Brainstem | Anatomy2Medicine
brainstem function

Brainstem

    • The Brainstem
      • includes the medulla, pons, and mesencephalon (midbrain).
      • gives rise to cranial nerve (CN) III to CN XII.

 

  • receives its blood supply from the vertebrobasilar system.
  • Medullar Oblongata (Myelencephalon)
  • contains autonomic centers that regulate respiration, circulation, and gastrointestinal motility.

 

      • gives rise to CN IX to CN XII.
      • The nuclei of CN V and CN VIII extend caudally into the medulla.

 

  • is connected to the cerebellum by the inferior cerebellar peduncle.
  • Ascending sensory pathways and relay nuclei
  • Fasciculus gracilis and fasciculus cuneatus

 

          • convey dorsal column modalities.
          • terminate in the nucleus gracilis and nucleus cuneatus.

 

  • Nucleus gracilis and nucleus cuneatus

 

          • contain second-order neurons of the dorsal column–medial lemniscus pathway.
          • give rise to internal arcuate fibers.

 

  • project via the medial lemniscus to the ventral posterolateral nucleus of the thalamus.
  • Internal arcuate fibers

 

          • arise from the nucleus gracilis and nucleus cuneatus
          • form the contralateral medial lemniscus.

 

  • Decussation of the medial lemniscus

 

          • is formed by decussating internal arcuate fibers.

 

  • Medial lemniscus

 

          • conveys dorsal column modalities to the ventral posterolateral nucleus.

 

  • Spinal lemniscus

 

          • contains the lateral and ventral spinothalamic tracts and the spinotectal tract.

 

  • Descending motor pathways
  • Pyramidal decussation
  • is located at the spinomedullary junction

 

          • consists of crossing corticospinal fibers.

 

  • Pyramids
  • constitute the base of the medulla.

 

          • contain uncrossed corticospinal fibers.

 

  • Cerebellar pathways and relay nuclei
  • Accessory (lateral) cuneate nucleus

 

          • contains second-order neurons of the cuneocerebellar tract.
          • projects to the cerebellum via the inferior cerebellar peduncle.

 

  • Inferior olivary nucleus

 

          • underlies the olive.

 

  • is a cerebellar relay nucleus that projects olivocerebellar fibers via the inferior
  • cerebellar peduncle to the contralateral cerebellar cortex and cerebellar nuclei.

 

          • receives input from the red nucleus.

 

  • Central tegmental tract

 

          • extends from the midbrain to the inferior olivary nucleus
          • contains rubro-olivary and reticulothalamic fibers.
          • contains taste fibers.

 

  • Lateral reticular nucleus

 

          • is a cerebellar relay nucleus
          • projects via the inferior cerebellar peduncle to the cerebellum.

 

  • Arcuate nucleus
  • Dorsal spinocerebellar tract

 

          • mediates unconscious proprioception from the lower extremities to the cerebellum
          • via the inferior cerebellar peduncle.

 

  • Ventral spinocerebellar tract

 

          • mediates unconscious proprioception from the lower extremities to the cerebellum
          • via the superior cerebellar peduncle.

 

  • Inferior cerebellar peduncle

 

          • connects the medulla to the cerebellum.
      • Cranial nerve nuclei and associated tracts

 

  • Medial longitudinal fasciculus (MLF)
  • contains vestibular fibers of CN VIII
  • coordinate eye movements via CN III, IV, and CN VI.

 

          • mediates nystagmus and lateral conjugate gaze.

 

  • Solitary tract

 

          • receives general visceral afferent (GVA) input from CN IX and CN X.

 

  • receives special visceral afferent (SVA) (taste) input from CN VII, CN IX, and CN
  • Solitary nucleus

 

          • projects GVA and SVA input ipsilaterally via the central tegmental tract to the

 

  • parabrachial nucleus of the pons and to the posteromedial nucleus of the thalamus.
  • Dorsal motor nucleus of CN X
  • gives rise to vagal preganglionic parasympathetic general visceral efferent (GVE) fibers that synapse in the terminal (intramural) ganglia of the thoracic and abdominal viscera.
  • Inferior salivary nucleus of CN IX
  • gives rise to preganglionic parasympathetic (GVE) fibers that synapse in the otic ganglion.
  • Hypoglossal nucleus of CN XII
  • gives rise to general somatic efferent (GSE) fibers that innervate the intrinsic and extrinsic muscles of the tongue.
  • Nucleus ambiguus of CN IX, CN X, and CN XI
  • represents a special visceral efferent (SVE) cell column whose axons innervate pharyngeal arch muscles of the larynx and pharynx.

 

          • These fibers contribute to parts of CN IX, CN X, and CN XI

 

  • Ventral horn of CN XI

 

          • is located at the level of the pyramidal decussation.
          • contains motor neurons of the spinal accessory nerve.

 

  • Spinal trigeminal tract

 

          • replaces the dorsolateral tract of Lissauer.

 

  • contains first-order neuron general somatic afferent (GSA) fibers that mediate pain, temperature, and light touch sensations from the face via CN V, CN VII, CN IX, and CN X.

 

          • projects to the spinal trigeminal nucleus.

 

  • Spinal trigeminal nucleus

 

          • replaces the substantia gelatinosa of the spinal cord.
          • gives rise to decussating axons that form the ventral trigeminothalamic tract.
          • This tract terminates in the ventral posteromedial nucleus of the thalamus.

 

  • Inferior and medial vestibular nuclei of CN VIII

 

          • receives proprioceptive (special somatic afferent [SSA]) input from the semicircular ducts, utricle, saccule, and cerebellum
          • project to the cerebellum and MLF.

 

  • Area postrema
  • lies rostral to the obex in the floor of the fourth ventricle.

 

          • is a circumventricular organ with no blood–brain barrier.

 

  • Pons

 

      • consists of

 

  • base

 

          • contains corticobulbar, corticospinal, and corticopontine tracts and pontine nuclei
        • tegmentum,
          • contains cranial nerve nuclei, reticular nuclei

 

  • major ascending sensory pathways.
  • is connected to the cerebellum by the middle cerebellar peduncle.
  • contains auditory relay nuclei and vestibular nuclei
  • vestibular nuclei regulate postural mechanisms and vestibulo-ocular reflexes.

 

      • contains, in its caudal portion, the facial motor nucleus of CN VII
        • it innervates the muscles of facial expression.
      • contains, in the mid pons, the trigeminal motor nucleus of CN V
        • its axons innervate the muscles of mastication.
      • contains a center for lateral gaze.
      • gives rise to CN V to VIII.

 

  • Ascending sensory pathways and relay nuclei
  • Dorsal and ventral cochlear nuclei
  • receive auditory input from the cochlea through SSA fibers via the cochlear branch of CN VIII.

 

          • are auditory relay nuclei

 

  • give rise to the ipsilateral and contralateral lateral lemniscus.
  • Trapezoid body

 

          • is formed by decussating fibers of the ventral cochlear nuclei.

 

  • contains the acoustic striae, medial lemnisci, exiting abducent (CN VI) fibers, and
  • aberrant corticobulbar fibers.
  • Superior olivary nucleus

 

          • is an auditory relay nucleus at the level of the trapezoid body. •
          • receives input from the cochlear nuclei.
          • contributes bilaterally to the lateral lemniscus.

 

  • Lateral lemniscus
  • is a pontine auditory pathway extending from the trapezoid body to the nucleus of the inferior colliculus.

 

          • conducts a preponderance of contralateral cochlear input.

 

  • Medial lemniscus
  • mediates contralateral dorsal column modalities to the ventral posterolateral nucleus of the thalamus.
  • Spinal lemniscus
  • contains lateral and ventral spinothalamic tracts and the spinotectal tract.

 

      • Descending motor pathways (base of the pons)

 

  • Corticobulbar tract

 

          • synapses in the motor nuclei of the cranial nerves except in the ocular motor nuclei of CN III, IV, and VI.

 

  • Corticospinal tract (pyramidal tract)
  • synapses in the ventral horn of the spinal cord.
  • Corticopontine tract

 

          • synapses in the pontine nuclei

 

  • Cerebellar pathways and relay nuclei
  • Central tegmental tract

 

          • extends from the midbrain to the inferior olivary nucleus.

 

  • contains rubro-olivary and reticulothalamic fibers.
  • Juxtarestiform body
  • forms part of the inferior cerebellar peduncle.
  • contains vestibulocerebellar, cerebellovestibular, and cerebelloreticular fibers.
  • Middle cerebellar peduncle
  • contains pontocerebellar fibers.
  • connects the pons to the cerebellum.
  • Superior cerebellar peduncle

 

          • connects the cerebellum to the pons and midbrain.

 

  • contains the dentatorubrothalamic fibers and the ventral spinocerebellar tract.
  • Pontine nuclei

 

          • are cerebellar relay nuclei in the base of the pons.

 

  • give rise to pontocerebellar fibers that constitute the middle cerebellar peduncle.
  • Cranial nerve nuclei and associated tracts
  • Dorsal and ventral cochlear nuclei of CN VIII

 

          • are found at the medullopontine junction.
          • Medial, lateral, and superior vestibular nuclei of CN VIII

 

  • receive proprioceptive (SSA) input from the semicircular ducts, utricle, saccule, and cerebellum.

 

          • project to the cerebellum and the MLF.

 

  • The lateral vestibular nucleus gives rise to the lateral vestibulospinal tract.
  • Medial longitudinal fasciculus

 

          • contains vestibular fibers of CN VIII
          • coordinate eye movements via CN III, CN IV, and CN VI.
          • mediates nystagmus and lateral conjugate gaze.

 

  • Abducent nucleus of CN VI
  • underlies, in the caudal medial pontine tegmentum, the facial colliculus of the rhomboid fossa.

 

          • projects exiting fibers through the trapezoid body

 

  • gives rise to GSE fibers that innervate the lateral rectus muscle.
  • gives rise to fibers that project via the MLF to the contralateral medial rectus subnucleus of the oculomotor nucleus of CN III.
  • is the pontine center for lateral conjugate gaze, which receives commands from the contralateral frontal eye field (area 8).
  • It innervates via the MLF the contralateral medial rectus muscle and via abducent fibers the ipsilateral lateral rectus muscle to execute conjugate lateral gaze.
  • Facial nucleus of CN VII

 

          • gives rise to SVE fibers that innervate the muscles of facial expression.

 

  • receives bilateral input for upper facial muscles and contralateral input for lower facial muscles.
  • contains neurons that project axons dorsomedially, encircle the abducent nucleus as a genu
  • Superior salivatory nucleus of CN VII

 

          • includes the lacrimal nucleus.

 

  • gives rise to GVE preganglionic parasympathetic fibers that synapse in the pterygopalatine and submandibular ganglia.
  • Spinal trigeminal tract and nucleus of CN V
  • Motor nucleus of CN V

 

          • lies in the lateral midpontine tegmentum
          • lies medial to the principal sensory nucleus of the trigeminal nerve.
          • receives bilateral corticobulbar input.
          • gives rise to SVE fibers that innervate muscles of mastication.

 

  • Principal sensory nucleus of CN V

 

          • lies lateral to the motor nucleus of CN V.
          • receives discriminative tactile and pressure sensation input from the face.

 

  • gives rise to trigeminothalamic fibers that join the contralateral ventral trigeminothalamic tract.

 

          • gives rise to the uncrossed dorsal trigeminothalamic tract, which terminates in the ventral posteromedial nucleus of the thalamus.

 

  • Mesencephalic nucleus and tract of CN V

 

          • extend from the upper pons to the upper midbrain.
          • contain pseudounipolar neurons.
          • receive input from muscle spindles and pressure receptors (muscles of mastication and extraocular muscles).

 

  • Locus ceruleus

 

          • is a melanin-containing nucleus in the pons and midbrain.
          • is an important nucleus of the monamine system that projects noradrenergic axons to all parts of the central nervous system (CNS).

 

  • Mesencephalon (Midbrain)

 

      • mediates auditory and visual reflexes.
      • contains the oculomotor nerve (CN III) and the trochlear nerve (CN IV), which innervate the extraocular muscles of the eye.
      • contains a center for vertical conjugate gaze in its rostral extent.
      • contains the substantia nigra,
        • the largest nucleus of the midbrain;
        • degeneration of this extrapyramidal motor nucleus results in Parkinson disease.
      • contains the paramedian reticular formation
        • lesions of this formation result in coma.
      • gives rise to two cranial nerves: CN III (oculomotor) and CN IV (trochlear).
      • consists dorsoventrally of three parts

 

  • the tectum
  • the tegmentum
  • the base (basis pedunculi)

 

      • Structures of the midbrain

 

  • Tectum

 

          • is located dorsal to the cerebral aqueduct.

 

  • forms the roof of the midbrain

 

          • including the superior and inferior colliculi.

 

  • Tegmentum

 

          • is located between the tectum and the base (basis pedunculi).
          • contains cranial nerve nuclei and sensory pathways.

 

  • Basis pedunculi (crus cerebri)

 

          • forms the base of the midbrain
          • contains corticospinal, corticobulbar, and corticopontine tracts.

 

  • Pedunculus cerebri (cerebral peduncle)

 

          • includes the tegmentum and basis pedunculi.

 

  • Pretectum (pretectal area)

 

          • is located between the superior colliculus and the habenular trigone

 

  • Inferior collicular level of the midbrain
  • Inferior colliculus
  • Nucleus of the inferior colliculus
  • is an auditory relay nucleus that receives binaural input from the lateral lemniscus.

 

          • projects to the medial geniculate body via the brachium of the inferior colliculus.

 

  • Commissure of the inferior colliculus

 

          • interconnects the inferior collicular nucleus and its opposite partner

 

  • Brachium of the inferior colliculus
  • conducts auditory information from the inferior collicular nucleus to the medial geniculate body.
  • Lateral lemniscus

 

          • projects binaural auditory information to the inferior collicular nucleus.

 

  • Cerebral aqueduct
  • is located between the tectum and tegmentum.

 

          • is surrounded by the periaqueductal gray matter.
          • interconnects the third and fourth ventricles.

 

  • blockage (aqueductal stenosis) results in hydrocephalus.
  • Periaqueductal gray matter

 

          • is the central gray matter that surrounds the cerebral aqueduct
          • contains several nuclear groups.

 

  • Locus ceruleus
  • Mesencephalic nucleus and tract
  • Dorsal tegmental nucleus
  • contains enkephalinergic neurons that play a role in endogenous pain control.
  • Dorsal nucleus of raphe

 

          • contains serotonergic neurons.

 

  • Trochlear nucleus of CN IV

 

          • gives rise to GSE fibers, which encircle the periaqueductal gray matter
          • decussate in the superior medullary velum
          • exit the midbrain from its dorsal aspect
          • innervate the superior oblique muscle.

 

  • Medial longitudinal fasciculus

 

          • contains vestibular fibers that coordinate eye movements.

 

  • interconnects the ocular motor nuclei of CN III, CN IV, and CN VI.
  • Decussation of the superior cerebellar peduncles

 

          • is most conspicuous structure of this level.

 

  • Interpeduncular nucleus

 

          • receives input from the habenular nuclei via the habenulointerpeduncular tract (fasciculus retroflexus of Meynert).

 

  • Substantia nigra

 

          • is divided into the dorsal pars compacta, which contains large pigmented (melanin) cells, and the ventral pars reticularis.
          • receives gamma-aminobutyric acid–ergic (GABA-ergic) input from the caudatoputamen (striatonigral fibers).

 

  • projects dopaminergic fibers to the caudatoputamen (nigrostriatal fibers).

 

          • projects nondopaminergic fibers to the ventral anterior nucleus, ventral lateral nucleus, and mediodorsal nucleus of the thalamus (nigrothalamic fibers).

 

  • Medial lemniscus
  • mediates dorsal column modalities to the ventral posterolateral nucleus.
  • Spinal lemniscus

 

          • contains the lateral and ventral spinothalamic tracts and the spinotectal tract.

 

  • Central tegmental tract

 

          • contains rubro-olivary and reticulothalamic fibers.

 

  • Basis pedunculi (crus cerebri)

 

      • Superior collicular level of the midbrain

 

  • Superior colliculus
  • receives visual input from the retina and from frontal (area 8) and occipital (area 19) eye fields.

 

          • receives auditory input from the inferior colliculus to mediate audiovisual reflexes.

 

  • is concerned with detection of movement in visual fields, thus facilitating visual orientation, searching, and tracking.
  • Commissure of the superior colliculus

 

          • interconnects the two superior colliculi.

 

  • Brachium of the superior colliculus

 

          • conducts retinal and corticotectal fibers to the superior colliculus and to the pretectum, thus mediating optic and pupillary reflexes.

 

  • Cerebral aqueduct and periaqueductal gray matter
  • Oculomotor nucleus of CN III
  • gives rise to GSE fibers that innervate four extraocular muscles (medial, inferior, superi- or recti, and inferior oblique) and the superior levator palpebrae.

 

          • projects crossed fibers to the superior rectus.
          • projects crossed and uncrossed fibers to the levator palpebrae

 

  • Edinger-Westphal nucleus of CN III
  • gives rise to GVE preganglionic parasympathetic fibers that terminate in the ciliary ganglion.

 

          • Postganglionic fibers from the ciliary ganglion innervate the ciliary body (accommodation) and the sphincter muscle of the iris (pupillary light reflex).

 

  • Medial longitudinal fasciculus
  • Central tegmental tract

 

          • contains rubro-olivary and reticulothalamic fibers.

 

  • Red nucleus

 

          • is located in the tegmentum at the level of the oculomotor nucleus (the level of the
          • superior colliculus).
          • receives bilateral input from the cerebral cortex.
          • receives contralateral input from the cerebellar nuclei.

 

  • gives rise to the crossed rubrospinal tract.

 

          • gives rise to the uncrossed rubro-olivary tract.
          • exerts facilitatory influence on flexor muscles

 

  • Medial lemniscus
  • Spinal lemniscus
  • Substantia nigra
  • Basis pedunculi (crus cerebri)

 

      • Posterior commissural level (pretectal region)
        • is a transition area between the mesencephalon and the diencephalon

 

  • Posterior commissure

 

          • marks the caudal extent of the third ventricle.
          • marks the rostral extent of the cerebral aqueduct.

 

  • interconnects pretectal nuclei, thus mediating consensual pupillary light reflexes.
  • Pretectal nucleus

 

        • receives retinal input via the brachium of the superior colliculus.
        • projects to the ipsilateral and contralateral Edinger-Westphal nucleus, thus mediating the pupillary light reflexes.

 

 

  • Corticobulbar (Corticonuclear) Fibers

 

      • arise from precentral and postcentral gyri.

 

  • may synapse directly on motor neurons or indirectly via interneurons (corticoreticular fibers).

 

      • innervate sensory relay nuclei (gracile, cuneate, solitary, and trigeminal).

 

  • innervate cranial nerve motor nuclei bilaterally, with the exception of part of the facial nucleus (CN VII).

 

        • The upper face division of the facial nucleus receives bilateral input;
        • the lower face division of the facial nucleus receives only contralateral input
      • innervate the ipsilateral spinal nucleus of CN XI, which supplies the sternocleidomastoid muscle

 

  • innervate the contralateral spinal nucleus of CN XI, which innervates the trapezius muscle.

 

    • The orbicularis oculi muscle receives a variable number of crossed and uncrossed fibers; the paresis therefore varies from patient to patient.