Cranial Nerves and Nuclei I
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formation | Main Anatomy Index | Cranial nerves and nuclei II
Last updated 30 March 2006
Cranial Nerves and Nuclei I
Click here for the gross anatomy of the cranial nerves
(First Year Medicine ANAT1006).
- Although the tracts and nuclei of the cranial nerves are intricate, they can be
systematised in terms of their function components
contained within each nerve.
- Spinal nerves contain both motor and sensory
- Some of each kind may be related to visceral or somatic structures.
- A given nerve fibre can thus be put into the following categories:
General Somatic Afferent (GSA)
These fibres are related to the receptors for pain, temperature, and mechanical stimuli
in somatic structures such as skin, muscle
Special Somatic Afferent (SSA)
- These fibres are related to the special senses of sight,
hearing and equilibrium.
General Somatic Efferent (GSE)
These fibres innervate skeletal muscle (i.e., they are
the axons of the alpha and gamma motor neurons).
General Visceral Afferent (GVA)
These fibres are related to the receptors of the visceral
structures such as the walls of the digestive tract.
Special Visceral Afferent (SVA)
- These fibres are related to the special senses of smell
Special Visceral Efferent (SVE)
General Visceral Efferent (GVE)
- These are fibres that are the preganglionic autonomic fibres.
- These fibres innervate certain striated muscles with a special embryological origin.
- They are referred to as the branchiomeric muscles.
- Structures that develop into gill arches in fish
develop instead into various structures near or in the head
and neck (muscles of the face, larynx and pharynx).
- Although these muscles are identical to normal striated muscle, neurons
for branchiomeric muscles have a distinctive
location in the brainstem.
Locations of the Cell Bodies of the
The locations of the cell bodies where cranial nerve afferents
terminate or cranial nerve efferents originate,
can be predicted to some extent from the embryology of the brainstem.
The walls of the neural tube spread apart
- The sulcus limitans runs longitudinally
along the floor of the adult
- It still separates the sensory alar plate derivatives
(now lateral) and the motor basal plate derivatives (now
- The cell concerned with visceral function tend to be
located nearer the sulcus limitans.
- Ideally, the cell columns of the special components of the cranial
nerves would be located adjacent to those for
the corresponding general components.
- The actual arrangement is not so simple, however, because:
- The cell columns are not continuous with those of the spinal cord.
In a few instances, portions of the cell columns migrate away from their expected locations.
- They are interrupted and form a series
- All components may not be present in one
- For example, most SVE neurons are located in the ventrolateral
part of the tegmentum rather than on the floor of the ventricle, adjacent to
the other efferent neurons.
Function Components of the
contains all seven functional
There are 3 types of cranial nerves:
- Those that contain GSE fibres and little or nothing else (e.g., CN
III, IV, VI and XII) and are referred to as somatic efferent nerves;
- Those that contain special sensory fibres (SSA or SVA) and
nothing else (e.g., CN I, II and VIII);
- And the remaining nerves that are more complex, contain several
components including the innervation of the branchial
arch musculature (e.g., CN
V, VII, IX, X and XI). These nerves are called branchiomeric nerves.
Table is from Nolte, The Human Brain 3rd Ed. p. 180 Table 6
termination within CNS
sensory or motor ending
||Originates in olfactory
||Lateral geniculate nucleus and
||Originates in ganglion cells of
||SR, IR, MR, IO, levator palpebrae
||Sphincter pupillae, ciliary
||Cochlear and vestibular nuclei
||Organ of Corti, cristae of
semicircular canals, maculae of utricle and saccule
||Muscles of the tongue
- Cranial nerves I and II are not attached to the brainstem, but are directly related to the forebrain.
Somatic Efferent Nerves
This includes the oculomotor (CN III), trochlea (CN IV), abducens (CN
VI) and hypoglossal (CN XII) nerves.
These nerves are the simplest of the cranial nerves as
each contains only one functional component (GSE fibres).
The exception is for CN III, which has a small but
important complement of GVE fibres.
Oculomotor Nerve (CN III)
This supplies the levator palpebrae superioris and all
the internal and external muscles
of the ipsilateral eye.
The exception are the lateral rectus, the superior oblique and dilator pupillae.
- The fibres originate from the wedge-shaped oculomotor nucleus.
- This is located at the ventral edge of the periaqueductal grey in the rostral
- They then proceed ventrally and arch
through the midbrain tegmentum in several
- These join the nerve as they emerge into the interpeduncular fossa.
This consists of a series of longitudinal
cell columns or subnuclei.
The column supplying:
- The levator palpebrae superioris is located in the
midline and innervates this muscle on both
- The superior rectus projects to the contralateral
- The medial rectus, inferior
rectus, and inferior oblique all project to
the ipsilateral eye;
This column includes preganglionic parasympathetic neurons
It straddles the midline and projects to the ipsilateral ciliary ganglion.
The ciliary ganglion then innervates the sphincter pupillae
and the ciliary muscle.
Clinical Significance of the Oculomotor
Although the oculomotor nerve has a partly-crossed, uncrossed nature, this is of limited clinical significance.
This is as the two nuclei are so close together, that a
central lesion in this vicinity is likely to damage both nuclei.
- However, once a given oculomotor nerve leaves the brainstem, it supplies only ipsilateral muscles.
- Thus, a lesion to the nerve affects only one eye.
- Therefore, unilateral nuclear damage is extremely rare
(paralysis of superior rectus on one side and of the other extraocular muscles on the
- Damage to the oculomotor nerve cause:
- Lateral strabismus, as medial rectus is paralysed and the lateral rectus is
- Diplopia, double-vision as one of the eye deviates from the midline;
- Inability to move the eye medially
- Ptosis as the ipsilateral levator palpebrae superioris is paralysed;
- Mydriasis (dilated pupil of affected side) and unresponsiveness
to light as the sphincter pupillae is non-functional and the dilator pupillae
- Inability for the affected eye to focus on near objects
as the ciliary muscles have also been paralysed.
- Along the course of the oculomotor nerve from the brainstem to the orbit, the GVE fibres of the Edinger-Westphal
nucleus travel in a superficial location.
- They are especially susceptible to external
- Thus a dilated pupil and unresponsiveness
to light may be the first clinically detectable sign of something pressing on
the third nerve.
- Note: on Third Nerve Damage vs. Horner's Syndrome.
- Third Nerve Damage: ptosis and same side larger pupil.
- Horner's Syndrome: ptosis and same side smaller pupil.
- Also, with third nerve damage, ptosis is more pronounced,
and is usually accompanied with defective eye movements
and lateral strabismus.
This supplies the superior oblique muscle.
Its cell bodies are located in the contralateral
- This is a small nucleus located at the level of the inferior colliculus.
- Here, it indents the medial
longitudinal fasciculus (MLF).
- Fibres leaving the nucleus turn caudally in the periaqueductal grey.
- They then arch dorsally to decussate
and leave the brainstem at the pons-midbrain junction.
- The trochlear nerve is unique in that:
- It is the only cranial nerve attached to the dorsal aspect
of the brainstem;
- It is the only one to originate completely from the contralateral nucleus.
Clinical Significance of the Trochlear
Damage to the trochlear nerve result in much less
drastic and noticeable deficits than damage to the oculomotor
or abducens nerves.
- The superior oblique muscle helps to move the eye downward and laterally.
- Attempted movements in these directions (e.g., reading or walking down stairs) may cause
This supplies the lateral rectus muscle.
The fibres originate from the ipsilateral abducens nuclei.
This is located in the caudal
pons, beneath the floor of the 4th
- Medial to the nucleus is two bundles of fibres.
- The more medial of the two is the MLF.
- Between the MLF and the abducens
nuclei are the motor fibres of the facial nerve.
- The facial fibres project dorsomedially
and wrap around the abducens nucleus.
- They then turn back ventrally to exit from the brainstem.
- Here, the fibres are known as the internal genu of the facial nerve.
Clinical Significance of the Abducens Nerve
Damage to the Abducens Nerve
This causes medial strabismus (the affected eye
deviates medially by the unopposed action of the medial rectus muscle).
The individual may be able to move the affected eye to the midline, but no further, by
relaxing the medial rectus muscle.
Damage to the Abducens Nuclei
This has the same deficit as damage to the sixth nerve with a significant
Not only is it impossible to move the ipsilateral eye laterally,
but now also the contralateral medially.
This is called lateral gaze paralysis.
- This is as internuclear neurons are also affected.
- These neurons normally project to the MLF to the motor neurons controlling the contralateral
medial rectus muscle.
- This is so that we normally have co-ordinated lateral gaze
(i.e., if we want to look left, our left eye turns laterally and simultaneously, our right
eye turns medially).
- This condition is known as internuclear ophthalmoplegia
This nerve supplies the muscles of the tongue.
The fibres originate in the ipsilateral hypoglossal nucleus.
- Hypoglossal axons proceed ventrally and emerge as a series of rootlets
in the groove between the pyramid
and the olive.
Clinical Significance of the Hypoglossal
This will cause weakness of one
side of the tongue.
As this is a lower motor neuron lesion, that side of
the tongue will also atrophies.
- This weakness is easily demonstrated by the patient
protruding his or her tongue.
- The tongue is deviated to the side
of the lesion.
- Bilateral hypoglossal lesions may cause difficulties in both speaking and eating.