Brainstem Cross Sections
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Direct answer
A brainstem cross-section is identified by its ventral surface, the shape of its central cavity, and the signature structures of each level. The midbrain shows paired crura cerebri with the substantia nigra and a small cerebral aqueduct; the pons shows a bulky basis pontis of transverse fibres with the fourth ventricle behind; the medulla shows pyramids and an olive, the central canal reopening into the fourth ventricle below. Naming the tracts and nuclei at each level is a favourite practical and viva task.
What you must remember
- Midbrain at superior colliculus: cerebral aqueduct in the central grey, oculomotor nucleus with the Edinger-Westphal nucleus beneath it, red nuclei in the tegmentum, substantia nigra, and the crus cerebri divided into medial one-sixth frontopontine, middle three-fifths corticospinal and corticobulbar, lateral one-sixth temporopontine fibres.
- Midbrain at inferior colliculus: trochlear nucleus in the central grey, decussation of the superior cerebellar peduncles filling the tegmentum, lateral lemniscus ending in the inferior colliculus.
- Basis pontis: corticospinal and corticobulbar fibres cut into small bundles among the pontine nuclei; transverse pontocerebellar fibres cross to form the opposite middle cerebellar peduncle.
- Caudal pons tegmentum: facial colliculus (abducens nucleus capped by looping facial fibres) in the ventricular floor, vestibular nuclei lateral to it, spinal nucleus of V ventrally, and the trapezoid body marking the auditory pathway.
- Open (upper) medulla: inferior olivary nucleus inside the olive, pyramid, medial lemniscus beside the midline, spinal nucleus of V laterally, nucleus ambiguus dorsal to the olive, hypoglossal and vagal nuclei in the ventricular floor.
- Closed (lower) medulla: pyramidal decussation ventrally; gracile and cuneate nuclei dorsally, whose internal arcuate fibres cross as the sensory decussation to form the medial lemniscus.
- Syndromes: Weber syndrome at the basis pedunculi — ipsilateral third nerve palsy with contralateral hemiplegia; lateral medullary (Wallenberg) syndrome after posterior inferior cerebellar artery occlusion.
Common confusion
The commonest error is placing a nucleus where its nerve emerges: the facial nucleus lies deep in the pons though the nerve leaves at the pontomedullary junction, and the spinal nucleus of V runs into the upper cervical cord. Keep the medial lemniscus paramedian throughout — dorsal to the pyramids in the medulla, lateral to the midline higher up.
Exam-focused takeaway
In practicals, identify spotter sections by the crus, basis pontis and pyramid, then name the nuclei level by level. Theory papers ask for a labelled transverse section at the inferior colliculus or mid-pons, or the tracts damaged in medial and lateral medullary syndromes. In the viva, trace the medial lemniscus from its formation to the thalamus.
Frequently asked questions
How is a midbrain section recognised at a glance?
By the crus cerebri, substantia nigra and cerebral aqueduct; at superior colliculus level add the red and oculomotor nuclei, at inferior colliculus the decussating superior cerebellar peduncles.
What forms the facial colliculus?
The abducens nucleus in the floor of the fourth ventricle with facial nerve fibres looping over it; a lesion there causes ipsilateral lateral rectus palsy with facial weakness.
Which decussations are seen in the lower medulla?
The pyramidal decussation of corticospinal fibres ventrally, and the sensory decussation of internal arcuate fibres from the gracile and cuneate nuclei dorsally, forming the medial lemniscus.
What is the composition of the crus cerebri?
Medial one-sixth frontopontine, middle three-fifths corticospinal and corticobulbar, and lateral one-sixth temporopontine fibres, with the substantia nigra behind them.