Central chemoreceptors- where and what are they ?

Question 1

What are the 2 systems contributing to the respiratory chemoreflex ?

Answer 1

Peripheral chemoreceptors- sensitive to arterial oxygen levels and proton concentrations in the arterial blood
central chemoreceptors- sensitive to central proton concentration changes

Question 2

Where are most of the peripheral chemoreceptors?

Answer 2

within the carotid sinus

Question 3

What did a working heart brain stem prep demonstrate about the peripheral respiratory chemoreflex?

Answer 3

the peripheral chemoreceptors can be activated by sodium cyanide- a very powerful stimulant
it causes an increase in BP likely due to peripheral vasoconstriction
respiration increases dramatically
reduction in HR which is likely due to only activating the peripheral chemoreceptors
increase sympathetic nerve activity which would contribute to vasoconstriction

Question 4

Where in the carotid sinus are the chemoreceptors?

Answer 4

they are a specialised ball of cells within the carotid body and they receive sensory innervation from carotid sinus nerve which is a branch of the glossopharyngeal nerve ]
the type 1 cells respond to hypoxia causing a reduction in activity of potassium channels to induce depolarisation

Question 5

What is interesting about the central cardiovascular pathways involved in the respiratory chemoreflex?

Answer 5

both the vagal and sympathetic activity are enhanced

Question 6

In a cat model of the ventral medulla, what did application of nicotine to the rostral and caudal chemosensitive areas do ?

Answer 6

caused an increase in ventilation

Question 7

What happened when they cut the midline of the ventral medulla in cats?

Answer 7

it abolished most of the central chemosensitivity and then subsequent cuts abolished the remainder of activity
found that even if you apply nicotine to one side but cut the link between both sides then the response to chemosensitivity is lost

Question 8

What do the central chemoreceptors do when brain PCO2 increases ?

Answer 8

they recognise the decline in proton concentration and therefore stimulate breathing
they maintain the arterial PCO2 within a few mmHg of 40mmHg
it is quite slow though, as it takes 50 seconds
a 2mmHg rise of brain PCO2 will increase ventilation by 50%
detects increased PCO2 by CO2 + h20 producing protons + bicarbonate ions

Question 9

What are the 4 theories of central chemoreceptors?

Answer 9

1) specicialised central respiratory chemoreceptors
2) broad- spectrum central respiratory chemoreceptors
3) ubiquitous chemoreception
4) pH- insensitive neurones

Question 10

What is the specialised central respiratory chemoreceptor theory ?

Answer 10

neurones that detect protons and drive respiration of central pattern generators

Question 11

What is the broad-spectrum CRCs theory ?

Answer 11

proton sensitive neurones that modulate activity of many targets not just central pattern generators

Question 12

What is the ubquitous chemoreception theory ?

Answer 12

chemoreception is due to summation of small effects of pH all over the respiratory network

Question 13

What is the pH-insensitive neurone theory ?

Answer 13

not central respiratory chemoreceptors are not directly affected by protons

Question 14

What are the main areas considered to contain the central chemoreceptors?

Answer 14

retrotrapezoid nucleus
raphe nuclei
distributed throughout CNS

Question 15

Where is the RTN and how are they defined?

Answer 15

it is rostral to the BotC
-putative central chemoreceptors in RTN comprise about 2000 chemically defined neurones in rats and 800 in mice located amongst other cells

defined by phox2b, neurokinin-1 R, vesicular glutamate transporter 2 mRNA, TASK2
they dont have ChAT, TH or GAD

Question 16

What experimental procedure was carried out in the RTN ?

Answer 16

anaesethetised rats and recorded action potentials from the RTN while changing the inspired carbon dioxide levels
sometimes difficult to find RTN so they stimulate the facial nerve to help localise it

Question 17

What was shown by the experimental procedure in the RTN ?

Answer 17

are the CO2 levels were increased the phrenic nerve activity increased and the activity of the neurones in the RTN was increased so RTN neurones must respond to CO2 or changes in proton concentration

Question 18

What was shown about the action potentials of the RTN neurones when CO2 levels were increased ?

Answer 18

they done fire in a particular rhythmic fashion and therefore it would appear that they are not involved in setting the rhythm of firing but it may tweak the pre-BotC rhythm

Question 19

What ontogenetic approach was carried out in the RTN?

Answer 19

they took RTN cells and expressed ChR2 under the control of the phox2b promoter
they then activated the ChR2 with blue laser in constant CO2 levels and the light causes these cells to be excited, recorded a large increase in their activity
they then put a low end tidal CO2 level to switch off the natural drive of these neurones and this showed that the light could overcome the switching off- demonstrating that these cells are powerfully involved in controlling respiration

Question 20

Why does guyenet believe the ccRTN cells are not part of the rhythm or pattern generating for respiration ?

Answer 20

• they dont exhibit much entrainment of firing rate with respiration, only when co2 levels are extreme
• even when the ccRTN cells are activated by optogenetics at a constant rate, respiration continues to be rhythmic as if normal central chemoreceptor stimulation
• the excitatory effects of ccRTN stimulation takes a long time

Question 21

Where do the RTN neurones project to ?

Answer 21

pre-BotC, pons, VRG and NTS- all areas you would expect if this nucleus is involved in respiration

Question 22

What happens in the RTN when the CSF pH declines?

Answer 22

It closes the potassium channels to induce excitation to drive respiration

Question 23

What other cells may be influencing the RTN?

Answer 23

glial cells

they can detect declines in pH

Question 24

Where are the raphe nuclei located?

Answer 24

located along the midline and contain serotonergic neurones projecting throughout the CNS - supply 5ht to every area of the CNS- clearly involved in many different systems

Question 25

What is richerson’s hypothesis ?

Answer 25

raphe nuclei as chemoreceptors

project to the BotC to increase activity and generate respiratory activity upon hypercapnic acidosis

Question 26

What was shown about serotonergic neurones when they recorded from brain slices?

Answer 26

recorded action potentials of raphe nuclei neurones and made them bathe in acidic CSF
this caused increased firing of action potentials due to activation of chemo sensitive neurones

activated by both a decrease in extracellular pH and an increase in carbon dioxide at constant extracellular pH - this indicates that the stimulus is probably a decrease in intracellular pH

Question 27

What is another element that would help serotonergic neurones be chemo-sensitive?

Answer 27

they are closely associated with arteries
therefore they can be easily influence by whats in the blood
not all 5ht cells are like so maybe it is just a subset that are chemo-sensitive

Question 28

What in vivo evidence do they have about serotonergic cells having a chemoreceptive role ?

Answer 28

as co2 increase there was increased firing in some 5ht cells
focal acidosis of some raphe sites increases phrenic nerve discharge
lesioning 5ht cells decreases respiratory responses to co2

Question 29

Where do the 5ht cells project to and what does it indicate ?

Answer 29

project to the NTS, motor neurones and pre-BotC and excites all these different areas which are all involved in respiration and therefore indicates its involvement in respiration

Question 30

What did guyenet think about the involvement of the serotonergic cells in respiration ?

Answer 30

didn’t really believe it so he took expressed ChR2 in 5ht cells in raphe obscurus and excited them with a laser and then recorded from the muscle cells - diaphragmatic EMG
the EMG amplitude increased upon light stimulation and therefore increased respiratory frequency - showed they could increase respriation upon activation, but he showed that the cells were not activated by hypercapnia and therefore they were not intrinsically chemo-sensitive so they dont fill the criteria of being central chemoreceptors

Question 31

Overall what did guyenet state about the serotongeric cells involvement in respiration ?

Answer 31

he thinks they dont drive respiration as they are chemo-sensitive but they maybe innervated by chemo-sensnitve cells, therefore not sure about the raphe nucleus’ involvement in this process

Question 32

What is nattie’s idea?

Answer 32

a distributed central chemoreceptor system
there are many areas that are CO2 responsive by focal acidification in vivo- NTS, CVLM, RTN, rostral and caudal MR and PBC
maybe the change in respiration is a summation of all their influences
maybe you need more than one area controlling such an important activity

Question 33

What is Gourine’s ATP hypothesis ?

Answer 33

he thinks the glial cells are central chemoreceptors
he took rats and changes the co2 levels to induce a chemoreceptove stimulus
then monitored whats released from the chemoreceptive zone and noticed that ATP is released in line with respiration
its only released in chemoreceptive areas
noticed that by blocking ATP p2x receptors the response o CO2 is attenuated but appluing ATP mimics the effects

Question 34

Where do they think the ATP comes from ?

Answer 34

think it comes from astrocytes
they are near the ventral surface and are very pH sensitive
the astrocytes are often closely associated to blood vessels and a decrease in pH causes an increase in calcium

Question 35

How did the measure the activity of the astrocytes ?

Answer 35

carried out in vivo calcium imaging of pH evoked astrocytes
filled the astrocytes with a calcium sensor and as they made the pH more acidic there was a greater rise in calcium in the astrocytes - only in ventral medulla - no effect in astrocytes in cortex

Question 36

Where are astrocytes located?

Answer 36

aligned with blood vessels therefore they can detect the pH in blood vessels

Question 37

When recording from RTN phox2b neurones what happened when ATP was blocked?

Answer 37

normally a decreased pH causes phox2b neurones to get excited however when they blocked the ATP it reduced their excitation

Question 38

What optogenetic experiment was carried out on glial cells ?

Answer 38

they expressed ChR2 in the glial cells and recorded from the phox2b neurones
depolarisation of the glial cells by light activation caused depolarisation of phox2b neurones
this effect was abolished when the ATP antagonist MRS2179 was applied

Question 39

What other parameters were recorded showing that glial cells could be central chemoreceptors?

Answer 39

shone light on astrocytes and it caused increased phrenic nerve activity and increased respiration and again ATP blocker was able to block this effect