A A MesencephalMesencephalon Decathlonon 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 anatomy4. 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