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 the corticospinal tract → indirect corticospinal tract
Purves, et al, Neuroscience, 3rd ed. TheThe CNCN IIIIII motormotor fiberfiber passpass throughthrough thethe redred nucleusnucleus
Lesions of the red nucleus result in contralateral tremor StructuresStructures inin thethe mesencephalicmesencephalic basisbasis
NeuronalNeuronal axonsaxons CorticoCortico --pontoponto --cerebellarcerebellar fibersfibers CorticospinalCorticospinal fibersfibers corticofugal fibers CorticobulbarCorticobulbar fibersfibers NeuronalNeuronal bodiesbodies -- SubstantiaSubstantia nigranigra CCorticospinalorticospinal andand corticobulbarcorticobulbar tractstracts CorticospinalCorticospinal tracttract
Origin: Primary motor cortex (BA 4) Course: Post Limb IC, Pyramids Termination: Spinal cord ventral horn Laterality: CONTRA (lateral), IPSI (anterior) Corticopontine tract
Origin: Premotor, primary motor, and primary somatosensory cortex Course: Internal Capsule (A/P), cerebral peduncles, Pons Termination: Pontine nuclei Laterality: Ipsilateral Somatotopic organization of corticospinal & corticobulbar tracts SubstantiaSubstantia nigranigra (SN)(SN)
Pars compacta
Pars reticulata NeuronsNeurons inin SNSN
ParsPars compactacompacta (SNpc)(SNpc) ––dorsal,dorsal, containingcontaining neuromelaninneuromelanin →→ dopaminedopamine ParsPars reticulatareticulata (SNpr)(SNpr) –– ventral,ventral, ironiron compoundscompounds →→ Ach,Ach, GABA;GABA; SNprSNpr extendsextends rostrallyrostrally asas farfar asas thethe subthalamicsubthalamic region,region, andand isis consideredconsidered toto bebe homologoushomologous withwith thethe medialmedial segmentsegment ofof thethe globusglobus pallidus,pallidus, whichwhich itit resemblesresembles structurallystructurally TH staining of DAergic neurons in SN pars compacta SubstantiaSubstantia nigranigra modulatesmodulates thethe loopsloops betweenbetween cortexcortex --basalbasal gangliaganglia
Cortex
(+)
Substantia (+) nigra Thalamus
(-)
Basal ganglia DamageDamage ofof SNpcSNpc neuronsneurons leadsleads toto ParkinsonParkinson ’’ss diseasedisease
Parkinson Normal
Lewy bodies in SNpc neuron, H&E Upper midbrain
Pars compacta
Pars reticulata LowerLower midbrainmidbrain
Pars compacta
Pars reticulata BloodBlood supplysupply ofof midbrainmidbrain
Basilar artery → paramedian branch oculomotor (III) nerve nucleus red nucleus (RN) Superior cerebellar artery → tectum Posterior cerebral artery basal branch → cerebral peduncle (CP) dorsolateral branch → spinothalamic tract (ST), medial lemniscus (ML), and SCP BloodBlood supplysupply ofof midbrainmidbrain –– clinicalclinical syndromessyndromes Inability to move the eyes up or down
Tremor + sensory deficiency
Hemiparesis + CN III paralysis