The Recurrent Laryngeal Nerve

Figure 1: Paths of the Laryngeal Nerves in the Neck

As noted in the critique below, the superior laryngeal nerve (SLN) comes off the vagus nerve high up in the neck and goes directly to the larynx whereas the recurrent laryngeal nerve (RLN) comes off the vagus nerve further down, wraps around the aortic arch, and comes back up (recurs) to innervate the larynx. The RLN is said to be too long, so represents “bad design”.

As you read this article your optic nerves are taking the photoreceptor information from your retinas to your occipital lobes to interpret them as vision. You’re using the third, fourth and sixth cranial nerves to move your eyes to scan this sentence. If you move your head even slightly, the “doll’s eyes reflex” that’s dependent on the eighth cranial nerves sending information about head angular movement to your brain stem from the semicircular canals in your ears keeps everything in focus. And the radial, ulnar and median nerves on your dominant side are telling the muscles in that hand and wrist where to move your mouse. But that’s not all.

While you’re seated there at rest, to get enough O₂ and get rid of enough CO₂ to keep you alive you’re using the phrenic and thoracic nerves to tell your diaphragm and intercostal muscles to contract so you can breathe air. And, assuming you’re not walking around while reading this or overly excited, your vagus nerves are telling your heart not to pump too quickly.

Of course, your motor cortices, basal ganglia, cerebellum, brainstem and spinal cord, are all working together to send coordinated messages through many of your other nerves to keep you sitting up straight and balanced in position with your feet planted firmly on the floor and your knees flexed at about ninety degrees. And the higher ordered interconnected language centers in your brain are processing the black dots and squiggles on the screen and interpreting them as words which if properly phrased hopefully express ideas you can understand.

Just to sit there and read these words requires dozens of different nerves telling dozens of your muscles what to do. And if you want to speak to someone, then it’ll be your laryngeal nerves (fig. 1) moving your vocal cords to be able to do that trick. But of course the larynx does do much more than just allow you to talk. It’s also equipped to keep the airway open for you to breathe properly and protect it from aspirating things into your lungs when you swallow something.

It would seem that this incredibly complex neuromuscular set-up that allows you to do what you need to do to survive—breathe air, safely swallow liquids and solids, move around and handle things, and besides talking, gives you other ways to communicate—would point to a system with at least some sort of purposeful design, right? Wrong!

Since one particular nerve in your body—the recurrent laryngeal nerve—is thought by the self-described experts to be longer than it should be, they tell us that this represents “bad design” which then mysteriously (but not surprisingly) becomes for them evidence for neo-Darwinism. You know, the concept that life came about solely from the unguided and purposeless forces of natural selection acting on random genetic mutation, rather than there having been any intelligent design involved. The one critique below is a good summary of this “bad design” argument.

The Critic Speaks

“The larynx is the voice box, in the throat. It’s supplied by two nerves from the brain called the laryngeal nerves. One of these, the superior laryngeal is sensibly wired up directly from the brain to the voice box. The other one, the recurrent laryngeal is crazy. It goes down the neck from the brain, shoots straight past the larynx (the place where it is supposed to end up) way down into the chest. There it loops around one of the main arteries attached to the heart, then whizzes straight back up the neck and finally ends up in the larynx, where it should have stopped on the way down. This is obviously bad design.” (Outgrowing God: A Beginner’s Guide: Richard Dawkins: Bantam Press: 2019)

Filling in the Gaps

The Nerve Cell (Neuron): How it works

Since this article is about a nerve with supposedly “bad design” it seems to make sense to begin to debunk this opinion by explaining how neurons (which make up nerves) actually work.

At rest, certain ions (electrically charged atoms) move in and out through the cell membrane. This causes the inside of the cell membrane to have a negative charge and the outside of the cell membrane to have a positive charge (fig. 2). The difference in electrical charge (voltage) across the cell membrane is measured in millivolts (mV) and is called the resting membrane potential (RMP). The RMP varies from cell to cell; for a liver cell it’s -40 mV, a nerve cell, -70 mV and for a ventricular muscle cell, -90 mV.

Figure 2: Resting Membrane Potential (RMP)

The rectangular orange boxes marked A represent membrane-impermeable anions like proteins.

Nerve cells are said to be excitable because with adequate stimulation the sodium ion (Na+) channels within the cell membrane open to allow Na+ ions to flood into the nerve cell, thereby reversing its polarity—making the charge inside the cell membrane become positive and the outside become negative—the opposite of what is taking place at rest. This process of reversing the negative charge inside and the positive charge outside the cell to a positive charge inside and a negative charge outside the cell is called depolarization.

The nerve cell (fig. 3) looks like an octopus with a central body (soma) containing the nucleus. There are also several projecting dendrites and one axon which may or may not be covered by a fatty myelin sheath (produced by Schwann cells) which aids in electrical conduction.

Depolarization generally takes place when one of the dendrites is sufficiently stimulated and as a wave (action potential) moves to the soma and from there down the axon to the nerve endings. The reversal of polarity (the inside of the cell going from negative to positive) in the nerve endings signals local calcium ion (Ca2+) channels there to open up and allow Ca2+ ions to flood into the cell. This sudden rise of Ca2+ ions within the nerve endings triggers them to release the cell’s neurotransmitter (over one hundred different types) into a space called the synapse.

The neurotransmitter then moves across the synapse to affect a target cell (e.g. a nerve or muscle or gland cell) on the other side by attaching to a specific receptor like a key in a lock. Some neurotransmitters, like acetylcholine and norepinephrine, are excitatory. They make the target cell’s RMP less negative (i.e. heading toward positivity) thereby facilitating depolarization. Other neurotransmitters, like gamma-aminobutyric acid (GABA) and serotonin, are inhibitory. They make the target cell’s RMP more negative (heading away from positivity), suppressing depolarization. This inhibitory effect is also called hyperpolarization.

Figure 3: Parts of the nerve cell (see text)

The Nervous System’s Division of Labor

The nervous system is organized into two main divisions; the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS is made up of the brain and spinal cord neurons which receive sensory information from, and send motor orders to, the body to maintain control. The PNS is made up of neurons that send the sensory information to the CNS and take the motor orders from the CNS to the body (fig. 4).

Figure 4: The CNS and PNS

The PNS is further subdivided into the somatic nervous system and the autonomic nervous system (ANS). The former is involved in voluntary (conscious) actions, like the head turning, and the latter is involved in involuntary (unconscious) actions, like the stomach churning (fig. 5). The ANS is itself subdivided into the sympathetic nervous system, which speeds things up to promote action (“flight or fight”) and the parasympathetic nervous system which slows things down to promote relaxation (“rest and digest”).

Figure 5: The Nervous System’s Division of Labor

There are mainly two types of nerve cells in the PNS, sensory and motor. The sensory nerves send information to the brain about what’s going on outside and inside the body from somatic (surface), visceral (internal) and special sensory organs, like the eyes and ears. This sensory information is processed and analyzed by the brain. For example, sensory input related to the position of your head as you look in one direction while you experience a light touch on your shoulder right after you’ve swallowed some food.

The brain then makes a decision and sends out instructions along the somatic and autonomic motor nerves telling the body what to do. The somatic motor nerves control skeletal muscles, those attached to your bones. And the autonomic motor nerves control smooth muscles, those surrounding the blood vessels and the gastrointestinal and genitourinary tracts. In addition, the autonomic motor nerves control organs like the eye, heart, lungs, liver and kidneys in addition to glands like the pancreas, adrenal, salivary and sweat glands.

So, in response to what was sensed above, the somatic motor nerves tell your head to turn to let your eyes see who (or what) just touched your shoulder and the autonomic motor nerves make your stomach start to churn after food has entered it.

Now that you understand how the nervous system works let’s cone down on the subject at hand.

The Larynx: What’s it all about?

The larynx (voice box) is a midline structure located in the front of the neck. It connects the pharynx, the common pathway to the lungs and gastrointestinal system, to the trachea, which leads into the lungs, and sits in front of the esophagus (fig. 6). Due to its anatomical position the larynx has several functions. It allows vocal communication, keeps the airway (glottis) open for breathing and with swallowing closes the glottis to protect the airway from aspiration i.e. letting food and drink go into your lungs (not a good thing for survival).

Figure 6: Lateral view of the head and neck

From the larynx air enters the trachea below (not labelled).

The larynx is made up of nine cartilages, three are single and three are paired. The single cartilages are the thyroid and cricoid cartilages and the epiglottis. The paired cartilages are the arytenoids, corniculates and cuneiforms. They are joined together by strong ligaments which together give the larynx very sturdy support (fig. 7).

Figure 7: The Cartilages of the Larynx

The hyoid bone is seen at the top of the left image. Note the ligaments between the hyoid bone and the thyroid cartilage below it and the thyroid cartilage and the cricoid cartilage below it. The cuneiform cartilages, which sit on the arytenoids in front of the corniculates are not shown.

The larynx is equipped with extrinsic and intrinsic muscles. The extrinsic muscles move the whole laryngeal apparatus up and down (which you can feel for yourself) during swallowing to protect the airway against aspiration (fig. 8). The intrinsic muscles move the individual parts of the larynx affecting the diameter of the glottis and tension of the vocal cords (fig. 9).

Swallowing first triggers the appropriate intrinsic muscles to contract to close the glottis, stop breathing and prevent aspiration. Then the extrinsic muscles move the larynx up and forward to hide it under the floor of the mouth and the base of the tongue while being protected by the epiglottis. This and other muscle action then allows what you swallow to bypass the larynx (and the airway) and slide into the esophagus. Finally, once everything has passed by and the coast is clear, the extrinsic muscles lower the laryngeal apparatus back to its former position.

Figure 8: Extrinsic Muscles of the Larynx

During swallowing the muscles in the left figure raise the larynx and the ones in the right figure lower the larynx. These muscles are not controlled by either of the laryngeal nerves (see below).

Figure 9: Some of the intrinsic muscles of the larynx

The vocal cords (folds) are formed by the thyroarytenoid muscles. The posterior cricoarytenoid muscles are the only ones that abduct the vocal cords to open the glottis. The other three muscles tend to relax and/or adduct the vocal cords to close the glottis. The RLNs control all of them.

The RLNs: What types of neurons do they have and what do they do?

Now that you know what makes up the larynx, how it works and what kinds of nerve cells can be in a peripheral nerve, let’s consider the contents of the different laryngeal nerves.

First, it’s important to realize that neither of the laryngeal nerves (SLNs or RLNs) controls the extrinsic laryngeal muscles—the ones that lift it up and down during swallowing. The extrinsic muscles of the larynx are controlled by three upper cervical nerves and four cranial nerves, but this does not include the tenth (vagus) cranial nerve.

Second, in addition to there being right and left RLNs, there are also right and left superior laryngeal nerves (SLNs). As noted above, the SLNs are the nerves our critic says are “sensibly wired”. That’s because the SLNs come off the vagus nerve in the upper neck and go directly to the larynx (fig. 1).

The SLNs provide sensation from the mucous membrane in the epiglottis to the vocal cords (upper half of the larynx) which is involved in the cough reflex and helps to protect the airway during swallowing. The SLNs also provide motor and sensory innervation of one of the intrinsic laryngeal muscles, the cricothyroid muscle (not shown in fig. 7), which increase the amplitude of phonation and the pitch of the voice.

Injury of the SLNs can lead to difficulties with breathing, raising the voice, especially while trying to sing, in addition to a diminished cough reflex and adequate protection of the airway causing an increased risk for aspiration.

In contrast to the SLNs, the RLNs come off the vagus nerve much further down. The L RLN wraps around the aortic arch and the R RLN wraps around the right subclavian artery. Then they both travel back up to the larynx. So, one can see that the RLNs are longer than the SLNs and the L RLN is longer than the R RLN (fig. 10).

Figure 10: RLNs and the major arteries

Notice that the L RLN wraps around the lower placed aortic arch and then turns back up to the larynx whereas the R RLN wraps around the higher placed right subclavian artery instead.

The RLNs provide sensation to the laryngeal mucous membrane below the vocal cords (lower half of the larynx), motor and sensory innervation of all, except one, of the intrinsic laryngeal muscles, motor and sensory control of the inferior constrictor muscle (swallowing), is involved in the gag, swallow and cough reflex, and provides parasympathetic control of the heart (slows it down) along with visceral sensory and parasympathetic motor control of the upper trachea and upper esophagus. All the nervous control provided by the RLNs affords us the ability to breathe adequately at rest and with activity, have coordinated swallowing, proper protection of the airway and sufficient phonation.

So, we can then see that the RLNs do much more than what the critic seems to be concerned with—speech and motor control of the larynx (voice box). In fact, technically, the RLNs don’t have total control of the larynx because the SLNs control the cricothyroid muscles to help with phonation and other nerves control the extrinsic laryngeal muscles to move the larynx up and down during swallowing. For some reason total laryngeal function has been divvied up between many different nerves.

Injury to the RLNs can cause problems with phonation, swallowing, breathing and protection of the airway. Moreover, a defect in development, involving aberrant placement of the right subclavian artery, resulting in the R RLN not diving down and around it but coming off the vagus nerve higher up and going directly to the larynx, as preferred by our critic (i.e. a non-recurrent laryngeal nerve (NRLN), often results in swallowing problems (fig. 11).

Since this NRLN comes off the right vagus nerve too high to provide innervation of the trachea and esophagus on that side, one has to wonder if the reason why these people have swallowing problems is because the right vagus nerve that continues beyond the high take off of the NRLN does not adequately service the visceral sensory and parasympathetic motor needs of the trachea and esophagus on that side? Nobody seems to know the answer to this question, but it would certainly explain a lot, wouldn’t it?

Figure 11: The Non-Recurrent R RLN (NRLN)

This aberration has the right subclavian artery coming off the aortic arch downstream from the left subclavian artery and then twisting back to the right side of the body. There are three variations of the NRLN (types I, IIa, IIb). One can see by where the NRLN comes off the right vagus nerve that it will not be involved with innervation of the heart, trachea and esophagus. After takeoff of the NRLN, the right vagus nerve travels down to innervate the tissues below.

All of this only goes to show how important it is for laryngeal function and survival to have a normal RLN on each side of the body and why their recurrent routes are necessary to allow them to do what they’re supposed to do. Things seem to be just a bit more complicated than our critic realizes or is willing to admit or reveal.

The R& L Recurrent Laryngeal Nerves: How did they get that way?

In the beginning of the embryonic stage, the head and neck and the heart are situated very close to each other (so is everything else). The tissues in this region are designated as the five pharyngeal arches numbering I to VI as V degenerates later (fig. 12). Of course, right from the beginning all of the tissues that make up the pharyngeal arches need to develop a blood supply to keep them alive along with associated nerves to eventually control their function.

Figure 12: The Five Embryonic Pharyngeal Arches

The larynx derives from pharyngeal arches IV and VI—see text

As the embryo develops further into a fetus the neck lengthens northward and the heart migrates southward further away from the neck taking its blood supply and associated nerves with it. The larynx derives from the stretching of pharyngeal arches IV and VI.

The SLN derives from pharyngeal arch IV and so comes off the vagus nerve higher up than the RLN. As already noted, it then goes directly to the larynx supplying it with specific sensory and limited motor function. In contrast, the RLN derives from pharyngeal arch VI which makes it responsible for servicing not only the larynx but also the heart, trachea and esophagus. In order to accomplish these extra tasks, the RLN must stay closer to these vital structures which means diving down and around the aortic arch (left) and subclavian artery (right) and then going back up to the larynx (recurrent).

It’s also important to note here that on the left side where the L RLN wraps around the aortic arch it actually does it at the site of the ligamentum arteriosum, which during intrauterine life was the ductus arteriosus, a very important shunt between the fetal pulmonary artery and the aorta. The ductus arteriosus allowed newly oxygenated blood from the placenta to bypass the closed and “not ready for prime time” lungs for fetal survival. Once delivered from the mother and loss of the placenta had taken place, certain chemical changes in the newborn signalled the ductus arteriosus to close off, degenerate and later become the ligamentum arteriosum (fig. 13).

Figure 13: The L RLN and the Fetal Ductus Arteriosus

Notice how the L RLN comes right under and supports the ligamentum arteriosum which during intrauterine life was the ductus arteriosus—the shunt that allowed newly oxygenated blood from the placenta to bypass the closed lungs and travel from the pulmonary artery (the purple T structure) to the aorta (the red curved structure)—without it, no human life.

The key thing to keep in mind is that for a major blood vessel like the ductus arteriosus to have been able to shut down completely soon after birth so that the newborn could have normal cardiopulmonary function, required that its vessel walls contain mostly smooth muscle rather than the elastic tissue present in similarly sized major blood vessels like the aorta and pulmonary artery. Recent discovery that surrounding connective tissue affects cell differentiation has resulted in it being postulated that, in contrast to the unsupported aortic arch, the presence of the RLN acting as a sling to support the ductus arteriosus during development may have signalled it to form mostly smooth muscle rather than elastic tissue in its vessel walls. This may be one of the main developmental reasons for the L RLN, to dive down and wrap around the aortic arch (and the ductus arteriosus).

Two Questions

Whew! That was a lot of stuff to cover wasn’t it? A lot more than what our critic seems to consider (or at least expresses) before he and many of his like-minded colleagues come to the inevitable conclusion that the RLN is evidence for “bad design”. Of course, how this actually supports the neo-Darwinian position of the origin and diversity of life seems to be a bit odd doesn’t it? After all, what we’ve just covered above should engender a lot of very important questions that first need to be answered before anyone should agree with this critic’s position of how it came about in the first place. Here are just two of them. I’m sure you can come up with more, all of which would need to be actually answered, not explained away by “just so” stories.

In what order and from where did the new information come that specified the formation and organization of enough of the right nerve cells to develop into the central and peripheral nervous systems (including the somatic and autonomic nervous systems) to allow for adequate neuromuscular control? And given that each part of each nervous system is absolutely needed for survival what is the real probability that they could have all come about by purposeless and undirected forces while providing adequate neuromuscular control in intermediate organisms each step along the way?

In what order and from where did the new information arise that specified the formation of each part of the larynx and their organization into a coherent functional system allowing for adequate phonation, breathing and safe swallowing? And given that each part of the larynx and its neuromuscular control is absolutely needed for survival what is the real probability that they could have all come about by purposeless and undirected forces while remaining functional in intermediate organisms each step along the way?

So, are you willing to accept the undirected forces of natural selection acting on random variation as the definitive answer to the above questions? The “smoke and mirrors” of neo-Darwinism which doesn’t even try to account for the simultaneous development of all the different nerve cells, their organization into the CNS and PNS (including the somatic and autonomic nervous systems) to provide adequate neuromuscular control in general, and in particular, the larynx and its triple functions of breathing, swallowing and speaking?

It’s important to realize that natural selection acting on random variation (genetic mutation) means exactly what it says. Over time, life required a gazillion bits of new genetic, and possibly other, information (not natural selection) to bring about new structures with new functions. All natural selection did was preserve the life that was up and running and able to survive due to these gazillion undirected mutations. But keep in mind, natural selection cuts both ways.

Based on what we know about how life actually works neo-Darwinism may explain the survival of the fittest but not the arrival of the fittest. That’s because when it comes to survival, logic tells us that the same power that natural selection had to preserve human life when the CNS and the PNS, along with the larynx, were working right would have also prevented it from surviving if any one of the parts of these systems were missing, misplaced or defective.

The known engineering principles needed to bring about the functional capacities of all of these parts that resulted in human existence means that, in principle, not only does Darwin’s theory of gradualism fail, but so do all the other neo-Darwinian attempts to replace it. What do you think?

Comments

The critic over-simplifies the larynx (voice box) by making it sound like a monolithic structure with only one motor function—phonation. However, the larynx consists of many parts, mostly several cartilages held together by many ligaments and moved by many different muscles, each with the right shape, specifications and interfaces, having been precisely assembled under the control of many nerves which affords it a functional capacity above and beyond mere phonation, including life-giving respiration, swallowing and protection of the airway.

The critic also errs by making it sound like the two laryngeal nerves on each side of the body come directly from the brain. That’s just not true. As noted above, both the SLNs and RLNs come off the vagus (tenth cranial) nerve which comes from the brain stem. This isn’t just being picky it’s an important anatomical and functional distinction. The vagus nerve, like some of the other cranial nerves, provides much more than just skeletal muscle control.

The third (oculomotor) cranial nerve controls several eye muscles but also has autonomic control of the iris, allowing the pupil to control light entry, and the ciliary body, which allows the lens to adjust its shape so as to see things close up. The seventh (facial) cranial nerve controls the facial muscles but also has autonomic control of the some of the salivary glands and provides taste for the front two-thirds of the tongue. The ninth (glossopharyneal) cranial nerve controls most of the muscles for swallowing but also has autonomic control of some of the salivary glands and taste for the back one-third of the tongue.

The tenth (vagus) cranial nerve, through the SLNs and RLNs, controls the intrinsic muscles of the larynx resulting in phonation, and also aids in swallowing with protection of the airway in addition to controlling the opening of the glottis to aid in respiration, especially with activity. But the vagus nerve also helps to control cardiac, respiratory and gastrointestinal function in addition to reflexes such as coughing, sneezing and vomiting.

That’s a lot of function to be responsible for and, as the critic neglects to mention, a lot of that function is placed under the purview of the RLNs. So, to get their jobs done, the RLNs have to dive down deeper than the SLNs. None of this is even mentioned by the critic so he doesn’t have to explain it, does he?

Finally, it must be noted that since the SLNs are much shorter than the RLNs and the R RLN is much shorter than the L RLN, why then don’t we experience poorly coordinated laryngeal function? Why don’t the cricothyroid muscles (controlled by the SLNs) contract a split second before the rest of the other intrinsic muscles (controlled by the RLNs)? And why don’t the rest of the intrinsic muscles on the right side of the larynx (controlled by the R RLN) contract a split second before the ones on the left side (controlled by the L RLN)? Wouldn’t that make it harder to coordinate speaking and singing, never mind breathing and swallowing?

Given this significant length asymmetry between the SLNs and RLNs and the R RLN and the L RLN why are we able to have apparently symmetrical laryngeal muscle function? Our critic doesn’t point out the fact that despite his alleged “bad design” claim, none of this seems to affect laryngeal function or for that matter, survival, in the least. And because neo-Darwinism assumes “bad design” automatically means “no design” (an obvious non sequitur), science tends to ignore how this can be?

But other people have wondered about this. Some have wondered if the nerve conduction velocity in the R RLN may be slower than in the L RLN due to differences in formation. Others have wondered if the nucleus in the brain stem from where intrinsic muscle control originates is able to compensate for these differences in nerve length. All of this would seem to have been too much to think about for our critic. It’s just a lot easier to criticize about how life looks without having to account for how it actually works.

Laufmann’s Triple Filter

Not understanding the objectives of the designer

Based on the anatomical position of the larynx (voice box) common sense tells us that besides phonation (the apparent main concern of the critic) it’s also involved in the vital functions of breathing and swallowing. This means that any nerve that services the larynx is likely to include one or more of these three functions within its purview. However, and this is important to keep in mind, this service includes not only motor function but also sensory function.

Motor function, includes things like contracting the extrinsic and intrinsic laryngeal muscles to protect the airway, breathe and vocalize. Sensory function includes things like sending sensory information to the brain, telling it what’s going on within the mucous membrane of the larynx and what the extrinsic and intrinsic muscles of the larynx are doing.

By providing sensory data to the brain about what’s going on within the mucous membrane of the larynx below the vocal cords, and motor and sensory innervation of most of its intrinsic muscles, the RLNs service all three functions of the larynx. But that’s not all! The RLNs also provide innervation for cardiac, tracheal and esophageal function.

Given that these last three structures are located near to where the RLNs swing around the aortic arch on the left, and the subclavian artery on the right, and having provided support during intrauterine life, the L RLN likely facilitated proper development of the ductus arteriosus, the circuitous route of the RLNs seems justified. And when you realize that the differences in length between the SLNs and RLNs and the R RLN and L RLN have no functional significance, then one can see that the critic’s position is one of “much ado about nothing” (as usual).

So, what’s the big deal? Does the critic even have a point? Well, he does have one, although given the anatomical and developmental considerations, it’s a foolish one because to make it, he has to ignore what’s required not only for adequate laryngeal function but also the function of other vital organs too. But of course, this is the usual method for the neo-Darwinism enterprise—describe what you want but explain nothing—talk about how life looks but not how it actually works—spin “just so” stories of origin which have no connection to what we now know about the built-in engineering that is really needed for life to survive—so why should we be surprised?

The question that has to be asked is “After coming off the vagus nerve where they service the heart, trachea and esophagus, why did the RLNs have to come back up, from near the aortic arch on the left, and the subclavian artery on the right, to also service laryngeal function? Of course, if they hadn’t have done this then they would have been given a different name, right? Maybe they would have been called the cardiotracheoesophageal nerves (CTEs) instead.

So, what the critic is essentially asking is “Why couldn’t we have the CTE nerves branching off the vagus nerve further down to service just the heart, trachea and esophagus and have the SLNs on their own, or maybe an additional one (two, three, four) more laryngeal nerves branching off the vagus nerve higher up so they would have a more direct route to the larynx?”

It’s a good question. Let’s consider the answer in the next section below.

Not accounting for the functional requirements, constraints and trade-offs

The functional requirements of the nervous system mean that it has to work like a military operation. The generals providing leadership in headquarters must receive correct and precise intelligence about the enemy to be able to decide how best to apply their strategy to win.

To make the right decisions about what should be done at any given point in time the brain must have correct and precise moment to moment information about what’s going on outside and inside the body. This information is sent to the brain by electrical impulses being transmitted along the axons of the sensory neurons. After analyzing the sensory information and having made a decision about what to do, the brain must then send out instructions to the muscles by transmitting electrical impulses along the axons of the motor neurons.

But how should the axons of these sensory or motor neurons be organized? Should each of them be independent of each other by making its way to the brain or back to the muscles on its own? Would it be best to have “billions and billions” of axons, going from the sensory organs to the brain or from the brain to the muscles, freely travelling loosely throughout the body without any support or protection?

It would be like having a thousand individual delicate glass optical fibers coming in to service your home’s communication equipment rather than being all bundled together into a solid fiber-optic cable. Imagine what it would be like to protect each of them from your children or your dog, besides how to connect them up to your systems.

That’s exactly how your peripheral nerves are constrained. The axons of the sensory and/or motor neurons within a peripheral nerve are bundled together in fascicles which separate them from each other (fig. 14). Each axon within a fascicle (whether myelinated or not), is supported by a surrounding sheath of connective tissue called endoneurium. In turn, each fascicle is supported by a surrounding sheath of connective tissue called perineurium. And all of the fascicles are bundled together to make up the peripheral nerve which itself is supported by a surrounding connective tissue called epineurium. It’s a very interesting set up and you have to wonder from where the blueprints and assembly instructions came, besides the whereabouts of this information within the genome and/or maybe places of epigenetic storage.

Figure 14: The Microanatomy of a Peripheral Nerve

Well, now that we know the microanatomy of a peripheral nerve there are a few more questions that we need to ask. Which types of neurons (somatic sensory, visceral sensory, somatic motor, autonomic motor) should be in which peripheral nerves? Which peripheral nerves should service which specific territories of the body for their sensory input and/or motor function? How many sensory and/or motor neurons are needed to properly service a given territory of the body? How many total peripheral nerves does the body need to do the job right? How much material and energy is needed to make and maintain all of these peripheral nerves and how is it provided?

Now, let’s come down to providing the sensory input from, and motor instructions to, the larynx.

Remember, the larynx has three basic jobs to do. It not only affords us the ability to vocalize and verbalize, but it’s also vital for adequate respiration and safe swallowing. To accomplish all of these things adequate sensory information must come to the brain from, not only all of the parts of the larynx, but also the tongue, pharynx, epiglottis, lower constrictor muscle, trachea and esophagus. Moreover, to do what it’s supposed to do properly, the larynx uses many different muscles, controlled by millions of neurons, carried within many different peripheral nerves. How many? Well, don’t forget, based on what we know, not only does it need the SLNs and RLNs but also the upper cervical and several cranial nerves too.

The critic doesn’t give any counterproposal for how to service the larynx other than to complain that the RLNs having an overly long route which is “obviously bad design”. But what would he propose instead? One option would be to add the laryngeal innervation done by the RLNs to the SLNs. Another would be having, one, or more, new nerves coming off the vagus nerve higher up, on either side of the body (for a more direct route to the larynx) which would be responsible for the same laryngeal innervation done by the RLNs.

But, does it really matter how many different nerves are needed and their different routes as long as the brain gets the right sensory information it needs from the larynx and sends out the right orders to its muscles in a timely manner to allow us to breathe, swallow and speak properly?!

The SLNs being totally dedicated to the larynx can travel directly to its target tissue. However, the RLNs, which innervate more sensory and motor neurons than the SLNs and service the heart, trachea and esophagus, along with providing physical support to the ductus arteriosus in-utero, must take a circuitous path so it can do is designated jobs.

Finally, one has to consider the trade-offs involving the nervous system developing at the same time as the cardiovascular system, because without a blood supply, all cells, even neurons, die. As noted above, early in development, the larynx sits very close to the major blood vessels. As development progresses, the aortic arch and subclavian artery migrate south into the chest cavity while the larynx stays up in the neck. The specific nerves having been designated to service the different sensory and motor aspects of the larynx (SLNs, RLNs) must then accommodate to these developmental changes.

So, one can see that the functional requirements of the nervous system, in general, and servicing the larynx, in particular, results in real constraints when it comes to how many and what types of neurons are needed to do what it’s supposed to do for breathing, swallowing and phonation, the materials and energy needed to preserve and assemble them into specific peripheral nerves, and the specific work they must do within a specific territory. As to the possible trade-offs, despite what’s mentioned above regarding embryonic development, there really are none in this situation because despite the perceived “bad design” of the RLNs, particularly the left one, the whole system functions well enough for survival. The only conceivable “trade off” here is that the master engineer-designer was left open to neo-Darwinian criticism, one that in its expression requires a very low threshold and an absence of rigor in its analysis of the details.

Failure to acknowledge user abuse and degradation over time

A common reason for realizing the importance of a specific organ or tissue and so then trying to figure out how it actually works happens when it malfunctions. The circuitous route of the RLN makes it prone to injury resulting in malfunction during neck and thoracic surgery. In addition, a mass pressing on the RLN in the neck (thyroid cancer) or the chest (lung cancer) or even a large thoracic aortic aneurysm (Ortner’s Syndrome—look it up online) can first manifest as RLN malfunction with a weak voice and hoarseness along with breathing and swallowing problems. None of these represent user abuse or degradation but they do demonstrate what can happen in the course of a lifetime.

Conclusion

The neuromuscular system in general, and how it manages the three aspects of laryngeal function in particular, is a marvel of engineering. To claim, as the above critic and many of his colleagues do, that the RLNs, which do much more than help control the larynx, is “obvious bad design” is not only misguided but, based on engineering principles, totally absurd. What do you think?

Also see Dr. Glicksman's Series on

"Beyond Irreducible Complexity"

"Exercise Your Wonder"

"On Being Alive"

Howard Glicksman M. D. graduated from the University of Toronto in 1978. He practiced primary care medicine for almost 25 years in Oakville, Ontario and Spring Hill, Florida. He now practices palliative medicine for a Hospice organization in his community. He has a special interest in how the ethos of our culture has been influenced by modern science’s understanding and promotion of what it means to be a human being.