Brainstem

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  • 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)
    • containsautonomic centers that regulate respiration, circulation, and gastrointestinal motility.
    • gives rise to CN IX to CN XII.
    • The nuclei of CN V and CN VIIIextend caudally into themedulla.
    • is connected to the cerebellum by the inferior cerebellar peduncle.
    • Ascending sensory pathways and relay nuclei
      • Fasciculus gracilis and fasciculus cuneatus
        • conveydorsal column modalities.
        • terminate in the nucleus gracilis and nucleus cuneatus.
      • Nucleus gracilis and nucleus cuneatus
        • containsecond-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
        • conveysdorsal 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.
        • containuncrossed corticospinal fibers.
    • Cerebellar pathways and relay nuclei
      • Accessory (lateral) cuneate nucleus
        • containssecond-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
        • containsrubro-olivary and reticulothalamic fibers.
        • containstaste fibers.
      • Lateral reticular nucleus
        • is a cerebellar relay nucleus
        • projects via the inferior cerebellar peduncle to thecerebellum.
      • Arcuate nucleus
        • Dorsal spinocerebellar tract
        • mediatesunconscious proprioception from the lower extremities to the cerebellum
        • via the inferior cerebellar peduncle.
      • Ventral spinocerebellar tract
        • mediatesunconscious 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)
        • containsvestibular fibers of CN VIII
        • coordinate eye movements via CN III, IV, and CN VI.
        • mediatesnystagmus 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
        • projectsGVA 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) fibersthat 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 andextrinsic 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 ofCN IX, CN X, and CN XI
      • Ventral horn of CN XI
        • is located at the level of the pyramidal decussation.
        • containsmotor 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 substantiagelatinosa of the spinal cord.
        • gives rise to decussating axons that form the ventral trigeminothalamic tract.
        • Thistract terminates in the ventral posteromedial nucleus of the thalamus.
      • Inferior and medial vestibular nuclei of CN VIII
        • receivesproprioceptive (special somatic afferent [SSA]) input from the semicircularducts, 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
        • containscorticobulbar, corticospinal, and corticopontine tracts and pontine nuclei
      • tegmentum,
        • containscranial nerve nuclei, reticularnuclei
      • major ascending sensory pathways.
    • is connected to the cerebellum by the middle cerebellar peduncle.
    • containsauditory 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
      • itinnervates the muscles of facial expression.
    • contains, in the mid pons, the trigeminal motor nucleus of CN V
      • its axons innervate themuscles 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
        • receiveauditory input from the cochlea through SSA fibers via the cochlear branch of CN VIII.
        • areauditory 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
        • aberrantcorticobulbar 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 ofthe inferior colliculus.
        • conducts a preponderance of contralateral cochlear input.
      • Medial lemniscus
        • mediatescontralateral dorsal column modalities to the ventral posterolateral nucleus of the thalamus.
      • Spinal lemniscus
        • containslateral 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.
        • containsrubro-olivary and reticulothalamic fibers.
      • Juxtarestiform body
        • forms part of the inferior cerebellar peduncle.
        • containsvestibulocerebellar, cerebellovestibular, and cerebelloreticular fibers.
      • Middle cerebellar peduncle
        • containspontocerebellar 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
        • arecerebellar 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, andcerebellum.
        • project to the cerebellum and the MLF.
        • The lateral vestibular nucleus gives rise to the lateral vestibulospinal tract.
      • Medial longitudinal fasciculus
        • containsvestibular fibers of CN VIII
        • coordinateeye movements via CN III, CNIV, and CN VI.
        • mediatesnystagmus and lateral conjugate gaze.
      • Abducent nucleus of CN VI
        • underlies, in the caudal medial pontinetegmentum, the facial colliculus of therhomboid 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 fromthe 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.
        • receivesbilateral input for upper facial muscles and contralateral input for lowerfacial muscles.
        • contains neurons that project axons dorsomedially, encircle the abducent nucleus asa 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
        • liesinthelateralmidpontinetegmentum
        • lies medial to the principal sensory nucleus of the trigeminal nerve.
        • receivesbilateral 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.
        • receivesdiscriminative 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 theventral posteromedial nucleus of the thalamus.
      • Mesencephalic nucleus and tract of CN V
        • extend from the upper pons to the upper midbrain.
        • containpseudounipolar neurons.
        • receive input from muscle spindles and pressure receptors (muscles of masticationand 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)
      • mediatesauditory and visual reflexes.
      • contains the oculomotor nerve (CN III) and the trochlear nerve (CN IV), which innervatethe extraocular muscles of the eye.
      • contains a center for vertical conjugate gaze in its rostral extent.
      • contains the substantianigra,
        • the largest nucleus of the midbrain;
        • degeneration of thisextrapyramidal 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).
      • consistsdorsoventrally of three parts
        • thetectum
        • thetegmentum
        • the base (basispedunculi)
      • 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).
            • containscranial nerve nuclei and sensory pathways.
          • Basis pedunculi (crus cerebri)
            • forms the base of the midbrain
            • containscorticospinal, corticobulbar, and corticopontine tracts.
          • Pedunculuscerebri (cerebral peduncle)
            • includes the tegmentum and basis pedunculi.
          • Pretectum (pretectal area)
            • is located between the superior colliculus and the habenulartrigone
        • 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
            • projectsbinaural 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
            • containsenkephalinergic neurons that play a role in endogenous pain control.
          • Dorsal nucleus of raphe
            • containsserotonergic neurons.
          • Trochlear nucleus of CN IV
            • gives rise to GSE fibers, which encircle the periaqueductal gray matter
            • decussate in thesuperior medullary velum
            • exit the midbrain from its dorsal aspect
            • innervate the superior oblique muscle.
          • Medial longitudinal fasciculus
            • containsvestibular fibers that coordinate eye movements.
            • interconnects the ocular motor nuclei of CN III, CN IV, and CN VI.
          • Decussation of the superior cerebellar peduncles
            • ismost conspicuous structure of this level.
          • Interpeduncular nucleus
            • receives input from the habenular nuclei via the habenulointerpeduncular tract (fasciculus retroflexus of Meynert).
          • Substantianigra
            • is divided into the dorsal pars compacta, which contains large pigmented (melanin) cells, and the ventral pars reticularis.
            • receivesgamma-aminobutyric acid–ergic (GABA-ergic) input from the caudatoputamen (striatonigral fibers).
            • projectsdopaminergic fibers to the caudatoputamen (nigrostriatal fibers).
            • projectsnondopaminergic fibers to the ventral anterior nucleus, ventral lateral nucleus, and mediodorsal nucleus of the thalamus (nigrothalamic fibers).
          • Medial lemniscus
            • mediatesdorsal column modalities to the ventral posterolateral nucleus.
          • Spinal lemniscus
            • contains the lateral and ventral spinothalamic tracts and the spinotectal tract.
          • Central tegmental tract
            • containsrubro-olivary and reticulothalamic fibers.
          • Basis pedunculi (crus cerebri)
        • Superior collicular level of the midbrain
          • Superior colliculus
            • receivesvisual input from the retina and from frontal (area 8) and occipital (area 19) eye fields.
            • receivesauditory 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
            • conductsretinal 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 levatorpalpebrae.
            • projectscrossed fibers to the superior rectus.
            • projectscrossed and uncrossed fibers to the levatorpalpebrae
          • 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
            • containsrubro-olivary and reticulothalamic fibers.
          • Red nucleus
            • is located in the tegmentum at the level of the oculomotor nucleus (the level of the
            • superiorcolliculus).
            • receivesbilateral input from the cerebral cortex.
            • receivescontralateral input from the cerebellar nuclei.
            • gives rise to the crossed rubrospinal tract.
            • gives rise to the uncrossed rubro-olivary tract.
            • exertsfacilitatory influence on flexor muscles
          • Medial lemniscus
          • Spinal lemniscus
          • Substantianigra
          • 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.
            • interconnectspretectal nuclei, thus mediating consensual pupillary light reflexes.
          • Pretectal nucleus
            • receivesretinal 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 postcentralgyri.
        • may synapse directly on motor neurons or indirectly via interneurons (corticoreticular fibers).
        • innervatesensory relay nuclei (gracile, cuneate, solitary, and trigeminal).
        • innervatecranial nerve motor nuclei bilaterally, with the exception of part of the facial nucleus (CN VII).
          • innervate the ipsilateral spinal nucleus of CN XI, which supplies the sternocleidomastoid muscle
          • innervatethe 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.


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