Control of breathing

Question 1

Describe the major cell groups in the medulla involved in the control of breathing

Answer 1

The central pattern for the basic breathing rhythm is in the medulla.

The dorsal respiratory group is located in the nucleus tractus solitarii, which contains cells that are active during inspiration. Firing rate controls the depth of breathing. Receives sensory input from CN IX and CNX which carry information from peripheral chemoreceptors, baroreceptors and lung receptors

The ventral respiratory group is a column of cells that contains both inspiratory and expiraratory neurones. Sends signals to accessory muscles of respiration.

Question 2

Explain how the control of breathing is affected by changes in pH

Answer 2

Chemoreceptors monitor blood gases and pH and alter ventilation to meet the metabolic demands of the body.

Central chemoreceptors are responsible for the majority of the respiratory drive and respond to changes in the CSF pH.

CO2 crosses the BBB and increases [H+]. Low pH increases ventilation. If [H+] falls then ventilation decreases.

Question 3

Describe the location and function of central chemoreceptors

Answer 3

Chemosensitive neurones at the ventrolateral medulla respond to changes in [H+].

The pH of the CSF around the chemoreceptor is determined by the blood pCO2 and HCO3. Dissolved [CO2] is determined by the plasma pCO2 and HCO3- in the CSF is determined by the activity of cells in the choroid plexus. Bicarbonate from the plasma cannot cross the BBB

A rise in blood pCO2 makes the CSF more acidic. This is detected by the chemoreceptors which increases ventilation to blow off excess CO2.

Response is delayed because CO2 has to diffuse across the BBB

Question 4

Describe the location and function of peripheral chemoreceptors

Answer 4

Peripheral chemoreceptors are located in the carotid and aortic bodies and detect changes in arterial blood plasma pO2 and pCO2. Response is rapid (1-3s).. Activation increases rate and depth of breathing.

Carotid bodies are located near the bifurcation of the common carotid. Afferent fibres travel to the CNS in the glossopharyngeal nerve.

Aortic bodies are located along the ascending aorta and is innervated by vagal afferents.

Question 5

Describe the response of chemoreceptors in hypoxia

Answer 5

Peripheral chemoreceptors are resposible for 40% of the effect of PaCO2 on ventilation. The rest is controlled by central receptors.

Peripheral receptors are sensitive to changes in rising arterial H+ and falling pO2. Response of peripheral receptors depends on arterial pO2. Minute ventilaiton increases in response to hypoxia.

Question 6

Define hypoxia

Answer 6

Fall in PaO2 <8.3

Question 7

Define hyercapnia

Answer 7

Rise in PaCO2 over 6kPa

Hypocapnia fall below 4.5kPa

Question 8

Define hyperventilation

Answer 8

Increased alveolar ventilation which greater than what is required to meet metabolic needs

Question 9

Define hypoventilation

Answer 9

When alveolar ventilation is less than required to meet metabolic needs.

Question 10

Role of pneumotaxic centre in breathing

Answer 10

Receives input from the hypothalamus and higher centres,

Coordinates medullary homeostatic functions
with factors such as emotion and temperature

affects the pattern of breathing by inhibiting the dorsal respiratory group.

Question 11

Voluntary control of breathing

Answer 11

Voluntary control is mediated by cortical motor neurones
in the pyramidal tract, which by-passes the respiratory neurones in the brainstem.

The action of higher, “conscious” centers in the cerebral cortex which permit voluntary control of ventilation by interacting with and over-riding the autonomic centers in the medullary rhythmicity area

Question 12

What is the main buffer in the CSF?

Answer 12

Bicarbonate ions

The pH of is held within narrow limits. During chronic acid-base disturbances, there is a small change in [HCO3-] of the CSF compared to the blood. In respiratory disturbance, changes in the blood and CSF are the same.

Question 13

Define metabolic acidosis

Answer 13

Caused by an increase of H+ (ingestion or production)

Results in depletion of [HCO3-] which acts as a buffer to produce water and CO2 to restore the pH to normal.

Respiratory rate increases to remove excess CO2 in the blood so the ratio of HCO3- and Co2 is restored.

Question 14

Blood gases in compensated metabolic acidosis

Answer 14

pH: normal

pO2: normal

pCO2: lower

HCO3: lower

Bicarbonate reacts with excess H+, causing a base defecit. Reaction produces CO2 which is excreted by increasing ventilation. HCO3 and CO2 are lower than their normal values. pH is returned to normal.

Question 15

Describe the changes that occur in metabolic alkalosis

Answer 15

When there is an excess loss of H+, [HCO3} increases, meaning there is a base excess.

Respiratory mechanisms compensate by increaing pCO2 above its normal physiological value by excreting less from the lung. This restores pH. HCO3- and CO2 levels are higher than normal

Question 16

Hering Breur reflex

Answer 16

Inspiratory reflex which involves inhibiting respiration once the lungs are inflated.

Inflation of the lung activates stretch receptors in the bronchial walls which provide negative feedback to the medulla via the vagus nerve and inhibits inspiratory neurones.

This prevents over-inflation of the lungs and enables expiration to occur.

Question 17

How do chemoreceptors respond to changes in pCO2 in respiration?

Answer 17

Small changes in arterial pCO2 produce very rapid changes in breathing which stabilise the pCO2. The principal source of this is from central chemoreceptors. If the pCO2 remains altered for a long period of time, activity of the choroid plexus adapts by pumping out more HCO3- to rest the system and exert control at a differnt pCO2. Seen in COPD patients.