Cranial Nerves and Nuclei II
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and nuclei I | Main Anatomy Index | The auditory system
Last updated 30 March 2006
Cranial Nerves and Nuclei II
The Branchiomeric Nerves
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:
- 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:
- The ophthalmic [V1];
- The maxillary [V2];
- And the mandibular [V3] divisions.
Trigeminal Motor Nucleus
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.
Sensory Nuclei of the Trigeminal Nerve
There are 3 sensory nuclei:
- The main (or principle) sensory nucleus;
- The spinal nucleus;
- 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.
- Entering trigeminal afferent (GSA) fibres
(cell bodies are the trigeminal ganglion) do one of 3 things:
- 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;
- Or they don't bifurcate and either terminate directly
in the main sensory nucleus;
- Or terminate directly in the spinal
tract after turning caudally.
Spinal Trigeminal Nucleus
The primary afferent collaterals are arranged somatotopically; with the mandibular
fibres most dorsal, and the ophthalmic fibres most
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
- The caudal nucleus also gives rise to a crossed ascending pain pathway analogous to the spinothalamic tract.
- It is called the ventral trigeminal (ventral
- 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
- In the somatotopic arrangement of the rostral-caudal distribution of endings:
- Trigeminal fibres representing areas near the midline
end near the obex;
- Fibres representing areas near the back of the head end
near the upper cervical cord.
Main Sensory Nucleus
This is located near the motor nucleus and is
analogous to the posterior column nuclei.
It is therefore concerned with discriminative touch and
- It receives large-diameter, heavily
myelinated tactile afferents, and gives rise to two
- 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
- 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
Mesencephalic Trigeminal Nucleus
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
- 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.
Clinical Significance of the Trigeminal
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:
- Sectioning the nerve root;
- Destroying or mechanically disturbing the trigeminal ganglion;
- 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.
Cranial nerves VII, IX
and X contain fibres belonging to several different function
General Afferents of the Facial Nerve
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.
Solitary Nucleus and Facial GVA Fibres
These three nerves also contain GVA
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
- The caudal levels deal with vagal
- Between these two, the glossopharyngeal afferents
- There is, however, considerable overlap.
- The solitary nucleus in turn projects:
- To the reticular formation;
- To brainstem visceral motor nuclei;
- And to the intermediolateral cell column of the spinal cord.
Taste and SVA Fibres
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
|Facial (CN VII)
||Anterior 2/3 of tongue and palate
||Posterior 1/3 of tongue and pharynx
|Vagus (CN X)
Solitary Nucleus and Facial SVA Fibres
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:
- Participate in reflex activities, such as swallowing or
coughing (by way of the motor nuclei of branchiomeric nerves).
- 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.
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.
- 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
Efferents of the Facial Nerve
Superior Salivatory Nucleus and
Facial GVE Fibres
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
- The submandibular gland;
- The sublingual gland;
- And the lacrimal gland.
Facial Motor Nucleus and SVE Fibres
Most of the fibres of the facial nerve are SVE
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
- The facial motor nucleus is involved in the corneal blink reflex, which is a reflex of considerable
- If either cornea is touched with a foreign object, both eyes automatically blink.
- 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:
- Both trigeminal nerves;
- Both facial nerves;
- And some of their central connections.
Clinical Significance of the Facial Nerve
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:
- The ipsilateral upper face;
- The contralateral upper face;
- 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.
Glossopharyngeal GVA Fibres
This nerve contains a number of GVA
Among them are the afferents from:
- The carotid body;
- The carotid sinus;
- The medial surface of the tympanic
- The posterior 1/3 of the tongue;
- And the walls of the pharynx.
- Most of these enter the solitary tract and terminate in the solitary
- 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.
Inferior Salivatory Nucleus and
Glossopharyngeal GVE Fibres
A small group of GVE fibres arise in this
nucleus (a.k.a. inferior drool nucleus).
These fibres supply the parotid gland.
Nucleus Ambiguus and Glossopharyngeal SVE
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
Glossopharyngeal GSA and SVA Fibres
The GSA (skin of outer ear) and SVA (taste buds) fibres of CN IX is similar to that of CN VII and CN X.
Clinical Significance of the
This is a rare but particularly
- 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
- 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.
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.
Vagal GVA Fibres
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).
Dorsal Motor Nucleus of the Vagus and GVE Fibres
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.
- Other GVE fibres, particularly those to
the heart, originate in the nucleus
Nucleus Ambiguus and Vagal SVE
These innervate most of the striated muscles of the larynx and pharynx.
A clinically useful reflex is the gag
Touching the wall of one side of the pharynx
elicits an unpleasant bilateral response.
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
- 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