Brainstem

Last updated: July 19, 2023

Summary

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.

Overview

Structures (superior to inferior)
Embryogenesis
- Mesencephalon → midbrain
- Rhombencephalon
  - Metencephalon → pons and cerebellum
  - Myelencephalon → medulla oblongata
Composition
- Nuclei of CN III–XII
- Major ascending (sensory) and descending (motor) pathways
- Reticular formation (responsible for alertness)
Function
- Regulation of awareness and sleep cycle
- Regulation of autonomic functions (e.g., circulation, respiration, gastrointestinal activity, salivation, lacrimation)
- Conduction
  - Ascending sensory tracts via cranial nerves (face) and spinal cord (rest of the body)
  - Descending motor tracts from forebrain and cerebellum to neck and face (via cranial nerves) and the rest of the body (via spinal cord)
  - Local pathways (e.g., visual and auditory reflexes)

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. Regions of the brain with embryological derivation Brainstem and diencephalon Brainstem and cerebellar peduncles Cranial nerve nuclei

Midbrain

Characteristics

Location: rostral to the pons, ventral to the cerebellum, caudal to the thalamus
Function
- Processing of auditory and visual information
- Coordination of eye movement
- Modulation of extrapyramidal motor function
- Conduction of fiber tracts
- Nociception
- Arousal and alertness
Composition: three main components
Embryogenesis: derived from mesencephalon

Components of the midbrain

Overview of the midbrain
Structure	Location	Components	Function/Characteristics
Tectum	Posterior to the cerebral aqueduct Forms the roof of the midbrain	Superior colliculi	Relay voluntary and involuntary eye movements (e.g., saccades) Facilitate optic reflexes Receive input from the retina and the pretectal area Project to the lateral geniculate bodies
Tectum		Inferior colliculi	Part of the auditory pathway Receive input from the lateral lemniscus Project to the medial geniculate bodies
Tegmentum	Extends from midbrain to pons Between the tectum and the base of the midbrain	Sensory pathways and nuclei of oculomotor nerve (CN III) and trochlear nerve (CN IV) Mesencephalic nucleus of trigeminal nerve (CN V) Substantia nigra (see below) Red nucleus (see below)	Involved in modulation of extrapyramidal motor function
Substantia nigra	Within the tegmentum	Pars compacta: contains dopaminergic neurons with high levels of melanin Pars reticularis: contains mainly GABAergic neurons	Involved in initiating and coordinating movements Receives inhibitory input (GABA) from the striatum Projects dopaminergic fibers to the striatum (↑ motor function) Projects GABAergic fibers to motoric nuclei (e.g., ventral lateral, ventral anterior) of the thalamus (↓ motor function) For more information on the basal ganglia, see also “Basal ganglia” in “The cerebral cortex, meninges, basal ganglia, and ventricular system.” For information on associated disorders, see “Parkinson disease” and “Parkinson-plus syndromes.”
Red nucleus	Within the tegmentum, at the level of the oculomotor nerve nucleus	Caudal magnocellular part Rostral parvocellular part	Coordination of movement and gait Preservation of muscular tone Receives and relays input from the cerebral cortex and the cerebellum Origin of the rubrospinal tract and rubroolivary tracts
Crura cerebri	Form the base of the midbrain	Corticospinal tracts Corticopontine tracts Corticobulbar tracts	Voluntary movement of the contralateral side of the body Coordination of planned movement Voluntary movement of muscles of the face and neck For more information on spinal cord tracts, see “Spinal cord tracts and reflexes.”
Cerebellar peduncles	Arise at the lateral ventral aspect of the pons	Superior cerebellar peduncles Middle cerebellar peduncles Inferior cerebellar peduncles	Relay motor planning signals from the cerebellum Relay proprioceptive signals to the cerebellum Connection of Pons to cerebellum Cerebellum to midbrain Cerebellum to medulla and spinal cord For more information on the cerebellar peduncles, see “Cerebellar peduncles” in “Cerebellum.”
Pretectal area (pretectum)	Dorsal side of the midbrain in close proximity to the tectum Between thalamus and superior colliculus	Seven nuclei (e.g., olivary pretectal nucleus, optic tract nucleus)	Part of the subcortical visual system Modulates behavior in reaction to light (e.g., pupillary reflex and accommodation) Receives input from the retina and occipital cortical fields Projects to the Edinger-Westphal nucleus and the superior colliculus
Cerebral aqueduct	Between the tegmentum and the tectum Surrounded by periaqueductal gray matter	Cerebrospinal fluid	Connects the 3^rd and 4^th ventricles Stenosis of this structure causes non-communicating hydrocephalus
Periaqueductal gray matter	Surrounds the cerebral aqueduct	Dorsal tegmental nucleus Dorsal raphe nucleus	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	Found on each side near the midline ventral to the fourth ventricle (in the medulla and pons) and cerebral aqueduct (in the midbrain)	White matter fiber bundles (ascending and descending)	Coordinate eye movement Highly myelinated (to enable fast coordinated eye movements) Connect CN III, CN IV, and CN VI Lesions lead to internuclear ophthalmoplegia Common site of involvement in multiple sclerosis (esp. bilateral lesions)
Central tegmental tract	From the midbrain to the inferior olivary nucleus in the medulla	Ascending reticulothalamic fibers Descending rubroolivary fibers Taste fibers ascending from solitary nucleus to the VPM nucleus of thalamus	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 of the midbrain

Tectum: posterior cerebral artery
Tegmentum
- Posterolateral part: superior cerebellar artery
- Posteromedial part: penetrating interpeduncular branches of the basilar artery
- Anterior part: penetrating peduncular branches of the posterior cerebral artery
Crura cerebri
- Lateral parts posterior cerebral artery
- Middle parts penetrating peduncular branches of the posterior cerebral artery
- Medial parts posterior choroidal artery (arising from the P2 segment of the posterior cerebral artery)

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

Brainstem and diencephalon Brainstem and cerebellar peduncles Mesencephalon (midbrain) Pupillary light reflex Brainstem and ventricular system Cortico-basal ganglia-thalamo-cortical loop (CBGTC) Brainstem and origin of cranial nerves II-XII

Pons

Characteristics

Location
- Rostral to the medulla, caudal to the midbrain
- Connected to the cerebellum via the middle cerebellar peduncles
Function
- Conduction of nerve fibers and tracts (e.g., transmission of pain, temperature, proprioception, and auditory signals)
- Regulation of autonomic functions
Composition: divided into 2 parts (dorsal to ventral)
- Pontine tegmentum: contains CN nuclei
  - Trigeminal (CN V)
  - Abducens (CN VI)
  - Facialis (CN VII)
  - Vestibulocochlear (CN VIII)
  - For information on the cranial nerves, see “Cranial nerve types and functions” in “Cranial nerve palsies.”
- Basilar pons: contains relay nuclei (e.g., pontine nuclei) and nerve tracts
Embryogenesis: rhombencephalon → metencephalon → pons and cerebellum

Components of the pons

Overview of the pons
Structure	Location	Components	Function
Cochlear nuclei	Dorsolateral aspect of the pons	Ventral cochlear nuclei Dorsal cochlear nuclei	Part of the auditory pathway (second-order neuron) Receive auditory signals from the cochlear branch of the vestibulocochlear nerve (CN VIII) via special somatic afferent fibers Fibers project to the ipsilateral and contralateral lateral lemniscus via acoustic striae
Trapezoid body	Ventral aspect of the pons, posterior to the pyramidal tracts Formed by a decussation of the ventral cochlear fibers	Medial lemniscus (see below) Fibers of CN VI Acoustic striae: axons originating from the cochlear nuclei	Part of the auditory pathway Sound localization
Superior olivary nuclei	At the level of the trapezoid body	Brainstem nuclei	Part of the auditory pathway (third-order neuron) First major point of integration of auditory information from both ears Relay auditory signals from the cochlear nuclei Join the lateral lemniscus (see below) and synapse on the neurons of the inferior colliculus (fourth-order neurons).
Corticopontine tract	Along the pons	Frontopontine fibers Parietopontine fibers Temporopontine fibers Occipitopontine fibers	Part of the corticopontocerebellar pathway Synapses in the pontine nuclei
Pontine nuclei	Basilar pons	Projection neurons GABAergic interneurons	Part of the corticopontocerebellar pathway Modulation of movement Relay cerebellar signals
Locus coeruleus	Posterior portion of the rostral pons	Melanin-containing nuclei	Part of the reticular activating system Site of norepinephrine production Projects axons to numerous parts of the brain
Medial lemniscus	Medial aspect of the midbrain, pons, and cranial medulla	Formed by the decussation of internal arcuate fibers: myelinated axons that arise from the gracile and cuneate nucleus	Pathway for proprioception Transmits vibration and fine touch Projects to: ventral posterolateral nucleus of the thalamus
Lateral lemniscus	From trapezoid body to nucleus of inferior colliculus	Axons from the cochlear nuclei and superior olive	Part of the auditory pathway Projects to the inferior colliculus
Spinal lemniscus	Dorsolateral aspect of basilar pons	Ascending afferent nerve fibers	Part of the spinothalamic tract Nociception and heat sensation

Blood supply of the pons

The main part of the pons is supplied by pontine arteries (penetrating branches of the basilar artery).
The smaller, upper portion of the pons receives its blood supply from the superior cerebellar artery and the anterior inferior cerebellar artery.
For information on the clinical relevance, see “Pontine syndromes” in “Overview of stroke.”

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

Pons (transverse plane) Brainstem and diencephalon Brainstem and origin of cranial nerves II-XII Brainstem and cerebellar peduncles

Medulla oblongata

Characteristics

Location
- Rostral to the spinal cord, caudal to the pons
- Connected to the cerebellum via the inferior cerebellar peduncles
Function
- Regulation of autonomic functions (e.g., circulation, respiration, gastrointestinal activity)
- Contains centers for swallowing, sneezing, coughing, and vomiting
- Conduction of motor and sensory information
Composition
- CN nuclei
  - Part of the sensory trigeminal nerve nucleus (CN V)
  - Glossopharyngeal (CN IX)
  - Vagus (CN X)
  - Accessorius (CN XI)
  - Hypoglossal (CN XII)
  - For information on the cranial nerves, see “Cranial nerve types and functions” in “Cranial nerve palsies.”
- Fissures
  - Anterior median fissure: extends along the entire length of the medulla
  - Ventrolateral sulcus (preolivary sulcus)
    - Separates the olives from the pyramids
    - Contains rootlets of CN XII
  - Posterolateral sulcus: contain rootlets of CN IX–XI
- Pyramids
- Area postrema
- A number of brainstem nuclei (e.g., nucleus gracilis, nucleus cuneatus, inferior olivary nucleus)
Embryogenesis: myelencephalon → medulla oblongata

Components of the medulla oblongata

Overview of the medulla oblongata
Structure	Location	Fiber tracts and course	Function
Gracile nucleus and cuneate nucleus	At the junction between the spinal cord and the medulla oblongata in the dorsal column	Contain second-order neurons of the dorsal column-medial lemniscus pathway Axons from these neurons form the internal arcuate fibers in the pons	Both nuclei relay proprioception and fine touch from first-order neurons located in the dorsal root ganglia Gracile nucleus: lower trunk and legs Cuneate nucleus: upper trunk and arms
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	Contain the motor fibers of the corticospinal and corticobulbar tracts	Voluntary motor control
Accessory cuneate nucleus	Lateral to the cuneate nucleus	Contains second-order neurons of the cuneocerebellar tract Fibers travel to the cerebellum via the inferior cerebellar peduncle	Proprioception (upper extremities)
Inferior olivary nucleus	Forms a swelling on the anterior surface of the medulla oblongata, lateral to both pyramids, called olivary bodies (or olives)	Receives input from various sources including the red nucleus, cerebral cortex, and spinal cord Axons travel to cerebellum via ascending fibers in the contralateral inferior cerebellar peduncle	Relays motor and sensory information to facilitate motor coordination
Lateral reticular nucleus	Located at the caudal, ventrolateral medulla oblongata	Receives input from the spinal cord, the cerebral cortex, the red nucleus, and superior colliculus Fibers travel to the cerebellum via the inferior cerebellar peduncle	Relay nucleus for cerebellar signals Integration motor information Adjustment and regulation of motor activity
Arcuate nucleus (medulla)	Anterior to the pyramids	Receives input from the corticospinal tract Forms the arcuatocerebellar tract of the striae medullares (in the rhomboid fossa) Fibers travel to the cerebellum via the inferior cerebellar peduncle	Chemoreception Breathing control (e.g., modulation of respiratory rate)
Solitary nucleus	Located within the dorsomedial medulla	The solitary tract contains axons of the cranial nerves that provide input to the solitary nucleus: General visceral afferent signals from the glossopharyngeal (CN IX) and vagus nerves (CN X) Special visceral afferent signals from the facial (CN VII), glossopharyngeal (CN IX), and vagus nerves (CN X) Projects ipsilateral fibers to the posteromedial nucleus of the thalamus and the pons via the central tegmental tract Connected to the area postrema	Relay nucleus for general visceral and taste afferents Blood pressure homeostasis (mediates the baroreceptor reflex)
Area postrema (vomiting center)	Located anterior to the 4^th ventricle on the floor of the medulla	Receives input from the: Nucleus tractus solitarius (main coordinator) GI tract via vagus nerve Vestibular system CNS	Chemoreception Control of autonomic functions (e.g., blood pressure, appetite) Triggers vomiting Contains the chemoreceptor trigger zone, a region with no blood-brain barrier and, accordingly, in direct contact with the bloodstream that can trigger vomiting when stimulated by certain circulating substances (e.g., toxins, hormones) Receptors Muscarinic (M1) Histamine (H1) Neurokinin (NK–1) Dopamine (D2) Serotonin (5–HT3) Antagonism of these receptors is used to treat: Chemotherapy-induced vomiting (NK–1, D2, and 5–HT3) Motion sickness (M1 and H1) Hyperemesis gravidarum (H1)

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	Anterior median fissure Pyramids	Posterior median fissure Gracile fasciculus (posterior) and nucleus (anterior) Cuneate fasciculus (posterior) and nucleus (anterior)	Spinal nucleus of the trigeminal nerve (CN V) Posterior spinocerebellar tract Lateral and anterior spinothalamic tracts (spinal lemniscus)	Central canal Decussation of pyramids Medial longitudinal fasciculus (lateral to the decussation of pyramids)
Decussation of medial lemnisci	Anterior median fissure Pyramids			Central canal Hypoglossal nucleus (CN XII) Medial longitudinal fasciculus (medial to the decussation of medial lemnisci) Internal arcuate fibers (decussation of medial lemnisci) Inferior olivary nucleus (lateral to the decussation of medial lemnisci)
Olives	Pyramidal tracts Hypoglossal nerve (CN XII) rootlet Olivary complex	4^th ventricle Nuclei Hypoglossal nerve (CN XII) Dorsal nucleus of vagus nerve (CN X) Solitary nucleus Medial and inferior vestibular nuclei (CN VIII) Posterior cochlear nucleus (CN VIII)	Inferior cerebellar peduncle Anterior cochlear nucleus (CN VIII) CN rootlets: Vagus nerve (CN X) Glossopharyngeal nerve (CN IX, superior to CN X) Spinal nucleus of the trigeminal nerve Nucleus ambiguus Anterior and lateral spinocerebellar tracts	From posterior to anterior Medial longitudinal fasciculus Tectospinal tract Medial lemniscus

Blood supply of the medulla oblongata

Anteromedial part: anterior spinal artery
Posterolateral part
- Posterior inferior cerebellar artery (branch of the vertebral artery) at the level of the olives (rostral part of the medulla)
- Posterior spinal artery at the level of pyramidal decussation (caudal part of the medulla)
Lateral part: penetrating branches of the vertebral arteries
Clinical significance: medullary syndromes, bulbar palsy

Cranial nerve nuclei of the medulla oblongata Brainstem and diencephalon Brainstem and origin of cranial nerves II-XII Brainstem and cerebellar peduncles Medulla oblongata: olivary complex

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