Fish Out of Water

Human ailments as varied as hernias, hiccups, and choking are a legacy of our “fishy” ancestry.

shark gonads

Both shark and human embryos (a, b) have similar gill arches (brightly colored elements). In sharks, the cells in these arches become bones, nerves, arteries, and muscles that support the gills (c); in humans, they form the jaw, ears, larynx, and parts of the throat (d). Tim Gunther www.gunthergraphics.biz

Tim Gunther www.gunthergraphics.biz

Unfortunately, that flexible throat, so useful in talking, makes us susceptible to a form of sleep apnea that results from obstruction of the airway. During sleep, the muscles of the throat relax. In most people, this does not present a problem, but in some, the passage can collapse so that relatively long stretches pass without a breath. This, of course, can be very dangerous, particularly in people who have heart conditions. Snoring is a symptom of the same underlying problem.

Another trade-off of speech is choking. Our mouths lead both to the trachea, through which we breathe, and to the esophagus, so we use the same flexible passage to swallow, breathe, and talk. Those functions can be at odds, for example when a piece of food “goes down the wrong pipe” and gets lodged in the trachea; our fishy ancestors had no such worries. Other mammals, and reptiles too, use the same structures for eating, breathing, and communicating but the back of the mouth does not need to be so vertically spacious and flexible as ours. The basic mammalian structures are arranged so that nonhuman animals can safely swallow while breathing. Tweaking the engineering to enable us to talk has left us peculiarly vulnerable.

The annoyance of hiccups also has its roots in our fish and amphibian past. If there is any consolation, we share that misery with others. Cats and dogs, like many other mammals, also get hiccups. A small patch of tissue in the brain stem is thought to be the center that controls that complicated reflex.

The hiccup reflex is a stereotyped twitch that involves a number of muscles in the body wall, diaphragm, neck, and throat. A reflexive firing of one or two of the major nerves that control breathing causes those various muscles to contract. This results in a very sharp inspiration of air. Then, about thirty-five milliseconds later, a flap of tissue in the back of the throat (the glottis) closes the top of the airway. The fast inhalation followed by a brief closure of the air tube produces the “hic.”

Our tendency to develop hiccups is another influence of our past. There are two issues to think about. One is what causes the reflexive firing of nerves that initiates the hiccup. The other is what controls that distinctive hic—the abrupt inhalation and the glottis closure. The nerve action is a product of our fish history, while the hic is an outcome of the history we share with tadpoles.

Fish first. Our brains can control our breathing without any conscious effort on our part. Most of the work takes place in the brain stem, at the boundary between the brain and the spinal cord. The brain stem sends nerve impulses to the main breathing muscles. Breathing happens in a pattern: muscles of the chest, the diaphragm (the sheet of muscle that separates chest from abdomen), and the throat contract in a well-defined order. Consequently, the part of the brain stem involved is known as a “central pattern generator.” It can produce rhythmic patterns of nerve and, consequently, muscle activation. A number of such generators in the brain and spinal cord control other rhythmic forms of behavior, such as swallowing and walking.

The problem is that the brain stem, originally controlling breathing in fish, has been jury-rigged to work in mammals. Sharks and bony fish respire using muscles in the throat and around the gills. The nerves that control those areas all originate in a well-defined portion of the brain stem. We can even detect that nerve arrangement in some of the most primitive fish in the fossil record. Imprints of ancient ostracoderms, from rocks more than 400 million years old, preserve casts of the brain and cranial nerves, and just as in living fish, the nerves that control breathing extend from the brain stem.

That works well in fish, but it is a lousy arrangement for mammals. In fish, the nerves that control breathing do not have very far to travel from the brain stem. The gills and throat generally surround that area of the brain. We mammals have a different layout; our breathing is carried out by muscles much farther away. For example, the major nerve that controls contraction of the diaphragm—the phrenic nerve—exits the brain stem from the base of the skull, just as it does in fish. Then, however, it extends all the way down through the neck and the chest cavity to reach the diaphragm. That long path through soft tissue is exposed and vulnerable; a rational design would have the nerve travel through the protective spinal column and emerge nearer the diaphragm. Unfortunately, anything that interferes with one of these nerves, such as a tumor in the chest cavity, can block their function or cause reflexive firing.

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