Control of Breathing

Question 1

What are the 4 main aims of the regulatory system of breathing?

Answer 1

1. Achieve uninterrupted involuntary rhythmical breathing
2. Regulate PaCO2 and PaO2 within tight limits
3. Ensure minimal effort to obtain (close to) full saturation of haemoglobin
4. Allow involuntary/voluntary overriding in different situations

The basic breathing system is split into:

Sensors -> Medulla -> Effectors

Question 2

Where is the respiratory centre of the brain?

Answer 2

It is in the Medulla

It is split into the:
* Dorsal Respiratory Group (DRG)
* Ventral Respirtory Group (VRG)

It is also known as the central pattern generator (CPG)

Question 3

Describe the location and function of the dorsal respiratory group?

Answer 3

Dorsal Respiratory Group (DRG)
* Is located on the floor of the 4th ventricle near the tractus solitarius.
* Predominantly consists of inspiratory neurones (phrenic and intercostal muscle neurones).
* It sends UMNs to the anterior horn cells on the contralateral side of the spinal cord and is primarily concerned with the timing of the respiratory cycle.
* Almost all sensory inputs go to the DRG

Question 4

Describe the anatomy and function of the ventral respiratory group?

Answer 4

Ventral Respiratory Group (VRG)
Consists of 4 nuclei that are involved in controlling the muscles of respiration.
* Nucleus Ambiguus and nucleus para ambigualis are involved in inspiration
* Nucleus retro ambigualis and botzinger complex are involed in expiration

Question 5

Describe the function of the pontine respiratory group?

Answer 5

Pontine Respiratory Group (PRG)
Contributes to the fine control of respiratory rhythm through interaction with the medullary respiratory neurones in a multi- synaptic pathway.

Question 6

What would happen to breathing if there was a transection above pons?

Answer 6

Breathing is essentially normal. With vagotomy, there is removal of afferent input from stretch receptor, inspiration will be enhanced due to the Hering-Breuer response now being absent.

Question 7

What would happen to breathing if there was a transection mid pons?

Answer 7

Increased depth of breathing (loss of inhibitory signals from upper pons). With vagotomy, apneusis occurs (prolonged periods of inspiration interrupted by occasional expiration)

Question 8

What would happen to breathing if there was a transection just above the medulla?

Answer 8

Maintained breathing but irregular. Vagotomy has little effect – the basic rhythm generator is at the level of the medulla or below.

Question 9

What would happen to breathing if there was a transection below the medulla?

Answer 9

Cessation of breathing. Basic rhythm generator is in the medulla.

Question 10

How does the central pattern generator stimulate breathing?

Answer 10

With the I. augmentation neurone, there is a slow membrane depolarisation that allows a spontaneous discharge by a similar mechanism to pacemaker cells using a combination of potassium and calcium.

Switched off using the activation of calcium-dependent potassium channels.

Question 11

How do inhibitory respiratory neurones work?

Answer 11

Work through hyperpolarisation of the target cells making them much harder to depolarise.

Question 12

Describe the voluntary control of breathing?

Answer 12

The cortex can bypass the CPG altogether but not indefinitely as involuntary systems eventually force their way back in

I.e. in breath holding a combination of hypoxia, hypercarbia and impulses from the diaphragm muscle override.

Question 13

Describe the 3 functions of the respiratory motor neurones?

Answer 13

1. Comes from the DRG and VRG (CPG) – concerned with the inhibitory and excitatory output e.g the control of automatic breathing
2. Voluntary control of breathing
3. Involuntary non-rhythmic respiratory control (e. g. coughing, swallowing, hiccups)

Additional input to the respiratory motor neurones come from the cerebellum and from the reticular activating system, ‘informing’ the respiratory motor neurones of the body’s posture, and relaying information about level of arousal.

Question 14

What are the chemorecptors involved in the control of breathing?

Answer 14

Central chemoreceptors: Sensitive to PaCO2 through hydrogen ion concentration in CSF. This is the most important single driver of ventilation.

Peripheral Chemoreceptors: Sensitive to PaO2, PaCO2 and H+ ions in the periphery

Question 15

What is the location of the central chemoreceptors?

Answer 15

They are present 0.2mm below the antero-lateral (ventral) surface of the medulla (distinct from the DRG) close to the origin of cranial nerves IX and X.

Crossed by the ant. Inf. Cerebellar a. (AICA).

Contains 2 areas: one rostral and one caudal with an intermediate zone to connecting them .

Question 16

Describe the mechanism of action of central chemoreceptors?

Answer 16

1. CO2 diffuses through the blood brain barrier (BBB)
2. Dissociation to H+ ions occur (H+ are impermeable through the BBB)
3. H+ ion concentration sensed by the chemoreceptors

80% of response to CO2 occurs in central chemoreceptors and the response occur relatively slowly over 1-3mins from the time of change of PaCO2.

There may also be an additional effect from the indirect increase in cerebral blood flow in response to a fall in pH.

Question 17

What occurs to central chemoreceptor response in chronic CO2 retention?

Answer 17

With chronic raises in CO2 and CSF [H+] concentration increases, this results in HCO3- being transported across the BBB.

This may take hours – days to take place and eventually blunts the resultant increase in minute ventilation.

Eventually individuals lose their response to hypercapnia and rely on the response to hypoxia.

Question 18

What is the normal pH of CSF and why does it differ to blood?

Answer 18

Normal CSF pH =7.32

CSF contains less protein and thererfore has less of a buffering ability resulting in a slightly lower pH.

Question 19

Other than chronic CO2 retention what factors can reduce the central chemoreceptors response?

Answer 19

• sleep
• increased age
• genetic and racial factors
• training (athletes and divers have a low CO2 sensitivity)
• drugs (opiates, barbiturates).

Question 20

Where are the peripheral chemorecptors located?

Answer 20

Carotid bodies
Aortic bodies

Question 21

Describe the location and structure of the carotid bodies?

Answer 21

Located at the bifurcation of both carotid arteries as a small cluster of chemoreceptor cells.

It is made up of 2 types of glomus cells:
* Type I (chief) – Chemoreceptive cells in synaptic contact with afferent nerve endings of the sinus nerve (branch of CN IX)
* Type II (sheath) – Resemble glial cells that act as supporting cells

Question 22

What is the response of the carotid bodies to PO2?

Answer 22

Due to large blood flow (2L/100g/min) across the carotid body, there is a negligible arterial–venous O2 difference in spite of a high metabolic rate. It is therefore ideally situated to sample changes in PaO2.

Type I cells detect graded changes to ATP concentration from minor changes of PaO2 and responds within a few seconds to a fall of PaO2 < 7.89 kPa, increasing MV.

The increase in MV is augmeneted if there is an acidosis or raised PaCo2

Question 23

What is the response of the carotid bodies to an increase in PCO2 or hypotension?

Answer 23

Response to PaCO2 is dependent upon carbonic anhydrase which suggests it responds to [H+].

It accounts for less than 20% of responses to PaCO2 but is very rapid and is the only response to rapid alterations in PaCO2.

Hypotension increases ventilation through stagnant hypoxia.

Question 24

Where are the aortic bodies located and what do they respond to?

Answer 24

Located above and below the aortic arch.

They respond more weakly than the carotid bodies.

Respond to changes in oxygen content rather than changes in arterial oxygen tension. They are therefore more attuned to actual oxygen delivery including a reduced haemoglobin and hypoxaemia

Question 25

Describe the location and fucntion of the pulmonary stretch receptors?

Answer 25

Located in the smooth muscle cells of the large airways and transmit impulses via Vagus (X) nerve to the respiratory centres in response to sustained lung inflation.

They are an example of slowly adapting stretch receptors (SARs) – responds slowly to stimulation (distension of the lungs) and continues firing as long as the stimulus continues.

They function to:
* Terminate inspiration via the Hering-Breuer reflex
* Strong stimulation will excite expiratory neurones

Question 26

Why are pulmonary stretch receptors less important in adults than infants?

Answer 26

As they are not stimulated during normal tidal breathing in adults, only during vigorous exercise.

Question 27

Describe the location and function of the irritant receptors?

Answer 27

**Rapidly adapting receptors **which fire rapidly to stimulus but effects are **diminished if the stimulus is constant.
**
They respond to mechanical and chemical irritation such as noxious gases/cigarette smoke/dust/cold air.

Located in epithelial cells of large airways – especially at the level of the carina through the Vagus (X) nerve.

Also located in the nose and upper airwaysand the cause for laryngospasm.

They cause coughing, sneezing, and bronchoconstriction.

Question 28

Describe the location and function of the J (juxta capillary) receptors?

Answer 28

Located in alveolar cell walls close to the capillaries.

They respond quickly to chemicals in the pulmonary circulation and is also stimulated by oedema in the interstitium (may play a part in pulmonary oedema) through the Vagus (X) nerve.

They cause rapid, shallow breathing.

Question 29

Describe how the following receptor effect breathing: joint and muscle propioreceptors, muscle spindles, arterial baroreceptors, pain and temperature receptors?

Answer 29

1. Joint and muscle proprioceptors – thought to play a role during exercise where stimulation causes increased inspiration
2. Muscle spindles – responsible for the stretch reflex in muscle contraction. May influence the CPG * in the medulla
3. Arterial Baroreceptors – Decrease in BP may result in hyperventilation
4. Pain and temperature – when stimulated result in increased ventilation

*Central pattern generator