Neural Control Of Breathing

Question 1

How is breathing initiated? What causes respiratory muscles to contract?

Answer 1

Breathing is initiated by the neural activation of respiratory muscles, which provides the movement required for ventilation.
As respiratory muscles consist of skeletal muscle, they require neural inputs/ stimulations to contract.
Innervation from motor neurons synapsing from descending spinal tracts provide the contractile signal.

Question 2

Which muscles (including accessory muscles) are utilised in quiet/ forced inspiration and expiration?

Answer 2

INSPIRATION:

• quiet breathing: diaphragm
• increased/ forced ventilation: external intercostal muscles (pectorals, sternomastoid and scalene muscles as accessories)

EXPIRATION:

• quiet breathing: elastic recoil
• increased/ forced ventilation: elastic recoil and internal intercostal muscles (abdominal muscles as accessories)

Question 3

How does the central pattern generator (CPG) determine how often and hard to breathe?

Answer 3

Signals from various inputs provide feedback, which integrate to regulate breathing. Examples of such signals would be:

• the pH of arterial blood
• the amount of CO2 and O2 in arterial blood
• current lung volume
• how stretched the lungs are

The CPG integrates data from these various neuronal inputs to regulate ventilation.

Question 4

List the two main chemoreceptors, where they are located, and what changes they detect in the body

Answer 4

The two main chemoreceptors involved in respiratory feedback are:

• CENTRAL CHEMORECEPTORS: These are located on the ventrolateral surface of the medulla oblongata, and detect changes in the pH of the spinal fluid. They can be desensitised over time from chronic hypoxia (oxygen deficiency) and increased carbon dioxide.
• PERIPHERAL CHEMORECEPTORS: These are located in the aortic body, which detect changes in blood oxygen and carbon dioxide, but not pH, and in the carotid body, which detects all three. They do not desensitise, and have less of an impact on the respiratory rate compared to the central chemoreceptors.

Question 5

What are the hypercapnic and hypoxic drive?

Answer 5

The hypoxic drive is a form of respiratory drive in which the body uses oxygen chemoreceptors instead of carbon dioxide receptors to regulate the respiratory cycle.

Question 6

Describe what will come of a situation of hypocapnia before jumping into shallow waters.

Answer 6

OXYGEN DROPS:
As the breath hold begins, oxygen is metabolised and carbon dioxide levels increase. As the breath hold continues, the body becomes starved of oxygen.

UNCONCIOUSNESS:
Under normal circumstances, incresed carbon dioxide would trigger a breath, but because the CO2 levels were so low upon submersion (due to hyperventilation), there is not enough to initiate a breath, and the swimmer loses conciousness.

DROWNING:
Once the swimmer loses conciousness, the body reacts and forces a breath. That causes the lungs to fill with water and, without an immediae rescue, a drowning death is all but certain.

Question 7

Describe what sleep apnoea is, how it affects health and how it can be investigated.

Answer 7

Sleep apnoea is the temporary cessation of breathing during sleep.

EFFECTS OF HEALTH:

• tiredness (poor sleep quality)
• cardiovascular complications (due to stress and increased sympathetic nervous system tone)
• obesity and diabetes (due to inflammation and metabolic disfunction)

We can investigate someone’s sleeping pattern using a polysomnography. It measures different clinical aspects of breathing, such as how much you’re breathing, airflow through the nose, sensing abdominal movements, etc.

Question 8

What is obstructive sleep apnoea, and what are some risk factors?

Answer 8

Obstructive sleep apnoea is the blockade of the upper respiratory tract during sleep. The relaxation of the genioglossus muscle (the main muscle in the tonuge) can obstruct the upper airway.

Some risk factors include:

• obesity
• alcohol/ sedatives
• smoking

Question 9

What is central sleep apnoea, and what are some of its causes?

Answer 9

Central sleep apnoea is the dysfunction in the process that initiates breathing.

Causes:

• STROKE: damage to the respiratory centres in the brain
• DRUGS (EG. OPIODS): suppression of neuronal activity
• CENTRAL HYPOVENTILATION SYNDROME: may come about through injury/ trauma to the brain stem, or may be congenital (‘Ondine’s Curse’); results in respiratory arrest during sleep
• NEONATES: still continuing development of the respiratory centres in the brain
• ALTITUDE: eg. Cheyne-Stoke respiration

Question 10

Describe Cheyne-Stokes respiration.

Answer 10

Cheyne-Stokes respiration is oscillating apnoea and hyperpnoea.

First, there is a pathological stimulus (such as increased altitude, CR disfunction, heart failure, etc.) that results in hypercapnia and hypoxaemia. To compensate, the body starts hypervnetilating. This causes hypocapnia, and thus alkalosis. This causes the body to decrease its respiratory drive, and the body will compensate with hypoventilation. This causes hypercapnia and hypoxaemia, and the cycle continues.