Neural Control of breathing

Question 1

What normally contributes towards quiet (passive) expiration?

Answer 1

1. Elastic recoil of the lungs

2. Relaxation of the diaphragm

Question 2

What normally contributes towards quiet (passive) inspiration?

Answer 2

1. Contraction of the diaphragm

Question 3

What normally contributes towards forced expiration?

Answer 3

1. Elastic recoil of the lungs
2. Relaxation of diaphragm
3. Contraction of the internal intercostal muscles
4. Contraction of the abdominal muscles

Question 4

What normally contributes towards forced inspiration?

Answer 4

1. Contraction of the diaphragm

2. Contraction of the internal intercostal muscles

Question 5

How do respiratory muscles tissues contract?

Answer 5

All respiratory muscle tissues are of the skeletal type, and so require nervous stimulation in order to contract

Question 6

Where are contractile signals initiated and where does the pathway follow?

Answer 6

Initiated in the brain and descend via spinal tracts.

Signals then pass from synapse from descending upper motor neuron tracts within the spinal cord to lower motor neurons that innervate (respiratory) skeletal muscles causing them to contract.

Question 7

What determines how often to breathe (basic patterns of ventilation)?

Answer 7

Neurones within the brainstem (medulla and pons) called the central pattern generator (CPG)

Question 8

Describe how inputs from higher somatic and emotional centres also feed into the CPG

Answer 8

On image

Question 9

Describe the role of central respiratory chemoreceptors and where are they located?

Answer 9

Located in the medulla (part of the brainstem)

Indirectly monitor changes in PaCO2 by responding to change in pH in the cerebrospinal fluid (CSF).

An increase in PaCO2 decreases blood pH, H+ cannot pass across the blood-brain barrier as they are charged.

Therefore CRC do not respond to blood pH directly, however arterial CO2 can pass through the blood-brain barrier into the CSF, where it will then react to produce carbonic acid, and the resulting H+ activates CRCs. This CRC response to PaCO2 provides the predominant (physiological experiments estimate ≈ 70%) signal involved in regulating ventilation and initiating the urge to breathe.

Question 10

Describe the role of peripheral chemoreceptors and where are they located?

Answer 10

Located in the carotid and aortic bodies.

Detect levels of O2, CO2 and pH within arterial blood.

Peripheral chemoreceptors are activated by low O2, high CO2, and low pH, and signal to medullar centres to increase ventilation.

Question 11

What do stretch receptors do and where are they located?

Answer 11

Stretch receptors within the lungs that prevent damage due to over-inflation

Question 12

What do irritant receptors do and where are they located?

Answer 12

Irritant receptors within the airways that initiate cough.

Question 13

What is the level of ventilation proportional to?

Answer 13

Level of PaCO2

Question 14

How does pH and PaO2 also affect ventilation?

Answer 14

However pH is typically closely linked to CO2 levels, and hypoxic drive (the increased ventilation in response to decreased PaO2) only occurs at very low PaO2.

Question 15

What feedback response provides the predominant stimulus for respiration in healthy individuals?

Answer 15

Hypercapnic drive (i.e. increased ventilation in response to increased levels of CO2 within arterial blood) provides the predominant respiratory stimulus in healthy individuals.

Hypoxic drive (i.e. increased ventilation is response to decreased levels of O2 within arterial blood) only takes on a prominent role at very low PaO2 levels, or in chronically hypercapnic individuals (e.g. severe COPD).

Question 16

Describe 4 key central pathologies that affect breathing

Answer 16

• Inhibition the brainstem caused by drugs such as opioids and barbiturates
• Injury to the brainstem caused by stroke or trauma
• Congenital defects in brainstem signalling processes (central hypoventilation syndrome, in which individuals lack the capacity to breath whilst asleep)
• Insufficient development of the relevant structures and pathways in neonates (infantile central sleep apnoea)

Question 17

Describe neuromuscular pathologies (3)

Answer 17

Muscle wasting diseases (e.g. muscular dystrophy) - atrophy of respiratory muscles, resulting in failure to generate sufficient muscular contractile force to generate the movements required to breathe.

Motor neuron disease – degradation of the motor neurons required to conduct signals from the brainstem to respiratory muscles.

Spinal cord injury – damage to the pathway required to conduct signals from the brainstem to respiratory muscles.