All right everybody, welcome back. The next topic we're going to discuss is the
regulation of breathing. So, we're going to look at the nervous control of
breathing. So we're going to look and we see that in the brain we have two
respiratory control center. One is going to be in the medulla and the other is
going to be in the pons. We're going to look at where they are located. This is
the medulla over here and there is going to be a respiratory control center. We're
going to look at it in a minute . This is the pons and we're going to have another
respiratory control center but going to do things slightly differently as we are
going to see. We're going to start with the medullary control center. Now what
does it do? This is the area that sets the basic rhythm. So we call it actually
the rhythmicity center. Why do we call it that? Because it will send regular
rhythmic impulses to the breathing muscles to set the rate of breathing.
This is of course the resting respiration. There are two groups in the
medulla. One is located dorsal. We call it the dorsal respiratory group. The other
is located ventrally and we call it the ventral respiratory group. So this is
where we are looking. This is the dorsal respiratory group and this is the
ventral respiratory group. Let's look at each one individually. The dorsal
respiratory group mainly concern has inspiratory neurons which means its main
function is to send the neural stimuli via the nerves to the inspiratory
muscles, stimulating them at regular intervals to contract
for inspiration to occur. So it's going to send impulses to the external
intercostal muscles and the diaphragm at a regular rhythm so that we are
breathing at a rate of 12 to 15 breaths per minute.
Once this stops and shuts off, the stimulation stops, expiration will occur
as a passive process. How about the ventral group? The ventral group helps
the dorsal a little in the in the inspiration, but it also contains
expiratory neurons which means they will have a role in forced expiration.
So they will send stimulation to a different group of muscles and
those muscles are the internal intercostal that will allow us to do
forced expiration. Now we're thinking breathing in and out, resting. We
understand this when do we do force expiration. An example, if you go to the
doctor and he is examining your chest and putting a stethoscope and says
breathe in and breathe out. So you will force air in and force air out. This is
called forced expiration and this is where the ventral group is going to act.
So we're looking at both the ventral and the dorsal over here. We can see that the
dorsal is going to send stimulation to the external intercostals and the
diaphragm for the rhythmic inspiration. The ventral is only responsible for the
internal intercostals which is involved in force expiration. So this is the
center in the medulla. How about the one in the pons? What does it actually do?
It moderates the rate which means if there are certain
situations where the rhythm has to increase a little, or the rhythm will
decrease a little, the pons is responsible for doing that. And the
pons also does something. It does the flawless easy transition between
inspiration and expiration. So we're not like I'm breathing in, stopping, breathing
out. It gives this smooth transition between the phases of breathing. So in
the pons again. We're going to find two centers. We're going to look at them
briefly. The Apneustic Center and the Pneumotaxic Center. What does
the Apneustic Center do? It helps in the process of inspiration, helps
stimulate the medulla to keep on stimulating the inspiratory muscles. The
Pneumotaxic, however, does something completely different. It will inhibit
inspiration. Not shut it off, but limit how much inspiration should go for. So as
soon as we expand the lung and inspiration has reached its norm, the
Pneumotaxic is going to stop the process of inspiration and allow for the
passive exploration to occur. So it actually limits the inspiratory volume
and the inspiratory rate. So we're looking at these two centers. This is the
Apneustic Center and this is the Pneumotaxic. The Apneustic will
stimulate the medullary Center to continue the rhythmic inspiration. The
Pneumotaxic will limit inspiration so we don't over expand the lungs. All right.
So we know who controls the rate of normal resting breathing, air going in
and outside the lung. Are there factors that can change
into this normal rate? Yes, there are certain factors and we're going to look
at them together. We're going to look at chemoreceptors whether peripheral or
central. And then we're going to look at how the higher centers affect the rate
of breathing. Just a reminder, what are chemoreceptors? Chemoreceptors are
receptors that will notice any chemical changes in the ah, in the blood, so whether
there's change in the level of oxygen carbon dioxide or pH, chemoreceptors are
there to sense this change and to react accordingly. So we're going to start with
the chemoreceptors. We're going to look at the peripheral chemoreceptors. Where
are the peripheral chemoreceptors located? They are located in the bigger
arteries. So in the aorta we call them the aortic bodies. In the carotid, we
call them the carotid bodies. So they are sensing the chemical constituents of the
blood leaving the heart . What are they sensitive to? They are going to be
sensitive to any changes in partial pressure of oxygen, carbon dioxide or
change in ph. Particularly, if there is decrease in the partial pressure of
oxygen, increase in the partial pressure of carbon dioxide, decrease in pH which
means increase high hydrogen concentration or acidosis. Let's remember
all of these changes always happen together. If I have low oxygen, I have
high carbon dioxide. I have acidosis. Anytime these changes will occur my
peripheral chemoreceptors will send extra stimulation to the respiratory
center to increase the respiratory rate. Increase the
respiratory rate. What will this do? This will restore the normal homeostatic
level. So if I increase the respiratory rate I am increasing the oxygen input. I
am washing out the excess carbon dioxide and accordingly we are reversing the
acidosis. So when we increase the respiratory rate, we're restoring things
back to the norm. These are the peripheral chemoreceptors. The central
chemoreceptors. We call them central because they are in the central nervous
system. So where are they located? Actually, in the medulla oblongata. So
there is a respiratory center in the medulla. And I also have central
chemoreceptors there. What can they detect? What are they sensitive to? High
levels of carbon dioxide. So they are sensitive to high levels of PCO2 which
will result in acidosis. Why only that? Because carbon dioxide can cross the
blood-brain barrier. Let's look at that. So this is the blood. This is the brain.
This is the medulla oblongata. And the only gas that can cross the
blood-brain barrier is carbon dioxide. So if I have increased level, if I have high
levels of PCO2, CO2 will diffuse and will enter into the cerebrospinal fluid and
the carbon dioxide will mix with water forming carbonic acid and this will
break into bicarbonate and hydrogen ion which means acidosis. If this occurs, who
will detect this change? The chemo receptors.
And accordingly, what will the chemoreceptors do? They will stimulate the respiratory
center, increase the respiratory rate to restore back the norm. This is another
nice depiction of the central chemoreceptors of the cerebral
capillaries, and how when PCO2 rises CO2 diffuses. We end up with acidosis. The
central chemoreceptors will detect this abnormal change. This will stimulate the
respiratory control center in the medulla - it's right there in the medulla
and ventilation will increase and this will restore the normal CO2 and pH. All
right, so we spoke about the peripheral chemoreceptors, the central
chemoreceptors. What else can affect the respiratory rate? All right. These are two
together, combined, the peripheral are in the blood. It is response, it's going to detect
changes in O2, CO2, pH and will affect the respiratory center to increase the
ventilation. This is the central chemoreceptors. Once PCO2 rises it will
diffuse to the cerebrospinal fluid. Central chemoreceptors will detect it
and this will increase the ventilation. So this is the combined view of both of them.
Now how about the higher centers? Do the higher centers have any role in the
control of breathing? Yes. So we're going to talk about a few of these. First we're
going to look at the cerebral cortex. Cerebral cortex is right here and we
call it the higher center. It has a role in voluntary breathing. So it has the
ability of increasing or decreasing the rate of breathing voluntary. When does
this happen? I'm going to give you example. So if
somebody has certain lung disease and the doctor
says you need to do some breathing exercises. You need to breathe so many
minutes in and out in a certain routine. This is going to occur voluntarily. So a
person is going to say, okay it's time for my breathing exercise. I'm going to
start to breathe in and out at a certain rate. This is voluntary. Who does this? Who
controls this? It is definitely the cerebral cortex.
What other higher centered the limbic system -- if you remember the limbic system
it it's what we call the emotional brain. So sometimes when we are scared or
excited we breathe faster. So this will stimulate the respiratory center to work
at a higher rate. The hypothalamus also is where we have the regulating center
for temperature. And during fever breathing can increase and again it has
a good physiological role. So this concludes our talk on the control of
breathing. We've seen how it it occurs from the higher centers of the brain and
we cover different factors that can affect the rate of breathing. And thank
you.