Neural Control of Breathing

Question 1

Describe why there is a need for breathing to be modulated

Answer 1

• Breathing as hard as possible all the time is inefficient as it would be a too high of an energy expenditure
• It would also mean too much carbon dioxide would be expelled which would cause alkalosis as CO2 decreases pH
• It must be constantly adjusted as the body’s demand for oxygen and production of carbon dioxide consistently change - adequate absorption of oxygen and expulsion of carbon dioxide is achieved by maintaining pressure gradients between alveoli and blood

Question 2

State the circumstances in which oxygen demand and carbon dioxide production increase and why

Answer 2

• Demand for oxygen and carbon dioxide production both increase during physical activity as there is an increased need for ATP production hence oxygen consumed increases as it is needed as the final electron acceptor in oxidative phosphorylation
• Oxygen demand increases as well as carbon dioxide production during infection, injury or metabolic dysfunction

Question 3

Describe how breathing changes to modulate ventilation

Answer 3

• Breathing changes through changing the tidal volume and breathing frequency
• Both factors increase when oxygen demand and carbon dioxide production have increased

Question 4

Explain why the heart is also needed to increase total oxygen transported

Answer 4

• To maximise oxygen transport cardiac output it also increased
• Because haemoglobin is already 98% saturated with oxygen at rest, increasing the rate of ventilation alone has minimal effects on oxygen delivery
• Hence cardiac output must be increased by increasing heart rate and or stroke volume to ensure that oxygen is transported at a faster rate to meet higher demands

Question 5

Describe how breathing is physiologically initiated

Answer 5

• Respiratory muscles provide the movement required for ventilation
• As respiratory muscles consist of skeletal muscle they require neural inputs/stimulation to contract
• Innervation from motor neurons synapsing from descending spinal tracts provide the contractile signal

Question 6

Describe which muscles are responsible primarily for inspiration/expiration at rest and when it is forced

Answer 6

• Inspiration at rest is mainly controlled by the diaphragm
• Expiration at rest mainly relies on the elastic recoil of the lungs
• In forced inspiration the external intercostal muscles begin to be used as well as the pectoral muscles, sternomastoid and scalene muscles
• In forced expiration it is both elastic recoil as well as use of the internal intercostals and the abdominal muscles

Question 7

Describe how the rate and depth of breathing is determined by the central pattern generator

Answer 7

• Emotional signals are detected and voluntary control is held by higher brain centres e.g. hypothalamus with use the limbic system to send signals to the central pattern generator
• The central pattern generator consists of the pons as well as the medulla oblongata (dorsal and ventral respiratory groups) which exert involuntary control over breathing
• The central pattern generator receives signals from various sources in order to determine what the correct depth and rate of breathing should be for a particular demand
• Stretch receptors in the lungs will send signals as well as irritant receptors, those in muscles and joints and peripheral chemoreceptors like those in the carotid and aortic arteries. Somatic motor neurons are then innervated for both inspiration and expiration
• In expiration the somatic motor neurons innervate the internal intercostals and abdominal muscles
• In inspiration somatic motor neurons innervate the diaphragm, external intercostal muscles as well as accessory muscles like the pectorals

Question 8

Describe where central respiratory chemoreceptors are located, and what they respond to

Answer 8

• Central respiratory chemoreceptors are present in the medulla and indirectly monitor changes in arterial carbon dioxide
• Although central respiratory chemoreceptors respond to changes in [H+] within cerebrospinal fluid as H+ does not cross the blood brain barrier, central respiratory chemoreceptors do not directly respond to changes in the blood pH
• This is an indirect response to CO2 because when CO2 reacts with H2O it forms carbonic acid (H2CO3) which then dissociates to form H+ and HCO3-
• When a high [H+] in the cerebrospinal fluid is detected the chemoreceptors that detect it then send signals to the higher centres in the brain that increase ventilation to decrease [CO2] and thus [H+]

Question 9

Describe where peripheral chemoreceptors are found and what they respond to

Answer 9

• These respond to changes in arterial oxygen (low oxygen), carbon dioxide (high carbon dioxide) and pH (low pH) - present in aortic and carotid bodies
• Activated by decreased partial pressures of oxygen and increased partial pressures of carbon dioxide
• These then signal to respiratory centres in the medulla via sensory nerves to increase ventilation (negative feedback)

Question 10

What is hypercapnic drive

Answer 10

• Ventilation is proportional to pressure of carbon dioxide
• Hypercapnia (elevation of the partial pressure of CO2) triggers a deep slow breathing pattern to increase tidal volume
• It is also proportional to the pressure of oxygen but to a lesser extent

Question 11

What is hypoxic drive

Answer 11

• Hypoxaemia (low partial pressure of oxygen) stimulates increased ventilation
• This only kicks in at very low pressures of oxygen though

Question 12

Describe what happens to ventilation in sleep and why the changes occur

Answer 12

• There is a decreased metabolic rate so decreased respiratory demands
• Postural changes alter mechanics of breathing
• There is less sympathetic and more parasympathetic nervous system tone which means heart rate, blood pressure and cardiac output decrease
• This means tidal volume, breathing frequency and minute volume all decrease
• Oxygen saturation decreases from 98 to around 96% as oxygen demand is lessened and the partial pressure of carbon dioxide increases

Question 13

State some pathology associated with dysfunction in central processes that initiate breathing

Answer 13

• Trauma - damage to respiratory centres in the brainstem
• Stroke - ischaemia induced brain stem tissue injury
• Drugs e.g. opioids that suppress neuronal activity
• Congenital central hypoventilation syndrome
• Neonates - incomplete development of respiratory centres prior to birth
• Altitude - control systems unable to cope with abnormal atmospheric environment e.g. low oxygen and carbon dioxide

Question 14

Describe what sleep apnoea is and the effects it has on health

Answer 14

Sleep apnoea - temporary cessation of breathing during sleep
- Characterised by more than 5 episodes per hour lasting 10 seconds
- Duration may be as long as 90 seconds and frequency can be as high as 160 an hour

Effects on health -
- Tiredness (poor quality sleep)
- Cardiovascular complications (stress + SNS tone)
- Obesity/diabetes (inflammation + metabolic dysfunction)

Question 15

Describe what cheyne stokes respiration is including its causes and what it causes

Answer 15

• This occurs at high altitudes, in heart failure or chemoreceptor dysfunction
• It consists of oscillating periods of apnoea and hyperpnoea
• Breathing rate initially decreases but overcompensates and the person can experience sleep apnoea (complete cessation of breathing for a short time)
• Following this as oxygen levels in the body decrease the peripheral chemoreceptors detect this and cause breathing rate to increase but again it increases too much causing hyperpnoea
• This causes carbon dioxide levels to greatly decrease which causes alkalosis (high pH) so breathing rate again drops but drops too much (and the cycle continues)