Mid Brain

Midbrain (Mesencephalon)

 Upper

and shortest part of brain stem(2.5cm long and 2.5cm wide).  Passes through tentorial notch.  Related on each side to optic tract, parahippocampal gyrus, posterior cerebral artery.

Anteriorly related to interpeduncular structures  the

optic chiasm, the tuber cinereum and the infundibulum, the mamillary bodies, the posterior perforated substance.  (in that order from before backwards) Interpeduncular fossa and contents

Posterior relations  Posterior

ends of thalami (pulvinar).  Pineal body.  Splenium of corpus callosum.

External features 





Ventrally presents two crura which converge to enter the pons and form the posterior boundaries of interpeduncular fossa. Oculomotor nerves emerge between the crura. Trochlear nerves curve round the lateral borders of crura.

Dorsal surface  Colliculi

(superior and inferior pairs).  Superior and inferior brachia.  Pineal body (gland).

Cerebral aqueduct(of Sylvius)

Tectum

Cerebral peduncle

?

TS of midbrain

Internal structure  tectum–

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

D

Tg SN

V

Crus cerebri  Contains

corticonuclear and corticospinal fibers in the middle2/3.  Medially frontopontine fibers and laterally fibers from other lobes of cerebrum.

cn,cs fp

Tectum and tegmentum differ in structure.  Therefore

the midbrain is studied at two

levels;  1.at the level of inferior colliculus  2.at the level of superior colliculus

TS at the level of inferior colliculus 





Grey matter around the aqueduct (central grey) contains, IV nerve nucleus and mesencephalic nucleus of V Trochlear nerve fibers decussate and emerge dorsally and enter the cavernous sinus. This is the only cranial nerve to emerge dorsally!!!

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





Decussation of superior cerebellar peduncle is the notable feature at this level. The lemnisci are arranged in a curved manner and lateral lemniscus ends in the inferior colliculus. MLF, tectospinal and rubrospinal tracts are close to the midline from dorsal to ventral.

T.S at the level of superior colliculus 



Grey matter here contains: Oculomotor nucleus which also has a parasympathetic nucleus called the “EdingerWestphal nucleus (pupillo constrictory center), mesencephalic nucleus of V, and the red nucleus.

contd  E-W

nucleus supplies the sphincter pupillae and ciliaris muscle (for accommodation).  Superior colliculus receives afferents from retinae and gives efferents called tectospinal tracts.

Pretectal nucleus  Small

group of neurons lying superior to superior colliculus. It gives fibers to the E-W nucleus of both sides.  It is an important part of the pathway for pupillary light reflex and consensual light reflex.  Any injury to this nucleus causes  “Argyll-Robertson’s pupil” where the light reflex is lost but accommodation reflex is present. (ARP)

White matter at this level 





Decussation of tectospinal tracts forms dorsal tegmental decussation (of Meynert). Decussation of rubrospinal tracts forms ventral tegmental decussation (of Forel). Tegmentum contains all the lemnisci except the lateral lemniscus.

MLF  Contains

fibers coming from vestibular nuclei (mainly) and interconnects the nuclei of 3rd,4th,5th,6th and spinal accessory.  Coordinates the movement of the eyes, head and neck in response to the stimulation of vestibulocochlear nerve.

Blood supply  Superior

cerebellar, basilar and posterior communicating arteries.

Clinical aspects  Weber’s

syndrome- injury to oculomotor nerve and the crus cerebri due to the occlusion of posterior cerebral artery.  Effects:  Ipsilateral lateral squint (unopposed action of the lateral rectus).  Contralateral hemiplegia  Contralateral paralysis of lower face and tongue (corticonuclear fiber damage)

Weber’s syndrome (contd)  Drooping

of upper eyelid (ptosis) due to paralysis of levator palpebrae superioris.  Dilated pupil and loss of accommodation reflex (damage to E-W nucleus).

Benedikt’s syndrome  Ischemia

of tegmentum involves the lemnisci, superior peduncle, red nucleus and 3rd nerve.  Effects:  Ipsilateral lateral squint.  Contralateral loss of pain and temperature  Contra lateral loss of tactile, muscle, joint and vibration sense.

Parinaud’s syndrome  Due

to compression of superior colliculus by a tumour of pineal gland.  Feature is the loss of upward gaze only other movements of the eye remain normal (anatomical basis is obscure).

Parinaud’s

Benediktt”s

Weber’s

Areas involved in these three syndromes