The midbrain connects the hindbrain and forebrain. It has three main subdivisions - the tectum, cerebral peduncles, and tegmentum. The midbrain receives blood supply from the basilar artery and branches of the posterior, superior, and anterior cerebral arteries. Key structures in the midbrain include the superior and inferior colliculi, substantia nigra, red nucleus, and nuclei of the oculomotor, trochlear and trigeminal nerves. Lesions of specific midbrain structures can cause syndromes such as Weber's, Benedict's, and Parinaud's.

1. Dept of Rachana Sharir
Dr Samadhan M Patil
MD Rachana Sharir

2. Aim
• To study Mid brain with special reference to Human
anatomy (B D Chaurasia – 7th edition)

3. Introduction
• The midbrain is also called the mesencephalon
• It is 2 cm long
• Connect the hind brain with the forebrain
• It passes through the tentorial notch
• It’s cavity is known as the cerebral aqueduct of sylvius
• Cerebral aqueduct connects the third ventricle with the fourth
ventricle

5. Relations
• Anterior: Interpeduncular structure
• Posterior: Splenium, great cerebral vein, pineal body, posterior
end of thalami
• Lateral: Parahippocampal gyri, optic tract, posterior cerebral
artery, basal vein, trochlear nerve, geniculate bodies

7. Subdivision
• The major subdivision of midbrain are as follows:
1. Tectum: It is posterior to aqueduct and made up of the right and
left inferior colliculi
2. Cerebral peduncle: each half present anterior to the aqueduct.
It is subdivided into a) Crus cerebri – anteriorly b) Substantia
nigra – in the middle c) Tegmentum - posteriorly

9. External features
It has two surfaces: 1) Ventral and 2) Dorsal
1. Ventral surface:
• Crus cerebri is present
• Crossed by optic tract, basal vein, posterior cerebral and superior
cerebellar artery
• From the Medial side of this occulomotor nerve and lateral side
trochlear nerve emerges

10. 2. Dorsal surface:
• Superior and inferior colliculi are present (called corpora
quadrigemina)
• Superior colliculus is connected to the lateral geniculate body
by the superior brachium
• Inferior colliculus is connected to the medical geniculate body
by the inferior brachium

11. Internal structure of midbrain
• Transverse section at the level of inferior colliculi
1. Grey matter
• Central grey matter: nucleus of trochlear nerve,
mesencephalic nucleus Of trigeminal nerve
• Inferior colliculus: center for auditory reflexes and help in
localising the source of sound
• Substantia nigra: concerned with muscle tone

12. 2. White matter:
1. Crus cerebri- Corticospinal tract (middle), Frontopontine fibre
(medial 1/6), Temporopontine, Parietopontine and
Occipitopontine fibre (lateral 1/6)
2. Tegmentum contains ascending tracts as follows:
• Medial/ Trigeminal/ Spinal/ Lateral lemniscus
• Decussation of the superior cerebellar peduncles
• Medial longitudinal bundle
• Tectospinal and rubrospinal tract
3. Trochlear nerve: decussates in superior medullary velum and
emerges lateral to the frenulum veli

13. • Transverse section at the level of superior colliculi
1. Grey matter:
1. Central grey matter- Nucleus of oculomotor with Edinger-westphal
nucleus (ventromedial part), Mesencephalic nucleus of trigeminal
nerve (Lateral part)
2. Superior colliculus: controls reflex movements of the eyes, head
and neck in response to visual stimuli
3. Pretectal nucleus: important part of the pathway for light reflex
and the consensual reflex, present superolateral part of the
superior colliculus
4. Red nucleus: It has an inhibitory influence on muscle tone
5. Substantia nigra

14. 2. White matter:
1. The Crus cerebri
2. The Tegmentum –
• Same lamnisci except lateral lemniscus
• Dorsal tegmental decussation (decussation of the
Tectospinal and tectobulbar tracts)
• Ventral tegmental decussation (decussation of the
rubrospinal tract)
• Medial longitudinal bundle
• Emerging fibres of oculomotor nerve
3. Tectum shows posterior commissure

17. Medial longitudinal bundle
• Fibres from left and right medial vestibular nuclei
• These fibres reach upto interstitial nucleus of Cajal at upper end
of aqueduct
• The lower end of fibres reaches up to cervical segments of spinal
cord
• Connections: cranial nerve nuclei III, IV, VI, VIII, spinal root of XI,
also connected to the anterior horn cells

18. Blood supply
• Derived from branches of the basilar artery
• Also receives from
1. Posterior cerebral
2. Superior cerebral
3. Posterior communicating
4. Anterior choroidal

19. Clinical anatomy
1. Weber’s syndrome:
• Involves III Nerve nucleus and Corticospinal fibres
• Pupil point downwards and laterally due to paralysis of III nerve nucleus
• Hemiplegia on the opposite side due to involvement of Corticospinal
fibres
2. Benedict’s syndrome: damaged Tegmentum
• Loss of proprioception – lesion of Medial lemniscus
• Pupil point downwards and laterally
• Tremors and twitching of opposite side – damage to red nucleus and
superior cerebellar peduncle

20. 3. Parinaud’s Syndrome:
• Lesion of superior colliculi
• Weackness of upward gaze and vertical nystagmus
4. Argyll Robertson Pupil:
• Lesion in the vicinity of pretectal nucleus
• Light reflex is lost but accommodation reflex is retained