The neural control of breathing

Question 1

what is breathing?

Answer 1

a rhythmic process that maintains O2 and CO2 pressure gradients between alveoli and blood

Question 2

define the following:
VA
PA
Pa
Pv

Answer 2

VA - alveolar ventilation
PA - alveolar partial pressure
Pa - arterial partial pressure
Pv - venous partial pressure

Question 3

equation involving diffusion:

Answer 3

diffusion = (surface area x pressure difference)/ distance^2

Question 4

what does ventilation have to maintain:

Answer 4

the adequate partial pressure gradient between the air in the alveoli and capillary blood

Question 5

as ventilation becomes deeper/greater, what happens?

Answer 5

air within the alveoli will resemble atmospheric air more and more

Question 6

why is ventilation regulated?

Answer 6

to meet varying o2 demand and co2 production

Question 7

how is ventilation increased?

Answer 7

by changing tidal volume (depth of breath) and the frequency (rate of breaths per minute)

Question 8

in what situations is ventilation increased?

Answer 8

during infections like sepsis, increased acid production in the body and increased metabolism, and also in trauma

Question 9

why is respiratory muscle contraction and relaxation is required in inspiration?

Answer 9

in order to expand and compress the thoracic cavity

Question 10

what type of muscle tissue are the respiratory muscles (e.g. diaphragm) made up of?

Answer 10

skeletal muscle tissue

Question 11

what causes skeletal muscle to contract?

Answer 11

neural innervation- i.e. it needs to be stimulated by a neural input.
-innervation from motor neurons synapsing from descending spinal tracts provide the contractile signal.

Question 12

how is breathing initiated?

Answer 12

neural activation of respiratory muscles, which provide the movement required for ventilation.

Question 13

what is the effect of spinal cord injury/motor neuron disease/muscular dystrophy?

Answer 13

Respiratory failure

-the person will not be able to breathe

Question 14

difference between quiet/passive breathing and forced breathing?

Answer 14

quiet breathing is breathing at rest

forced is big breathing manoeuvres

Question 15

what happens in quiet breathing?

Answer 15

contraction and relaxation of the diaphragm that creates that force needed for the movement of the thoracic cavity and movement of the lungs

Elastic recoil of the lungs is sufficient to compress the lungs again

Question 16

when are further muscles recruited in breathing?

Answer 16

If we need to increase the force of breathing (greater speed or depth)

Question 17

define the term “accessory muscles”, and give an example:

Answer 17

muscle tissues which don’t have a primary role in respiration, but contribute in forced breathing manoeuvres

eg. scalene muscles in the neck which can be used if you really need to expand the thoracic cavity volume.

Question 18

what is the basic breathing pattern generated by?

Answer 18

neuronal systems within the brainstem

Question 19

where are signals initiated and sent to when respiratory muscles need to contract?

Answer 19

signals are initiated from the brain stem, where complex groups of neural networks are located that function all together to take various inputs from different parts of the body and accordingly send different signals to different respiratory muscles in a rhythmic pattern in order to breathe in and out for a certain amount of time

signals are sent from the brain to motor neurons via spinal cord, skeletal muscle then contracts

Question 20

what is the role of The Central Pattern Generator (CPG)?

Answer 20

it integrates data from various neuronal inputs to regulate ventilation (ie. determines how often and hard to breathe)

Question 21

list some neuronal inputs which send signals to provide feedback (which are integrated to regulate breathing):

(look at diagram in notes)

Answer 21

• higher brain centres
• respiratory centres (medulla and pons)
• pain receptors acting through the hypothalamus
• peripheral chemoreceptors
• central chemoreceptors
• receptors in muscles and joints
• stretch receptors in lungs
• irritant receptors in lungs and bronchi

Question 22

what questions does the body “ask” in order to determine ventilation in the body, and what does it do with the info?

Answer 22

• What is the pH in the CSF? (typically determined by PaCo2)
• How much H+, o2 and co2 are in arterial blood?
• What is the current lung volume? How stretched are the lungs?
• Is there stimulation from higher emotion centres or ANS?

The body takes info from these questions to determine the depth and rate of breathing, and creates a pattern based on that

Question 23

what do central respiratory chemoreceptors (CRC) present in the medulla indirectly monitor and respond to?

Answer 23

changes in arterial (P)CO2

Question 24

what does it mean when there are high levels of CO2 within arterial blood?

Answer 24

means the level of ventilation within the body isn’t meeting the metabolic demands of the body- because CO2 is because produced but not it’s not being gotten rid of

Question 25

in the brain, where will CO2 present within arterial blood go?

Answer 25

CO2 is a small uncharged gas and so can pass through the blood brain barrier, and will diffuse into the cerebrospinal fluid

Question 26

once in the CSF, what happens to the CO2 and chemoreceptors?

Answer 26

• CO2 reacts with water within the CSF to produce carbonic acid
• carbonic acid then dissociates, producing H+ ions
• CRC within the medulla tissue are activated by the presence of increased H+ ions
• the chemoreceptors signal to the brainstem telling it to increase ventilation
• increased ventilation removes more CO2, which gets rid of the original stimulus and everything goes back to normal

Question 27

explain why CRC’s don’t directly respond to changes in blood pH (except via CO2)?

Answer 27

because H+ ions are charged so they don’t naturally diffuse through the blood brain barrier, unlike CO2

Question 28

are are peripheral chemoreceptors located and what do they respond to?

Answer 28

• located outside of the CNS, in the arterial system, within the aortic and carotid bodies
• respond to changes in arterial o2, co2 and pH

Question 29

what are aortic and carotid bodies activated by?

Answer 29

• decreased partial pressure of oxygen
• increased partial pressure of carbon dioxide
• acidaemia (high conc of [H+] in blood)

Question 30

what happens when the aortic and carotid bodies activated?

Answer 30

signals are sent to the respiratory centres in the medulla via sensory nerves to increase ventilation (negative feedback) and get rid of the initial feedback

Question 31

what is the predominant influence on the rate of ventilation at a given time?

Answer 31

the level of co2 within the body

o2 has a certain effect, but it is generally second to co2 unless o2 levels are extremely low - hypoxic drive

Question 32

what is hypercapnia?

Answer 32

hypercapnia is abnormally high levels of co2 in the blood

Question 33

so, what is ventilation is generally proportional to?

Answer 33

PaCO2

Question 34

what is hypoxaemia?

Answer 34

low PaO2

Question 35

what does hypoxaemia stimulate?

Answer 35

increased ventilation

Question 36

what is hypoxic drive?

Answer 36

when there are very low partial pressures of o2, which have a substantial effect on the rate of ventilation

Question 37

in what type of situation does hypoxic drive start to take on a greater role?

Answer 37

in situations where patients have chronic high levels of co2 because of chronic respiratory conditions (eg. COPD)

this is because the person becomes tolerant to high co2 levels

Question 38

what is Central Sleep Apnoea?

Answer 38

temporary cessation of breathing during sleep caused by dysfunction of the processes that initiates breathing

Question 39

potential negative effects of Central Sleep Apnoea on health?

Answer 39

• tiredness
• cardiovascular complications
• metabolic dysfunction

Question 40

what are the causes of Central Sleep Apnoea?

Answer 40

• stroke, damage to respiratory centres in brain. The area of the brain affected will determine the effect of the stroke.
• drugs (eg. opoids), suppression of neuronal activity
• central hypoventilation syndrome-injury/trauma to brainstem, or congenital (‘Ondine’s curse)
• altitude eg. Cheyne-Stokes respiration

Question 41

so what happens in CSA?

Answer 41

person stops breathing consistently during the night for 10-20 seconds, at which point they usually wake up (causing tiredness and emotional difficulties as sleep is disrupted)

this 10-20 second period where they don’t get oxygen and they don’t get rid of carbon dioxide causes stress on the body

metabolic complications involve metabolic imbalances, affecting peoples diabetic control or obesity

Question 42

what is Cheyne-Stokes respiration?

Answer 42

oscillating apnoea and hypernoea

Question 43

when does Cheyne-Stokes respiration occur and what happens?

Answer 43

at altitude, where there is a change in the levels of partial pressures of different gases in the environment

the person starts to hyperventilate which means they get rid of an excess amount of co2, leading to an decrease in pH - repertory alkalosis

as a result, o2 levels go down and they body goes too far in the opposite direction as it tries to compensate, resulting in a wobbly line

Question 44

hypercapnia drive vs. hypoxic drive?

Answer 44

hypercapnia drive is the predominant stimulus underlying the urge to breathe

hypoxic drive occurs at very low PaO2