Lecture 30: Generation and Control of Rhythmic Breathing

Question 1

What type of muscle are respiratory muscles and do they contract spontaneously?

Answer 1

Skeletal muscles and they do not contract spontaneously

Question 2

How is rhythmic breathing generated?

Answer 2

Generated involuntarily in the respiratory centres in the medulla oblongata and CVS centres

Question 3

Describe the action of neurons in the inspiratory centre during rhythmic breathing:

Answer 3

• Spontaneously discharge - limited in duration
• Initiate APs in motor neurons to inspiratory muscles and induce muscle contraction

Question 4

Describe the action of neurons in the expiratory centre during rhythmic breathing:

Answer 4

• Normally quiescent → don’t discharge spontaneously
• Expiration occurs passively due to the elastic recoil of the lungs
• During forced expiration the expiratory centre fires to activate expiratory muscles

Question 5

Describe breathing control during restful breathing and exercise within the medulla oblongata:

Answer 5

• Restful breathing: Inspiratory centre sends signals to inspiratory muscles and diaphragm to contract via spinal cord and motor neurons
• Exercise: Expiratory centre sends signals to expiratory muscles via spinal cord and motor neurons
• These centres have a negative feedback mechanism towards eachother

Question 6

Where does the medulla oblongata receive input from?

Answer 6

• Pons and higher brain centres
• Chemo/mechano receptors in body

Question 7

What 5 voluntary actions of breathing an be performed?

Answer 7

• Breath deeply and slowly
• Breath shallowly and rapidly
• Hold breath
• Forced expiration
• Speak and sing

Question 8

Where does voluntary control come from?

Answer 8

Cerebral cortex
- Sends signals directly to respiratory muscle motor neurons in the spinal cord, bypassing respiratory centres

Question 9

How do inspiratory and expiratory centres receive sensory input?

Answer 9

Via Nucleus Tractus Solitarius (NTS) from a variety of sensory receptors:
* Mechanoreceptors e.g. irritants
* Chemoreceptors e.g. arterial gas

Question 10

What do sensory inputs cause?

Answer 10

Reflex adjustment in the breathing response to:
* Exercise
* Airway irritants
* Environmental changes

Question 11

Describe the protective reflex of a sneeze:

Answer 11

• Irritation of the nasal mucosa stimulates mechanoreceptors
• Several inspirations are followed by a powerful expiration - sneeze
• High expiratory airflow velocity, helps to remove the irritant

Question 12

Describe the protective reflex of a cough:

Answer 12

• Mechanoreceptors in the larynx are stimulated by irritants
• Long slow inspiration followed by a rapid and powerful expiration, initially against a closed glottis
• High expiratory airflow velocity helps to remove the irritant

Question 13

How are arterial gases altered?

Answer 13

By alteration of alveolar ventilation

Question 14

What monitors the chemical composition of arterial blood?

Answer 14

Chemoreceptors - peripheral and central

Question 15

Where are peripheral chemoreceptors located?

Answer 15

• In arteries
• Aortic bodies; signal via vagus nerve
• Carotid bodies; signal via glossopharyngeal nerve
• Near to the baroreceptors (CVS lectures)

Question 16

What are the stimuli for peripheral chemoreceptors?

Answer 16

1. Decrease in arterial PO2 - hypoxia
2. Increase in arterial PCO2 - hypercapnia
3. Increase in arterial [H+] - acidosis

Question 17

What happens when peripheral chemoreceptors are stimulated?

Answer 17

Increase firing causing an increase in ventilation
- complete breathing responses to hypoxia and acidosis are dependent on peripheral chemoreceptors

Question 18

Where are central chemoreceptors located?

Answer 18

Medulla oblongata

Question 19

What are central chemoreceptors insensitive to?

Answer 19

1. Hypoxia
2. Arterial acidosis; H+ ions cannot pass from arterial blood into ISF in brain, due to blood-brain barrier

Question 20

What are central chemoreceptors sensitive to?

Answer 20

[H+] in the brain ECF
* Source of H+ ions at the central chemoreceptors is CO2
* CO2 can pass through the blood-brain barrier

Question 21

What happens to CO2 in the ISF in the brain?

Answer 21

CO2 is converted into bicarbonate and H+ ions
CO2 + H2O → H2CO3 → HCO3- + H+

Question 22

What happens when central chemoreceptors are stimulated?

Answer 22

Ventilation is increased

Question 23

Describe the overall response to hypoxia:

Answer 23

1. ↓ Inspired PO2 - high altitude
2. ↓ Alveolar PO2
3. ↓ Arterial PO2
4. ↑ Peripheral chemoreceptor firing -> NTS -> medullary respiratory centres
5. ↑ Respiratory muscle contraction
6. ↑ Ventilation
7. Return of alveolar and arterial PO2 to normal

Question 24

When does ventilation response occur for hypoxia?

Answer 24

Only occurs when arterial PO2 falls below 60 mmHg

Question 25

How much effect does each chemoreceptor have in response to hypercapnia?

Answer 25

* Peripheral receptors: 30% of effect * Central chemoreceptors: 70% of effect

Question 26

What happens to peripheral chemoreceptors in response to hypercapnia?

Answer 26

1. Increased PCO2 increases arterial blood [H+] 2. Stimulates chemoreceptors, increasing ventilation

Question 27

What happens to central chemoreceptors in response to hypercapnia?

Answer 27

1. CO2 rapidly diffuses across the blood-brain barrier 2. Increased PCO2 increases the [H+] in the ISF in the medulla 3. H+ ions directly stimulate chemoreceptors, increasing ventilation

Question 28

When does ventilation response occur for hypercapnia?

Answer 28

* Small increase in arterial PCO2 leads to a large increase in ventilation * 4 mmHg change in PCO2 doubles ventilation

Question 29

What does the ventilation response for hypercapnia allow for?

Answer 29

Close control of arterial PCO2 - As CO2 in the body increases, breathing is stimulated e.g. a breath-hold or emphysema

Question 30

What is metabolic acidosis?

Answer 30

Exercise → lactic acid → lactate + H+ (metabolic acidosis)

Question 31

Describe the overall response to metabolic acidosis:

Answer 31

1. ↑ arterial [H+] - can't cross BBB 2. ↑ Peripheral chemoreceptor firing to send signals to medullary respiratory centres 3. ↑ Respiratory muscle contraction 4. ↑ Ventilation - hyperventilation; CO2 "blown off" 5. ↓ Alveolar PCO2 6. ↓ Arterial PCO2 7. Return of arterial [H+] to normal

Question 32

What type of acidosis does breathing resolve?

Answer 32

Respiratory and non-respiratory acidosis

Question 33

What is metabolic alkalosis?

Answer 33

Increased pH * Loss of H+ ions can occur following vomiting * Loss of H+ ions increases the pH of blood

Question 34

How is metabolic alkalosis detected?

Answer 34

1. Reduced [H+] detected by the peripheral chemoreceptors and ventilation is reduced - hypoventilation 2. Elevation in arterial PCO2 3. Formation of H+ ions CO2 + H2O <-> H2CO3 <-> HCO3 - + H+ 4. [H+] returns toward normal