Anatomy chapters
Brainstem
The three parts of the brainstem, the cranial nerve nuclei and tracts in each, and the classic crossed syndromes that let you localise a brainstem lesion.
- The brainstem has three parts: midbrain (III, IV), pons (V to VIII) and medulla (IX to XII).
- It carries all the long tracts and the cranial nerve nuclei, so a lesion gives same-side cranial nerve signs with opposite-side limb signs (crossed signs).
- Classic syndromes: Weber (midbrain), medial and lateral medullary (Wallenberg), locked-in (pons).
On this page
Overview
The brainstem joins the cerebrum to the spinal cord. It has three parts, from above down: midbrain, pons and medulla oblongata.
It contains:
- The nuclei of cranial nerves III to XII.
- All the ascending and descending tracts between brain and cord.
- Centres for breathing, heart rate, blood pressure and consciousness (reticular formation).
Small lesions here cause serious and characteristic problems.
Cranial nerves by level
| Level | Cranial nerves attached |
|---|---|
| Midbrain | III (front), IV (back) |
| Pons | V (side of pons) |
| Pontomedullary junction | VI, VII, VIII |
| Medulla | IX, X, XI (between olive and inferior cerebellar peduncle), XII (between pyramid and olive) |
The "rule of 4": 4 cranial nerves above the pons (I–IV), 4 in the pons (V–VIII) and 4 in the medulla (IX–XII); the 4 motor nuclei whose numbers divide evenly into 12 (III, IV, VI, XII) lie near the midline.
The trochlear nerve (IV) is the only cranial nerve that leaves the back of the brainstem, and the only one that crosses to the other side.
Midbrain
External features: the two cerebral peduncles in front, with the oculomotor nerves between them; the superior and inferior colliculi (tectum) behind.
Internal features:
- Crus cerebri: carries the corticospinal and corticobulbar tracts.
- Substantia nigra: dopamine-producing neurons; lost in Parkinson's disease.
- Red nucleus: involved in motor control.
- Cerebral aqueduct, surrounded by periaqueductal grey matter.
- Superior colliculus: visual reflexes. Inferior colliculus: part of the hearing pathway.
- Oculomotor and trochlear nuclei. The Edinger-Westphal nucleus gives parasympathetic fibres to the pupil.
Pons
External features: a bulging front with transverse fibres that pass into the middle cerebellar peduncles. The trigeminal nerve attaches at the side.
Internal features:
- Basilar part (front): corticospinal fibres and the pontine nuclei that relay to the cerebellum.
- Tegmentum (back): nuclei of V, VI, VII, VIII, the medial lemniscus and the spinothalamic tracts.
- Facial colliculus: a bump on the floor of the fourth ventricle made by facial nerve fibres looping around the abducent nucleus.
A lesion at the facial colliculus damages both the abducent nucleus and the facial nerve fibres on that side.
Medulla
External features:
- Pyramids in front (corticospinal fibres), which cross at the pyramidal decussation.
- Olives beside the pyramids (inferior olivary nucleus, linked to the cerebellum).
- Inferior cerebellar peduncles behind and at the sides.
Internal features:
- Nucleus gracilis and cuneatus (dorsal column relay) and the sensory decussation forming the medial lemniscus.
- Nucleus ambiguus: motor to the muscles of the pharynx and larynx (IX, X, XI).
- Nucleus of the solitary tract: taste and visceral sensation.
- Dorsal motor nucleus of the vagus: parasympathetic to the heart, lungs and gut.
- Hypoglossal nucleus.
- Centres for breathing and circulation.
The nucleus ambiguus is motor to the throat muscles; the solitary nucleus is sensory (taste). Do not mix them up.
Reticular formation
A network of neurons running through the core of the brainstem. Its reticular activating system keeps the cortex awake and alert. Damage can cause coma.
It also contains centres that control breathing, heart rate and blood pressure.
Brainstem syndromes
All cause crossed signs: cranial nerve signs on the same side as the lesion, and limb weakness or sensory loss on the opposite side (because the long tracts cross lower down).
| Syndrome | Level and artery | Same side | Opposite side |
|---|---|---|---|
| Weber | Midbrain base (branches of PCA) | Oculomotor palsy (eye down and out, ptosis, dilated pupil) | Hemiplegia (crus cerebri) |
| Benedikt | Midbrain tegmentum | Oculomotor palsy | Tremor and involuntary movements (red nucleus), sensory loss |
| Parinaud | Dorsal midbrain (often a pineal tumour) | Paralysis of upward gaze, pupils react poorly to light | - |
| Medial pontine | Pons (basilar branches) | Abducent palsy (± facial) | Hemiplegia, loss of vibration sense |
| Locked-in | Base of the pons (basilar artery) | - | Paralysis of all four limbs and the lower cranial nerves; patient awake, communicates by vertical eye movements |
| Medial medullary | Anterior spinal / vertebral artery | Tongue deviates to the lesion (XII) | Hemiplegia; loss of vibration and position sense |
| Lateral medullary (Wallenberg) | PICA or vertebral artery | Loss of pain and temperature on the face, Horner's, hoarseness and difficulty swallowing (nucleus ambiguus), ataxia, vertigo | Loss of pain and temperature on the body |
Lateral medullary syndrome is the classic "alternating sensory loss": same-side face, opposite-side body. The motor tracts are usually spared because they lie in front.
Clinical relevance
Brain herniation
How it happens: raised pressure pushes the brain down. The uncus of the temporal lobe can herniate over the tentorium and press on the oculomotor nerve and the midbrain. Tonsils of the cerebellum can herniate through the foramen magnum and press on the medulla.
What you see: first a dilated pupil on the side of the mass, then falling consciousness; tonsillar herniation compresses the breathing centres and can kill.
Parkinson's disease
How it happens: loss of dopamine neurons in the substantia nigra.
What you see: resting tremor, rigidity, slow movement and stooped, shuffling gait. See basal ganglia.
Expect vignettes with crossed signs. Know which nerves arise at each level, the facial colliculus, the nuclei in the medulla (ambiguus vs solitary), and the features and arteries of Weber, medial medullary, lateral medullary and locked-in syndromes.
Quick revision
- Midbrain: III, IV. Pons: V–VIII. Medulla: IX–XII.
- IV: only nerve from the back of the brainstem; crosses.
- Facial colliculus: VII fibres around the VI nucleus.
- Nucleus ambiguus: throat muscles. Solitary nucleus: taste.
- Weber: same-side III palsy, opposite hemiplegia.
- Lateral medullary (PICA): same-side face and opposite-side body pain loss, Horner's, hoarseness.
- Locked-in: basilar artery, vertical eye movements preserved.
Written by The House Job from standard neuroanatomy references (Snell's Clinical Neuroanatomy).
FAQ
What are the parts of the brainstem?
The midbrain, the pons and the medulla oblongata.
What is Weber syndrome?
A midbrain lesion causing an oculomotor nerve palsy on the same side and weakness of the opposite side of the body.
Which artery is blocked in lateral medullary syndrome?
Usually the posterior inferior cerebellar artery (PICA), or the vertebral artery.
What is locked-in syndrome?
Paralysis of all four limbs and the lower cranial nerves from a lesion in the base of the pons, with consciousness and vertical eye movements preserved.