Brainstem and Cerebellum
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Direct answer
Ten of the twelve cranial nerves emerge from the brainstem, and their levels map the stem from above downwards: third and fourth in the midbrain (the fourth the only nerve to emerge dorsally, decussating in the superior medullary velum), fifth to eighth in the pons with sixth, seventh and eighth crowded at the pontomedullary junction, and nine to twelve on the medulla. The named crossed syndromes sell the same map clinically — Weber's third nerve palsy with contralateral hemiplegia, Millard-Gubler's sixth and seventh with contralateral weakness, and the lateral medullary syndrome of Wallenberg. The cerebellum, connected by the inferior, middle and superior peduncles, runs on a single output neuron: the Purkinje cell, inhibitory and GABA-ergic, acting through the deep nuclei, and because its fibres cross twice — spinocerebellar input and cerebellothalamic output — cerebellar signs appear ipsilaterally.
What you must remember
- Midbrain landmarks: superior colliculus with the third nerve nucleus and the Edinger-Westphal nucleus, inferior colliculus with the fourth nerve nucleus emerging below; the cerebral peduncles ventrally, substantia nigra between peduncle and tegmentum, red nucleus in the tegmentum.
- Facial colliculus: the abducens nucleus with facial nerve fibres looping over it forms the swelling in the floor of the fourth ventricle — a lesion here gives ipsilateral lateral rectus palsy with failure of conjugate gaze toward that side.
- Medulla landmarks: pyramids (corticospinal fibres decussating caudally), olives, gracile and cuneate tubercles; the ninth, tenth and eleventh nerves share the nucleus ambiguus of the lateral medulla; the area postrema on the floor of the fourth ventricle is the chemoreceptor trigger zone outside the blood-brain barrier.
- Wallenberg (lateral medullary) syndrome: occlusion of the posterior inferior cerebellar artery or vertebral artery — ipsilateral facial pain and temperature loss (spinal trigeminal tract), nucleus ambiguus signs of dysphagia and hoarseness, vertigo, ipsilateral Horner syndrome and cerebellar ataxia, with contralateral loss of body pain and temperature from the lateral spinothalamic tract.
- Named midbrain syndromes: Weber — ipsilateral third nerve palsy with contralateral hemiplegia (crus involvement); Benedikt — third nerve palsy with contralateral tremor from red nucleus involvement.
- Cerebellar Organisation: flocculonodular lobe and vermis for equilibrium and truncal control (midline lesions produce gait ataxia — the paediatric medulloblastoma pattern); hemispheres for limb coordination; deep nuclei from lateral to medial — dentate, emboliform and globose (interposed), fastigial.
- Peduncle traffic: inferior peduncle mainly input (spinocerebellar and olivocerebellar fibres), middle peduncle purely input from the contralateral pontine nuclei, superior peduncle the output highway of the dentatorubrothalamic pathway.
- Signs of hemisphere lesions: dysmetria, dysdiadochokinesia, intention tremor, nystagmus with the fast phase toward the lesion, scanning dysarthria and hypotonia — all ipsilateral to the lesion.
Decoding a lateral medullary infarct
An elderly hypertensive develops sudden vertigo, hiccups, a hoarse voice and unsteadiness, and finds he cannot feel the right side of his face while his left trunk is numb to pinprick. The crossed sensory dissociation is the signature: the spinal trigeminal tract and nucleus serving the right face and the lateral spinothalamic fibres from the left body — already crossed in the cord — lie side by side in the lateral medulla, so one infarct strikes both. The hoarseness and dysphagia localise to the nucleus ambiguus; the vertigo to vestibular nuclei; the ipsilateral Horner syndrome to descending sympathetic fibres; the ataxia to the inferior cerebellar peduncle. The vessel is the posterior inferior cerebellar artery or its parent vertebral. Set beside it the medial medullary syndrome of the anterior spinal artery — ipsilateral twelfth nerve palsy with contralateral weakness — and the whole medullary map has been examined at the bedside.
Crossing points examiners probe
Three facts anchor the screening questions here. The trochlear nerve is the only cranial nerve to emerge from the dorsal aspect of the brainstem, the only one to decussate completely, and therefore supplies the contralateral superior oblique — a triple one-liner NBE enjoys. The facial colliculus is composite, so a sixth nerve palsy with a facial weakness on the same side means a pontine lesion, not two peripheral nerve diseases. And the ipsilaterality of cerebellar signs — explained by the double crossing of input and output — is tested as a laterality question, where candidates trained on pyramidal crossing predict the wrong side.
Frequently asked questions
Which cranial nerve emerges from the dorsal aspect of the brainstem?
The trochlear (fourth) nerve, which decussates in the superior medullary velum and supplies the contralateral superior oblique.
What forms the facial colliculus?
The abducens nucleus with facial nerve fibres looping over it, in the floor of the fourth ventricle; lesions cause ipsilateral lateral rectus palsy with conjugate gaze palsy and facial weakness.
Which artery is occluded in lateral medullary (Wallenberg) syndrome?
The posterior inferior cerebellar artery or the vertebral artery, producing ipsilateral facial sensory loss, nucleus ambiguus signs, Horner syndrome and ataxia with contralateral body pain and temperature loss.
What is the sole output neuron of the cerebellar cortex?
The Purkinje cell, inhibitory and GABA-ergic, projecting to the deep nuclei whose fibres leave through the superior peduncle.
Why are cerebellar signs ipsilateral to the lesion?
Because cerebellar input crosses to and output crosses back from the contralateral cortex and the deep nuclei exit through crossed pathways, netting one functional side.