A
Mesencephalon
Decathlon
Jim Thorpe
Gold medal in the
1912 Olympic
decathlon
Questions
• What are the 3 primary brain vesicles?
• What are the corpora quadrigemina?
• Which anatomic structures comprise the basis
pedunculi?
• What is Claude syndrome?
– What is a rubral tremor?
Outline
1. Embryology
2. External anatomy
3. Internal anatomy
4. Vascular supply
5. Stroke syndromes
6. Herniation syndromes
Embryology
• 1 of 3 primary brain vesicles
– Prosencephalon
– Mesencephalon
– Rhombencephalon
• Intermediate zone gives rise to alar and basal plates
– Alar = colliculi, red nucleus and substantia nigra
– Basal = general somatic efferent (CN III & IV) and general
visceral efferent (E-W nucleus)
• Crus cerebri arise from cells outside the mesencephalon
External
Anatomy
• Crus cerebri
– Bordered anteriorly by optic tract
• CN III exit medial edge of crus cerebri and pass through
interpeduncular fossa
• Corpora quadrigemina = 4 colliculi
• CN IV marks midbrain/pons junction
• SC brachium leads to pulvinar nucleus
• IC brachium leads to MGB
• Anterior subarachnoid space = interpeduncular cistern
• Posterior subarachnoid space = quadrigeminal cistern
Internal
Anatomy
• 3 divisions
– Tectum (roof)
– Tegmentum (floor)
– Basis pedunculi (crus cerebri + substantia nigra)
• Cerebral peduncle = crus +basis pedunculi
• Ascending and descending pathways
• Caudal Midbrain
– Inf Colliculi receive auditory input from lateral lemniscus
– PAG involved in pain modulation (connections to thalamus,
hypothalamus and somatosensory input)
– Fronto-, parieto-, occipito- & temporopontine fibres project to
pons and enter MCP
• Caudal Midbrain
– CN IV axons pass postero-lateral, crossing midline
• Somatotopographic organization of the
medial lemniscus
• Rostral Midbrain
– SN
• Pars compacta = output to corpus striatum
• Pars reticulata = output to thalamus
• Rostral Midbrain
– RN
• Input from contra cerebellum & ipsi cortex
• Rubrospinal and rubro-olivary tracts
• Diencephalon-mesencephalon junction
– Edinger-Westphal nucleus
• Output to ciliary ganglion
• Input from pretectal neuclei
• Diencephalon-mesencephalon junction
• Reticular nuclei
– Part of ascending reticular activating system
– Responsible for alert, wakeful state
• Raphe nuclei
– Modulate activity in sleep/dream cycles
Vascular Supply
Stroke Syndromes
Herniation Syndromes
• Vascular supply
– Branches of SCA and PCA
– Lateral midbrain also supplied by anterior choroidal
artery (branch of ICA)
• Weber
– Ipsi CN III, contra bulbar motor
• Claude
– Ipsi CN III, contra tremor, ataxia and incoordination
• Benedikt
– Weber + Claude
• Central/transtentorial herniation
• Upward cerebellar herniation
– May lead to
• Cerebellar stroke from SCA occlusion
• Hydrocephalus from aqueduct compression
• Uncal herniation
– Lesion most often in temporal lobe
– Ipsi CN III is often earliest sign
Questions
• What are the 3 primary brain vesicles?
• What are the corpora quadrigemina?
• What anatomic structures comprise the basis
pedunculi?
• What is Claude syndrome?
– What is a rubral tremor?
• Rubral tremor (aka Holme’s tremor)
– A coarse, slow (4Hz) tremor, especially
present in the upper extremities, that is found
at rest, postural and intention.
The End