Midbrain

Midbrain

1. Midbrain
Visual & auditory processing and reflexes
Fine-tuning of voluntary movements

2. Midbrain•Shortest part of brain stem
–2 cm in length
•Connects
–Pons (post. Cranial fossa) & Diencephalon
(mid. cranial fossa)
–Through tentorimcerebelli

3. External features
•Surfaces
–Ventral
–Dorsal
•Ends
–Superior (rostral)
–Inferior (caudal)

4. Midbrain –Ventral surface
•Features
–Peduncles
–Interpeduncular fossa
–Emerging structure
•3
rd
cranial nerve
–On medial aspect of
peduncle
•4
th
cranial nerve
–On lateral aspect of peduncle
–Only nerve emerging from
dorsal aspect of brain stem

5. Midbrain –Ventral surface
•Crossing structures of
peduncle anteriorly
–Optic tract
–Posterior cerebral artery
•Terminal branch of basilar
–Superior cerebellar artery
•Branch of basilar
3
rd
and 4
th
cranial nerves are lying between the
Posterior cerebral and superior cerebellar arteries

6. Dorsal surface-
External Features
•Has4 colliculi (corpora
quadrigemina)
–2 superior
–2 inferior
•Separated by
–Cruciform sulcus
•Vertical limb ends superiorly in
–Pineal body (part of epithalamus)
•Vertical limb ends inferiorly in
–Frenulum veli(median elevation
on superior medullary velum)
•Trochlear nerve emerges below
the inferior colliculus

7. Lateral aspect of Midbrain
•Superior brachium
–Passes from
•Superior colliculus
–To
•Lateral geniculate body
•Inferior brachium
–Passes from
•Inferior colliculus
–To
•Medial geniculate body

8. Cerebral aqueduct
•Cavity of Midbrain
•Connects
–III ventricle
•Above
–IV ventricle
•Below
•Acts an important
landmark
–To divide midbrain
•Into subdivisions

9. Internal structure -Midbrain
•Mid-brain is divided by
–A line drawn through
cerebral aqueduct
–Dorsal to the line
•Tectum
–Ventral to the line
•Cerebral Peduncle

10. Internal structure -Midbrain
•Cerebral peduncle part
–Has a pigmented area
•Rich in melanin
•Known as substantia nigra
•Wider medially narrow
laterally
•Horizontally placed
•Helped to divide peduncle into
3 areas
–Tegmentum (posteriorly)
–Cruscerebri(anteriorly)

11. Transverse section of midbrain
•Common features in
inferior and superior
colliculus are
–Crus cerebri(or basis
pedunculi)
–Substantia nigra

12. •Pigmented layer of gray matter
–Present in midbrain
–Lies between pons and subthalamus
–Part of extrapyramidal system
–Contains nerve cells with melanin
pigments
–Involves in dopamine synthesis
•2 parts
–Dorsal compact part (smooth)
–Ventral reticular parts (spiked)
Degeneration of Subtantianigra
results Parkinson’s Disease
Substantia nigra

13. Main connections of substantia nigra
•Efferent fibres
–Mainly to striatum
•Which is caudate and
lentiformnucleus
–Constitutes nigrostriatal
pathway
•Rich in dopamine
•Control of muscle tone
•Prevents tremor & other
involuntary movements by
•Inhibiting Globaspallidus
& caudate nucleus

14. CS of Midbrain
Crus cerebri–Lies anterior to substantia nigra
–Contains descending tracts
–Medial 1/5
•Frontopontine fibres
–Middle 3/5
•Corticospinal & cortico-nuclear fibres
–Lateral 1/5
•Parieto-pontine
•Occipito-pontine fibres
•Temporo-pontine
•Above continuous with corona radiata&
below with longitudinal fibresof basilar
part of pons

15. •White matter
•Cortico-ponto-cerebellar
fibers
–Synapse with pontine nuclei
–Reaches cerebellum
•Through middle cerebellar
peduncle
•Corticonuclearfibers
–To motor nuclei of cranial
nerve
•Corticospinal fibers
–Form the pyramids
CS of Midbrain
Crus cerebri

16. Internal features –Inferior collicularlevel
Tectalpart -Grey matter
•Nucleus of Inferior
colliculus
•Reflex center for auditory
stimuli

17. Internal features –Tectalpart
Inferior collicularlevel•Inferior colliculus
–Act as relay station for auditory pathway
–Reflex centrefor response to auditory
stimuli
•Afferent fibres
–Lateral lemniscus
–Cortical fibres
•Efferent fibres
–Medial geniculate body
•By inferior brachium
•Connections for co-ordination
–Efferent to
•Superior colliculi
•Motor nuclei of cranial nerves
–Tectobulbar
•Spinal motor neurons
–Tectospinal
»Seen in middle part of tegmentum

18. Internal features –Tegmental & tectalpart
Lower midbrain
•Trapezoid body
–Lies in lower part of pons
–Decussation of the fibresfrom cochlear
nuclei
–Ascend as lateral lemniscus
•Lateral lemniscus
–Seen
•Upper pons
•Lower midbrain
•End in inferior colliculus

19. Internal features –Tegmental part
Inferior colliculus level
•Contains
–Grey matter
•Cranial nerve Nuclei
–White matter
•Ascending
•Descending
•Crossing

20. Internal features –Inferior collicularlevel
Tegmentum
•Grey matter
–Cranial nerve nuclei
•Trochlear nucleus
–Lies in ventral part of
central grey matter
•Mesencephalic nucleus of
trigeminal
–Lies in lateral part of
central grey matter
–Proprioceptive fibresfrom
muscles of mastication

21. Internal features
Inferior collicularlevel
Tegmentum
•Ascending tracts
•Has 4 lemnisci
–Lateral lemniscus conveys
•Auditory sensation to inferior
colliculus from two ears, mainly
the opposite ear
–Spinal lemniscus conveys
•Pain, temperature, touch and
pressure sensations from
opposite side of body
–Trigeminal lemniscus conveys
•Sensations from opposite side of
face
–Medial lemniscus conveys
•Discriminative touch and
vibration sense from opposite
side of body

22. Internal features –Tegmental part
Lower midbrain
•Lateral lemniscus
–Seen
•Upper pons
•Lower midbrain

23. Anterolateral system
•Spinal lemniscus
–Carry contralateral
•Anteiorspinothalamic
–Crude touch
•Lateral spinothalamic
–Pain & temperature
–Can be seen
•Medulla
•Pons
•Midbrain

24. •First order neuron
–Located in
•Trigeminal ganglion
•Second order neuron
–Located in
•Spinal nucleus
–(Pain & temperature)
•Main pontine nucleus
–(touch & pressure)
•Trigeminal lemniscus
–Fibresfrom second order neuron
–Cross opposite side
–Form trigeminal lemniscus
•Third order neuron
–Lies in thalamus
Trigeminal lemniscus

25. Dorsal system
•Medial lemniscus
–Carry dorsal column
modalities
•From contralateral side
–Can be seen
•Upper medulla
•Pons
•Midbrain

26. CS of Midbrain
Inferior collicularlevel
•Descending tracts
•Present in peripheral
part of tegmentum
–Lateral lemniscus
–Spinal lemniscus
–Trigeminal lemniscus
–Mediallemniscus

27. Internal features –Tegmental part
Lower midbrain
•Decussating fibres
–Decussation of superior
cerebellar peduncle
•Mainly dentate rubralfibres
–Fibresarising from
•Dentate nucleus
•Passing through superior
cerebellar peduncle
•Main destination is Red
nucleus
•Those fibresdecussate here
–In the midline
–At anterior part of
Tegmentum

28. Internal features –Tegmental part
Inferior collicularlevel
•White matter
–Fibresinterconnecting
cranial nerve nuclei
•Medial longitudinal fasciculus
–Descending tracts
•Tectospinal
•Rubrospinal
–Lies ventral to decussation of
superior cerebellar peduncle

29. Medial Longitudinal Fasciculus
•Formed of
–Descending & ascending fibers
•Has two components
–Ascending component
•Establishes connections with the
nucleiof
–Occulomotor, Trochlear &
Abducentnerves (motor nuclei
for extraoccularmuscles) for
coordination of head & eye
movements
–Descending component
•Extends into the spinal cord as the
medial vestibulospinaltract

30. RubrospinalTract
•Origin
–From red nucleus
–Located in midbrain
•Termination
–Lateral column of spinal
cord
•Seenat
–Lower midbrain
–Pons
–Medulla

31. TectospinalTract
•Origin
–Superior colliculus of
midbrain
•Termination
–Anterior Column
•Function
–Motor function of the
Skeletal muscles of the
head and eyes in response
to visual stimuli

32. Internal features –Tegmental part
Inferior collicularlevel
•White matter
–Fibresinterconnecting
cranial nerve nuclei
•Medial longitudinal fasciculus
–Descending tracts
•Tectospinal
•Rubrospinal
–Lies ventral to decussation of
superior cerebellar peduncle

33. Internal features –Tegmental part
Superior colliculus level
•Contains
–Grey matter
•Cranial nerve Nuclei
•Red nucleus
–White matter
•Ascending
•Descending
•Crossing

34. Internal features –Tegmental part
Superior colliculus level
•Grey matter
–Cranial nerve
•III nerve nuclei
–Nerve emerging through anterior
aspect
–Passing through Red nucleus
–Supplies all extraocular muscles
»Except
•Lateral rectus
•Superior obliquus
•Edingerwestphalnucleus (dorsal
part of oculomotor complex)
–Fibrespassing along with III N
–Supplies intra ocular muscles
»Sphincter pupillae
»Ciliaris
•Mesencephalic nucleus of V
–Lies in lateral part of central grey
matter
–Proprioceptive fibresfrom
muscles of mastication

35. Red Nucleus
•Ovoid mass of grey matter
•Involved in motor control
•Pink in colour–in fresh state
•Has multipolar neurons
•Situation
–In tegmentum
–Dorsal to medial part of
substantia nigra

36. Red nucleus -connections
•Afferents
–Corticorubraltract:
•from motor and premotor areas of
cerebral cortex
–Dentatorubraltract:
•from opposite dentate nucleus of
cerebellum
•Forms decussation in lower midbrain

37. Red nucleus -connections
•Efferent
•Cross in ventral tegmental
decussation
•Forms Rubrospinaltract
•Can be seen in lower midbrain &
lower brainstem
–Rubrothalamictract
•To thalamus (VA & VL nuclei)
•Lesion
–results in signs of cerebellar damage
•contralateral tremor and ataxia

38. Internal features –Tegmental part
Superior colliculus level
White matter
•In midline
–Medial longitudinal
fasciculus
–Decussating fibres
•Dorsal tegmental decussation
–Fibresarising from superior
colliculus forms
–Continues as tectospinal &
tectobulbartracts
•Ventral tegmental decussation
–Fibresarising from red
nucleus forms
–Then continues as
rubrospinaltract

39. Internal features –superior collicularlevel
Tectalpart -Grey matter
•Superior colliculus
–Part of visual system
–Optic reflex center
•Changing the position of eyes &
head in response to moving objects
•Afferents are
–From retina
•Through superior brachium (visual)
–From spinal cord
•Through Spinotectal(touch sense)
–From inferior colliculus
•Auditory impulses
–Corticotectalfibres
•From
–Visual cortex of occipital lobe
–Frontal eye field of frontal lobe

40. Internal features –superior collicularlevel
Tectalpart -Grey matter
Superior colliculus
•Efferent fibres
–Tectospinal
–Tectobulbar
•Forms dorsal tegmental
decussation

41. Internal features –superior collicularlevel
Tectalpart -Grey matter
•Pretectalnucleus
–Response for pupillary
light reflex
•Situation
–Close to superior
colliculus
–At junction of midbrain
& diencephalon
•Afferents
–Retina, superior
colliculus, visual cortex
•Efferent
–Edingerwestphal

42. Arterial supply
•Basilar arteries
•Branches of basilar
–Posterior cerebral
–Superior cerebellar
•Posterior communicating
•Anterior choroidal

43. Weber’s syndrome
•Lesion in ventral part of
midbrain
–Mainly due to occlusion of
posterior cerebral artery
•Involves oculomotor nerve &
basis pedunculi
–Upper motor neuron paralaysis
of opposite side
–Paralysis of all muscles
supplied by oculomotor
•Lateral squint

44. BenediktSyndrome
•Necrosis involves
medial lemniscus, spinal
lemniscus and red
nucleus
–Ipsilateral lateral squint
•Due to oculomotor
involvement
–Contralateral pain &
temperature loss
•Due to spinal lemniscus
involvement