The neural control of breathing Flashcards
what is breathing?
a rhythmic process that maintains O2 and CO2 pressure gradients between alveoli and blood
define the following: VA PA Pa Pv
VA - alveolar ventilation
PA - alveolar partial pressure
Pa - arterial partial pressure
Pv - venous partial pressure
equation involving diffusion:
diffusion = (surface area x pressure difference)/ distance^2
what does ventilation have to maintain:
the adequate partial pressure gradient between the air in the alveoli and capillary blood
as ventilation becomes deeper/greater, what happens?
air within the alveoli will resemble atmospheric air more and more
why is ventilation regulated?
to meet varying o2 demand and co2 production
how is ventilation increased?
by changing tidal volume (depth of breath) and the frequency (rate of breaths per minute)
in what situations is ventilation increased?
during infections like sepsis, increased acid production in the body and increased metabolism, and also in trauma
why is respiratory muscle contraction and relaxation is required in inspiration?
in order to expand and compress the thoracic cavity
what type of muscle tissue are the respiratory muscles (e.g. diaphragm) made up of?
skeletal muscle tissue
what causes skeletal muscle to contract?
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.
how is breathing initiated?
neural activation of respiratory muscles, which provide the movement required for ventilation.
what is the effect of spinal cord injury/motor neuron disease/muscular dystrophy?
Respiratory failure
-the person will not be able to breathe
difference between quiet/passive breathing and forced breathing?
quiet breathing is breathing at rest
forced is big breathing manoeuvres
what happens in quiet breathing?
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
when are further muscles recruited in breathing?
If we need to increase the force of breathing (greater speed or depth)
define the term “accessory muscles”, and give an example:
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.
what is the basic breathing pattern generated by?
neuronal systems within the brainstem
where are signals initiated and sent to when respiratory muscles need to contract?
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
what is the role of The Central Pattern Generator (CPG)?
it integrates data from various neuronal inputs to regulate ventilation (ie. determines how often and hard to breathe)
list some neuronal inputs which send signals to provide feedback (which are integrated to regulate breathing):
(look at diagram in notes)
- 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
what questions does the body “ask” in order to determine ventilation in the body, and what does it do with the info?
- 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
what do central respiratory chemoreceptors (CRC) present in the medulla indirectly monitor and respond to?
changes in arterial (P)CO2
what does it mean when there are high levels of CO2 within arterial blood?
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
in the brain, where will CO2 present within arterial blood go?
CO2 is a small uncharged gas and so can pass through the blood brain barrier, and will diffuse into the cerebrospinal fluid
once in the CSF, what happens to the CO2 and chemoreceptors?
- 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
explain why CRC’s don’t directly respond to changes in blood pH (except via CO2)?
because H+ ions are charged so they don’t naturally diffuse through the blood brain barrier, unlike CO2
are are peripheral chemoreceptors located and what do they respond to?
- located outside of the CNS, in the arterial system, within the aortic and carotid bodies
- respond to changes in arterial o2, co2 and pH
what are aortic and carotid bodies activated by?
- decreased partial pressure of oxygen
- increased partial pressure of carbon dioxide
- acidaemia (high conc of [H+] in blood)
what happens when the aortic and carotid bodies activated?
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
what is the predominant influence on the rate of ventilation at a given time?
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
what is hypercapnia?
hypercapnia is abnormally high levels of co2 in the blood
so, what is ventilation is generally proportional to?
PaCO2
what is hypoxaemia?
low PaO2
what does hypoxaemia stimulate?
increased ventilation
what is hypoxic drive?
when there are very low partial pressures of o2, which have a substantial effect on the rate of ventilation
in what type of situation does hypoxic drive start to take on a greater role?
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
what is Central Sleep Apnoea?
temporary cessation of breathing during sleep caused by dysfunction of the processes that initiates breathing
potential negative effects of Central Sleep Apnoea on health?
- tiredness
- cardiovascular complications
- metabolic dysfunction
what are the causes of Central Sleep Apnoea?
- 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
so what happens in CSA?
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
what is Cheyne-Stokes respiration?
oscillating apnoea and hypernoea
when does Cheyne-Stokes respiration occur and what happens?
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
hypercapnia drive vs. hypoxic drive?
hypercapnia drive is the predominant stimulus underlying the urge to breathe
hypoxic drive occurs at very low PaO2
