Basic Control

Back in the nervous system webpages (on the brain), you learned about the respiratory center found in the medulla and pons. Remember? The medulla contains the main region for respiration-- the region responsible for causing the normal, resting inspiration. This region is called the dorsal respiratory group. If you think about it, the most basic need in respiration to merely survive is the ability to inhale a normal breath. Once inspiration has occurred, expiration can happen passively. In this manner, we can obtain our tidal volume of air simply by getting inspiration to occur.

The dorsal respiratory group cycles through activity. When active, its neurons are firing in bursts that cause the contraction of inspiratory musculature. When it stops being active, those muscles can relax, and expiration occurs.

The ventral respiratory group is only active when you need to breathe more actively. For example, when you are talking and forcing air out, you are using your ventral respiratory group. Forced expiration requires the ventral respiratory group. Your book tells you that the term "medullary rhythmicity area" is used to describe the dorsal and ventral respiratory groups; I don't care if you know that one term, just that you do know the dorsal and ventral respiratory groups and what they do.

Another important area of the respiratory center is the pneumotaxic area. This area is in the pons and is important for regulating the amount of air one takes in with each breath. You see, if you could examine the activity of the dorsal respiratory group in total isolation, you would see that its rhythmic bursts of activity are of constant duration and at a constant interval. But you know that we don't breathe that way; we don't have a totally steady breathing rate-- it is always adjusting to our situation. Yet, the inspiratory musculature is controlled by the dorsal respiratory group. So, this is where the pneumotaxic area comes into play. The pneumotaxic area alters the bursting pattern of the dorsal respiratory group. When we find ourselves needing to breath faster, the pneumotaxic area tells the dorsal respiratory group to speed it up. And when we need to take longer breaths, the pneumotaxic area tells the dorsal respiratory group to prolong its bursts. All the information from the body that needs to feed into the control of our breathing converges in the pneumotaxic area, so that it can properly adjust our breathing.

Dynamic Control

Minute by minute we have to be able to alter the amount of air we breathe. For example, if I get up and walk across the room, I need to get more than my tidal volume of air. If I continue to move around by exercising for a while, I need a lot more air. Then, when I return into a resting state (not even moving my fingers on the keyboard!), less air is needed again. How can we control how much air we need all the time? (In other words, what tells the pneumotaxic area what it needs to tell the dorsal respiratory group?)

	central chemoreceptors in chemosensitive areas These areas are found in the brainstem, and contain neurons within them, central chemoreceptors, that detect changes in our carbon dioxide levels. The way they do this is somewhat indirect. You see, if our carbon dioxide levels rise, that means that we have to increase our breathing rate, getting rid of the carbon dioxide and taking in more oxygen. You learned that carbon dioxide does not tend to remain CO₂ in water. Instead, it changes into a bicarbonate ion, producing hydrogen ions as a byproduct of this conversion. Well, in blood when carbon dioxide is converted into bicarbonate ions, the hydrogen ions are not a problem because they immediately associate with hemoglobin (the globins act to buffer the hydrogen ions). However, in the brain, in the chemosensitive areas, there is no hemoglobin. The cerebrospinal fluid of the brain does not have proteins to buffer the hydrogen ions. So when levels of CO₂ in the brain begin to increase, much of it is converted into bicarbonate ions and hydrogen ions. The central chemoreceptors are sensitive to hydrogen ion levels. So they indirectly recognize the increase in carbon dioxide levels.
	peripheral chemoreceptors in carotid and aortic bodies These are not quite as important as the central chemoreceptors. Where the common carotid artery branches into the internal and external carotid arteries, there is a small swelling. This swelling is the carotid sinus, and it contains regions called carotid bodies within it. The aorta contains regions called aortic bodies within it. These regions contain our peripheral chemoreceptors, which detect oxygen levels directly. Exactly how these neurons can be sensitive to oxygen isn't the issue here, but instead, it is interesting to note that these neurons can only detect large decreases in oxygen levels... so they are only activated when oxygen levels drop to very low, life-threatening levels.
	inflation reflex This reflex, like most, is a type of negative feedback. You don't want your lungs to inflate beyond their maximum! The alveoli would explode! So, as your lungs expand, there are sensory neurons that detect lung stretching; your book calls these neurons stretch receptors, but they are not at all like the stretch receptors in muscle-- OK? We'll call these neurons sensory neurons that detect stretch. Anyway, the more these neurons are active, the more they feed into the pneumotaxic area and tell it to end this round of inspiration. That prevents our lungs from ever overinflating.