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

What effect does spinal chord injury, motor neuron disease, muscular dystrophy have on breathing?

Answer 2

• It can cause the person to be unable to breathe as the nerve impulses are not recieved by the respiratory muscles.
• So there is no muscle contration leading to failure in ability to breathe.

Muscular dystrophy (MD) meaning - It is a group of muscle diseases that results in increasing weakening and breakdown of skeletal muscles over time.

Question 3

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

Answer 3

Question 4

How is the basic breathing pattern generated by neuronal systems in the brainstem?

Answer 4

• The breathing pattern is initiated in the brainstem.
• Here you have an extremely complex series of different neural networks that function together to take various signals from different parts of the body and decide the breathing pattern.
• The breathing rate, depth of breath to meet the metabolic demands of the body, and related factors are decided here and signals are accordingly sent to different respiratory muscles in a rhythmeic pattern in order to breathe.

Question 5

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

Answer 5

• Signals from various inputs provide feedback, which integrate to regulate breathing.
• Examples of such signals would be: -
  
  • What is the pH in the CSF (cerebrospinal fluid)? (typically determined by PaCO₂)
  • How much H+, CO2 and O2 is in arterial blood?
  • What is the current lung volume? How stretched are the lungs?
  • Is there stimualtion from higher emotion centres or ANS?

Question 6

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

Answer 6

• The two main chemoreceptors involved in respiratory feedback are: -

1. CENTRAL CHEMORECEPTORS: -
   
   • These are located on the ventrolateral surface of the medulla oblongata, and indirectly monitor changes in arterial CO2.
   • Although central respiratory chemo-receptors (CRC) respond to changes in [H+] within cerebrospinal fluid, as H+ does not cross the blood brain barrier (as it is charged), CRC do not directly respond to changes in blood pH (except via CO2).
   • They can be desensitised over time from chronic hypoxia (oxygen deficiency) and increased carbon dioxide.
2. 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 are activated by low PaO2 and high PaCO2 and acidaemia.
   • They signal to respiratory centres in medulla (via sensory nerves) to increase ventilation (negative feedback).
   • They do not desensitise, and have less of an impact on the respiratory rate compared to the central chemoreceptors.

• The two chemoreceptors then send the signals to the respiratory pattern generator and alter the rate of ventilation.

Ventrolateral meaning - situated towards the junction of the ventral and lateral sides.

Question 7

What is the hypercapnic drive?

Answer 7

• Ventilation is generally proportional to the PaCO2.
• When the PaCO2 increases you feel the urge to breathe.

Question 8

What is the hypoxic drive?

Answer 8

• 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.
• So respiration is only undertaken when there is a decrease in PaO2.
• The situation where the hypoxic drive start to take on a greater importance than the hypercapnic drive would be where a patient is suffereing from high levels of CO2 due to COPD.
• Since there people have a high level of CO2 on a chronic basis due to insufficient ventilation they start to become tolerant to high levels of CO2.
• This is when hypoxic drive begins to take on a bigger role compared to hypercapnic drive.

Question 9

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

Answer 9

• Central sleep apnoea is the temporary cessation of breathing during sleep caused by dysfunction of the processes that initiates breathing.
• Potential negative effects on health include tiredness, cardiovascular complications, and metabolic dysfunction.
• Causes:
  
  • Stroke – damage to respiratory centres in brain.
  • Drugs (e.g. opioids) – suppression of neuronal activity.
  • Altitude – e.g. Cheyne - Stokes respiration.
  • Central hypoventilation syndrome – injury/trauma to brainstem, or congenital (‘Ondine’s curse’).

Question 10

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

Answer 10

• 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 11

How do we investigate someones sleeping pattern?

Answer 11

• We can investigate someone’s sleeping pattern using a polysomnography.
• Polysomnography records your brain waves, the oxygen level in your blood, heart rate and breathing, as well as eye and leg movements during the study.

Question 12

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

Answer 12

1. HYPERVENTILATION:
   
   • Overbreathing, either consciously or as a results of overexertion, artificially lowers carbon dioxide levels (hypocapnia).
2. 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.
3. UNCONCIOUSNESS:
   
   • Under normal circumstances, increased 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.
4. 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 13

Describe Cheyne-Stokes respiration.

Answer 13

• Cheyne-Stokes respiration is oscillating apnoea and hyperpnoea (Hyperpnoea is increased depth and rate of breathing).
• Cheyne–Stokes respiration is an abnormal pattern of breathing characterized by progressively deeper, and sometimes faster, breathing followed by a gradual decrease that results in a temporary stop in breathing called an apnea. The pattern repeats, with each cycle usually taking 30 seconds to 2 minutes

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