Human Anatomy, Physiology, and Medicine. Anything human!
Healthcare professionals have to learn , by rote if necessary, the distribution of the nervous supply within the human body.
The spinal cord is divided into anatomical sections, which are, in turn, numbered. The anatomical sections are referred to by their initial.
The cervical cord is protected by the cervical spine. C1 and C2 are its highest points, and intact functioning is crucial for life itself. C1 is right at the very top of the spinal cord, next to the brian stem. If an accident is severe enough, the cord can be snapped through, and the fractured boney prominence, (hitherto offering a protective function), can penetrate the medulla oblongata.
Thus, a healthcare professional should know how to assess and treat a possible boney fracture at the C1 and C2 level, because, if they do the wrong thing, (e.g. hyperextend the head), they could snap the cord.
They need to know about the innervation of the diaphragm in order to assess the breathing of a patient. If there is some kind of lesion in the spinal cord at the C3, C4 or C5 level, the functioning of this organ would be compromised.
If a person gets hiccups and wants to know what has set them off, there is a long list of medical or physiological disorders that are associated with hiccups and seem to cause them. The most common by far is distension or expansion of the stomach and movement of stomach acid into the esophagus. After that, a disease or irritation in the thorax could be to blame. Irritation of the phrenic nerve (the nerve to the diaphragm) or the diaphragm is often cited as a cause of hiccups, but this is only speculation about the exact mechanism. Hiccups can also arise from a variety of neurological lesions, many of them involving the brain stem, or some metabolic disorders (particularly renal failure). Medications, often ones that promote acid reflux into the esophagus, and a variety of other disorders have also been linked to hiccups.
Hiccups are seen in a wide variety of animals and are very common in the fetus. They have been recorded physiologically in animal studies and are easily recognized on ultrasounds of human babies. Hiccups appear before breathing movements as the fetus develops and are common in newborns but gradually disappear over the next few months.
These observations suggest that hiccup CPG may be left over from a previous stage in evolution. Searching through the animal kingdom for a recurring, rhythmic activity that resembles a hiccup turns up a few candidates. None of them looks exactly like a hiccup, but that is not really surprising given all the changes that have occurred over the eons. One candidate is the CPG for gasping, which is a sudden inspiration and can be rhythmic. In a recent paper, my colleagues and I argued that a better candidate is the CPG used by tadpoles for gill ventilation. The unique feature of hiccups is a big inspiratory effort while closing the glottis, which completely blocks air from being inspired. Halfway through its development a tadpole has both lungs that breathe air and gills for breathing water. It does not have a diaphragm and cannot suck air into its lungs and instead pushes fluids with its mouth. To breathe air, it fills its mouth cavity with air, then closes its nose, mouth and the passage to the gills and compresses the mouth cavity, forcing the air into its lungs. To breathe water, it fills its mouth with water and then closes the glottis and forces the water out through the gills. The positive pressure pump action of the mouth is synchronous with filling the lung or pushing water through the gills. The gill-breathing tadpole is thus inspiring while closing the glottis, the same action as hiccuping.
Of course, this hypothesis remains rather speculative. We do know, however, that the hiccup generator in mammals is located in the brain stem not far from where the separate gill CPG and lung CPG are found in frogs. In addition, the motion of the tongue in human hiccups is reminiscent of the tadpole compressing its mouth contents.
Evolutionary theory says that items from earlier stages are not likely to be preserved for long unless they continue to serve some function. The function of hiccups is again a matter for speculation. One possibility is that the CPG has been given the job of controlling suckling in infants, which uses primarily mouth muscles and ensures that milk does not get into the lungs. Another possibility is that the CPG has found a use for clearing gas out of an overfilled stomach. Contraction of the diaphragm makes negative pressure in the chest but positive pressure in the abdomen and thus tends to force stomach contents into the esophagus, which may set off backwards peristalsis and a burp. Closing the glottis keeps any liquids that come up from falling back into the lungs.
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