Describe the parts of the brainstem and the arrangement of the cranial nerve nuclei. Add a note on the brainstem syndromes.

Parts of the Brainstem

• Rule of four — four nuclei in the midbrain, four in the pons, four in the medulla; and the motor nuclei whose number divides into 12 (III, IV, VI, XII) lie medially, the rest laterally

Midbrain

• Divided by the cerebral aqueduct into the tectum behind and the cerebral peduncles in front
• Tectum — the corpora quadrigemina: the superior colliculi (visual reflexes) and inferior colliculi (auditory relay)
• Peduncle = crus cerebri + substantia nigra + tegmentum
• Contains the red nucleus, substantia nigra, oculomotor and trochlear nuclei, and the Edinger–Westphal nucleus
• The trochlear is the only cranial nerve to emerge from the dorsal surface, and the only one to cross completely

Pons and Medulla

• The medulla contains the vital centres — respiratory, cardiovascular and vomiting — in the reticular formation; this is why medullary compression is rapidly fatal

Functional Columns of Cranial Nerve Nuclei

Brainstem Syndromes

• The rule of crossed findings — a lesion causing cranial nerve signs on one side and long tract signs on the other is in the brainstem, and the cranial nerve involved gives the level

The Fourth Ventricle and the Reticular Formation

• The fourth ventricle is diamond-shaped, between the pons and medulla in front and the cerebellum behind; its floor is the rhomboid fossa
• Landmarks on the floor — the facial colliculus (the abducent nucleus with the facial nerve looping over it), the hypoglossal and vagal trigones, and the stria medullares
• Reticular formation — a diffuse network through the whole brainstem tegmentum
• Its ascending reticular activating system maintains consciousness and arousal; damage to it causes coma
• Its descending parts modulate muscle tone, pain (the descending inhibitory pathway), respiration and cardiovascular control

Applied Aspects

• Wallenberg syndrome is the commonest brainstem stroke, and its dissociated sensory loss — face on one side, body on the other — is entirely explained by the position of the spinal nucleus of V and the spinothalamic tract
• "Locked-in" syndrome — a ventral pontine lesion; the patient is conscious and can move only the eyes vertically, since the upper midbrain is spared
• Brainstem death is diagnosed by the absence of brainstem reflexes — pupillary, corneal, oculocephalic, vestibulo-ocular, gag and respiratory
• Acoustic neuroma at the cerebellopontine angle involves VIII, then V and VII, and later the cerebellum — giving deafness, a lost corneal reflex and ipsilateral ataxia
• Uncal herniation compresses the third nerve against the tentorium, giving a fixed dilated pupil on the side of the lesion — an urgent sign of a mass
• Pseudobulbar palsy — a bilateral upper motor neurone lesion of the corticobulbar tracts; the tongue is small and spastic, the jaw jerk brisk, with emotional lability — the opposite of the wasted fasciculating tongue of a bulbar palsy
• Medial longitudinal fasciculus lesion gives internuclear ophthalmoplegia, with failure of adduction and nystagmus in the abducting eye — classic in multiple sclerosis
• The brainstem is small but densely packed, so a lesion of a few millimetres produces a named syndrome, and the combination of signs localises it precisely
• Bulbar palsy from motor neurone disease or brainstem stroke causes nasal speech, nasal regurgitation and aspiration — the airway must be protected
• Basilar artery thrombosis carries a very high mortality, since the brainstem has little collateral supply