Control of Respiration

Peripheral chemoreceptor properties

20% of breathing; main stimulus is Pa,O2 (but it still gets a little from low pH and high Pa,CO2); fast non-linear response rate; can adapt.

Central chemoreceptor properties

80% of breathing; main stimulus is Pa,CO2 and pH in the CSF; slow linear response; can adapt

Key things to remember about central chemoreceptor stimulation

It is DIRECTLY stimulated by H+ but this is in response to changes in PCO2! The response is linear in response to Pa,CO2 The response is very sensitive to changes in Pa,CO2 If hypoxia is present, the response is even more sensitive and greater for a given Pa,CO2 (the linear response is shifted to the left and upward)

When is the expiratory center of the medulla active?

It is usually not, but when it is, is when the inspiratory center is silent. it then stimulates muscles of expiration: internal intercostals and some abdominal wall muscles

How does the medulla control breathing?

Its got an inspiratory and expiratory center. The inspiratory center has neurons that have intrinsic neural AP firings (rhythm generator). Increased APs within a burst means deeper stronger inspiration. Increased frequency of bursts means increase ventilation rate. Usually both go together.

Features of the peripheral chemoreceptor responses

Response is non linear, small changes in Pa,O2 elicit small responses, but if Pa,O2 drops like below 60torr, the response is great. The response is to PaO2 not O2 content; so no response to anemia There is no adaptation Hypercapnia and acidosis help modulate the response

Explain how the central chemoreceptors can adapt

The CSF pH regulators is regulated by a non-respiratory mechanism; therefore, after some time, that mechanism will restore pH in the CSF even though the PCO2 is high. This means that the receptors now think that the high PCO2 is normal and no longer increase ventilation. It takes a couple of days to adapt.

Feedback on breathing comes from where?

The main ones are the central chemoreceptors and the peripheral chemoreceptors (carotid and aortic bodies) stretch receptors in SM of the airways, irritant receptors btw airway and epithelial cells, joint and muscle receptors (which stimulate breathing in response to limb movement), and juxtacapillary receptors in the alveolar walls (these sense the engorgement of the pulmonary capillaries and cause rapid shallow breathing)

Describe the different components of the peripheral chemoreceptors

They are made up of the carotid and aortic bodies. The cells in these bodies doing the sensing are called Glomus cells (anoxia ihibits their K+ channels, causing depolarization, causing bursts of AP, causing Ca influx, causing release of NTs. They respond to hypoxia, hypercapnia (high Pa,CO2), and low pH. The main response however is to Pa,O2

Higher Brain centers influence on ventilation

They can basically override all the brain stem. They can recruit accessory respiratory muscles, and even control the diaphragm to change ventilation.

How do the central chemoreceptors sense PCO2?

Through [H+] in the CSF. CO2 can diffuse across the BBB, there it spontaneously turns into H+ and HCO3-. Also there is no CA in CSF or proteins, so any change in PCO2 is translated into changes in [H+] in the CSF.

Under which conditions will the central chemoreceptor response be reduced?

during sleep, in older people, athletes, drugs like morphine, barbiturates, and anesthetics

Apneustic breathing

prolonged inspiratory efforts interrupted by the occasional expirations. This pattern is induced by stimulation of the apneustic center or if the connection b/w the apneustic and pneumotaxic center is severed.

Apneusic Center of the Pons

stimulates respiratory neurons in the medulla an increases inspiration.

Which areas comprise the central controller?

the medulla, the pons, and the cerebral cortex

What is interesting about ventilation and exercise?

there aren't significant changes to Pa,O2, PaCO2, pHa during exercise. The central control works even during imaginary exercise. The exercise response starts also before it can change blood gases. Since Pa,O2 levels don't change significantly during exercise, it is not the trigger for increased ventilation.

Irritant receptors

these are located near the airway epithelium. They send afferent signals along the vagus nerve. They are stimulated by smoke, dust, rapid inflation, histamine. Their responses include, coughing, sneezing, or bronchoconstricition

Hering-Breuer reflex and stretch receptors

when lung inflates to 50% of capacity, the stretch receptors inhibit further inflation. However, if there is abrupt deflation of the lungs, these same receptors respond by increasing the ventilation rate.

Describe %Hba vs %Hbv O2 saturation

%Hba>%Hbv due to the fact that muscles are using more O2. Decreased CO makes %Hbv even lower, therefore in cases of anoxia, you should be checking mixed venous O2 levels.

What happens when PCO2 goes up?

Both central and peripheral chemoreceptors increase their firings!

Pneumotaxic center of the Pons

inhibits the apneustic center, clinically terminating inspiration it controls the depth of the respiratory response to CO2