RESPIRATORY & CARDIORESPIRATORY INTEGRATION

Identify the three phases of respiratory rhythm

• inspiration

• post inspiration

• late expiration

Describe inspiration including the nerves and muscles that are activated

Inspiration:

• Diaphragm contracts, activated by the phrenic nerve, expanding the thoracic cavity (also intercostal muscles fire)

• The recurrent laryngeal nerve (vagus nerve) activates the abductor muscles of the upper airways to contract, reducing subglottal pressure

• Thoracic cavity and airway expands → intrapulmonary pressure decreases below atmospheric pressure → air flows into lungs

Describe post-inspiration including the nerves and muscles that are activated

Post-Inspiration:

• The phrenic nerve activity stops, relaxing the diaphragm

• The recurrent laryngeal nerve increases its activity even more, activating the adductor muscles, constricting the upper airways

• Increased subglottal pressure → slows the outflow of air → increases diffusion as air can remain in lungs for slightly longer

  • note: this phase is when most modulation occurs (e.g. when talking)

Describe late expiration including the nerves and muscles that are activated

Late Expiration

• Both nerves have very little activity, relaxing the diaphragm, upper airway and intercostal muscles

• This increases the rate of air outflow, enabling intra-alveolar pressure to return to atmospheric pressure and the intra-plural pressure to return to resting

• This increases the rate of outflow of air

Describe the process by which the pre-Bötzinger complex was identified as the potential respiratory rhythm generator

Found synchronous activity in a number of nerves but wanted to locate the source of the respiratory rhythm

• Explored by slicing sections of the brainstem and measuring the pattern of response

• Slicing where we now know is the VRG, caused respiratory rhythm of all nerves to disappear

  • thus this is the area that controls respiratory rhythm

To learn specifically where: looked at and recorded the sagittal section of the rat

• Cells associated with inspiration were found on the pre-Bötzinger complex that aligned with the respiratory rhythm

• Blocking the activity of neurons in this complex also lead to the loss of respiratory rhythm

Describe how Pre-BötC neurons can be characterised functionally

• Pre-BötC are pre-inspiratory neurones

• Little to no expiratory neurones

• Glutamatergic

  • Subset that is glycinergic

  • Thus is functionally excitatory and inhibitory

Describe how Pre-BötC neurons can be characterised chemically

There are no unique markers that define these neurones

• NK1-receptor expressing

• Somatostatin (SST) expressing

• Dbx-1 (transcription factor) expressing

None of these markers are exclusive to this complex, so when these are expressed, it is only likely that they are part of this complex

Describe how Pre-BötC neurons can be identified

Selective lesion of > 80% of NK1 receptor expressing cells in Pre-BötC using Saporin-Substance P

• Lead to irregular breathing patterns with large periods of apnea

• Thus NK1-R expressing cells in Pre-BötC are required for normal breathing rhythm generation

Confirmed more recently, using allatostatin:

• allatostatin inhibits the Pre-Bötzinger Complex, which causes breathing to stop, meaning it is essential for respiratory rhythm generation

Describe the properties of the Pre-Bötzinger complex

Pre-Bötzinger neurons are pacemakers that have intrinsic bursting properties

• if you block neurotransmitter receptors, action potentials of these cells still continue

The intrinsic bursting properties are reliant on persistent sodium currents in the Pre-BötC neurones

• if you block sodium currents (through riluzole), the rhythmic bursts stop

Also reliant on calcium ion activated non-selective cation currents

• if you block calcium activated non-selective cation currents there are a smaller intrinsic bursts

What happens if you destruct or silence neurons in the Pre-BötC?

Destruction or silencing of neurons in the Pre-BötC leads to severe disruptions in respiratory rhythm including apnea and altered breathing, demonstrating the critical role of these neurons in generating normal breathing.

Describe a key characteristic of the Pre-BötC in terms of activity generation

Respiratory rhythm is an emergent property

In an experiment it took 9 small bursts it takes to get a big burst of activity

Identify briefly evidence from humans that supports the importance of the Pre-BötC in respiratory function

Looked at the Pre-BötC in humans and found:

• the number of cells in the pre BötC is much smaller for someone with multiple system atrophy (which is sleep apnea and dysregulated breathing)

Identify other rhythmogenic sites and explain the current view of respiratory rhythm generation

Pre-Bötzinger Complex drives inspiration and is still the pacemaker, interacting with both other complexes to set the base respiratory rhythm:

• PiCo (post inspiratory complex)

• pFL (parafacial nucleus)

  • whether these complexes communicate is not clear

• RTN/pFRG activation drives active expiration

Describe the organisation of the respiratory neural network

The respiratory neural network is organized into distinct rhythmic centers, including the Pre-Bötzinger Complex, the Bötzinger Complex, and the Pons, which coordinate and modulate the rhythmic pattern of breathing through complex interactions.

Identify the sub-regions of the ventral respiratory group (VRG)

There are four different regions of the VRG

From rostral to caudal:

• BötC

• pre-BötC

• rVRG

• cVRG

Describe the different types of neurons within each region of the VRG

There is some topographical organisation of the respiratory neurons

• Within the BötC, we have post-inspiratory and augmenting expiratory neurons (as one fades the other picks up through post-inspiratory and late expiratory phases)

  • These tend to be inhibitory glycinergic

  • help terminate inspiration and reinforce expiration phase (respectively) by inhibiting inspiratory neurons and activity

• In the pre BötC, there are pre-inspiratory neurons (excitatory glutaminergic) and early inspiratory neurons (inhibitory)

  • decrease in activity over the period of inspiration

  • trigger the onset of inspiratory bursts and shape the pattern of inspiratory activity

• In rVRG there are augmenting inspiratory (excitatory to motor outputs)

  • increase in activity over the period of inspiration

  • drive motor output to diaphragm and intercostals

What happens if you inject DLH to the Pre-BötC and BötC?

Injecting DLH = glutamate receptor agonist

• If you excite the BötC it will excite expiratory neurons, extending the expiratory phase

• If you excite the Pre BötC it will excite pre-inspiratory neurons, initiating the rhythm bursts sooner

What happens to respiratory rhythm if you activate different regions within the respiratory network?

• Pre-BötC initiates and drives inspiratory bursts.

• PiCo is activated slightly after inspiration and shapes post-inspiratory airflow by slowing expiratory recoil.

• RTN/pFRG becomes active during metabolic/physical demand, recruiting active expiration.

• BötC provides inhibitory control, terminating inspiratory drive and regulating transitions between phases.

What are the three important regions that generate the respiratory rhythm?

• Pontine respiratory group important for post-inspiratory phase

• BötC important for expiratory phase

• Transection caudal of the pre-BötC abolished respiratory rhythm

  • Remove pre-BötC = a short decrementing pattern (only inspiratory phase too)

Describe how the three phase rhythm looks from a nerve activation perspective

• The three phase rhythm which is most clearly identified in the cervical vagus nerve (cVN), which is the main nerve of the recurrent laryngeal nerve (RLN)

• Inspiration defined by the phrenic nerve

• Post inspiration which is the decrementing component of the recurrent laryngeal nerve

• Short phase called late expiration (which is short in this case) that is defined by then the recurrent laryngeal nerve has almost no activity to when the phrenic nerve activity starts

• The hypoglossal nerve activity occurs pre inspiration to ensure the tongue and upper airways are open before the phrenic nerve is active

What happens if you remove the pontine respiratory group?

Pontine respiratory group important for post-inspiration

The pons provides an excitatory drive to post-i neurons in the BötC

• the bursting is still there

  • post-inspiratory phase burst is lost

  • Inspiration and expiration are longer, and it has changed into a two phase rhythm

• What changed most is the cVN activity, the timing of the burst is now synchronised with the phrenic nerve, however there is now no more big burst after the phrenic burst

Identify scenarios when respiratory rhythm is modulated from the basal rhythm

When talking, exercising etc

Describe how pulmonary stretch receptors can modulate respiratory rhythm

Pulmonary stretch receptors are sensory receptors located in the lung's smooth muscle that help regulate the breathing pattern by detecting lung inflation. When activated, they send signals to the brain to inhibit further inspiration, thus preventing overinflation of the lungs.

Activation of PSRs will result in what changes in the respiratory phases?

• Inhibition and termination of inspiration

• Slowing of respiratory rhythm

• Prolongation of expiration

What will the effect of activation of PSRs be on respiratory neurons?

• Inhibit pre-inspiratory neurons

• Inhibit inspiratory neurons

• Excite post-inspiratory neurons

• Inhibit augmenting expiratory neurons

Describe the chemoreceptor response to hypoxia

The glomus cells (type I), located in the carotid bodies and aorta, detect low oxygen levels (hypoxia) in the blood (but also high CO2 and glucose levels)

Glomus cells get activated by depolarisation of the cell membrane through different mechanisms such as inhibition of potassium leak channels

• they transmit their signals via paracrine actions (using ATP) to the glossopharyngeal nerve which terminates at the NTS, activating the autonomic nervous system to increase respiration and cardiac output

• Hypoxia response is to increase sympathetic nerve activity which manifests through an increase in HR and a small increase in BP

Describe the location of central and peripheral chemoreceptors

Peripheral Chemoreceptors

• Location: Carotid bodies (mainly) and aortic bodies

Central Chemoreceptors

• Location: Retrotrapezoid nucleus (RTN)

Describe how hypercapnia stimulates RTN neurons

• CO2 that's in the blood will enter the capillaries within the brain, diffuse across the BBB into the CSF where it undergoes a chemical reaction with water creating hydrogen ions

• The hydrogen ions activate the RTN neurons through activating TASK-2 (ion channel) and GPR4 (GCPR) channels, increasing neuronal activity through an unknown mechanism

  • astrocytes also increase the activity of RTN neurons

• Increase in activity of RTN neurons will increase breathing and sympathetic nerve activity

• It will also evoke active expiration (through aug-e excitation) during late expiratory phase

  • Breaths become bigger and the abdominal nerve now has bursts

Provide an overview of the modulation of the respiratory rhythmogenic network by peripheral afferent inputs and central chemoreceptors

The respiratory rhythmogenic network is modulated by peripheral afferent inputs from chemoreceptors detecting changes in blood O2 levels and by central chemoreceptors responding primarily to CO2 levels.

Peripheral chemoreceptors respond to low oxygen in the blood

Central chemoreceptors respond to high carbon dioxide in the cerebrospinal fluid

• Peripheral inputs, such as those from carotid and aortic bodies, synapse onto the NTS which projects to the VRG, enhancing respiratory drive through an increase in sympathetic activity

• Central chemoreceptors in the RTN respond to hypercapnia, altering breathing patterns (switching from tonic to phasic) and increasing sympathetic activity

  • activates aug-e neurons that drive the expiratory motor neurons for expiratory muscle activity

Describe how muscle contraction and exercise modulates respiratory rhythm and pattern and how is this mediated?

There is a 1:1 coupling between step frequency and breathing

• Muscle contractions = somatic afferent stimulation

Afferent feedback from contracting muscles resets respiratory rhythm

• Stimulation of somatic afferents leads to shortening of post-I phase due to inhibition of Post-I neurons (in the BötC) + excitation of aug-E neurons

• This shortens expiration and leads to earlier inspiration

• The rhythm resets at the next inspiration, which is mediated by the PRG, which is mediated by substance P in the BötC

What does resetting result in relating to the respiratory rhythm

Resetting results in early onset of inspiration

• occurs with shortening of expiration by inhibition of post-inspiration

Describe the relationship between sympathetic activity and respiration

Sympathetic Nerve Activity and Heart Rate are Modulated by Respiration

Describe what happens to the relationship between sympathetic activity and respiration during hypertension

It is altered

• Altered respiratory sympathetic interaction is a driver for the development of hypertension

• In a hypertensive animal, the sympathetic activity is much more associated - the bursts are bigger and everything seems to be phase locked to respiration

• In a normotensive animal, more sympathetic nerve activity occurs between bursts and the bursts aren't as large

Describe how the sympathetic respiratory coupling interaction occurs

The heart is in the thoracic cavity so is subject to sudden thoracic pressure changes, impacting venous flow and heart rate

• This is one way that breathing impacts heart rate

There is another reflex that links respiratory rate to heart rate, the Hering-Brauer Reflex

• When you breathe in, the stretch receptors increase in activity and send information via the NTS, changing the activity of sympathetic nerves impacting both respiration and HR

This relationship between respiration and sympathetic activity is totally reliant on the output of the c1 neurons in the RLVM which are recieving respiratory input

What is the relevance of the respiratory coupling in terms of hypertension?

Sympathetic activity is time-locked with respiration, particularly in hypertensive rats. The resulting bursts of activity result in increased noradrenaline release and NPY co-transmission in postganglionic sympathetic neurons.

Explain how hypertension occurs in relation to respiratory coupling and how this is relevant in treatment

• When respiratory modulation is exaggerated, it leads over time to blood pressure escaping from homeostatic control, being elevated, and hypertension (seen through amplified Traube-Hering waves)

• If we can identify these people, if we inhibit the C1 neurons, we can reverse this phenotype, stopping the development of the hypertensive phenotype

  • The C1 neurons being catecholaminergic, there are drugs that are relatively selective at inhibiting these types of cells