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Structure

Brainstem

Not medically reviewed · Content last updated · 2 min read ·

This is the date the draft was written, not a medical review date.

The brainstem comprises the midbrain, pons and medulla oblongata. It carries all ascending and descending tracts between the cerebrum, cerebellum and spinal cord, contains the nuclei of cranial nerves III to XII, and houses centres for consciousness, breathing and cardiovascular control.

Terminologia Anatomica (Latin): Truncus encephali · Open in the 3D atlas

Location

In the posterior cranial fossa, continuing the spinal cord at the foramen magnum and lying anterior to the cerebellum and the fourth ventricle; the midbrain passes through the tentorial notch.

Structure

Midbrain: tectum (colliculi), tegmentum and cerebral peduncles, with nuclei of CN III and IV. Pons: basilar part and tegmentum, with nuclei of CN V to VIII. Medulla: pyramids, olives and nuclei of CN IX to XII (the spinal accessory nucleus lies in the upper cervical cord). The reticular formation runs through all three.

Blood supply

The vertebral arteries unite to form the basilar artery; paramedian, short circumferential and long circumferential branches supply the brainstem. PICA supplies the lateral medulla, AICA the lower lateral pons, and the superior cerebellar and posterior cerebral arteries supply the midbrain.

Innervation

Connections: corticospinal and corticobulbar tracts descend through the cerebral peduncle, basilar pons and pyramids; the medial lemniscus and spinothalamic tract ascend; cranial nerve nuclei send and receive fibres; cerebellar peduncles link it to the cerebellum.

Function

It conducts motor and sensory pathways, controls eye movements, facial and bulbar functions, hearing and balance relays, and contains the reticular activating system for arousal, together with respiratory and cardiovascular centres in the pons and medulla.

Clinical correlations

Brainstem strokes produce crossed signs (ipsilateral cranial nerve deficit with contralateral long-tract signs). Examples: Weber syndrome (midbrain; ipsilateral CN III palsy with contralateral hemiparesis) and lateral medullary (Wallenberg) syndrome (PICA or vertebral territory; ipsilateral facial pain and Horner syndrome with contralateral loss of pain and temperature in the body). Basilar occlusion is life-threatening.

Common exam points

  • Three parts: midbrain, pons, medulla; nuclei of CN III-XII (CN I and II are not brainstem nerves).
  • CN III and IV at midbrain, V-VIII at pons, IX-XII at medulla (XI from upper cervical cord).
  • Crossed syndromes: ipsilateral cranial nerve sign plus contralateral long-tract signs.
  • Weber: midbrain, ipsilateral CN III palsy and contralateral hemiparesis.
  • Wallenberg: lateral medulla (PICA), ipsilateral face and contralateral body pain and temperature loss.
  • Blood supply: vertebral and basilar arteries; basilar occlusion is an emergency.

References

  1. Kandel ER, Koester JD, Mack SH, Siegelbaum SA. Principles of Neural Science. McGraw-Hill, 6th edition (2021) — Tier 3 (textbook)
  2. Haines DE, Mihailoff GA. Fundamental Neuroscience for Basic and Clinical Applications. Elsevier, 5th edition (2018) — Tier 3 (textbook)
  3. Drake RL, Vogl AW, Mitchell AWM. Gray's Anatomy for Students. Elsevier — Tier 3 (textbook)

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