Cranial Nerves and Nuclei II

Cranial nerves and nuclei I | Main Anatomy Index | The auditory system

• This includes the trigeminal (CN V), facial (CN VII), glossopharyngeal (CN IX), vagus (CN X) and accessory (CN XI) nerves.

• All these nerves innervate striated muscle of branchial arch origin (i.e., they all contain SVE fibres).
• With the possible exception of CN XI, they all contain other components as well.

• The trigeminal system is ultimately responsible for the transmission of information of:

1. Tactile;
2. Proprioceptive;
3. Pain;
4. And temperature.

• This information travels from the head to the cerebral cortex, cerebellum and reticular formation.
• The primary afferent fibres are distributed peripherally in the 3 divisions of the trigeminal nerve:

1. The ophthalmic [V₁];
2. The maxillary [V₂];
3. And the mandibular [V₃] divisions.

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• There is one motor nucleus, a special visceral efferent (SVE) nucleus, associated with the trigeminal nerve.

• It innervates the muscles of the first branchial arch, which consists mostly of the muscles of mastication.
• They also include the tensor tympani and several other small muscles.

• The nucleus is located in the midpons at the level of attachment of the trigeminal nerve to the brainstem.

• Fibres of the trigeminal motor nucleus emerge as a separate motor root.
• They are distributed peripherally with the mandibular division.

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• There are 3 sensory nuclei:

1. The main (or principle) sensory nucleus;
2. The spinal nucleus;
3. And the mesencephalic nucleus.

• They form a long, almost continuous column of cells.
• This extends from the rostral midbrain to the upper cervical spinal cord.

• The main sensory nucleus forms an enlargement in this column in the midpons, slightly lateral to the trigeminal motor nucleus.

• The spinal nucleus extends caudally from this level.
• The mesencephalic nucleus is very slender and extends rostrally.

• Entering trigeminal afferent (GSA) fibres (cell bodies are the trigeminal ganglion) do one of 3 things:

1. Most fibres bifurcate and send a very short ascending branch into the main sensory nucleus and a longer descending branch into the spinal trigeminal tract;
2. Or they don't bifurcate and either terminate directly in the main sensory nucleus;
3. Or terminate directly in the spinal tract after turning caudally.

• The primary afferent collaterals are arranged somatotopically; with the mandibular fibres most dorsal, and the ophthalmic fibres most ventral.
• They terminate in the medially adjacent spinal trigeminal tract.

• Both nucleus and tract extend caudally to the 3rd cervical segment of the spinal cord.
• The nucleus blends into the posterior horn.
• The tract blends into Lissauer's tract.

• The spinal trigeminal nucleus is subdivided into 3 regions.
• The most caudal part, extending from the spinal cord to the obex, is the caudal nucleus.
• The most rostral part, extending from the main sensory nucleus to about the pontomedullary junction, is the oral nucleus.
• Between the two is the interpolar nucleus.

• The caudal nucleus is known to be particularly important in the processing of pain and temperature information in the head.
• This fits with its appearance.
• The caudal nucleus looks much like the posterior horn of the spinal cord with a cap of substantia gelatinosa.

• The caudal nucleus also gives rise to a crossed ascending pain pathway analogous to the spinothalamic tract.
• It is called the ventral trigeminal (ventral trigeminothalamic tract).
• This tract terminates in the ventral posteromedial nucleus of the thalamus (VPM), which is adjacent to the VPL.

• Trigeminal pain fibres also reach the thalamus indirectly.
• This is through relays in the reticular formation (in a manner similar to spinoreticulothalamic fibres).

• In the somatotopic arrangement of the rostral-caudal distribution of endings:

1. Trigeminal fibres representing areas near the midline end near the obex;
2. Fibres representing areas near the back of the head end near the upper cervical cord.

• This is located near the motor nucleus and is analogous to the posterior column nuclei.
• It is therefore concerned with discriminative touch and proprioception.

• It receives large-diameter, heavily myelinated tactile afferents, and gives rise to two ascending pathways.

• One is a collection of fibres that cross the midline and joins the medial lemniscus.
• It terminates in the VPM.
• These fibres are generally considered to be part of the ventral trigeminal tract.

• The other one is completely ipsilateral.
• It projects from the dorsomedial portion of the main sensory nucleus.
• In this area, there is no projection into the ventral trigeminal tract.
• This tract is known as the dorsal trigeminal tract (dorsal trigeminothalamic tract).
• It travels through the dorsomedial part of the brainstem tegmentum and ends in its own separate portion of the VPM.

• These fibres to this nucleus carry proprioceptive fibres.
• These afferents from muscle spindles in the muscles of mastication and some from mechanoreceptors of the gums, teeth and hard palate don't have their cell bodies in the trigeminal ganglion.
• Instead, they are located in the slender column of cells called the mesencephalic trigeminal nucleus.

• This nucleus extends all the way rostrally to the posterior commissure.

• The cells of this nucleus are pseudounipolar (analogous to dorsal root ganglion cells) and their myelinated processes collect in a bundle, called the mesencephalic trigeminal tract or root.
• This lies adjacent to the nucleus.
• The central processes of these neurons end in the motor and main sensory nuclei and a few other brainstem sites.

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• This is characterised by brief (usually < 1 minute) attacks of excruciating pain in the distribution of one, or more than one, divisions of the trigeminal nerve.
• Between attacks, no sensory abnormalities can be found.
• There is frequently, a "trigger zone" where tactile stimulation may precipitate an attack.

• The mechanism is unknown.

• Most cases can be treated pharmacologically.
• A number, however, require surgery.
• These include:

1. Sectioning the nerve root;
2. Destroying or mechanically disturbing the trigeminal ganglion;
3. Or sectioning the trigeminal spinal tract slightly caudal to the obex.

• The last procedure retains tactile sensibility but abolishes pain sensations over one half of the face.
• This is evidence that the caudal part of the spinal trigeminal nucleus deals with pain and afferents from all three divisions of the trigeminal nerve.

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• Cranial nerves VII, IX and X contain fibres belonging to several different function components.

• All three nerves contain GSA fibres from the skin of the outer ear and the immediate vicinity.
• The GSA fibres of CN VII, IX and X all enter the spinal trigeminal tract.
• They behave exactly like trigeminal afferents.
• They are the most dorsomedial fibres of the spinal tract and occupy a position adjacent to the mandibular division of trigeminal.

• These three nerves also contain GVA fibres.
• In the facial nerve, this is a small collection of afferents that innervate parts of the nasal cavity and soft palate.
• They are of little clinical importance.

• The GVA fibres enter the solitary tract and terminate in the solitary nucleus.
• Both tract and nucleus extend through most of the medulla.

• The rostral levels deal with facial afferents.
• The caudal levels deal with vagal afferents.
• Between these two, the glossopharyngeal afferents terminate.
• There is, however, considerable overlap.

• The solitary nucleus in turn projects:

1. To the reticular formation;
2. To brainstem visceral motor nuclei;
3. And to the intermediolateral cell column of the spinal cord.

• Taste buds are usually associated with the tongue, although they are widely distributed over the palate and pharynx.
• The receptor cells synapse on SVA fibres from the facial, vagus and glossopharyngeal nerves.

• The solitary nucleus is the principle visceral afferent nuclei.
• It receives both GVA fibre and SVA fibres.

• However, the gustatory SVA fibres, as well as the chemical-sensitive trigeminal fibres, end separately.
• This is in lateral and rostral portions of the solitary nucleus.

• Second-order taste fibres do 3 things:

1. Participate in reflex activities, such as swallowing or coughing (by way of the motor nuclei of branchiomeric nerves).
2. Project to the cerebral cortex via the thalamus.
• The projection is uncrossed and the fibres travel ipsilaterally through the central tegmental tract to the most medial part of the VPM.
• They end in adjacent to the uncrossed dorsal trigeminal tract.
• This medial part of the VPM then projects into the gustatory cortex, which is adjacent to the somatosensory representation of the tongue.
• It is located in the frontal-parietal operculum and the insula.
3. The hypothalamic-limbic projection of visceral and perhaps gustatory information that is presumably involved with autonomic reflexes and our subjective sense of the pleasantness of things we ingest.

• A small collection of GVE fibres originate in this nucleus (a.k.a. superior drool nucleus) and travel in the facial nerve.
• This nucleus is located near the internal genu of the facial nerve.

• They innervate:

1. The submandibular gland;
2. The sublingual gland;
3. And the lacrimal gland.

• Most of the fibres of the facial nerve are SVE fibres.
• These innervate the muscle so the second branchial arch.
• These are the muscles of facial expression and stapedius.

• The large facial motor nucleus is located in the ventrolateral tegmentum of the caudal pons.

• The facial motor nucleus is involved in the corneal blink reflex, which is a reflex of considerable clinical importance.
• If either cornea is touched with a foreign object, both eyes automatically blink.

• The sensory afferent is by way of the ophthalmic division of the trigeminal nerve.
• The afferents enter the spinal trigeminal tract and synapse on interneurons in the spinal trigeminal nucleus, mostly rostral to the obex.
• They also synapse on the interneurons of the main sensory nucleus.

• These interneurons project bilaterally to the motor neurons of the facial motor nucleus, which forms the efferent limb.

• Thus, by touching each of the individuals corneas in turn, it is possible to test, in a crude fashion, the integrity of:

1. Both trigeminal nerves;
2. Both facial nerves;
3. And some of their central connections.

• The corticobulbar fibres (upper motor neurons of the cranial nerves as the corticospinal fibres are the UMN of the motor neurons of the spinal cord) from one frontal lobe contact 3 groups of facial motor neurons:

1. The ipsilateral upper face;
2. The contralateral upper face;
3. And the contralateral lower face.

• The consequence of this arrangement is that the lesion of the corticobulbar fibres of one side will result in weakness of contralateral lower face muscles only.
• This is useful in distinguishing supranuclear lesions from nuclear or root lesions.
• In the later, the entire ipsilateral half of the face will be affected.

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• This nerve contains a number of GVA fibres.
• Among them are the afferents from:

1. The carotid body;
2. The carotid sinus;
3. The medial surface of the tympanic membrane;
4. The posterior 1/3 of the tongue;
5. And the walls of the pharynx.

• Most of these enter the solitary tract and terminate in the solitary nucleus.
• However, there is evidence to show that the fibres conveying information of pain and temperature enter the spinal trigeminal tract and terminate in the spinal nucleus.

• A small group of GVE fibres arise in this nucleus (a.k.a. inferior drool nucleus).
• These fibres supply the parotid gland.

• These SVE fibres which arise from the nucleus ambiguus, innervate the muscles of the pharynx.
• The glossopharyngeal nerve specifically innervates the stylopharyngeus.
• The remainder of the pharyngeal musculature is innervated by the vagus nerve.

• The nucleus ambiguus is embedded in the medullary reticular formation.
• It is located in the ventrolateral medullary tegmentum.
• It is just dorsal to and roughly coextensive with the inferior olivary nucleus.

• The GSA (skin of outer ear) and SVA (taste buds) fibres of CN IX is similar to that of CN VII and CN X.

• This is a rare but particularly distressing condition.

• Attacks of pain usually begin in the posterior tongue or walls of the pharynx.
• It then radiates to the vicinity of the ear.

• This condition is so distressing as the trigger zone is often on the tongue or pharyngeal wall.
• Swallowing and talking and set off the attacks.

• Pharmacological relief is usually available.
• If not, the dorsomedial portion of the spinal trigeminal tract may be sectioned in the caudal medulla.
• The efficacy of this procedure provides evidence that the involved pain fibres travel in the spinal trigeminal tract.

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• This nerve has components and connections similar to and partially overlapping those of the glossopharyngeal nerve.
• The GSA and SVA components are similar to those of CN VII and CN IX.

• The vagus contains large collections of GVA fibres.
• These fibres innervate the thoracic and abdominal viscera.
• This includes the pressure receptors and chemoreceptors of the aortic arch.

• The vagal GVA fibres enter the solitary tract and terminate in the caudal solitary nucleus.
• The GVA fibres innervating the larynx, oesophagus and lower pharynx are thought to enter the spinal trigeminal tract and terminate in the spinal trigeminal nucleus (like those of CN IX).

• This large collection of fibres travels to the thoracic and abdominal viscera from the DMnX.
• This nucleus is the principle parasympathetic nucleus of the brain.

• It is located in the floor of the 4th ventricle just lateral to the hypoglossal nucleus.
• It underlies the vagal trigone.

• Other GVE fibres, particularly those to the heart, originate in the nucleus ambiguus.

• These innervate most of the striated muscles of the larynx and pharynx.
• A clinically useful reflex is the gag reflex.
• Touching the wall of one side of the pharynx elicits an unpleasant bilateral response.

• The afferent limb is via the glossopharyngeal nerve.
• The efferent limb is via the vagus nerve.

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• This nerve consists of fibres that originate from the very caudal medulla and the anterior horn of the upper 5 cervical segments.
• It exits just posterior to the denticulate ligament.

• This nerve innervates the sternocleidomastoid and part of the trapezius.
• These muscles are considered to be part of branchial arch origin so accessory nerve fibres are of the SVE category.

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