L29. Generation and Control of Rhythmic Breathing

Question 1

Generation of rhythmic breathing - involuntary control - inspiratory centre?

Answer 1

• The respiratory muscles, which are skeletal muscles, do not contract spontaneously
• Our rhythmic breathing is generated involuntarily in the respiratory centres in the medulla oblongata (also cardiovascular (CVS) centre)
• The neurons in the inspiratory centre spontaneously discharge
• When discharging, they initiate action potentials in the motor neurons to the inspiratory muscles and induce muscle contraction
• The spontaneous discharge of the inspiratory centre is limited in duration and it soon ceases (at end of inspiration)

Question 2

Involuntary control - expiratory centre?

Answer 2

• Neurons in the expiratory centre are normally quiescent; they do not discharge spontaneously
• Thus, when the inspiratory centre stops firing contraction stops and the inspiratory muscles relax
• Expiration occurs passively due to the elastic recoil of the lungs
• During forced expiration/exercise/disease: the expiratory centre fires to activate expiratory muscles

Question 3

Medulla processes?

Answer 3

The medulla receives constant input from higher brain areas and chemo/mechano receptors in the body

Question 4

Voluntary control?

Answer 4

By choice we can:
- Breathe deeply and slowly
- Breathe shallowly and rapidly
- Hold our breath
- Perform a forced expiration
- Speak and sing

Voluntary control comes from the cerebral cortex
- Sends signals directly to the respiratory muscles’ motor neurons in the spinal cord, bypassing the respiratory centres

Question 5

Changes in involuntary control?

Answer 5

The inspiratory and expiratory centres also receive sensory inputs (via the Nucleus Tractus Solitarius (NTS)) from a variety of sensory receptors including:
- Mechanoreceptors (e.g. irritants)
- Chemoreceptors (e.g. arterial gas)

These sensory inputs cause reflex (re)adjustment in the breathing response to:
- Exercise, airway irritants, environmental changes

Question 6

Sneeze?

Answer 6

Positive reflex and mechanoreceptors
- Irritation of the nasal mucosa stimulates mechanoreceptors
- In response, several inspirations are superimposed followed by a powerful expiration
- This powerful expiration is a sneeze
- The high expiratory airflow velocity achieved, helps to remove the irritant

Question 7

Cough?

Answer 7

Positive reflex and mechanoreceptors
- Mechanoreceptors in the larynx are stimulated by irritants
- In response, a long slow inspiration occurs
- This is followed by a rapid and powerful expiration, initially against a closed glottis
- The high expiratory airflow velocity achieved, helps to remove the irritant

Question 8

Control of breathing by chemoreceptors?

Answer 8

• Breathing is adjusted automatically in order to maintain normal levels of arterial PO2, PCO2, and [H+]
• Arterial gases are altered by alternation of alveolar ventilation
• In order to be able to achieve homeostasis of arterial PO2, PCO2, and [H+], the chemical composition of arterial blood is closely monitored by chemoreceptors
• Have both peripheral and central chemoreceptors

Question 9

Carotid bodies?

Answer 9

Peripheral chemoreceptors - O2 supply to the brain

Carotid bodies are located in arteries and signal via glossopharyngeal nerve

Question 10

Aortic bodies?

Answer 10

Peripheral chemoreceptors - O2 supply to the brain

Aortic bodies are located in arteries and signal via vagus nerve

Question 11

Peripheral chemoreceptors stimuli?

Answer 11

• A decrease in arterial PO2 (called hypoxia)
• An increase in arterial PCO2 (called hypercapnia)
• An increase in arterial H+ concentration (reduced pH, called acidosis)
• When peripheral chemoreceptors are stimulated by the above stimuli, they increase firing causing an increase in ventilation
• The complete breathing responses to hypoxia (reduced PO2) and arterial acidosis are dependent on the peripheral chemoreceptors

Question 13

Central chemoreceptors?

Answer 13

• Central chemoreceptors are located in the medulla oblongata

They are insensitive to:
* Hypoxia (low PO2)
* Arterial acidosis (because H+ ions cannot pass from arterial blood into the interstitial fluid in the brain, due to the presence of the blood-brain barrier, BBB)

They are sensitive to:
* Concentration of H+ in the brain extracellular fluid
1. The source of H+ ions at the central chemoreceptors is CO2
2. CO2 can pass through the blood-brain barrier
- In the interstitial fluid in the brain, CO2 is converted into bicarbonate and H+ ions
- The newly formed H+ ions are detected by the central chemoreceptors and ventilation is increased

Question 14

Hypoxic stimulus?

Answer 14

The ventilation response only occurs when arterial PO2 falls below 60mmHg
- Peripheral chemoreceptors are relatively insensitive to a hypoxic stimulus where PO2 > 60

Question 15

Response to hypercapnia?

Answer 15

Peripheral chemoreceptors:
- Increased PCO2 leads to a rise in arterial blood [H+] which stimulates the chemoreceptors, leading to increased ventilation
- Peripheral receptor stimulation by CO2 is less prominent than stimulation of central receptors (only 30% of effect)

Central chemoreceptors:
- (70% of effect)
- CO2 rapidly diffuses across the blood-brain barrier
- Increased PCO2 increases the [H+] in the interstitial fluid in the medulla
- The H+ ions directly stimulate the central chemoreceptors and increase ventilation

Question 16

PCO2 vs ventilation - response to hypercapnia?

Answer 16

• Even a small increase in arterial PCO2 leads to a large increase in ventilation –> CO2 is “blown off”, reducing the PCO2
• 4mmHg change in PCO2 doubles ventilation
• Response allows close control of arterial PCO2
• As CO2 in the body increases, breathing is stimulated
  E.g. a breath-hold or lung disorder (emphysema)

Question 17

Response to metabolic acidosis?

Answer 17

Exercise –> lactic acid –> lactate + H+ (metabolic acidosis)
H+ ions cannot cross the blood-brain barrier
A signal is sent to the medullary respiratory centres
Ventilation is increased –> hyperventilation

CO2 is “blown off”
The PCO2 of alveolar air is reduced
The PCO2 of arterial blood is reduced

Breathing resolves respiratory and non-respiratory acidosis

Question 18

Response to metabolic alkalosis?

Answer 18

Metabolic alkalosis (increased pH)
- Loss of H+ ions can occur following vomiting
- Loss of H+ ions increases the pH of blood (alkalosis)
- The reduced [H+] is detected by the peripheral chemoreceptors and ventilation is reduced
- Induces hypoventilation
- Hypoventilation leads to an elevation in the arterial PCO2
- The elevated PCO2 leads to the formation of H+ ions

Question 19

The Bohr effect alters the carriage of O2 BECAUSE the Bohr effect changes the affinity of Hb for O2 in response to CO2

A. If both statements are true, and the second causes the first
B. If both statements are true, but the second does not cause the first
C. If the first is true and the second is false
D. If the first is false and the second is true
E. Both statements are false

Answer 19

A

Question 20

O2 diffusion into tissue is increased when:

A. The diffusion distance increases
B. The surface area decreases
C. The diffusion constant decreases
D. The pressure gradient increases

Answer 20

D

Question 21

Describe hypoxia, hypercapnia, and hypocapnia?

Answer 21

Hypoxia –> low PO2
Hypercapnia –> high PCO2
Hypocapnia –> low PCO2