The Medulla

The medulla is the caudal-most division of the brainstem, the transition zone between the spinal cord and the rest of the brain. It is small — about 3 cm long — but it contains the centers controlling the most fundamental functions of life: breathing, heart rate, blood pressure, swallowing. Lesions of the medulla can be immediately fatal if they involve these centers. The medulla also houses four cranial nerve nuclei (IX, X, XI, XII), the principal motor decussation of the corticospinal tract, and the sensory decussation of the medial lemniscus. The lateral medullary syndrome (Wallenberg) is perhaps the most pathognomonic stroke syndrome in clinical neurology — once you know the constellation, it is unmistakable.

This page covers medullary anatomy at cross-section, the cranial nerve nuclei and the long tracts, and the major syndromes that arise from medullary lesions.

External Anatomy

The medulla connects the pons (above) to the spinal cord (below). External landmarks on the ventral surface:

• Pyramids: paired longitudinal ridges on either side of the ventral midline, formed by the corticospinal tracts.
• Pyramidal decussation: at the cervicomedullary junction, where the corticospinal tracts cross to the opposite side.
• Olives: prominent oval swellings lateral to the pyramids, formed by the inferior olivary nuclei.
• CN XII (hypoglossal): exits between the pyramid and the olive (preolivary sulcus).
• CN IX, X, XI rootlets: emerge in a vertical row lateral to the olive (postolivary sulcus).

External landmarks on the dorsal surface:

• Gracile tubercles: small swellings formed by the nuclei gracilis.
• Cuneate tubercles: small swellings formed by the nuclei cuneatus.
• Obex: the inferior point of the fourth ventricle.
• Hypoglossal trigone and vagal trigone: small triangular elevations on the floor of the fourth ventricle marking the underlying nuclei.

Internal Architecture by Level

The medulla is conventionally divided into:

• Closed (lower) medulla: where the central canal is still closed, similar to the spinal cord.
• Open (upper) medulla: where the central canal expands into the floor of the fourth ventricle.

Closed (Lower) Medulla

Below the obex. Key structures:

• Nuclei gracilis and cuneatus: dorsally, receiving the dorsal column fibers from the cord. Second-order neurons project across the midline as internal arcuate fibers to form the medial lemniscus on the opposite side — the sensory decussation of the dorsal column–medial lemniscus pathway.
• Pyramidal decussation: ventrally, where the corticospinal fibers cross to the opposite side and enter the lateral corticospinal tract of the cord. About 85% of fibers cross here; the remaining 15% descend uncrossed as the anterior corticospinal tract.
• Spinal trigeminal nucleus and tract: laterally, extending down from the pons into the upper cervical cord.
• Spinothalamic tract: more anteriorly placed in the lateral medulla.

Open (Upper) Medulla

Above the obex, with the floor of the fourth ventricle dorsally. Key structures:

• CN XII (hypoglossal) nucleus: in the dorsomedial floor, just lateral to the midline.
• Dorsal motor nucleus of CN X (vagus): just lateral to the hypoglossal nucleus, in the floor of the fourth ventricle. Provides parasympathetic outflow to thoracic and abdominal viscera.
• Solitary nucleus and tract: more laterally. Receives visceral sensory input from CN VII, IX, X (including taste in its rostral portion and visceral sensation in its caudal portion).
• Nucleus ambiguus: more ventrolaterally, near the lateral reticular formation. Contains motor neurons for muscles of the pharynx, larynx, and palate, providing motor output for CN IX, X, and the cranial portion of XI.
• Vestibular nuclei: laterally, near the floor of the fourth ventricle. Receive input from CN VIII and project widely.
• Cochlear nuclei: at the lateral pontomedullary junction.
• Inferior cerebellar peduncle: emerging dorsolaterally to enter the cerebellum.
• Inferior olivary nucleus: a prominent folded gray matter structure in the ventrolateral medulla. Major input to the cerebellum via the inferior cerebellar peduncle; receives input from the red nucleus, the spinal cord, and the central tegmental tract.
• Spinal trigeminal nucleus and tract: continuing through the lateral medulla.
• Spinothalamic tract: in the lateral medulla.
• Medial lemniscus: after decussating in the lower medulla, ascends in the medial paramedian region.
• Medial longitudinal fasciculus (MLF): near the midline dorsally.
• Pyramid (corticospinal tract): in the ventral midline, before the decussation.
• Reticular formation: throughout the medullary tegmentum.

The Cardiovascular and Respiratory Centers

The medulla contains the centers controlling vital functions:

• Respiratory centers: in the dorsal and ventral medulla. The dorsal respiratory group (in the solitary nucleus) generates the basic rhythm of respiration; the ventral respiratory group modulates it. The pre-Bötzinger complex (in the rostral ventrolateral medulla) is thought to be the principal rhythm generator.
• Cardiovascular centers: integration of input from baroreceptors (via the carotid sinus and aortic arch through CN IX and X), and output through the dorsal motor nucleus of the vagus (parasympathetic) and through the rostral ventrolateral medulla (sympathetic).
• Swallowing centers: in the nucleus ambiguus and adjacent reticular formation.
• Vomiting center: in the dorsal medulla, with input from the area postrema (a circumventricular organ on the floor of the fourth ventricle).

Damage to these centers can be immediately life-threatening. Bilateral medullary infarction or hemorrhage often produces death from respiratory failure.

The Inferior Olive

The inferior olivary nucleus is unique among brainstem nuclei. It sends climbing fibers to the contralateral cerebellar Purkinje cells — one of the two principal cerebellar inputs (the other being mossy fibers, mostly from the pontine nuclei). Each Purkinje cell receives input from only one climbing fiber, but that single fiber synapses extensively along the Purkinje cell’s dendritic tree, producing a powerful “complex spike” response. The climbing fiber system is thought to provide error signals for motor learning, modifying Purkinje cell responses to mossy fiber input.

Lesions involving the central tegmental tract (which carries input from the red nucleus to the inferior olive) produce hypertrophy of the inferior olive — a peculiar enlargement of the deafferented nucleus rather than the more typical atrophy. This may be associated with palatal myoclonus, rhythmic involuntary movements of the soft palate (and sometimes adjacent muscles) that persist during sleep — one of the few movement disorders that does not stop in sleep.

The Area Postrema

A small structure on the floor of the fourth ventricle at the level of the obex. One of the circumventricular organs lacking a blood-brain barrier. Serves as the chemoreceptor trigger zone for emesis — sensing circulating emetic stimuli (drugs, toxins, cytokines) and inducing vomiting. The pharmacological basis of many antiemetic drugs (5-HT3 antagonists, dopamine antagonists, neurokinin-1 antagonists) targets this region.

The area postrema is also involved in the regulation of fluid balance and is a site of pathology in neuromyelitis optica spectrum disorder (NMOSD), where it can produce intractable vomiting and hiccups as initial symptoms.

Vascular Supply

The medulla receives blood from:

• Vertebral arteries: paramedian and lateral branches supplying most of the medulla.
• Anterior spinal artery: a small branch from each vertebral, joining in the midline to supply the medial medulla (pyramids, medial lemniscus, hypoglossal nucleus).
• Posterior inferior cerebellar artery (PICA): from the vertebral, supplying the lateral medulla, the inferior cerebellum, and adjacent regions.

Medullary Syndromes

Lateral Medullary Syndrome (Wallenberg, PICA Syndrome)

The classical medullary stroke syndrome, from occlusion of the vertebral artery (more common than PICA itself, which often comes from the proximal vertebral). The lateral medullary infarct involves multiple structures:

The combination — ipsilateral facial sensory loss + contralateral body sensory loss + ipsilateral Horner + ipsilateral cerebellar signs + bulbar findings — is essentially pathognomonic. The corticospinal tract is not affected (it is in the medial medulla), so there is no hemiparesis. Vertebral artery dissection should always be considered, especially in young patients with neck pain.

Medial Medullary Syndrome (Dejerine Syndrome)

Infarction of the medial medulla from paramedian vertebral or anterior spinal artery occlusion:

• Contralateral hemiparesis sparing the face (corticospinal tract before decussation; corticobulbar to face nuclei has already left).
• Contralateral loss of vibration and joint position sense (medial lemniscus).
• Ipsilateral tongue weakness with atrophy and fasciculations (CN XII nucleus).

The combination of contralateral hemiparesis sparing the face plus ipsilateral tongue weakness is characteristic. Recognition matters because the face-sparing hemiparesis can be mistaken for a cervical cord lesion if the tongue is not examined.

Hemimedullary Syndrome (Babinski-Nageotte)

Extensive medullary infarction involving both medial and lateral territories combines features of Wallenberg and medial medullary syndromes. The patient has all the Wallenberg features plus contralateral hemiparesis sparing the face.

Bilateral Medial Medullary Syndrome

Rare syndrome from bilateral vertebral or anterior spinal artery disease. Quadriparesis with face sparing, bilateral dorsal column sensory loss, bilateral tongue weakness. Can be life-threatening from respiratory involvement.

Anterior Spinal Artery Syndrome (Medullary Level)

Occlusion of the anterior spinal artery at the medullary level produces the medial medullary syndrome plus the spinal anterior cord syndrome below.

Neuromyelitis Optica Spectrum Disorder (NMOSD) Area Postrema Syndrome

Inflammation of the area postrema in NMOSD produces intractable vomiting, hiccups, and nausea. Sometimes this presents as the first manifestation of NMOSD, before other characteristic features (optic neuritis, transverse myelitis). MRI shows T2 hyperintensity in the area postrema. The diagnosis is suggested by aquaporin-4 antibody positivity.

🔍 Did You Know?

The Wallenberg syndrome, described by Adolf Wallenberg in 1895, is often called the most clinically distinctive stroke syndrome in neurology because the constellation of findings — once you know it — is essentially diagnostic. The combination of ipsilateral facial pain/temperature loss, contralateral body pain/temperature loss, ipsilateral Horner syndrome, ipsilateral cerebellar ataxia, and bulbar dysfunction with dysphagia in a patient without hemiparesis (because the corticospinal tract is in the medial medulla, away from the lesion) is essentially pathognomonic. The cause is most often vertebral artery atherosclerosis in older patients and vertebral artery dissection in younger ones. The clinical sign that should always raise the question of vertebral artery dissection is neck pain or a history of cervical trauma — even apparently trivial trauma like a chiropractic manipulation or a bumpy bus ride.

Pitfalls and Pearls

• The medulla houses four cranial nerve nuclei: IX (nucleus ambiguus, solitary, salivatory), X (dorsal motor nucleus, ambiguus, solitary), XI (cranial portion in ambiguus), XII (hypoglossal).
• The pyramidal decussation is at the cervicomedullary junction, with about 85% of corticospinal fibers crossing.
• The sensory decussation (internal arcuate fibers) is in the lower medulla, where the dorsal column nuclei project to the contralateral medial lemniscus.
• The lateral medullary (Wallenberg) syndrome is one of the most distinctive in neurology: ipsilateral face + Horner + ataxia + bulbar findings, contralateral body sensory loss, no hemiparesis.
• Always consider vertebral artery dissection in young patients with Wallenberg, especially with neck pain or trauma history.
• The medial medullary syndrome produces face-sparing hemiparesis plus ipsilateral tongue weakness. Easy to miss if the tongue is not examined.
• The medulla contains the centers for breathing and cardiovascular control. Bilateral medullary lesions can be immediately fatal.
• The area postrema is the chemoreceptor trigger zone for vomiting. Many antiemetics work here.
• Palatal myoclonus reflects denervation hypertrophy of the inferior olive after central tegmental tract injury. Persists during sleep.
• NMOSD area postrema syndrome produces intractable vomiting and hiccups, sometimes as the first manifestation.

References

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3. Caplan LR. Caplan’s Stroke: A Clinical Approach. 5th ed. Cambridge University Press; 2016.
4. Wingerchuk DM, Banwell B, Bennett JL, et al. International consensus diagnostic criteria for neuromyelitis optica spectrum disorders. Neurology. 2015;85(2):177-189.
5. Wallenberg A. Acute Bulbäraffection (Embolie der Art. cerebelli post inf sinistr?). Arch Psychiatr Nervenkr. 1895;27:504-540.
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