Brainstem Overview

Lubdha M. Shah, MD

IMAGING ANATOMY

Overview

Main components
- Cranial nerve nuclei
- Long tracts
- Cerebellar connections
- Reticular formation
Midbrain
- Rostral limit: Brainstem meets thalamus and hypothalamus at level of tentorium cerebelli
- Tectum “roof”: Superior and inferior colliculi
  - Lies dorsal to cerebral aqueduct
- Tegmentum lies ventral to cerebral aqueduct in midbrain and ventral to 4th ventricle in pons and medulla
- Basis: Most ventral part
  - Location of corticospinal and corticobulbar tracts
Pons
- Pontomesencephalic junction: Midbrain meets pons
- Pontomedullary junction: Pons meets medulla
Medulla oblongata
- Caudal limit of brainstem in cervicomedullary junction at level of foramen magnum and pyramidal decussation

Cranial Nerves

Motor nuclei ventrally located
Sensory nuclei dorsally located
3 longitudinal columns of motor nuclei
- Somatic motor nuclei: Adjacent to midline
  - General somatic efferent
  - Oculomotor (CN3): Rostral midbrain, ventral to periaqueductal gray matter
  - Trochlear (CN4): Caudal midbrain, ventral to periaqueductal gray matter
  - Abducens (CN6): Floor of 4th ventricle in lower pons, helps form facial colliculus
  - Hypoglossal (CN12): Floor of 4th ventricle in medulla
- Brachial motor nuclei
  - Special visceral efferent
  - Trigeminal motor nucleus (CN5): Upper to mid pons, ventral to main sensory trigeminal nucleus
  - Facial nucleus (CN7): Pontine tegmentum, looping fibers of facial colliculus
  - Nucleus ambiguus (CN9, CN10): Runs longitudinally through medulla
  - Spinal accessory nucleus (CN11): Located in upper 5 segments of cervical cord, between dorsal and ventral horns of spinal cord gray matter
- Parasympathetic nuclei
  - General visceral efferent
  - Edinger-Westphal nuclei (CN3): Curve over dorsal and rostral aspect of CN3
  - Superior salivatory nuclei (CN7): Pontine tegmentum
  - Inferior salivatory nuclei (CN9): Pontine tegmentum
  - Dorsal motor nucleus of CN10: Rostral to caudal medulla, lateral to CN12
3 columns of sensory nuclei
- General somatic sensory afferent
  - Trigeminal nuclear complex: Inputs from CN5, CN7, CN9, CN10
  - Runs from midbrain to upper cervical cord
  - Mesencephalic: Proprioception, lateral border of periaqueductal gray matter in midbrain
  - Main sensory: Upper to mid pons, dorsolateral to trigeminal motor nucleus
  - Main sensory subserves fine discriminative touch
  - Spinal trigeminal tract (STT): Lateral pons and medulla, extending to dorsal horn of spinal cord
  - STT subserves pain and temperature sensation for face and body
- Special somatosensory afferent: Inputs for hearing (cochlear) and vestibular sense (vestibular)
  - Many fibers from cochlear nuclei decussate in trapezoid body of caudal pons
  - Some fibers synapse in superior olivary nuclear complex
  - Auditory information ascends in lateral lemniscus to inferior colliculus → medial geniculate nuclei of thalamus → primary auditory cortex
  - 4 vestibular nuclei lie along floor of 4th ventricle in pons and medulla
  - Convey perception of head position and acceleration to cerebral cortex via relays in thalamus
  - Medial and lateral vestibulospinal tracts involved in posture and muscle tone
  - Reciprocal connections with cerebellum, particularly inferior vermis and flocculonodular lobes
- Visceral afferents travel to nucleus solitarius
  - Located lateral to dorsal motor nucleus of CN10
  - Rostral (gustatory nucleus): Special visceral afferents from CN7, CN9, CN10
  - Rostral extension of taste path via central tegmental tract → ventral posterior nucleus of thalamus → parietal operculum and insula
  - Caudal: General visceral afferents from cardiorespiratory and gastrointestinal systems (CN9, CN10)
Medial longitudinal fasciculus (MLF): Ventral to CN3, CN4
- Interconnects CN3, CN4, CN6, CN8: Mediates vestibulo-ocular reflexes

Long Tracts

Corticospinal tract (CST) and corticobulbar tracts travel in middle 1/3 of cerebral peduncles
- CST runs through basis pontis → pyramids in ventral medulla
- Pyramidal decussation occurs at cervicomedullary junction → lateral CST
Posterior columns carry axons subserving vibration, joint position, fine touch → posterior column nuclei
- Nucleus gracilis: Legs
- Nucleus cuneatus: Arms
- Internal arcuate fibers cross and ascend as medial lemniscus → ventral posterior lateral nucleus of thalamus
Anterolateral spinothalamic tract
- Pain, temperature, crude touch
- Decussate in spinal cord
- Descending sympathetic pathway in lateral brainstem: Injury can cause Horner syndrome