at very low PaO2

Question 8

describe ventilation during sleep

Answer 8

• ↓metabolic rate = ↓respiratory demands
• Postural changes alter mechanics of breathing
• ↓SNS & ↑PNS tone = ↓HR, BP & CO.
• ↓tidal volume, ↓breathing frequency, ↓minute volume
• ↓SaO2 (≈96%), ↑PaCO2 (≈7kPa)
• ↓upper airway calibre

Question 9

describe some Pathologies associated with dysfunction in central processes that initiate breathing

Answer 9

• Trauma – damage to respiratory centres in the brainstem
• Stroke – ischaemia-induced brainstem tissue injury
• Drugs (e.g. opioids) – suppression of neuronal activity
• Congenital central hypoventilation syndrome
• Neonates – incomplete development of respiratory centres prior to birth
• Altitude – control systems unable to cope with abnormal atmospheric environment (i.e. low O2 and low CO2), e.g. Cheyne-Stokes respiration

Question 10

what is sleep apnea ?

Answer 10

• temporary cessation of breathing during sleep
• Characterised by >5 episodes per hour lasting >10 seconds.
• Durations of apnoeas may be as long as 90 seconds and the frequency of episodes as high as 160 per hour.

Effects on health:
* Tiredness (poor sleep quality)
* Cardiovascular complications (stress + ↑SNS tone)
* Obesity/Diabetes (inflammation + metabolic dysfunction)

Question 11

what is cheyne-stokes respiration ?

Answer 11

• Hypoxia and hypercapnia (high CO₂) trigger hyperventilation, reducing CO₂ levels too much.
• Low CO₂ levels suppress breathing, leading to apnea.
• Apnea causes CO₂ to build up, triggering hyperventilation again.
• The cycle repeats every 30 seconds to 2 minutes.

Question 12

what is hypercapnic drive ?

Answer 12

increased ventilation in response to increased levels of CO2 within arterial blood