ELEVENTH (ACCESSORY) CRANIAL NERVE
This is a purely motor nerve supplying the stcrnomastoid and trapezius muscles.
The cranial portion of the accessory nerve arises from the lowest part of the nucleus ambiguus in the medulla.
The spinal part arises in the ventral grey matter of the upper five cervical segments, ascends alongside the spinal cord and passes through the foramen magnum. After joining with the cranial portion it exits as the accessory nerve through the jugular foramen.
The supranuclear connections act on the ipsilateral sternomastoid (turning the head to the contralateral side) and on the contralateral trapezius. This results in:
- head turning away from the relevant hemisphere during a seizure
- head turning towards the relevant hemisphere with cerebral infarction.
Unilateral lower motor neurone weakness produces a lower shoulder on the affected side (trapezius) and weakness in turning the head to the opposite side (sternomastoid).
Clinical examination (see page 17) Causes (see page 175)
TWELFTH (HYPOGLOSSAL) CRANIAL NERVE
This is a purely motor nerve which supplies the intrinsic muscles of the tongue.
The nucleus lies in the floor of the IV ventricle and fibres pass ventrally to leave the brain stem lateral to the pyramidal tract.
Since each nucleus is bilaterally innervated, a unilateral supranuclear lesion will not produce signs or symptoms. A bilateral supranuclear lesion results in a thin pointed (spastic) tongue which cannot be protruded.
^Olivary nucleus XII nerve
A lesion of the hypoglossal nerve results in atrophy and deviation of the tongue to the weak side
Clinical examination (see page 18) Causes (see page 175)
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