Brainstem Anatomy

• The brainstem is divided transversely into the midbrain, pons and medulla
• All the cranial nerve nuclei save I and II lie in the brainstem
• Midbrain:
  • III (oculomotor)
  • Pre-tectal and Edinger-Westphal nucleus (consensual light reflex)
    • II → pre-tectal nucleus → projections to bilateral Edinger-Westphal nuclei (parasympathetic nuclei of III) → ciliary ganglion → pupil constriction
  • IV (trochlear), the only cranial nerve which decussates
• Pons
  • V (trigeminal)
  • VI (abducens)
  • VII (facial)
  • VIII (vestibulocochlear)
• Medulla
  • IX (glossopharyngeal)
  • X (vagus)
  • XI (spinal accessory)
  • XII (hypoglossal)
• The midline structures of the brainstem are:
  • Motor (pure) nuclei: III, IV, VI and XII
  • Medial longitudinal fasciculus
  • Motor (corticospinal) tract, which decussates in the lower medulla
  • Medial lemniscus (proprioception and vibration), which decussates in the middle medulla
• The lateral structures of the brainstem are:
  • Sensory nucleus of Vth nerve
  • Spinothalamic tract (pain and temperature, decussates immediately in lower cord)
  • Spinocerebellar tract (proprioception from limbs to cerebellum), ipsilateral (does not cross)
  • Sympathetic pathway, ipsilateral

Differential Diagnosis of Brainstem Syndromes

• Vascular (ischaemic / haemorrhagic stroke)
• Inflammatory (e.g. demyelination in multiple sclerosis, acute disseminated encephalomyelitis)
• Brainstem encephalitis (Bickerstaff’s encephalitis)
• Central pontine myelinolysis
• Trauma
• Neoplastic (brainstem gliomas / ependymomas)
• Arnold-Chiari malformation
• Syringobulbia