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Brainstem

Last updated: May 4, 2021

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The brainstem is the caudal part of the brain and consists of the midbrain, pons, and medulla oblongata. The brainstem regulates autonomic functions (respiration, circulation, lacrimation, salivation), controls visual and auditory reflexes, and maintains vigilance. It is also a hub through which run all ascending sensory pathways, descending motor pathways, and other local pathways of the central nervous system.

Each of the 3 parts of the brainstem contains CN nuclei: 2 (CN III and IV) in the midbrain, 4 (CN V–VIII) in the pons, and 4 (CN IX–XII) in the medulla oblongata.

4 CN nuclei are medial: CN III, IV, VI, and XII.

Characteristics

Components of the midbrain

Overview of the midbrain
Structure Location Components Function/Characteristics
Tectum
  • Superior colliculi
  • Inferior colliculi
Tegmentum
  • Involved in modulation of extrapyramidal motor function
Substantia nigra
Red nucleus
Crura cerebri
Cerebellar peduncles

Pretectal area (pretectum)

Cerebral aqueduct
Periaqueductal gray matter
  • Part of the antinociceptive system
  • Learning of defensive/aversive behavior
  • Response to perceived threat (e.g., increased alertness, initiation of escape actions)
Medial longitudinal fasciculus
Central tegmental tract
  • Coordination of movement
  • Relay of taste fibers

The inferior colliculi are the part of auditory pathway while the superior colliculi are the part of the visual pathway: ears are below the eyes.

Blood supply

For information on the clinical relevance of the blood supply, see “Midbrain syndromes” in “Overview of stroke.”

Characteristics

Components of the pons

Overview of the pons
Structure Location Components Function
Cochlear nuclei
  • Dorsolateral aspect of the pons
Trapezoid body

Superior olivary nuclei

Corticopontine tract
  • Along the pons
  • Frontopontine fibers
  • Parietopontine fibers
  • Temporopontine fibers
  • Occipitopontine fibers
Pontine nuclei
  • Basilar pons
Locus coeruleus
Medial lemniscus
Lateral lemniscus
Spinal lemniscus
  • Dorsolateral aspect of basilar pons
  • Ascending afferent nerve fibers

Blood supply

Pontine stroke can cause ipsilateral loss of function of the facial and abducens nerves with contralateral hemiparesis (Millard-Gubler syndrome or Foville syndrome).

Characteristics

Components of the medulla oblongata

Overview of the medulla oblongata
Structure Location Fiber tracts and course Function

Gracile nucleus and cuneate nucleus

Pyramids (brainstem)

  • Form the base of the medulla
  • Pyramidal decussation: crossing of the fibers of the corticospinal tract from one side to the contralateral one, which occurs at the most caudal part of the medulla
  • Voluntary motor control

Accessory cuneate nucleus

Inferior olivary nucleus
  • Relays motor and sensory information to facilitate motor coordination
Lateral reticular nucleus
  • Relay nucleus for cerebellar signals
  • Integration motor information
  • Adjustment and regulation of motor activity

Arcuate nucleus (medulla)

Solitary nucleus

  • Located within the dorsomedial medulla

Area postrema

(vomiting center)

  • Chemoreception
  • Control of autonomic functions (e.g., blood pressure, appetite)
  • Triggers vomiting

The gracilis muscle is a muscle of the lower body, the gracile nucleus relays fine touch and proprioception from the lower body.

Cross-sectional layers of the medulla oblongata

Overview of cross-sectional layers in the medulla oblongata
Level (from inferior to superior) Anterior (from medial to lateral) Posterior (from medial to lateral) Lateral (from posterior to anterior) Central (from posterior to anterior)
Decussation of pyramids
Decussation of medial lemnisci
Olives

Blood supply