Page 392 - Clinical Anatomy

ECA6 7/18/06 6:54 PM Page 377

The cranial nerves 377

palsies on the one hand and ‘supranuclear’ palsies on the other. Both
nuclear and infranuclear palsies result in a facial paralysis which is com-
plete and which affects all the muscles on one side of the face. In supranu-
clear palsies there is no involvement of the muscles above the palpebral
fissure since the portion of the facial nucleus supplying these muscles
receives fibres from both cerebral hemispheres. Furthermore, in such cases
the patient may involuntarily use the facial muscles but will be unable to do
so on request.
2◊◊Supranuclear facial palsies most frequently result from vascular
involvement of the corticobulbar pathways, e.g. in cerebral haemorrhage.
Nuclear palsies may occur in poliomyelitis or other forms of bulbar paraly-
sis, while infranuclear palsies may result from a variety of causes including
compression in the cerebellopontine angle (as by an acoustic neuroma),
fractures of the temporal bone and invasion by a malignant parotid tumour.
However, by far the commonest cause of infranuclear facial paralysis is
Bell’s palsy, which is of unknown aetiology.
When the intracranial part of the nerve is affected or when it is involved
in fractures of the base of the skull there is usually loss of taste over the ante-
rior two-thirds of the tongue and an associated loss of hearing (8th nerve
damage).

The auditory (vestibulocochlear) nerve (VIII)
(Fig. 265)
The 8th nerve consists of two sets of fibres: cochlear and vestibular. The
cochlear fibres (concerned with hearing) represent the central processes of
the bipolar spiral ganglion cells of the cochlea which traverse the internal
auditory meatus to reach the lateral aspect of the medulla, at the cerebello-
pontine angle (together with VII), where they terminate in the dorsal and
ventral cochlear nuclei. The majority of the projection fibres from these
nuclei cross to the opposite side, those from the dorsal nucleus forming the
auditory striae in the floor of the 4th ventricle, those from the ventral
nucleus forming the trapezoid body in the ventral part of the pons. Most
of these efferent fibres terminate in nuclei associated with the trapezoid
body, either on the same or the opposite side, and then ascend in the lateral
lemniscus to the inferior colliculus and the medial geniculate body; from
the former, fibres reach the motor nuclei of the cranial nerves and form the
pathway of auditory reflexes; from the latter, fibres sweep laterally in
the auditory radiation to the auditory cortex in the superior temporal gyrus
(Fig. 247).
The vestibular fibres (concerned with equilibrium) enter the medulla just
medial to the cochlear division and terminate in the vestibular nuclei. Many
of the efferent fibres from these nuclei pass to the cerebellum in the inferior
cerebellar peduncle together with fibres bypassing the vestibular nuclei
and passing directly to the cerebellum.
Other vestibular connections are to the nuclei of III, IV, VI and XI and to
the upper cervical cord (via the vestibulospinal tract). These connections
bring the eye and neck muscles under reflex vestibular control.