Most nerves are either sensory (providing information through your five senses), motor (providing movement), or mixed, meaning they serve both functions. The greater petrosal nerve is a mixed nerve that carries sensory fibers and parasympathetic fibers, which are part of the parasympathetic nervous system. The primary functions of this nerve are taste and secretomotor (dealing with secretions from glands).
Anatomy
The nerves of the head and face—the cranial nerves—are unique in that they originate in the brain, while the rest of the body’s nerves come from the spinal cord.
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Each person has 12 cranial nerves, which exist as symmetrical pairs. (However, they’re typically referred to as a single nerve.) They arise from the back of the brain and wrap forward throughout your skull, exiting the skull through small holes called foramina so they can reach the muscles, skin, and other structures of the face.
The facial nerve is the seventh cranial nerve (CN7) and one of the more complex ones. It’s formed by two roots that emerge from the brainstem at a spot called the pontomedullary junction. CN7 and its branches control most of the facial muscles and glands that produce saliva and tears. This nerve also plays a role in taste.
Behind the ear, the facial nerve enters a bony structure called the facial canal. There, its roots are fused together and form a type of nerve relay station called the geniculate ganglion. The ganglion then distributes nerve fibers to several branches of the facial nerve that provide nerve function to glands, muscles, and structures of the mouth and ear.
These branches include:
Nerve to stapediusChorda tympaniGreater petrosal nerve
A lot of nerve signaling, called synapsing, happens inside ganglia (the plural of ganglion). However, the greater petrosal nerve doesn’t synapse inside the geniculate ganglion; it merely passes through.
Location
After leaving the geniculate ganglion, the greater petrosal nerve continues forward toward the cheek. It passes through an opening called the hiatus of the facial canal, then along a narrow passage called the middle cranial fossa. It then exits the skull through the foramen lacerum.
From there, the greater petrosal nerve runs alongside the petrosal branch of the middle meningeal artery. The nerve then enters the pterygoid canal and joins the deep petrosal nerve, which is a sympathetic nerve (it deals with heart rate, blood pressure, and blood vessel constriction). This is technically where the greater petrosal nerve ends, but its fibers continue on through various nerves and branches.
The combination of the greater petrosal and deep petrosal nerves is called the nerve of the pterygoid canal or the Vidian nerve. After the Vidian nerve leaves the pterygoid canal, it arrives at the pterygopalatine ganglion. This is where the greater petrosal nerve synapses with other nerves as they relay information back and forth from the brain to other body structures.
Structure
The parasympathetic fibers of the greater petrosal nerve leave the ganglion through several important branches:
Greater palatine nerveLesser palatine nerveNasopalatine and posterior nasal branchesPharyngeal branch
Anatomical Variation
With nerves, it’s imperative for surgeons to know about possible variations so they can avoid accidental injuries during procedures.
The greater petrosal nerve has some known variations:
Its distance from certain bony structures is different in some people. Its bony coverings vary considerably along its course, with a portion near the geniculate ganglion exposed in some people, which makes it vulnerable to injury. In some people, the greater petrosal nerve communicates with the glossopharyngeal cranial nerve.
Function
The primary function of the greater petrosal nerve is to supply parasympathetic fibers to the nerves that travel from the pterygopalatine ganglion to structures of the mouth, throat, nasal passages, and eyes.
The parasympathetic nervous system regulates activities that occur when your body is at rest, such as digestion, salivation, and tear production (which keeps your eyes moist at all times, not just when you cry).
The fibers from the greater petrosal nerve contribute to the function of numerous glands and tissues through the five branches that carry them. In order to do this, they join fibers from the maxillary division of the trigeminal nerve (CN5).
Greater and Lesser Palatine Nerves
The greater palatine nerve and two or three lesser palatine nerves carry specialized sensory fibers from the greater petrosal nerve.
They travel together from the ganglion to the mouth. There, the greater palatine nerve innervates the mucous membrane of the rear portion of the hard palate. The lesser palatine nerves innervate the soft palate. They both contribute to your sense of taste.
The greater palatine nerve also provides sensory fibers to the gums and a portion of your sinuses, and the lesser palatine nerves supply motor function to muscles of the uvula and tonsils.
Nasopalatine and Posterior Nasal Branches
The nasopalatine nerve and posterior nasal branches travel to the nasal cavity. The nasopalatine nerve supplies the nasal septum and the forward region of the hard palate, and communicates with the greater palatine nerve.
The posterior nasal branches innervate the mucous membranes in the nasal passages.
Pharyngeal Branch
The pharyngeal branch travels to the rear of the upper portion of the pharynx, which is the part of the throat behind the mouth and nose. It provides sensory function to the mucosal glands of the upper nasal pharynx.
Maxillary and Zygomatic Nerves
When the maxillary nerve leaves the ganglion, it carries secretomotor fibers from the greater petrosal nerve. The fibers then travel with the zygomatic nerve when it branches off from the maxillary.
Along with sensory nerves that provide feeling to some of the skin around the eye socket and cheek, the zygomatic nerve sends a communicating branch to the lacrimal nerve. Together, the zygomatic and lacrimal nerves provide secretomotor function to the lacrimal gland, meaning that they cause the gland to release tears.
Associated Conditions
The greater petrosal nerve is vulnerable to damage from disease, surgery, tumors, or trauma anywhere along its course, especially skull fractures. The function of its branches can be impaired by damage to the greater petrosal nerve itself, the facial nerve, or, because of their post-ganglionic relationship, the trigeminal nerve.
Xerotic Keratitis
One rare but possible consequence of greater petrosal nerve damage is reduced tear secretion, called xerotic keratitis, which can lead to extreme dryness of the eyes, ulceration of the cornea, and blindness. This condition is generally treated with eye drops or ointments to replace the moisture.
Crocodile Tears Syndrome
Lesions on the facial nerve (Bell’s palsy) can cause degeneration of the greater petrosal nerve. In some cases, the lesser petrosal nerve sometimes takes over the innervation of the lacrimal glands. Because the lesser petrosal nerve also innervates the salivary gland, when it takes on this extra function, salivation causes simultaneous eye watering.
This rare complication of Bell’s palsy is called crocodile tears syndrome because it involves false tears. In many cases, it doesn’t need to be treated since it doesn’t cause much distress. More commonly, the eye affected by Bell’s palsy is chronically dry.
Bell’s palsy is sometimes treated with corticosteroids, antiviral medications, electrical nerve stimulation, and facial exercises. Successful treatment of the condition may relieve crocodile tears syndrome as well.
Cluster Headaches
Parasympathetic dysfunction involving the greater petrosal nerve can cause cluster headaches. One type of cluster headache related to this nerve can cause excessive eye watering, nasal congestion, and runny nose.
Treatment of cluster headaches can include triptan drugs, injections of octreotide or dihydroergotamine, local anesthetics, or, in a hospital setting, oxygen.
