Brainstem Regions: Medulla, Pons, and Midbrain

• The brainstem serves as a connection point between the spinal cord caudally and the diencephalon rostrally, interconnecting to the cerebrum, and cerebellum.
• The brainstem handles information for the head, face and visceral organs and carries out functions essential for survival.
• Continuation of the 3 body pathways studied in the spinal cord (ALS, PC/ML, and voluntary motor pathway) are apparent in the brainstem in addition to nuclei and tracts associated with cranial nerves.
• Embryologically, the brainstem originates from the midbrain (mesencephalon) and hindbrain (rhombencephalon) enlargements after closure of the superior neuropore.
• The hindbrain divides into the metencephalon (pons and cerebellum) and the myelencephalon (medulla).
• Note that the cerebellum is not considered part of the brainstem itself, but is part of the metencephalon.
• Each of the 3 major areas of the brainstem has some gross structures associated with it on the external surface with transverse sections containing intrinsic structures, nuclei and tracts.

Brainstem Regions

• Medulla oblongata is the most caudal portion.
• Pons is the middle portion.
• Midbrain is the most rostral portion.
• The midbrain and pons contain the basilar part anteriorly
• The midbrain and pons contain the tegmentum posterior to the basilar zone.
• The midbrain has an additional zone called the tectum, meaning roof.
• The medulla is referred to as anterior or posterior.
• The cerebral aqueduct runs through the midbrain.
• The 4th ventricle is located between the cerebellum and the pons and rostral medulla.
• Zones or regions exist within the brainstem.

Reticular Formation

• The Reticular formation has no distinct boundaries, consisting of a network of nuclei and neurons (reticular and raphe nuclei)
• The reticular formation integrates information and relays it through neural pathways for survival functions.
• The term Reticular is derived from “reticulum”, which is Latin for “little net”.
• It spans the brainstem, contains over 100 individual nuclei and is located within the Raphe nuclei.
• The Raphe nuclei, placed bilaterally to the midline, contain serotonin (5 HT), enkephalin, and CCK.
• These help block pain information transmission to the cortex.
• The reticular formation controls vital functions like heart and respiratory rate, pain modulation, habituation, coordination of somatic motor movements, sleep-wake cycles, circadian rhythm, consciousness, and arousal.
• Ventral lateral reticular areas of the medulla help regulate heart rate and respiration.
• The ascending reticular activating system (ARAS) projects from the reticular formation to subcortical or cortical areas, and set levels of alertness, arousal, consciousness, sleep-wake cycles, and circadian rhythm.
• The ARAS is also responsible for habituation and contains dopaminergic, noradrenergic, serotonergic, histaminergic, cholinergic, and glutamatergic brain nuclei which project to the thalamus and cerebral cortex, particularly the prefrontal cortex.
• The lateral hypothalamus regulates the ARAS.
• Damage to the ARAS results in reduced consciousness and coma, with bilateral lesions at the midbrain level potentially causing death.
• Compression of the medulla can result in central apnea and possibly death.

Nuclei of Cranial Nerves

• The posterior view of the brainstem highlights the cranial nerve nuclei and tracts.
• Because some of these nuclei are very long, several of the same nuclei and tracts will be seen over several sections, i.e., the spinal trigeminal nucleus and tract.
• The roman numeral of the cranial nerve associated with each division of the brainstem decreases as we move caudal to rostral, i.e., from medulla to midbrain.

External Anatomy of Medulla Oblongata

• The spinal cord is caudal to and the pons is rostral to the medulla
• The foramen magnum is at the medulla's caudal most portion.
• The cranial nerves/nuclei associated with the medulla are XII, XI, X, IX, VIII, and part of V.
• The fourth ventricle is posterior to the rostral medulla.
• The central canal runs through the caudal medulla and ascending and descending tracts are traveling through the medulla.
• Anterior external features include the pyramids, olives (inferior olivary eminence), and cranial nerves VI and XII.
• Posterior external features include:
• Gracile tubercles - medial bumps which contain cell bodies which are part of a somatosensory pathway called the posterior columns/medial lemniscus
• Cuneate tubercles - lateral bumps which contain cell bodies of the posterior columns/medial lemniscus pathway
• Lateral external area: Inferior cerebellar peduncle, cerebellopontine angle, roots of cranial nerves VII, VIII, IX, X, and XI.
• The cerebellopontine angle is clinically important for acoustic neuromas tumors that affect CN VIII and eventually CN VII.
• Jugular foramen syndrome indicate damage to CN IX, X, XI and adjacent tumors may involve CN XII.

Internal Anatomy of the Medulla: Caudal Medulla Level of the Motor Decussation

• There are 4 classic transverse sections which highlight various sensory, motor, and autonomic nuclei and tracts
• Myelin-stained sections
  • bright areas represents nuclei
  • dark areas represent bundles of axons
• Previous structures seen in the spinal cord include
  • Fasciculus cuneatus - sensory axons relaying tactile, vibratory sense from the upper half or the body
  • Fasciculus gracilis - Sensory axons relaying tactile, vibratory sense from the lower half of the body
  • Nucleus cuneatus and nucleus gracilis are starting to show up in this section, but we will see them more prominently in the next section.
  • Anterolateral system (ALS) - contains the spinothalamic axons that are relaying pain and temp info from the contralateral body to the thalamus.
• Anterior & Posterior spinocerebellar tracts - relay muscle proprioceptive information to cerebellum
• New structures at the level of the motor decussation
  • Motor (pyramidal) decussation - axons of descending upper motor neurons in the corticospinal tract cross over to the contralateral side and become the lateral corticospinal tracts
  • Spinal trigeminal tract - axons relaying sensory information (pain and temperature) from the face
• Spinal trigeminal nucleus - The spinal trigeminal tract axons synapse upon the spinal trigeminal nucleus and pain and temp info from the face will be relayed to cortical areas.

Internal Anatomy of the Medulla: Caudal Medulla Level of the Sensory Decussation

• Previous structures include the spinal trigeminal tract and nucleus, the anterior and posterior spinocerebellar tracts, and the ALS-anterolateral system
• New structures include:
  • Nucleus Cuneatus - contains cell bodies innervated by axons in the fasciculus cuneatus, one of the two second order neurons in the PC / ML pathway, carrying tactile, vibratory sensation from T6 and above (“upper half of body"). Axons of neurons in the n. cuneatus travel through as the Internal Arcuate Fibers and cross to the contralateral side and both change their name to medial lemniscus which carries tactile, vibratory sensation from the body to the cortex.
  • Nucleus Gracilis – contains cell bodies innervated by axons in the fasciculus gracilis which carry tactile, vibratory sensation from T6 and below (“lower half of body"). Axons of n. gracilis neurons join those of n. cuneatus to form the medial lemniscus.
  • Pyramids - corticospinal axons descend through here

Internal Anatomy of the Medulla: Mid-Medulla Level of Cranial Nuclei IX, X, XII, Open Medulla

• Previous structures include the spinal trigeminal nucleus and tract, the anterolateral system, the anterior spinocerebellar tract, the medial Lemniscus and the Pyramids
• New structures at this level:
  • Hypoglossal nucleus - cell bodies of CN XII that innervate the ipsilateral tongue muscles
  • Hypoglossal nerves - exiting between the pyramid and Olive
  • Dorsal Motor nucleus of the Vagus - preganglionic parasympathetic cell bodies for CN X
  • Solitary nucleus and solitary tract - receives sensory input from CN VII, IX & X
  • MLF - medial longitudinal fasciculus - coordinates left and right eye movements; projects to cervical and upper thoracic spinal cord to control head-neck movements in order to coordinate head-eye movements
  • NA - nucleus ambiguus - cell bodies of the part of CN IX & X whose axons innervate Larynx and pharynx muscles
  • Inferior cerebellar peduncle - originate in the posterior spinocerebellar tract and will enter the cerebellum
  • Inferior Olivary nucleus - cell bodies receive multi-modal information and projects to the cerebellum

Internal Anatomy of the Medulla: Rostral Medulla Level of Cranial Nuclei VIII

• Old structures at this level: MLF, medial longitudinal fasciculus, spinal trigeminal nucleus and tract, NA, Nucleus ambiguus, ALS, anterolateral system, inferior cerebellar peduncle, anterior spinocerebellar tract, inferior Olivary nucleus, and pyramids
• New structures at this level:
  • Inferior and Medial Vestibular nuclei - cell bodies that are part of CN VIII vestibular portion
  • Dorsal and ventral cochlear nuclei - cell bodies that are part of CN VIII auditory portion

Blood Supply to the Medulla

• At levels of Caudal and Mid-medulla, the primary arteries perfusing the medulla include:
  • Posterior inferior cerebellar artery (PICA)
  • Vertebral artery (VA)
  • Anterior spinal artery (ASA)
• The anterior inferior cerebellar artery supplies blood to the rostral medulla
• A bleed or clot in the PICA leads to PICA syndrome or Wallenberg syndrome

Summary

• The brainstem is composed of the medulla, pons, and midbrain
• The brainstem consists of anterior and posterior zones in the medulla, basilar and tegmentum zones in the pons, and the tectum in the midbrain.
• The reticular formation regulates alertness, arousal, consciousness, sleep-wake cycles, and circadian rhythm
• The medulla is externally distinguished by the pyramids, olive, gracile and cuneate tubercles, and inferior cerebellar peduncle, cerebellopontine angle, roots of cranial nerves VII, VII, IX, X, XI with the 4th ventricle
• The cranial nerves and/or their nuclei associated with the medulla are XII, XI, X, IX, VIII, and part of V.
• The primary arteries perfusing the medulla are the posterior inferior cerebellar artery (PICA), VA – vertebral artery, ASA – anterior spinal artery, and AICA - anterior inferior cerebellar artery.