Midbrain

Features of the midbrain

Connects the pons and cerebellum with the forebrain

Traverses the tentorial notch

Lies in the posterior cranial fossa and overlaps with the parahippocampal gyrus

Cerebral aqueduct communicates the third ventricle with the fourth

Develops from the mesencephalon

External features of the midbrain

Crura cerebri

• Extends from the cranial border of pons to the undersurface of the cerebral hemisphere

• Forms posterolateral boundary of the interpeduncular fossa

• Medial sulcus separates the crus cerebri from the interpeduncular fossa

Ventral surface crossed by:

• Optic tract

• Posterior cerebral artery

• Superior cerebellar artery

• Teniae pontis

• Oculomotor and trochlear nerves pass through pontine branches and superior cerebellar artery

Dorsal surface presents:

• Superior and inferior colliculus

• Pineal gland

• Superior medullary velum

• Colliculi separated by cruciform sulcus

Interpeduncular fossa

Anteriorly- optic chiasma

Anterolaterally- optic tracts

Posterolaterally- crus cerebri

Posteriorly- cranial border of pons

Laterally overlapped by parahippocampal gyrus

Floor presents:

• Infundibulum

• Tuber cinereum

• Median eminence

• Mamillary bodies

• Posterior perforated substance

Colliculi of midbrain

Superior colliculus

• Larger and darker than inferior

• Concerned with optic senses

• Connected to lateral geniculate body

Inferior colliculus

• Smaller but more prominent

• Concerned with auditory senses

• Connected to medial geniculate body

Internal structure

Demarcated into:

• Ventral crus cerebri

• Substantia nigra

• Dorsal tegmentum

• Tectum

Studied at the level of:

• At the level of superior colliculus

• At the level of inferior colliculus

*Tegmentum differs but crus cerebri remains the same

Crus cerebri

Contains:

• Corticospinal fibres

• Corticonuclear fibres

• Corticopontine fibres

• Frontopontine fibres (medial 1/6)

• Temporoponitne, parietopontine and occipitopontine (lateral 1/6)

Substantia nigra

Pigmented sheet of multipolar motor neurons associated with the production of dopamine

Extends from the cranial border of the pons to the subthalamic region

Divided into:

• Dorsal pars compacta (rich in melanin, cells small)

• Ventral pars reticularis (rich in iron, larger cells)

• Pars lateralis insignificant in man

Afferents

• Nigro-stiratal fibres from putamen and caudate nucleus which convey GABA to the pars reticularis

• Also connected to the cerebral cortex, hypothalamus, subthalamus, spinal cord, thalamus (VA, VL, DM nucleus)

FUNCTION: Help in smooth and skillful performance of voluntary movements

Parkinson’s disease

Dopamine levels of the substantia nigra and corpus striatum dramatically decrease

Tegmentum at the level of the inferior colliculus

Trochlear nerve nucleI

• Nucleus extends throughout the caudal half of the midbrain

• Fibres pass dorsally to mesencephalic nucleus of V

• Thinnest cranial nerve and only one that exits from the dorsal aspect

• Only nerve that decussates before its target (expect optic)

Mesencephalic nucleus of trigeminal nerve

• Extends from main sensory nucleus of pons

• Cells of pseudounipolar

• Mesencephalic nucleus receives proprioceptive sensation

• Chief nucleus receives touch and pressure

• Spinal nucleus receives pain and temperature

Decussation of superior peduncle

• Efferents derived from dentate, globose and emboliform nucleus

• After decussation divide into ascending and descending fibres

• Ascending fibres terminate in VL nucleus and red nucleus (uncrossed end at midbrain and periaqueductal gray matter)

• Descending fibres terminate at reticular nuclei of pons and inferior olivary complex

Also present:

• Rubrospinal tract

• Tectospinal tract

• Median longitudinal fasciculus

• Medial lemniscus

• Lateral lemniscus (ends at inferior colliculus)

• Trigeminal lemniscus

• Spinal lemniscus

Tegmentum at the level of the superior colliculus

• Oculomotor nucleus

• Red nucleus

Occulomotor nucleus

• Consists of somatomotor and visceromotor nuclei (Edinger Westphal)

• Dorsolateral segment supplies inferior rectus

• Intermediate segment supplies inferior oblique

• Ventromedial segment supplies medial rectus

• Cuadal centre supplies levator palpebrae superioris

• Median raphe nucleus supplies superior rectus

• Nucleus of Darkschewitsch is an accessory nucleus that is concerned with eye movements and reflex gaze movements

• Edinger Westphal nucleus concerned with parasympathetic supply of ciliaris and sphincter pupillae

Red nucleus

• Ovoid mass containing iron

• Contains mostly multipolar cells

• Made up of caudal pars magnocellularis and cranial pars parvocellularis

Afferents

• Cerebellorubral (contralateral dentate, emboliform and globose)

• Corticorubral (ipsilateral from area 4&6)

• Pallidorubral (globus paliidus)

• Tectorubral (superior colliculus)

Efferents

• Rubrospinal

• Rubrobulbar

• Rubro-reticular fibres

FUNCTION: Forms a prominent and important motor nucleus concerned with the maintenance of posture and muscle tone

Tectum of midbrain

• Situated dorsal to the aqueduct

• Consists of inferior and superior colliculus

• Colliculi derived from dorsal lamina of the periaqueductal gray matter

Inferior colliculus

• Connected to the medial geniculate body

Afferents

• Lateral lemniscus

• Contralateral inferior colliculus

• Medial geniculate body

Efferents

• Medial geniculate body

• Opposite inferior colliculus

• To cerebellum as tectocerebellar fibres

• Ipsilateral superior colliculus

• Superior olivary nucleus and cochlear nuclei

FUNCTION: Acts as relay nucleus of auditory pathway

Superior colliculus

• Connected to the lateral geniculate body

Afferents

• Retinotectal fibres

• Corticotectal fibres (to visual asscoiation areas)

• Spinotectal fibres

• Inferior colliculus

Efferents

• Tectospinal

• Tectobulbar (to oculomotor, abducens and trochlear)

• Few tectothalamic

FUNCTION: Reflex and integrating centre of visual system

Pretectal nucleus

• Receives fibres from the optic tract

• Concerned with the pupillary light reflex

Reticular formation

Blood supply of midbrain

1. Posterior cerebral artery and superior cerebellar artery (branches of basilar artery)

2. Direct branches from basilar artery

3. Branches of posterior communicating

4. Branches from anterior choroidal artery

Venous drainage

• Into great cerebral vein and basal vein

Midbrain lesions

As a result of:

• Vascular occlusions of the branches of the posterior cerebral artery

• Aneurysm in the posterior part of the Circle of Willis

• Tumors of the pineal gland

• Hydrocephalus

Perinaud’s syndrome (dorsal midbrain)

Benedikt’s syndrome (paramedian midbrain)

Weber’s syndrome (medial midbrain)