MidbrainMidbrain

(Mesencephalon)(Mesencephalon)

Upper and shortest part Upper and shortest part of brain stem(2.5cm of brain stem(2.5cm long and 2.5cm wide).long and 2.5cm wide).

Passes through Passes through tentorial notch.tentorial notch.

Related on each side to Related on each side to optic tract, optic tract, parahippocampal gyrus, parahippocampal gyrus, posterior cerebral posterior cerebral artery.artery.

the optic chiasm, the tuber cinereum and the infundibulum, the mamillary bodies, the posterior perforated substance.

(in that order from before backwards)

Anteriorly related toAnteriorly related to interpeduncular structures interpeduncular structures

Interpeduncular fossa and contents

Posterior relationsPosterior relations

Posterior ends of Posterior ends of thalami (pulvinar).thalami (pulvinar).

Pineal body.Pineal body. Splenium of corpus Splenium of corpus

callosum.callosum.

External featuresExternal features

Ventrally presents two Ventrally presents two crura which converge to crura which converge to enter the pons and form enter the pons and form the posterior boundaries the posterior boundaries of interpeduncular fossa.of interpeduncular fossa.

Oculomotor nerves Oculomotor nerves emerge between the emerge between the crura.crura.

Trochlear nerves curve Trochlear nerves curve round the lateral borders round the lateral borders of crura.of crura.

Dorsal surfaceDorsal surface

Colliculi (superior and Colliculi (superior and inferior pairs).inferior pairs).

Superior and inferior Superior and inferior brachia.brachia.

Pineal body (gland).Pineal body (gland).

Tectum

Cerebral peduncle

Cerebral aqueduct(of Sylvius)

TS of midbrain

?

Tectum

D

V

Tg

SN

tectum– is the dorsal tectum– is the dorsal part-- made up of part-- made up of colliculi. colliculi.

cerebral peduncle— cerebral peduncle— is the ventral part -- is the ventral part -- (made up of (made up of tegmentum, tegmentum, substantia nigra and substantia nigra and crus cerebri)crus cerebri)

Internal structure

cn,cs

fp

Contains Contains corticonuclear and corticonuclear and corticospinal fibers corticospinal fibers in the middle2/3.in the middle2/3.

Medially Medially frontopontine frontopontine fibers and laterally fibers and laterally fibers from other fibers from other lobes of cerebrum.lobes of cerebrum.

Crus cerebri

Tectum and tegmentum differ in Tectum and tegmentum differ in structure. structure.

Therefore the midbrain is studied at two Therefore the midbrain is studied at two levels;levels;

1.at the level of inferior colliculus1.at the level of inferior colliculus2.at the level of superior colliculus2.at the level of superior colliculus

TS at the level of inferior colliculusTS at the level of inferior colliculus

Grey matter around the Grey matter around the aqueduct (central grey) aqueduct (central grey) contains, IV nerve contains, IV nerve nucleus and nucleus and mesencephalic nucleus of mesencephalic nucleus of VV

Trochlear nerve fibers Trochlear nerve fibers decussate and emerge decussate and emerge dorsally and enter the dorsally and enter the cavernous sinus.cavernous sinus.

This is the only cranial This is the only cranial nerve to emerge nerve to emerge dorsally!!!dorsally!!!

T.S at the level of inferior colliculus T.S at the level of inferior colliculus (contd)(contd)

Decussation of superior Decussation of superior cerebellar peduncle is the cerebellar peduncle is the notable feature at this notable feature at this level.level.

The lemnisci are The lemnisci are arranged in a curved arranged in a curved manner and lateral manner and lateral lemniscus ends in the lemniscus ends in the inferior colliculus.inferior colliculus.

MLF, tectospinal and MLF, tectospinal and rubrospinal tracts are rubrospinal tracts are close to the midline from close to the midline from dorsal to ventral. dorsal to ventral.

T.S at the level of superior T.S at the level of superior colliculuscolliculus

Grey matter here Grey matter here contains:contains:

Oculomotor nucleus Oculomotor nucleus which also has a which also has a parasympathetic nucleus parasympathetic nucleus called the “Edinger-called the “Edinger-Westphal nucleus (pupillo Westphal nucleus (pupillo constrictory center), constrictory center), mesencephalic nucleus of mesencephalic nucleus of V, and the red nucleus.V, and the red nucleus.

contdcontd

E-W nucleus supplies the sphincter pupillae E-W nucleus supplies the sphincter pupillae and ciliaris muscle (for accommodation).and ciliaris muscle (for accommodation).

Superior colliculus receives afferents from Superior colliculus receives afferents from retinae and gives efferents called tectospinal retinae and gives efferents called tectospinal tracts.tracts.

Pretectal nucleusPretectal nucleus

Small group of neurons lying superior to superior Small group of neurons lying superior to superior colliculus. It gives fibers to the E-W nucleus of colliculus. It gives fibers to the E-W nucleus of both sides.both sides.

It is an important part of the pathway for pupillary It is an important part of the pathway for pupillary light reflex and consensual light reflex.light reflex and consensual light reflex.

Any injury to this nucleus causes Any injury to this nucleus causes ““Argyll-Robertson’s pupil”Argyll-Robertson’s pupil” where the light reflex where the light reflex

is lost but accommodation reflex is present. is lost but accommodation reflex is present. (ARP)(ARP)

White matter at this levelWhite matter at this level

Decussation of Decussation of tectospinal tracts forms tectospinal tracts forms dorsal tegmental dorsal tegmental decussation (of Meynert).decussation (of Meynert).

Decussation of Decussation of rubrospinal tracts forms rubrospinal tracts forms ventral tegmental ventral tegmental decussation (of Forel).decussation (of Forel).

Tegmentum contains all Tegmentum contains all the lemnisci except the the lemnisci except the lateral lemniscus.lateral lemniscus.

MLFMLF

Contains fibers coming from vestibular Contains fibers coming from vestibular nuclei (mainly) and interconnects the nuclei (mainly) and interconnects the nuclei of 3nuclei of 3rdrd,4,4thth,5,5thth,6,6thth and spinal and spinal accessory.accessory.

Coordinates the movement of the Coordinates the movement of the eyes, head and neck in response to eyes, head and neck in response to the stimulation of vestibulocochlear the stimulation of vestibulocochlear nerve.nerve.

Blood supplyBlood supply

Superior cerebellar, basilar and posterior Superior cerebellar, basilar and posterior communicating arteries.communicating arteries.

Clinical aspectsClinical aspects

Weber’s syndrome- injury to oculomotor Weber’s syndrome- injury to oculomotor nerve and the crus cerebri due to the nerve and the crus cerebri due to the occlusion of posterior cerebral artery.occlusion of posterior cerebral artery.

Effects:Effects: Ipsilateral lateral squint (unopposed action Ipsilateral lateral squint (unopposed action

of the lateral rectus).of the lateral rectus).Contralateral hemiplegiaContralateral hemiplegiaContralateral paralysis of lower face and Contralateral paralysis of lower face and

tongue (corticonuclear fiber damage)tongue (corticonuclear fiber damage)

Weber’s syndrome (contd)Weber’s syndrome (contd)

Drooping of upper eyelid (ptosis) due to Drooping of upper eyelid (ptosis) due to paralysis of levator palpebrae superioris.paralysis of levator palpebrae superioris.

Dilated pupil and loss of accommodation Dilated pupil and loss of accommodation reflex (damage to E-W nucleus).reflex (damage to E-W nucleus).

Benedikt’s syndromeBenedikt’s syndrome

Ischemia of tegmentum involves the Ischemia of tegmentum involves the lemnisci, superior peduncle, red nucleus lemnisci, superior peduncle, red nucleus and 3and 3rdrd nerve. nerve.

Effects:Effects: Ipsilateral lateral squint.Ipsilateral lateral squint.Contralateral loss of pain and temperatureContralateral loss of pain and temperatureContra lateral loss of tactile, muscle, joint Contra lateral loss of tactile, muscle, joint

and vibration sense.and vibration sense.

Parinaud’s syndromeParinaud’s syndrome

Due to compression of superior colliculus Due to compression of superior colliculus by a tumour of pineal gland.by a tumour of pineal gland.

Feature is the loss of upward gaze only Feature is the loss of upward gaze only other movements of the eye remain other movements of the eye remain normal (anatomical basis is obscure).normal (anatomical basis is obscure).

Weber’s

Benediktt”s

Parinaud’s

Areas involved in these three syndromes