Central neuronal circuits controlling respiration

Question 1

Define: eupneic

Answer 1

normal breathing

Question 2

Where does the phrenic nerve arise from and run to ?

Answer 2

arises from the cervical spinal cord

runs from C3-C5

Question 3

What does the phrenic nerve do ?

Answer 3

it is the driver of respiration - respiration requires the brain to enable its function
the phrenic motor neurones innervate the diaphragm which when it is active it contracts to open up the rib cage to help pull air into the lungs

Question 4

What have recordings from the phrenic nerve enabled?

Answer 4

the extracellular recordings of action potentials from this nerve has helped researchers identify changes in the activity of the respiratory cycle and assign functional roles to neurones in the central respiratory network

Question 5

What happens to the phrenic nerve in normal activity ?

Answer 5

in normal breathing phrenic nerve increases its activity- the activity ramps up in intensity

Question 6

What are the sequences of breathing ?

Answer 6

3 essential phases
1) inspiration = phrenic nerve activity- action potentials coming down nerve
2) post inspiration = passive relaxation- no more breathing in
3) expiration = active muscular movements
each phase involves contraction of specific muscle groups and respiration requires functioning of many nerve and muscle structures including brainstem and respiratory muscles

Question 7

What happens in the proper 3 phase rhythm?

Answer 7

• augmenting shape of phrenic nerve bursts
• preinspiratory onset of hypoglossal nerve bursts (50-100ms before onset of phrenic nerve burst) = purpose is to move the tongue out of the way to enable breathing
• prominent stage of decrementing postinspiratory discharge in cervical vagus nerve - big peak post inspiration

Question 8

What brainstem regions are contributing to respiration ?

Answer 8

rVRG, cVRG, pre-Botzinger complex, botzinger complex and many regions in the pons
DRG- less studied as its less prominent in rats but it does project to the phrenic nerve
VRG- nearly continuous column in lateral rhombencephalon

Question 9

What is a key point about the rVRG?

Answer 9

rVRG is the source of excitatory drive to the phrenic motor neurones and so for inspiration
BUT the VRG cells dont have a rhythm of their own

Question 10

How did they know that the BotC, pre-BotC and VRG neurones were involved in respiration ?

Answer 10

these areas contain neurones that have neurones firing with a similar pattern to respiration- therefore firing pattern is related to respiration

Question 11

Why is the working heart-brainstem prep good?

Answer 11

it doesn’t require anaesthetic to be injected which is important when recording respiration because anaesthetic will alter breathing

• enables recordings from the BotC, pre-BotC and rVRG
• can also record from phrenic, cervical vagus and hypoglossal nerves

Question 12

What experiment did they do to decipher which areas of the brain are involved in the rhythm of respiration?

Answer 12

sliced off little bits of the brain until they got no activity to determine where the rhythm comes from

Question 13

What happened when sliced off all the way down to the medulla (cVRG and rVRG)?

Answer 13

once they were down just to the medulla there was no rhythm indicating that the drive cannot come from neurones in VRG alone

Question 14

What happened when the pre-BotC and medulla were present?

Answer 14

once the pre-BotC was added a rhythm was present but it was a 1 phase rhythm because it was decrementing - not quite the normal rhythm

• detected cervical vagal activity at the same time as phrenic which is NOT normal so rhythm was not proper
• hypoglossal and phrenic were also firing at the same time

Question 15

What happened in the medullary preparation with BotC, pre-BotC and VRG neurones?

Answer 15

a 2 phase inspiratory-expiratory rhythm pattern occurred
1) non-ramping square-wave motor profile
2) onset of activity synchronised in all nerves and by a lack of post- inspiratory discharge in cVN
still not a proper normal rhythm

Question 16

What are the neurons like in the BotC?

Answer 16

inhibit expiratory neurones

provide inspiratory-expiratory alterations during normal breathing

Question 17

What are the neurones like in the pre-BotC?

Answer 17

excitatory glutamate neurones, they are the primary source of inspiratory drive
does have some GABA cells too which may inhibit expiratory cells during inspiration

Question 18

What are the neurones like in the rVRG?

Answer 18

excitatory and they project to the spinal cord to drive inspiration in the phrenic motor neurones
they are excited by the pre-BotC in inspiration and inhibited by BotC in expiration

Question 19

What are the neurones like in the cVRG?

Answer 19

excitatory cells which shape patterns of excitatory drive to spinal motor neurones in expiration

Question 20

What is endogenous bursting activity based on ?

Answer 20

expression of persistent sodium channels

excitatory interactions within the population

Question 21

What happens when you record the activity in the pre-botzinger complex?

Answer 21

the activity relates to the activity of the phrenic nerve - fires in a rhythmic patter onto rVRG which projects to the phrenic motor neurones

Question 22

What happens when the pre-BotC is ablated and how did they do it ?

Answer 22

it disturbs respiration
noticed that the cells contained lots of substance P receptors - NK1R - they were fairly restricted to the pre-BotC complex
took antibody for receptor and tagged it with a poison saporin to kill the cells and it completely abolished respiration

Question 23

What was seen when recording from control and pre-BotC abolished mice?

Answer 23

the control rats breathing room air would spontaneously generate an occasional sigh followed by an extended expiratory pause which was absent in the pre-BotC abolished mice
they breathing movements weren’t occurring like normal

Question 24

What happened to the control and pre-BotC abolished mice in 5% co2 ?

Answer 24

normal animal had increased breathing rate and depth

whereas the abolished rat didn’t respond well, breathing didn’t increase

Question 25

What happened to the control and pre-BotC abolished mice in 100% o2?

Answer 25

in both control and abolished mice there was a decline in breathing

Question 26

What do the mice suffer from when the pre-BotC is abolished?

Answer 26

suffer from apnea - pauses breathing

this indicates that the pre-BotC is important for driving respiration

Question 27

What ontogenetic experiment was carried out in respiration?

Answer 27

injected channel rhodopsin into rats into pre-BotC
they proved it was the pre-BotC because they contained with EYFP for NK1R
shining the light on these channels demonstrated that they could drive respiration

Question 28

During optogenetic stimualtion, what did they record and what was seen?

Answer 28

monitored diaphragm activity and this demonstrated increased activity when laser is shone
measured airflow at trachea and when laser is shone it increased the number of breathes per min
shone laser at predefined intervals and respiration followed activation, therefore pre-BotC neurone activation drives respiration

Question 29

Why is the breathing pattern still not normal even with BotC?

Answer 29

the pattern is slightly flat
BotC is involved in inhibiting drive to breathe
requires pons activity and because it is absent it means the BotC is fully inhibited by Aug E drive

Question 30

What does the pons do?

Answer 30

it increases the excitability and firing frequency of pre-inspiratory neurones and therefore increases the amplitude of all motor outputs
excites BotC cells

Question 31

Why are the pons and retrotrapezoid nucleus important?

Answer 31

important in setting the normal drive

Question 32

What does the retrotrapezoid nucleus do to the Aug E ?

Answer 32

it is involved in shutting off respiratory drive

Question 33

How did they determining which cells types were important in which areas?

Answer 33

put electrodes into the VRG, BotC and pre-BotC to determine when the neurones in these areas fired

Question 34

What did they demonstrate about the neurones in the pre-BotC?

Answer 34

neurones fire in inspiration both early inspiration and pre-inspiration
fire onto rVRG which contains neurons that also fire in inspiration with a ramp like activity

Question 35

What did they demonstrate about the neurones in the BotC?

Answer 35

contains cells that fire in expiration, they are augmenting neurones
some fire in early inspiration
when these neurones fire the pre-BotC neurones dont fire

Question 36

Why do you get different phrenic firing when the BotC is removed?

Answer 36

you dont have the switch off so it causes differing firing activity