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Pons (Bridge)(Bridge) CNSCNS Divisionsdivisions Brainstembrainstem Divisionsdivisions BrainBrain stemstem PonsPons (bridge)(bridge) CNSCNS divisionsdivisions BrainstemBrainstem divisionsdivisions Midbrain Mesencephalon Pons Cerebellum Metencephalon Medulla Myelencephalon VentralVentral brainbrain stemstem cerebral peduncle middle cerebellar peduncle pyramid DorsalDorsal brainbrain stemstem Superior Cranial Nerve IV colliculus Inferior colliculus Middle Fourth ventricle cerebellar peduncle VentralVentral brainbrain stemstem Basilar Pons Cranial Nerve V Cranial Nerve VI Cranial Nerve VII Cranial Nerve VIII BrainstemBrainstem –– macroscopicmacroscopic overviewoverview Dorsal view Ventral view Bulbopontine sulcus NeuralNeural tubetube foldingfolding Day 21 → → day 26 OriginOrigin ofof CNSCNS subdivisionssubdivisions Primary/secondary brain vesicles & flexures that separate them Primary vesicles Cervical flexure Embryo axis Cephalic flexure Cephalic flexure Pontine flexure Cervical flexure Secondary vesicles NeuralNeural tubetube foldingfolding –– pontinepontine flexureflexure Neural tube folding (5 th -8th wk) AlarAlar vs.vs. BasalBasal plateplate derivativesderivatives -- medullamedulla dorsal somatic Alar plate = afferent = sensory visceral Basal plate = efferent = motor visceral somatic infeior olivary nucleus ventral AlarAlar vs.vs. BasalBasal plateplate -- ponspons Spinal cord Medulla Pons AlarAlar vs.vs. BasalBasal plateplate derivativesderivatives -- ponspons Pontine nuclei somatic are alar plate Alar plate = afferent = sensory visceral precursors migrated Basal plate = efferent = motor visceral ventrally somatic The basal plate → primarily efferent nuclei (CN V, VI, VII, superior salivatory nuclei) The alar plate → somatic and visceral sensory nuclei (CN V, VIII, pontine nuclei) EstimatedEstimated timetime ofof developmentdevelopment ofof variousvarious brainbrain regiregionsons Modified from Bayer SA et al. Neurotoxicology 14:83–144, 1993 LongitudinalLongitudinal zoneszones inin thethe brainbrain stemstem cranial tectum Structures in Pons: Nuclei → neuronal tegmentum bodies Reticular formation → neuronal bodies basis Tracts = white matter → neuronal axons passing through ending in pons caudal LevelLevel ofof CNCN VV -- PonsPons Superior Cerebellar Peduncle 4th Ventricle Middle Cerebellar Peduncle LevelLevel ofof CNCN VI,VI, VIIVII -- PonsPons Superior Cerebellar Peduncle Facial Colliculus Middle Cerebellar Peduncle Pyramids (beginning) TheThe brainstembrainstem isis connectedconnected toto thethe cerebellumcerebellum viavia thethe cerebellarcerebellar pedunclespeduncles superior cerebellar peduncle (SCP) Brachium conjunctivum MidbrainMidbrain ↔↔ CerebellumCerebellum middle cerebellar peduncle (MCP) Brachium pontis PonsPons →→ CerebellumCerebellum MCP is the principal input path of the cerebellum (it has no output) inferior cerebellar peduncle (MCP) MedullaMedulla oblongataoblongata ↔↔ CerebellumCerebellum AfferentAfferent cerebellarcerebellar pathwayspathways inin brainstembrainstem Saggital view Dorsal view StructuresStructures inin basisbasis pontispontis CorticoCortico --pontoponto --cerebellarcerebellar fibersfibers CorticospinalCorticospinal fibersfibers CorticobulbarCorticobulbar fibersfibers ((cortexcortex →→ cranialcranial nervenerve nucleinuclei )) directdirect cc orticobulbarorticobulbar indirectindirect cc orticoortico --reticuloreticulo --bulbarbulbar (( cortexcortex →→ RFRF →→ CNCN nucleinuclei )) NucleiNuclei pontispontis (gray(gray matter)matter) CorticopontocerebellarCorticopontocerebellar fibersfibers constituteconstitute thethe largestlargest groupgroup ofof fibersfibers inin thethe basisbasis pontispontis WWideide areasareas ofof cortexcortex →→ ipsilateralipsilateral pontinepontine nucleinuclei →→ contralateralcontralateral cerebellumcerebellum viavia MCPMCP (some(some 10%10% -- ipsilatipsilat .).) NumberNumber ofof fibersfibers -- 1919 millionmillion onon eacheach side/numberside/number ofof pontinepontine nucleinuclei neuronsneurons –– 2323 millionmillion onon eacheach sideside (~1:1)(~1:1) PrincipalPrincipal corticalcortical inputinput –– primaryprimary sensorimotorsensorimotor areasareas →→ correctioncorrection ofof movementsmovements AllAll projectionprojection ss areare somatotopicallysomatotopically organizedorganized CorticopontocerebellarCorticopontocerebellar pathwaypathway Purves, et al, Neuroscience, 3rd ed. Corticopontine tract – upper pons Origin: Premotor, primary motor, and primary somatosensory cortex Course: Internal Capsule (A/P), cerebral peduncles, Pons Termination: Pontine nuclei Laterality: Ipsilateral Fibers will then synapse on Corticopontine tract pontine nuclei, which then decussate in the transverse fibers of the pons and enter the MCP… Pontine Nuclei Origin: Premotor, primary motor, and primary somatosensory cortex Course: Internal Capsule (A/P), cerebral peduncles, Pons Termination: Pontine nuclei Laterality: Ipsilateral Inputs,Inputs, MCPMCP PontocerebellarPontocerebellar TractTract preprogram of fine motor movements, esp. upper extremity Origin: Pontine Nuclei Course: Immediately decussates and enters MCP Termination: granule cells of the cerebellar hemispheres Laterality: Contralateral CCorticospinalorticospinal andand corticobulbarcorticobulbar tractstracts CorticospinalCorticospinal tracttract –– upperupper ponspons Origin: Primary motor cortex (BA 4) Course: Post Limb IC, Pyramids Termination: Spinal cord ventral horn Laterality: CONTRA (lateral), IPSI (anterior) CorticospinalCorticospinal tracttract –– lowerlower ponspons Origin: Primary motor cortex (BA 4) Course: Post Limb IC, Pyramids Termination: Spinal cord ventral horn Laterality: CONTRA (lateral), IPSI (anterior) CorticobulbarCorticobulbar tractstracts MotorMotor inputinput to:to: ReticularReticular formationsformations –– pontinepontine CranialCranial NerveNerve motormotor nucleinuclei –– V,V, VI,VI, VII,VII, VIIIVIII Note – No direct connections from the primary motor cortex to CN VI; this nucleus gets input from frontal and parietal cortex StructuresStructures inin thethe tegmentumtegmentum pontispontis CranialCranial nervenerve nucleinuclei –– CNCN V,V, VI,VI, VII,VII, VIIIVIII ReticularReticular formationformation -- nucleusnucleus locuslocus ceruleusceruleus && otherother CrossingCrossing pathwayspathways ascending (sensory) – basal part of tegmentum lemniscal spinothalamic anterior spinocerebellar tract descending (motor) central tegmental tract (basal ganglia /midbrain → inferior olive ) rubrospinal tract tectospinal tract other – sympathetic fibers from hypothalamus; medial longitudinal fasciculus PositionPosition ofof ascendingascending toto descendingdescending fibersfibers inin tegmentumtegmentum ascending – ventral part of tegmentum descending – dorsal part of tegmentum DescendingDescending brainbrain stemstem pathwayspathways (to(to spinalspinal cord)cord) (tectospinal) Purves, et al, Neuroscience, 3rd ed. CranialCranial nervenerve nucleinuclei inin brainbrain stemstem Name Nerve Nuclei Oculomotor III Oculomotor, Edinger–Westphal midbrain Trochlear IV Trochlear Trigeminal V Main sensory, spinal (descending), mesencephalic, motor (masticatory) Abducens VI Abducens pons Facial VII Facial, superior salivatory, gustatory (solitary)** Vestibulocochlear VIII Cochlear (2 nuclei), vestibular (4 nuclei) Glossopharyngeus IX Ambiguus*, inferior salivatory, solitary** Vagus X Dorsal motor, ambiguus**, solitary* medulla Accessory XI Spinal accessory (C1–5), ambiguus** Hypoglossal XII Hypoglossal ** The solitary nucleus is common for CN VII, IX, and X * The ambiguus nucleus is common for CN IX, X, and XI Cranial nerves – sensory nuclei Cranial nerves – motor nuclei CNCN V:V: TrigeminalTrigeminal CNCN V:V: TrigeminalTrigeminal The “level” of CN V is determined by where the nerve fibers * exit * * * AAfferentfferent andand efferentefferent rootsroots ofof CNCN VV Mesencephalic nucleus is homologous to the dorsal root ganglion but centrally placed Trigeminal JawJaw jerkjerk reflexreflex –– CNCN VV Proprioceptive fibers to the mesencephalic nucleus convey pressure and kinesthesia from the teeth, periodontium, hard palate, and joint capsules as well as impulses from stretch receptors in the muscles of mastication. The mesencephalic nucleus is concerned with mechanisms that control the force of the bite. CNCN V:V: TrigeminalTrigeminal -- mesencephalicmesencephalic The mesencephalic Origin: Mesencephalic nucleus and tract run Nucleus of V above the level of CN V Course: Mesencephalic tract of V Termination: Motor Nucleus of V Laterality: Bilateral CNCN V:V: TrigeminalTrigeminal motormotor andand chiefchief sensorysensory (level(level ofof CNCN V)V) Chief Sensory Motor nucleus nucleus of V of V R A L L A A S A B Alar → sensory Basal → motor CornealCorneal reflexreflex CN V CN VII Touching the cornea elicits reflex closure of the eye → might be disrupted in pontine lesions CNCN V:V: TrigeminalTrigeminal SpinalSpinal nucleusnucleus andand tracttract atat levellevel ofof VI/VIIVI/VII Neuron #1 Origin: Trigeminal (Gasserian) Ganglion Course: All branches of Trigeminal Nerve, Spinal Trigeminal tract Termination: Spinal Nucleus of V Laterality: Ipsilateral CNCN VI:VI: AbducensAbducens Origin: Abducens Nucleus Course: Exit PM Jxn, ascend through Dorello’s canal, cavernous sinus, sup orb fissure Termination: Lateral Rectus Laterality: Ipsilateral CNCN VI:VI: AbducensAbducens CNCN VI:VI: AbducensAbducens -- paralysisparalysis Left eye can’t look to left. CNCN VII:VII: FacialFacial Origin: Geniculate Ganglion (sensory), Motor Nucleus of VII (motor), Salivatory Nucleus
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