Trigeminal Nerve

Generality

The trigeminal nerve is one of the twelve cranial nerves of the human being, to be precise the fifth.

From the site: doctorspiller.com

Present in pairs (or pairs), the cranial nerves are fundamental nervous structures, which arise at the level of the brain and can have a sensitive, motor or both (therefore mixed) function.

The trigeminal nerve is a nerve with mixed functions, but with a prevalence of sensitive fibers.

It originates at the level of the Varolio bridge (brainstem) and is subdivided, shortly after the point of emergence, into three large divisions (or branches): the ophthalmic nerve, the maxillary nerve and the mandibular nerve.

The ophthalmic nerve and the maxillary nerve play an exclusively sensitive role, while the mandibular nerve has both sensitive nerve fibers and motor nerve fibers.

The three divisions of the trigeminal nerve make three specific paths, along which they give rise to further branches.

Short review of what a nerve is

To fully understand what a nerve is, it is necessary to start from the concept of neuron .

Neurons represent the functional units of the nervous system . Their task is to generate, exchange and transmit all those (nervous) signals that allow muscle movement, sensory perceptions, reflex responses and so on.

Typically, one neuron consists of three parts:

A body, where the cell nucleus resides.
Dendrites, which are equivalent to receiving antennae for nerve signals from other neurons or receptors located in the periphery.
Axons, which are cellular extensions having the function of spreading the nervous signal. The axon covered with myelin (myelin sheath) is also called nerve fiber .

A bundle of axons forms a nerve .

Nerves can carry information in three ways:

From the central nervous system (CNS) to the periphery . The nerves with this property are called efferent. The efferent nerves control the movement of the muscles, so I am in charge of the motor sphere.
From the periphery to the CNS . Nerves with this capacity are called afferent. The afferent nerves signal to the CNS what they detected in the periphery, therefore they cover a sensitive (or sensory) function.
From the SNC to the periphery and vice versa . Nerves with this double capacity are called mixed. Mixed nerves have a dual function: motor and sensory.

What is the trigeminal nerve?

The trigeminal nerve is the fifth pair of cranial nerves of the twelve total, present in the human body.

It is a mixed nerve, but with a greater sensory function; moreover, it has nerve fibers that also play a role within the parasympathetic nervous system (parasympathetic nerve fibers).

It owes its particular name to the fact that, from it, three large nerve branches (also known as trigeminal divisions or branches ) derive: the ophthalmic nerve (V1), the maxillary nerve (V2) and the mandibular nerve (V3).

Before going into the complex anatomy and physiology of the trigeminal nerve, it is appropriate to review what the cranial nerves and the parasympathetic nervous system are.

WHAT ARE THE CRANIAL NERVES?

Cranial nerves (or encephalic nerves ) are a group of nerves that originate from some structures of the brain and that can carry motor, sensory or mixed information.

Present in pairs, they are altogether twelve pairs. To identify each pair, are the Roman numerals I to XII.

Except for the I and II pair - which arise, respectively, from the telencephalon and diencephalon - the remaining ten pairs emerge from the brainstem (a fundamental component of the central nervous system).

Through their axons, the cranial nerves establish contact with the musculature, the glands and the sense organs of the head and neck.

Within this framework, the vagus nerve (or X pair) is an exception: unlike the other cranial nerves it also makes contact with different thoracic and abdominal organs.

Please note: in medical language, it is customary to refer to the various pairs of existing cranial nerves with singular names. Thus, the use of the term "trigeminal nerve" to indicate the 5th pair of cranial nerves is completely normal, as is the use of the term "olfactory nerve" to refer to the first pair of cranial nerves and so on.

This is not a mistake, but a simple convention, which aims to facilitate the study of these important nervous structures.

WHAT IS THE PARASIMPATIC NERVOUS SYSTEM?

The parasympathetic nervous system constitutes, together with the sympathetic nervous system, the so-called autonomous or vegetative nervous system ( SNA ), which carries out a control action on involuntary bodily functions.

Originating in the central nervous system (some of its nerves originate from the brainstem, others from the spinal cord), the parasympathetic nervous system fulfills several functions: it stimulates rest, relaxation, rest, digestion and energy storage.

As can be appreciated from the figure below, he presides over the adaptation system known as " rest and digestion " (in English is " rest and digest "), which characterizes the moments of routine and the quiet activities of everyday life.

The stimulation of the parasympathetic nervous system can involve: the narrowing of the pupil (miosis), the increase of digestive secretions (salivary, gastric, biliary, enteric and pancreatic), the increase in peristaltic activity (greater intestinal motility), the decrease of heart rate, contraction of bronchial muscles, relaxation of the bladder sphincter (promotion of urination), dilation of blood vessels of skeletal muscles and stimulation of erection.

From a comparison between the parasympathetic and the sympathetic system, one can immediately notice that the latter carries out an activity diametrically opposed to the first, since it has stimulating, exciting and contracting functions (after all it presides over the adaptation system known as "attack and Run ").

Anatomy

The trigeminal nerve emerges at the lateral margin of the Varolio bridge, with two distinct roots: a sensitive, which is large and flat, and a motor, which is small and thin.

The sensitive root heads and penetrates into a semi-lunar structure, called the trigeminal ganglion (or Gasser's trigeminal ganglion ); the trigeminal ganglion resides in the so-called Meckel cord (or trigeminal cord).

The motor root follows a very similar path, enters the Meckel hollow, but, unlike the previous case, it remains outside the trigeminal ganglion; to be precise, it passes below it.

After passing through the Meckel cable, the trigeminal nerve gives rise to the large branches (or divisions) that characterize it and from which it derives its name: the ophthalmic nerve, the maxillary nerve and the mandibular nerve. It is necessary to specify that the motor root (which does not cross the trigeminal ganglion) participates in the formation of the mandibular nerve alone, thus adding to the latter's sensitive nerve fibers (which instead have crossed Gasser's ganglion).

The three branches emerge from the skull, in three different points: the ophthalmic nerve enters the upper orbital fissure, the maxillary nerve passes through the round hole and, finally, the mandibular nerve exits through the oval hole (of the sphenoid bone).

Where is Meckel's cable and what are the reports of Gasser's trigeminal ganglion?

The Meckel cord is a sort of pocket, formed by a splitting of the dura mater, that is the outermost meninge, placed in direct contact with the skull.

The trigeminal ganglion - which is located inside the Meckel cavity - is in close contact with the dura mater and confines with the temporal lobe, superiorly, and with the cavernous sinus and internal carotid artery, medially.

WHERE THE ROOTS COME FROM: THE TRIGEMINOUS NUCLEI

The two roots of the trigeminal nerve come from some nerve centers, called nuclei, which reside inside the brainstem.

Distributed between midbrain, bridge and Varolio and elongated marrow, these nuclei are in all 4: three sensory (mesencephalic, main and spinal) and a motor.

The mesencephalic sensory nucleus . Based in the midbrain, it contains the neurons responsible for the transport of proprioceptive impulses coming from the masticatory muscles and the ocular extrinsic muscles.
Proprioceptive impulses are sensitive type information.
The main sensitive core . Located in the highest part of the Varolio bridge, it contains the neurons that carry the finest tactile information of all three branches of the trigeminal nerve.
The spinal sensory core . Located in the medulla oblongata (inferior to the Varolio bridge), it contains the neurons used to transport the coarser tactile impulses and thermo-pains.
This nucleus also receives the sensitive fibers belonging to the glossopharyngeal nerve and the vagus nerve.
The motor nucleus . Located at the level of the Varolio bridge (near the main sensitive nucleus), it collects the cellular body of nerve fibers that innervate and control the movement of some muscles, including the so-called masticatory muscles.

Obviously the sensory nuclei are associated with the sensory root, while the motor nucleus is associated with the motor root.

Attention: a necessary clarification

The trigeminal nerve carries sensory information according to the scheme: sensory receptor located in the periphery → sensory nucleus → thalamus → cortex.

The first step (sensory receptor - sensory nucleus) occurs through primary neurons; the second passage (sensory-thalamus nucleus) occurs through secondary neurons; finally, the third passage (thalamus - cortex) occurs through tertiary neurons.

The cell bodies of primary neurons reside in Gasser's trigeminal ganglion (dendrites are in the periphery, while axons terminate in the brainstem); the cellular bodies of secondary neurons are located in the nuclei described above; finally, the cell bodies of tertiary neurons take place in the thalamus.

Within the aforementioned framework, the nerve path connected to the mesencephalic sensory nucleus is an exception. In fact, the cellular bodies of its primary neurons reside directly in the mesencephalic sensory nucleus and not in the trigeminal ganglion (which is only a transitional structure).

Note : neurologists often identify primary, secondary and tertiary neurons with singular terminology: the primaries become 1st neuron, the secondary become 2nd neuron and the tertiaries become 3rd neuron.

OPHTHALMIC NERVE

Exclusively sensitive, the ophthalmic nerve runs anterior to the lateral wall of the cavernous sinus, above the troclear nerve.

At the level of the upper orbital fissure, it separates into three further branches: the lacrimal nerve, the frontal nerve and the nasociliary nerve .

Along their path, the frontal nerve and the nasociliary nerve give life to other branches still: for example, the supratrochlear nerve, in the case of the frontal, and the posterior and anterior ethmoidal nerves, in the case of the nasociliary.

MASCELLAR NERVE

Exclusively sensitive, the maxillary nerve flows anterior to the lateral wall of the cavernous sinus (like the ophthalmic), under the trochlear nerve. Subsequently to the Gasser ganglion, it occupies an intermediate position between the ophthalmic and mandibular nerves.

After coming out of the cranial cavity (through the round hole), it enters the pterygopalatine fossa; from here, it reaches the orbit, passing through the sub-orbital groove (or lower orbital fissure) and the sub-orbital channel and leaving the so-called sub-orbital hole (or infraorbital hole).

Along this intricate path, it gives rise to numerous branches, including: the middle meningeal nerve, the zygomatic nerve, the sphenopalatine nerves, the upper alveolar nerves, etc.

The main branches of the maxillary nerve
seat	Nerve
Skull	Middle meningeal nerve
After the pterygopalatine fossa	Zygomatic nerve Sphenopalatine nerves (from which the postero-superior nasal nerves and palatine nerves derive) Upper alveolar nerves (from which the posterior superior alveolar nerve, the middle superior alveolar nerve and the anterior superior alveolar nerve derive) Infraorbital nerve Pterigopalatine nerve
In the suborbital channel	Anterior superior alveolar nerve Infraorbital nerve
On the face	Lower palpebral nerve Upper labial nerve Nasal lateral nerve

MANDIBULAR NERVE

The mandibular nerve is the division of the most voluminous trigeminal nerve.

Moreover, it contains a sensory component, comparable in volume to those of the ophthalmic nerve and the maxillary nerve, and a motor component (which is, in fact, the entire motor root leaving the Varolio bridge).

After its exit from the skull, at the level of the oval hole of the sphenoid bone, it immediately emits a small collateral branch, the so-called spinous nerve .

A few millimeters after this first branch, it gives rise to two series of branches, called, trunks: the front trunk and the rear trunk .

The anterior trunk forms 4 branches: the masseteric nerve, the buccal nerve, the deep temporal nerve and the lateral pterygoid nerve.

The posterior trunk, on the other hand, gives rise to 3 branches: the auricolotemporal nerve, the lingual nerve and the inferior alveolar nerve.

Function of the main branches of the mandibular nerve.
They have sensory function ...	They have motor function ...
Spiny nerve	Lateral pterygoid nerve
Buccal nerve	Deep temporal nerve
Auriculotemporal nerve	Masseteric nerve
Lingual nerve	Lower alveolar nerve (with only branch called the mylohyoid nerve)
Lower alveolar nerve (except for a branch)

From the site: msk-anatomy.blogspot.it

PARASIMPATIC FIBERS OF TRIGEMINUS

Taking up an initial statement, the trigeminal nerve also contains fibers with a parasympathetic function.

These fibers have a particular characteristic: they follow the path of some trigeminal branches, but belong to other cranial nerves, in this case the facial nerve (or VII cranial nerve) and the glossopharyngeal nerve (or IX cranial nerve).

Therefore, to include them in the trigeminal system is only the fact that they add up to the branches of the trigeminal nerve (ophthalmic nerve, maxillary nerve and mandibular nerve).

The most important contacts, which the parasympathetic fibers of the trigeminal nerve establish along their path, are with the lacrimal glands, the nasal glands, the sublingual and submandibular glands and, finally, the parotid glands.

Functions

With its sensitive component, the trigeminal nerve innervates: the skin of the face, the front part of the scalp, the buccal and nasal mucous membranes, the conjunctiva, the eyeball and teeth.

With its motor component, it provides innervation of the masticatory muscles and other specific muscle elements located near the jaw.

Finally, with its parasympathetic component, it controls nasal, lacrimal and salivary secretions.

OPHTHALMIC NERVE

Through its numerous branches, the ophthalmic nerve transports, to the central nervous system, sensitive information coming from:

The forehead and the front part of the scalp;
Frontal sinuses and ethmoid sinuses. The first are paranasal cavities located on the frontal bone, above the orbits; the latter are paranasal cavities located in the ethmoid bone, under the frontal bone and above the jaw;
Upper eyelids and associated conjunctivae;
The corneas;
The dorsal region of the nose.

The parasympathetic nerve fibers of the ophthalmic nerve innervate exclusively the lacrimal glands . These fibers originally belong to the facial nerve.

MASCELLAR NERVE

By means of its branches and sub-branches, the maxillary nerve leads to the central nervous system the sensory signals coming from:

The lower eyelids and the conjunctiva present therein;
The meningeal dura mater encephalic;
Cheeks and maxillary sinuses;
The nasal cavities and the lateral areas of the nose;
The upper lip;
The upper dental arch, made up of incisors, canines and molars, and the associated gingiva;
The upper palate.

The parasympathetic nerve fibers associated with the maxillary nerve innervate the lacrimal glands and the nasal glands . Like those of the ophthalmic nerve, even the aforementioned fibers belong, originally, to the facial nerve.

MANDIBULAR NERVE

With its sensitive branches, the mandibular nerve carries information from the central nervous system:

The mucosa of the floor of the mouth and the mucous membrane of the cheeks (buccal nerve)
The outer ear (deep temporal nerve).
The 2/3 anterior of the tongue (lingual nerve).
The lower lip (branches of the lower alveolar nerve).
The chin (branches of the inferior alveolar nerve).
The lower dental arch, consisting of incisors, canines and molars, and the associated gingiva (branches of the inferior alveolar nerve)

Instead, using motor fibers, check:

The muscles used for mastication, ie the medial pterygoid muscle (medial pterygoid nerve), the lateral pterygoid muscle (lateral pterygoid nerve), the masseter muscle (masseter nerve) and the temporal muscle (deep temporal nerve).
The tensor muscle of the palatal veil (group of nerves of the palatal vein tensor).
The tensor muscle of the eardrum (nerve of the tympanic tensor).
The anterior belly of the digastric muscle and the mylohyoid muscle (mylohyoid nerve).

Finally, the parasympathetic nerve fibers associated with the mandibular nerve innervate the submandibular and sublingual glands and the parotid glands .

The innervating submandibular and sublingual glands belong to the facial nerve, while those that control the parotid glands are part of the glossopharyngeal nerve.

Trigeminal diseases

The most common and well-known morbid condition, which involves the involvement of the trigeminal nerve, is the so-called trigeminal neuralgia .

Returning to the category of neuropathies, trigeminal neuralgia is a chronic syndrome, which causes the onset of severe pain in the areas of the face innervated by the trigeminal nerve, ie: forehead, eye, ear, nose, anterior region of the scalp, chin, mouth and jaw.

In general, subjects with trigeminal neuralgia suffer from pain attacks (in medical jargon, we speak of "crisis"), which are short-lived (1-2 minutes), but are very intense and appear without warning.

The precise cause of trigeminal neuralgia is still unknown. According to the most reliable hypotheses, it would be caused by the compression of the trigeminal nerve, exerted by one of the blood vessels that flow next to it (or that flow next to one of its main branches).

The compression of an important nerve determines its functional alteration and the sending of anomalous signals to the brain.

Often, trigeminal neuralgia is associated with aging and particular pathological conditions such as: diseases of the central nervous system (multiple sclerosis), brain lesions, traumas, blood vessel abnormalities (aneurysms), infections (shingles that cause so-called neuralgia post-herpetic) etc.

Living with trigeminal neuralgia can be quite difficult, especially when the symptoms are intense and the therapeutic remedies are not very effective.

In the most serious cases, failure to improve a particularly severe symptomatology can also lead to depression: despair and sadness are due to the fact that there is no improvement in health conditions.

Women suffering from trigeminal neuralgia are, for a reason that is still unknown, most of the time.

Curiosity

It will have happened to many readers to undergo some dental treatment and to receive, on the occasion, a local anesthesia against pain.

Anatomically, dentists practice this anesthesia at the points where the branches of the trigeminal nerve flow.

Using a concrete example, dentists dealing with a subject with dental problems in the lower arch perform anesthetic injection at the level of the inferior alveolar nerve, ie the branch of the mandibular nerve that runs in the mandibular canal and innervates the gums and teeth of the jaw.

Thanks to the possibility of desensitizing the trigeminal nerve it is possible to treat many dental disorders, without any particular suffering for the patient.