Cranial
Nerves
Olfactory nerve
The receptors for olfaction are found within the nasal mucosa, and their
axons project through the cribriform plate to the olfactory bulb on the
undersurface of the frontal lobe (see Chapter 30). This cranial nerve therefore
does not originate or pass through the brainstem and conveys information on
smell.
Optic nerve
The photoreceptors in the eye project onto bipolar cells to ganglion
cells and then to the CNS via the optic nerve. The nerve passes through the
optic canal at the back of the orbit into the brain and unites with the optic
nerve from the other eye to form the optic chiasm. The fibres from here pass
ultimately to the visual cortex as well as to a number of subcortical sites
(see Chapters 24–26).
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Damage to this nerve will affect vision,
although the extent and type of this visual loss depends on the site of injury
(see Chapter 25).
Oculomotor nerve
This originates in the midbrain at the level of the superior collicu- lus
and supplies all the extraocular muscles apart from the lateral rectus, and
superior oblique. It also carries the parasympathetic innervation to the eye
and provides the major innervation of levator palpebrae superioris.
•
A complete third nerve palsy causes the eye to
lie ‘down and out’ with a fixed dilated unresponsive pupil and ptosis (droopy
eyelid). Common causes of this are a posterior communicating artery
aneurysm or a microvascular insult to the nerve itself as occurs in
diabetes mellitus, for example.
Trochlear nerve
This nerve originates in the midbrain at the level of the inferior
colliculus, and exits out of the brainstem dorsally. It supplies the superior
oblique muscle.
•
Damage to this nerve causes double vision (diplopia)
when looking down. A common cause of fourth cranial nerve palsy is head trauma.
Fifth cranial or trigeminal nerve
The trigeminal nerve has both a motor and sensory function. The motor
nucleus is situated at the mid-pontine level, medial to the main sensory
nucleus of the trigeminal nerve, and receives an input from the motor cortex
(see Chapter 38 and 39). It supplies the muscles of mastication. Sensation from
the whole face (including the cornea) passes to the brainstem in the trigeminal
nerve, and synapses in three major nuclear complexes: the nucleus of the spinal
tract and the main sensory nucleus of the trigeminal nerve; and the
mesencephalic nucleus. Sensation from the face is relayed via three branches:
the ophthalmic division that supplies the fore-head; the maxillary division
that innervates the cheek; and the mandibular branch from the jaw–with the more
rostral fibres (ophthalmic branch fibres) passing to the lowest part of the
nucleus of the spinal tract in the upper cervical cord. These brainstem
trigeminal nuclei in turn project to the thalamus as part of the somatosensory
and pain systems (see Chapters 31 and 32).
•
Damage to the trigeminal nerve results in weak
jaw opening and chewing, coupled to facial sensory loss and an absent corneal
reflex.
Sixth cranial or abducens nerve
This originates from the dorsal lower portion of the pons and supplies
the lateral rectus muscle.
•
Damage to this nerve results in horizontal diplopia
when looking to the lesioned side and can be caused by local brainstem
path- ology or can be a false localizing sign in raised intracranial pressure.
Seventh cranial or facial nerve
This is predominantly a motor nerve, although it does carry parasympathetic
fibres to the lacrimal and salivary glands (the greater superficial petrosal nerve
and chorda tympani) as well as sensation from the anterior two-thirds of the
tongue (the chorda tympani). The motor nucleus for the facial nerve originates
in the pons, and supplies all the muscles of the face except for those involved
in mastication.
•
A lesion of this nerve produces a lower facial
nerve palsy with weakness of all the facial muscles ipsilateral to the side of
the lesion. In addition, there is a loss of taste on the anterior two-thirds of
the tongue if the lesion occurs proximal to the origin of the chorda tympani.
This is most commonly seen in Bell’s palsy. In contrast, damage
to the descending motor input to the facial nucleus from the cortex (an upper
motor neurone facial palsy) causes weakness of the lower part of the
contralateral face only, as the musculature of the upper part of the face has
upper motor neurone innervation from the motor cortex of both hemispheres.
Eighth
cranial or vestibulocochlear nerve
This conveys information from the cochlea (the auditory or cochlear
nerve; see Chapters 27 and 28) as well as the semicircular canals and otolith
organs (the vestibular nerve; see Chapter 29).
•
Damage to this nerve (e.g. in acoustic
neuromas) causes distur- bances in balance with deafness and tinnitus
(a ringing noise).
Ninth cranial
or glossopharyngeal nerve
The glossopharyngeal nerve contains motor, sensory and parasympathetic
fibres. The motor fibres originate from the rostral nucleus ambiguus and supply
the stylopharyngeus muscle, while the sensory fibres synapse in the tractus
solitarius (or nucleus of the solitary tract) and provide taste and sensation
from the poste- rior tongue and pharynx. The parasympathetic fibres originate
in the inferior salivatory nucleus and provide an input to the parotid gland.
•
Damage to this nerve usually occurs in
conjunction with the vagus nerve (see below).
Tenth cranial
or vagus nerve
This nerve provides a motor input to the soft palate, pharynx and larynx,
which originates in the dorsal motor nucleus of the vagus and nucleus ambiguus.
It also has a minor sensory role, conveying taste from the epiglottis and
sensation from the pinna, but has a significant parasympathetic role (see
Chapter 3).
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Damage to the vagus nerve causes dysphagia
and articulation disturbances and, as with glossopharyngeal nerve
lesions, there may be a loss of the gag reflex.
Eleventh cranial or spinal accessory nerve
This is purely motor in nature and originates from the nucleus ambiguus
in the medulla and the accessory nucleus in the upper cervical spinal cord. It supplies
the sternocleidomastoid and trapezius muscles.
•
Damage to the eleventh nerve causes weakness in
these muscles.
Twelfth cranial or hypoglossal nerve
The hypoglossal nerve provides the motor innervation of the tongue. Its
fibres originate from the hypoglossal nucleus in the posterior part of the
medulla.
•
Damage to this nerve causes wasting and weakness
in the tongue, which leads to problems of swallowing and speech, and is most
commonly seen in motor neurone disease (see Chapter 60). Isolated
damage of this nerve is rare and it is more commonly affected with other lower
cranial nerves (e.g. the ninth, tenth and eleventh cranial nerves) and in such
cases the patient may present with a bulbar palsy. A pseudobulbar
palsy, in contrast, refers to a loss of the descending cortical input
to these cranial nerve nuclei.
