Midbrain Anatomy And Clincial Correlation

Question 1

3 anatomically district components of the midbrain

Answer 1

Tectum (most dorsal part)

• includes superior and inferior colliculi
• separates via PAG from the tegmentum

Tegmentum (medial part)
- separated from the cerebral peduncle via the substantia Nigra

Cerebral peduncle (most ventral part)

Question 2

Caudal midbrain tectum portion

Answer 2

Contains the inferior colliculus which is integral for the auditory pathway (these inputs get to the inferior colliculus via the lateral lemniscus)
- relays auditory information from the cochlea

Question 3

Caudal midbrain tegmentum portion

Answer 3

Broken into both central and lateral portions

Central only contents:

• portion of the reticular formation
• raphe nuclei
• decussation of the superior cerebellar peduncle

Lateral only contents:

• lateral lemniscus
• spinothalamic tracts
• trigeminothalamic tract
• medial lemniscus

both parts contain the PAG and nucleus of the trochlear nerve

Question 4

PAG

Answer 4

Transitional region between the tectum and the tegmentum regions

Composed mainly of tightly packed cells surrounding the cerebral aqueduct

Has the following functions:

• regulation of autonomic functions
• affective and emotional processes
• modulate pain impulses

Question 5

Caudal mid brain Cerebral peduncle portion

Answer 5

Contain massive bundles of descending axons from the cerebral cortex

They are organized in the following manner

• medial 3/5ths = corticobulbar and corticospinal fibers
• lateral 1/5th = parietal/occipital/temporopontine fibers
• medial 15th = frontopontine fibers

Question 6

Substantia Nigra

Answer 6

Structures that is situated in the area between the cerebral peduncle and the tegmentum

Contains 2 sections

1) pars compacta:
- medially located
- functions is to synthesize and release dopamine

2) pars reticulata
- laterally located
- functions is to synthesize and release GABA

• Parkinson’s is caused by Pars compacta loss*

Question 7

Rostral midbrain tectum contents

Answer 7

Superior colliculus is here and functions to be a visual reflex center
- recieves retinal inputs and causes visual reflex to sudden movements in the visual field (I.e a bolt of lightening in the peripherals causes you to look directly at it out if instinct)

Question 8

Rostral midbrain tegmentum contents

Answer 8

Contains the following:

• oculomotor nucleus complex (general sensory and visceral efferent components)
• note CN3 passes through the red nucleus*
• red nucleus
• central tegmentum area which contains dopaminergic fibers

Question 9

Oculomotor nuclear complex

Answer 9

Is a V-shaped region on the rostral midbrain, located between he medial longitudinal fasciculus and the PAG

Consists of the following 2 divisions:
1) somatic efferent cells of the Oculomotor nucleus
(Functions in controlling 4/6 muscles of the eye)

2) visceral efferent cells of the E-W nucleus
* has two divisions*

Question 10

The two divisions of the E-W nuclei

Answer 10

1) preganglionic cells (EWpg)
- provides innervation for pupillary constriction and lens accommodation (parasympathetics)

2) centrally projecting cells (EWcp)
- provides innervation for modulating stress/eating and drinking

Question 11

Red nucleus

Answer 11

Presents in the rostral midbrain and caudal diencephalon

Functions to give rise to the rubrospinal tract (facilitates flexor motor neurons of the contralateral side) and supplies modulating activity to the contralateral cerebellar cortex

Question 12

How are the CN 3/4 nuclei orientated in the midbrain?

Answer 12

3 is more rostral (on level with red nucleus) ; 4 is more caudal

Question 13

Medial midbrain syndrome (Weber’s)

Answer 13

Primarily caused by a lesion with the paramedian branches of the PCA

Structures involved

• corticospinal fibers in crus cerebri
• corticobulbar fibers in the crus cerebri
• oculomotor nuclei

Clinical presentation:
- contralateral hemiplegia of the UE/trunk and LE

• tongue deviates AWAY from the lesion
• contralateral facial weakness
• ipsilateral partial paralysis of eye motions
  (eye is down and out and pupil is dilated/fixed)

Question 14

Central midbrain syndrome (Claude)

Answer 14

Occurs when lesions to branches of the PCA are present

Structures involved:

• oculomotor nerve
• cerebellothalamic tract
• potentially the red nucleus

Presentation
- ipsilateral paralysis of eye movement (eye is down and out and the pupil is dilated)

• contralateral ataxia and tremors of cerebellar origin

Question 15

Benedikt syndrome

Answer 15

A combination of central and midbrain syndromes and occurs usually with large strokes
- blood supply = branches of the PCA

Structures involved:

• corticospinal fibers
• oculomotor nerve
• red nucleus

Clinical presentation:

• contralateral hemiplegia
• ipsilateral partial paralysis of eye movement with pupil dilation
• ataxia

Question 16

Dorsal midbrain syndorme (parinuads)

Answer 16

Usually caused by tumors or demyelination of the dorsal midbrain

Structures involved

• superior colliculus
• cerebral aqueduct
• CN 3and 4

Presentation
- cant look up (usually bilateral)

• hydrocephalus
• ophthalmoparesis with large fixed pupil