this is the last unit in dr. kaminski's guest lecture on respiration and talks about the control and regulation of respiration. it focuses on the "respiratory group", which is an area in the upper medulla and contains four different groupings of neurons: the dorsal respiratory group, the ventral respiratory group, the pneumotaxic center, and the chemisensitive area. the dorsal group contains motor neurons which innervate the muscles involved in inspiration ("inspiratory neurons") as well as afferent sensory neurons. most of the impetus for inspiration comes from the dorsal respiratory group. during normal restful breathing, these neurons fire to stimulate the inspiratory muscles, and expiration occurs due to relaxation of the muscles and elastic recoil of the ribcage. in vigorous, more ribcage based breathing, the ventral respiratory center gets stimulated as well, which has inspiratory as well as expiratory neurons-- this causes both the inspiratory muscles to be activated during inspiration (external intercostals, SCM, scalenes, etc) as well as the expiratory muscles during expiration (rectus abdominis and internal intercostals).
we then look at the different factors that can affect this rhythm. the pneumotaxic center inhibits the neurons of the dorsal and ventral respiratory center, specifically causing the amplitude of the "inspiratory ramp signal" to decrease. this causes more shallow, rapid breathing. the central chemoreceptor (of the "chemosensitive area") detects levels of H+ in the cerebrospinal fluid-- H+ in the CSF is directly representative of CO2 levels because the only H+ that is beyond the blood brain barrier is that which is produced from the dissociation of carbonic acid, which is formed from CO2. if H+ levels rise, the chemosensitive area stimulates the dorsal inspiratory neurons to increase the ramp signal. peripheral chemoreceptors are a secondary chemical feedback system and are located near the aorta and carotid artery and measure O2 levels -- if PO2 falls below 100mmHg, they also activate the dorsal respiratory group.
a couple other ways in which ventilation can be regulated: via stretch receptors in the lungs, which inhibit the dorsal inspiratory neurons if the pressure from inspiration becomes too great-- this is called the hering breuer reflex. conscious control of breathing from higher brain centers can bypass the respiratory group completely and innervate the thoracic muscles directly.
questions
1. where is the "neurogenesis" of the respiratory drive?
2. what is the "respiratory control center" and where is it located?
3. what are the different neuronal groupings of the respiratory control center?
4. describe the dorsal respiratory group.
5. describe the ventral respiratory group.
6. describe the pneumotaxic center.
7. describe the chemosensitive area.
8. within the respiratory control center, where does the main respiratory drive come from?
9. describe how the inspiratory neurons control breathing.
10. describe the effect of the pneumotaxic center on the depth of breathing.
11. describe what happens in the respiratory center during vigorous breathing.
12. what are the three factors that can modify the ventilation cycle?
13. describe how the higher brain centers modify the ventilation cycle.
14. what is the hering breuer reflex?
15. where is the central chemoreceptor and what does it measure?
16. why does the central chemoreceptor measure H+ levels?
17. what does the central chemoreceptor stimulate?
18. describe the peripheral chemoreceptors.
answers
1. the medullary centers
2. a set of neuronal groupings that are found in the upper medulla and pons.
3. dorsal respiratory group, ventral respiratory group, pneumotaxic center, chemosensitive area.
4. contains inspiratory neurons and afferent sensory neurons.
5. contains both inspiratory and expiratory neurons.
6. inhibits the inspiratory neurons of the respiratory groups.
7. wired into the dorsal inspiratory neurons, has neurons that are sensitive to the chemistry of CSF.
8. inspiratory neurons of the dorsal respiratory group.
9. inspiratory neurons have a rhythm in the form of a "ramp signal" that signals the diaphragm. the faster the inspiratory neurons fire, the more the diaphragm is stimulated to contract via the phrenic nerve.
10. the pneumotaxic center inhibits inspiratory neurons and thus greater pneumotaxic activity facilitates shallow, rapid breathing.
11. during vigorous breathing, the ventral inspiratory and expiratory neurons get activated and both the inspiratory and expiratory muscles get activated.
12. voluntary override by higher centers of the brain, physical input from stretch receptors in the lung, evaluation of blood chemistry.
13. corticospinal and corticobulbar tracts connect directly to lower motor neurons of thoracic musculature, bypassing the respiratory control center.
14. when stretch receptors in the lung are activated by inspiration and inhibit the inspiratory neurons of the dorsal respiratory group.
15. in the "chemosensitive area" on the ventral surface of the medulla.
16. H+ is representative of CO2 levels since CO2 converts into carbonic acid, which dissociates into H+ and bicarbonate. furthermore, H+ does not cross the blood brain barrier, so it is directly representative of CO2.
17. the dorsal respiratory group.
18. the peripheral chemoreceptors are the aortic and carotid bodies, which sense O2 levels and begin firing if O2 levels fall below 100mmHg, stimulating the dorsal respiratory group.
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