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Lecture 14

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University of Toronto Scarborough
Biological Sciences
Stephen Reid

1    Lecture 14: The Control of Breathing 1. Summary of the Control of Breathing Breathing is produced in the respiratory control centres of the medulla oblongata and the pons, in the brainstem. A “basic rhythm of breathing” is produced and then modified by a host of inputs from various (central) chemoreceptors in the brain, peripheral chemoreceptors in the arteries, stretch receptors in the lungs receptors in the muscles and joints as well as irritant receptors in the lungs. We are not particularly concerned with the receptors in the muscles and joints, but in short, while we all know that breathing increases during exercise, there is no known respiratory control system that can account for the increase. It is thought that this increase in breathing may actually be caused by mechanoreceptors or proprioceptors, rather than receptors that are specifically respiratory-related in nature. The important central control centres for breathing are located in the brainstem, specifically the pons and medulla. If the brain is transected just above the midbrain the breathing continues completely unimpeded. There are three major groupings of neurons that are important for controlling breathing: in the pons we have the pontine respiratory group, and in the medulla we have the dorsal respiratory group and the ventral respiratory group. In most mammals, including humans, the dorsal respiratory group is the same as the nucleus tractus solitarius (or NTS). 2    The discovery of these groups arose from brainstem transection experiments in cats in 1929. If the midbrain is removed, regular breathing (eupnea) is instead replaced by apneustic breathing, in which the respiratory muscles remain active, but airflow stops. If the pons is cut away, gasping occurs (physiologically, this refers to the most basic, primitive form of breathing, with quick, harsh inhalations and exhalations). It is thought to be auto-resuscitative, and can also be caused by hypoxic or ischemic conditions. Finally, if the medulla is severed from the spinal cord, death results, because the respiratory control centers have been disconnected from the phrenic motor nucleus that drives the diaphragm as well as the motor nuclei that drive the intercostal muscles. 3    2. The Pontine Respiratory Group (PRG) The Pontine Respiratory Group (PRG) is located in the pons. It consists of two main nuclei, the Kölliker Fuse and the Nucleus Parabranchialis. It plays an important role in the termination of inspiration and the correct switching from inspiration to expiration. Later, we'll see that this is actually a good example of redundancy in respiratory control systems, because the stretch receptors in the lungs perform the same function. For important control systems, such as respiration or heart function, redundancy in control systems help to increase the chances of survival if one of the primary (or the primary) control systems should fail. The PRG is also called the apneustic centre because if the connection between the midbrain and the pons is severed, then apneustic breathing (prolonged inhalation) results. 3. The Dorsal Respiratory Group (DRG) The Dorsal Respiratory Group (DRG), which is approximately the same (in most animals, including humans) with the nucleus tractus solitarius in the medulla oblongata, is an important relay center for both respiratory and cardiovascular control system input. It is the site of the first synapse of carotid sinus baroreceptors and aortic arch baroreceptors, as well as carotid body O 2 chemoreceptors. Pulmonary stretch receptors also report to the NTS. The DRG is an important relay centre, taking in this respiratory-related information and sending signals to the ventral respiratory group as well as to some respiratory motor neurons. 4. The Ventral Respiratory Group (VRG) The Ventral Respiratory Group (VRG) consists of three main nuclei: the Bötzinger Complex, the Nucleus Ambiguous and the Nucleus Retroambiguous. It is essentially an integrative center, taking neural input from the DRG and the rhythm generator and then “modifies” this information to produce the motor output that ultimately drives the respiratory muscles. It is made up of inspiratory neurons some of which project to the respiratory motor neurons, some project within the VRG, and some fire only during active expiration (for example, during exercise). 4    5. The Pre-Bötzinger Complex The site of respiratory rhythm generation (the basic kernel of breathing) is the respiratory rhythm generator which appears to be located in two regions of the brain with the primary function being in the Pre-Bötzinger Complex. It is located in the ventral region of the brain, very close to the Bötzinger Complex of the VRG. The rhythm generator acts as the “pacemaker” for breathing. Neural activity generated in the Pre-Bötzinger Complex goes to the VRG for modification before being transmitted along the respiratory motor neurons to drive the respiratory muscles. The importance of the Pre-Bötzinger Complex can be seen when it is disabled - breathing becomes ataxic, and very irregular - and if the complex is completely destroyed, breathing ceases altogether. Generally, this would mean death - however, if an animal with a destroyed Pre- Bötzinger Complex is put on artificial ventilation for an hour or so (immediately following destruction of the Pre-Bot), breathing will eventually resume spontaneously. This indicates that there is a redundant rhythm generator located in the brain (though generally, if the Pre-Bötzinger Complex is destroyed, the redundancy cannot take over quick enough to prevent death). It is now known that the Pre-Bötzinger Complex works together with the Parafacial Nucleus (which is located right beside it) to act as a pacemaker, and given enough time the Parafacial Nucleus can take over as pacemaker if need be. 5    6. Models of Respiratory Rhythm Generation and Respiratory-Related Neurons Respiratory control is an extremely complicated and complex process, with many different nerves acting in different ways and firing at different times with respect to inspiration and expiration. The phrenic nerve, for example, ramps up in activity during inspiration, before reducing its activity during expiration and ending in a period of no activity before the next breath. Early Inspiration neurons are active during and slightly before inspiration, whilst ramp inspiration neurons are only active during inspiration. Late inspiration neurons are only active in a short burst towards the end of inspiration, whilst post-inspiration neurons are only active after inspiration. Type two early inspiration neurons only fire a while before a breath is taken, as do pre-inspiration neurons. All in all, there are perhaps twenty to twenty-five different types of such neurons, depending on how one classifies them. 6    7. Arterial Oxygen Chemoreceptors: The Carotid Body The primary site of oxygen sensing in the arterial blood are the arterial oxygen chemoreceptors located within the carotid body. The carotid body is found at the bifurcation of the carotid artery, and on a per-weight basis the carotid body receives more blood flow than any other part of the body. There are two cell types in the carotid body, of which Type I (glomus) cells are the O 2 sensors. The discovery of the carotid artery, by Corneille Heymans, occurred almost accidentally, when he injected cyanide into the carotid artery of an anesthetized dog. The cyanide stimulated the oxygen sensors of the carotid body, leading to a huge increase in breathing. For this discovery, Corneille Heymans received the 1938 Nobel Prize in medicine. The carotid body is innervated by the carotid sinus nerve, which joins the 9th cranial nerve before reporting to the brainstem. The carotid afferents report to the nucleus tractus solitarius in the medulla oblongata. The glomus cells, which are the oxygen-sensing cells in the carotid body, release a number of neurotransmitters, mainly acetylcholine and dopamine. Acetylcholine acts post-synaptically on t
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