The lesser petrosal nerve is not one of those areas of our bodies that is well-known to most people. Its lack of general popularity may be due in part to the shade of its better-recognized cousin, the greater petrosal nerve. Probably not, as that's pretty obscure to most people also.

Perhaps it is thanks to the relatively rare occurrence of medical complications. Regardless, the lesser petrosal nerve remains something of an enigma.

Indeed, the function of the nerve serves as an example of the complexity of the human body. Its path is convoluted and passes through several vital structures on its way to its final destination. To make things even harder, anyone wishing to learn more about the lesser petrosal nerve needs to digest a decent-sized chunk of medical terminology.

A breakdown of that information will be of some use, to uncover exactly what you need to know.

The Lesser Petrosal in Context

As is so often the case when discussing how the body and its parts work, it is helpful to start broad and then drill down to more specific functions.

The lesser petrosal nerve is part of the autonomic nervous system, which is responsible for a variety of functions, most of which occur automatically. Thanks to this system, the heart keeps beating, digestive processes self-regulate, blood vessels contract, and pupils dilate.

The nervous system as a whole shares many similarities, but in physiological terms splits into three distinct parts. These are known as the sympathetic, parasympathetic, and enteric systems.

The sympathetic nervous system

The sympathetic nervous system begins life in the spinal cord and reacts to the presence of danger. This fight or flight mechanism -- common to most species -- requires a ton of connected physiological responses. Faced with an imminent threat, the heart beats faster, blood pressure increases, pupils dilate, and so on. The sympathetic nervous system's job is to send the signals that kick those systems and more into high gear.

The parasympathetic nervous system

In physiological terms, the opposite of the fight or flight is "rest or digest." During times of relaxation, the parasympathetic nervous system kicks in, attempting to conserve energy in a variety of ways. The heart rate slows, blood pressure drops, and the digestive process begins.

Although also located in the spinal cord, certain parts of the parasympathetic nervous system  – including the lesser petrosal nerve – are known as cranial nerves due to beginning in the skull.

The enteric nervous system

The enteric nervous system is something of a hindbrain. Its primary function is to monitor gastrointestinal services, but it also controls certain motor functions, blood flow, and mucosal transportation.


Starting at the Top

To really understand the lesser petrosal nerve, you've got to know about this one:

The Glossopharyngeal nerve is part of the parasympathetic nervous system.

Located in the medulla oblongata of the brain, it controls the swallowing reflex and taste sensation on the rear third of the tongue. It also receives sensory information from the middle ear, tonsils, and pharynx. Significantly, it also connects to the parotid gland, the largest of the three salivary glands located in the mouth.

The lesser petrosal nerve is one of the components of this glossopharyngeal nerve.

Now, it gets interesting

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The glossopharyngeal nerve, however, as you might expect, branches off in several directions. It begins its journey by traveling through the posterior cranial fossa, the lowest curve of the skull; if you reach back and cup the back of your head, right above your neck, that's the area we're talking about.

It then exits via the jugular foramen, this is an opening in the base of the skull. That opening is located under the ear canal, and right behind the jaw bone and is where the jugular vein goes through.

Once there, it branches out. Now known as the tympanic or Jacobson's nerve, it passes into the tympanic cavity, also known as the middle ear.

A further branching then occurs with the anterior tympanic nerve moving towards a hollow structure known as the Eustachian tube. Meanwhile, the posterior nerve continues to an opening in the middle ear known as the round window.

Collectively, these two branches are known as the tympanic plexus, and it is from this plexus that the lesser petrosal arises.

The junction

The lesser petrosal nerve makes its way across the base of the middle cranial fossa, which is the portion behind your eyes -- it begins where the jaw meets the skull under your ear and ends right around the top of the eye openings. Then, it exits the skull, and ultimately junctions at the otic ganglion.

A ganglion is a "swelling of nerve cell bodies along the line of a nerve fiber," kind of like a junction box. The octic ganglion joins with the mandibular division of the trigeminal nerve, another cranial nerve that governs sensations relating to biting and chewing. But it's just a stop on the way through for the less petrosal nerve.

The lesser petrosal nerve continues via the auriculotemporal nerve before arriving at its ultimate destination, the parotid gland. It is here that it performs its primary duty: a secretomotor effect.

About that secreto...whatsit

Since the entire glossopharyngeal system concerns itself with the palate and throat, it probably comes as no surprise that the lesser petrosal nerve is involved with similar functions.

This next paragraph is a little complex, don't worry, we break it down.

The task in question is the transportation of parasympathetic fibers, taking them from the tympanic plexus to the parotid gland. And if that sounds like something of a mouthful to you, worry not, you’re not alone in thinking so.

In simple terms, this is how the nerve does it's job as we described above. A nerve fiber or "axon" is a slim projection of a nerve cell that transmits information to glands, neurons, and muscles. This information comes in the form of electrical impulses. These electrical impulses act as catalysts to a variety of different functions.

In the case of the lesser petrosal nerve, this function is secretomotor. As the name suggests, these secretomotor nerve endings induce glands to secrete substances such as mucus or, as in the case of the tear duct, actual tears.

In layman's terms, that means it's the type of nerve that causes a gland to secrete whatever it is that it makes -- in this case, saliva.


How Does the Lesser Petrosal Nerve Work?

The process is, thankfully, relatively straight forward. Secretomotor innervation is a fancy term for when electrical messages go through the nerve and to the organ, making it do its job. Such messages come in two flavors, sympathetic and parasympathetic.

As we already established, the lesser petrosal nerve is part of the parasympathetic nervous system. As such, the nerve performs only parasympathetic functions.

The origin point -- in this case, the medulla oblongata -- coordinates the response. It sends signals down the network of nerves towards the parotid gland. When it arrives, it stimulates the production of acetylcholine.

What is Acetylcholine?

Acetylcholine is a neurotransmitter, a chemical compound that is released by the nervous system to stimulate certain functions. Generally, this chemical stimulates the cells required for us to activate muscles.

In the case of the parotid gland, however, the presence of acetylcholine stimulates the production of saliva. In the reverse direction, it carries information relating to taste sensations on the back third of the tongue.

The nerve itself then can be thought of as part of a chain of command.

The Medulla decides that a little saliva is called for and sends a message down a network of nerve fibers. The signal branches off down the lesser petrosal nerve until it arrives at the parotid gland.

Here it stimulates the production of acetylcholine, which in turn encourages the production of saliva.

The gland then delivers the saliva to the mouth.

So, what does the greater petrosal nerve do?

As is the case with its lesser brethren, the greater petrosal nerve acts as a conduit for messages to induce secretion of a gland. The path that it takes, however, differs from that of the lesser petrosal nerve. It ultimately junctions with the tear ducts and the mucosal glands of the nose, palate, and pharynx.

As such, it stimulates the production of mucus in much the same way as the lesser petrosal nerve. It also controls the output of tears.


Clinical Conditions

It's helpful to remember that the path that the petrosal nerve takes is complicated. As such, identifying clinical conditions arising from damage to the nerve is no easy task. Even so, since the petrosal forms part of the facial nerve, injury, or infection to this area might act as a catalyst for Bell's palsy. A condition of facial paralysis.

The sudden paralysis brought on by said palsy, however, is self-correcting and often treatable via facial massage. Nasal obstruction can also occur during an episode of Bell palsy although, it too is both temporary and easily treated.

Still, the actual cause of the condition is unknown, and any direct link is not yet established.

Crocodile tears

A condition known as crocodile tear syndrome, however, most certainly does have a close correlation with damage to the lesser petrosal nerve. The name derives from an ancient and entirely incorrect belief that crocodiles shed tears shortly after making a kill.

The syndrome is rare and generally manifests in patients recovering from Bell's palsy. Characterized by the shedding tears while eating, the condition is thought to result from:

“An ephaptic union of the central portion of the damaged lesser superficial petrosal nerve.”

Put simply, that means that a short circuit of some kind has occurred due to trauma, infection, or swelling across the facial nerve. Signals sent to initiate saliva must somehow re-route and end up stimulating tears. Again though, the actual causes of Bell palsy are not fully understood.

Some Nerve - The Simplest Explanation

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The complicated paths and functions of the lesser petrosal nerve are hard to digest. After all, not everyone is an expert on human physiology. Even for experts, the fact remains that the nerve is not well understood.

But for those able to look past the jargon and focus on the known truths of this facial nerve, the basics are within reach.

When it’s time to eat, the nerve switches the largest of the salivary glands located within the mouth into high gear.

There are, of course, many secondary functions, as there often are in the human body. But, primarily, it's job is to stimulate saliva, and that's really all you need to know about it.

Did you learn what you wanted to know from this article? Let us know, down in the comments, we'd love to hear from you!

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