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Mesencephalon (Midbrain)(Midbrain) CNSCNS Divisionsdivisions Midbrainmidbrain –– Boundariesboundaries && Sizesize

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Mesencephalon (Midbrain)(Midbrain) CNSCNS Divisionsdivisions Midbrainmidbrain –– Boundariesboundaries && Sizesize BrainBrain stemstem MesencephalonMesencephalon (Midbrain)(Midbrain) CNSCNS divisionsdivisions MidbrainMidbrain –– boundariesboundaries && sizesize CConnectsonnects thethe ponspons andand cerebellumcerebellum withwith thethe forebrainforebrain TheThe smallestsmallest regionregion ofof thethe brainstembrainstem –– 66--7g7g TheThe shortestshortest brainstembrainstem segmentsegment << 22 cmcm longlong BrainstemBrainstem divisionsdivisions Midbrain Mesencephalon Pons Cerebellum Metencephalon Medulla Myelencephalon VentralVentral brainstembrainstem cerebral peduncle interpeduncular fossa middle cerebellar peduncle pyramid VentralVentral midbrainmidbrain Cranial Nerve 3 Cranial Nerve 4 Cerebral Peduncle DorsalDorsal brainbrain stemstem Superior Cranial Nerve IV colliculus Inferior colliculus Middle Fourth ventricle cerebellar peduncle DorsalDorsal midbrainmidbrain Brachium of the Inferior Colliculus Brachium of the Superior Colliculus Superior medullary velum Dorsal surface of the mesencephalon BrainBrain stemstem –– macroscopicmacroscopic overviewoverview Dorsal view Ventral view posterolateral sulcus median sulcus anterolateral sulcus OriginOrigin ofof CNSCNS subdivisionssubdivisions SecondarySecondary vesiclesvesicles MesencephalonMesencephalon isis largerlarger duringduring brainbrain developmentdevelopment thanthan inin postnatalpostnatal brainbrain Neural tube folding (5 th -8th wk) 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) AlarAlar vs.vs. BasalBasal plateplate derivativesderivatives -- midbrainmidbrain Sensory Motor visceral somatic 12 weeks 6 weeks Neural canal Basal plate motor nuclei of CN III & CN IV → extraocular muscles (somatic motor) Edinger-Westphal nucleus → intraocular muscles (visceral motor) Alar plate → tectum (colliculi) Mixed basal & alar plate → substantia nigra, red nucleus TThehe midbrainmidbrain hashas aa dorsoventraldorsoventral organization,organization, likelike thethe spinalspinal cord.cord. ThisThis isis becausebecause thethe 44th ventricleventricle doesdoes notnot extendextend intointo thethe midbrainmidbrain .. Spinal cord Midbrain Neural canal medulla AlarAlar vsvs basalbasal inin midbrainmidbrain alar basal EstimatedEstimated timetime ofof developmentdevelopment ofof variousvarious brainbrain regiregionsons 2 mo 6 mo Modified from Bayer SA et al. Neurotoxicology 14:83–144, 1993 LongitudinalLongitudinal zoneszones inin thethe brainbrain stemstem cranial Structures in Midbrain: Tectum ( =quadrigeminal plate) → superior & inferior colliculi + cerebral aqueduct Tegmentum → neuronal bodies + tracts passing Basis = crus cerebri + substantia nigra ( “black substance ”) neuronal axons – corticospinal/corticobulbar fibers neuronal bodies – substantia nigra caudal LongitudinalLongitudinal zoneszones inin thethe midbrainmidbrain cranial dorsal cerebral tectum aqueduct tegmentum basis ventral caudal MesencephalicMesencephalic tectumtectum ((==quadrigeminalquadrigeminal plate)plate) SuperiorSuperior colliculicolliculi InferiorInferior colliculicolliculi CerebralCerebral aqueductaqueduct MidbrainMidbrain -- levellevel ofof superiorsuperior colliculuscolliculus Superior Colliculus Periaqueductal Gray Cerebral Peduncles Red Nucleus ConnectionsConnections ofof superiorsuperior colliculuscolliculus (visual)(visual) cortexcortex thalamusthalamus eyeseyes superior colliculus cerebellumcerebellum spinalspinal cordcord TectospinalTectospinal tracttract -- reflexive movement of head and neck ; eye movements Origin: Superior Colliculus Course: Ventral to MLF, to ventromedial Spinal Cord Termination: Cervical Spinal cord ventral horn Laterality: CONTRA (crosses immediately in the dorsal tegmental decussation) Tectospinal tract The tectospinal tract carries impulses that control reflex movements of the upper trunk, neck, and eyes in response to visual stimuli. Purves, et al, Neuroscience, 3rd ed. MidbrainMidbrain -- levellevel ofof inferiorinferior colliculuscolliculus Inferior Colliculus Periaqueductal Gray Decussation of the Superior Cerebellar Peduncles Cerebral Peduncles The nucleus of the inferior colliculus is a major relay nucleus in the auditory pathway Afferent connections Efferent connections Brachium of the Superior Colliculus Lateral Superior geniculate colliculus vision body Brachium of the Inferior Colliculus Medial Inferior geniculate colliculus hearing body The inferior colliculi are involved in auditory reflexes and in determining the side on which a sound originates MidbrainMidbrain colliculicolliculi && brachiabrachia Brachium of the Superior Colliculus Superior Brachium of the colliculus Inferior Colliculus Inferior colliculus CerebralCerebral aqueductaqueduct (Sylvius)(Sylvius) ~15 mm long contains no choroid plexus 3rd Ventricle 4th Ventricle Obstruction of the cerebral aqueduct may occur congenitally, resulting in a non-communicating hydrocephalus. Pain control pathways & periaqueductal gray cerebral aqueduct Ascending pain pathways StructuresStructures inin thethe mesencephalicmesencephalic tegmentumtegmentum CranialCranial nervenerve nucleinuclei –– CNCN III,III, IVIV ReticularReticular formationformation -- nucleusnucleus locuslocus ceruleusceruleus && otherother CrossingCrossing pathwayspathways ascending (sensory) superior cerebellar peduncle lemniscal spinothalamic descending (motor) central tegmental tract (basal ganglia /midbrain → inferior olive ) rubrospinal tract tectospinal tract other – sympathetic fibers from hypothalamus; medial longitudinal fasciculus TheThe brainstembrainstem isis connectedconnected toto thethe cerebellumcerebellum viavia thethe cerebellarcerebellar pedunclespeduncles superior cerebellar peduncle (SCP) Brachium conjunctivum MidbrainMidbrain ↔↔ CerebellumCerebellum SCP is the principal output path of the cerebellum middle cerebellar peduncle (MCP) Brachium pontis PonsPons →→ CerebellumCerebellum inferior cerebellar peduncle (MCP) MedullaMedulla oblongataoblongata ↔↔ CerebellumCerebellum The midbrain is connected with the cerebellum via SCP SCP represents the main cerebellar SCP output to the thalamus and red nucleus (ICP) MCP MMajorajor ascendingascending andand descendingdescending tractstracts inin thethe mesencephalicmesencephalic tegmentumtegmentum Lemniscal system CorticobulbarCorticobulbar tractstracts MotorMotor inputinput to:to: ReticularReticular formationformation –– mesencephalicmesencephalic RedRed NucleusNucleus CranialCranial NerveNerve motormotor nucleinuclei –– III,III, IVIV Note – No direct connections from the primary motor cortex to III, IV, and VI; these nuclei get input from frontal and parietal cortex Cranial nerves – sensory nuclei Cranial nerves – motor nuclei 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 CNCN III:III: OculomotorOculomotor Origin: Oculomotor nucleus (motor), Edinger-Westphal nucleus (Psym) Course: Interpeduncular Fossa, Cavernous Sinus, Sup Orb Fissure Termination: MR, SR, IR, IO; Levator Palpebrae, Constrictor Pupillae (Psym) Laterality: Ipsilateral to extraocular muscles; bilateral to parasympathetic ganglia CNCN III:III: OculomotorOculomotor Edinger–Westphal motor CNCN III:III: OculomotorOculomotor Motor Sensory somatic visceral visceral somatic extraocular skeletal muscles MotorMotor innervationinnervation ofof thethe eyeeye Striated (extraocular muscles) Striated (extraocular muscles) - innervation CN IV CN VI All other – CN III CNCN III:III: OculomotorOculomotor CNCN IV:IV: TrochlearTrochlear Origin: Trochlear Nucleus Course: Quadrigeminal Cistern, Ambient Cistern, Cavernous Sinus, Sup Orb Fissure Termination: Superior oblique muscle Laterality: Contralateral CNCN IV:IV: TrochlearTrochlear The nucleus is at the The nerve exits caudal to level of the SCP the inferior colliculus decussation Contralateral CNCN IV:IV: TrochlearTrochlear RedRed nucleusnucleus OOvoidvoid massmass ~~55 mmmm inin diameterdiameter DDorsomedialorsomedial toto thethe substantiasubstantia nigranigra PinkishPinkish inin freshfresh preparationspreparations →→ itsits neuronsneurons containcontain ironiron pigmentpigment CConnectionsonnections ofof thethe redred nucleusnucleus Afferent connections Efferent connections RubrospinalRubrospinal TractTract –– movementmovement ofof upperupper limbslimbs Origin: Red nucleus Course: Dorsolateral white matter with LCST in spinal cord Termination: Cervical Spinal cord ventral horn Laterality: CONTRA (crosses immediately in the ventral tegmental decussation) Rubrospinal tract The red nucleus receives cortical input; Rubrospinal fibers project on the same spinal cord laminae as does
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