LAB ANATOMY #3
This lab includes 2 sections for the Mid Brain: (plz read drs slide for extra
notes not mentioned in the lecture)
Midbrain At the level of inferior colliculus = Mid Brain at the
decussation of superior cerebellar peduncle= Mid Brain section in the
abducent Nucleus (the dr said it's important to know all the previous
- Basic Structures of this section from anterior to posterior:
1- 2 crus cerebri = (cerebral peduncles).
2- Substantia Nigra.
3- Tegmentum (core) which contains Ascending &descending tracts +
Reticular formation + nuclei of certain cranial nerves.
4- Cerebral aqueduct that separates tegmentum anteriorly from the
5- Tectum and its 2 inferior colliculi
Let's start with the crus cerebri:
Don’t forget that the Crus cerebri contains only descending fibers
(motor) including corticobulbar, corticospinal and corticopontine Fibers
& doesn’t have ascending fibers at all ( Ascending fibers are found in the
* Crus cerebri contains the following fibers from medial to lateral:
1- Frontopontine Fibers (corticopontine Fibers from the frontal lobe)
2- corticobulbar fibers.
3- corticospinal Fibers of the upper limb.
4- corticospinal Fibers of the trunk .
5- corticospinal Fibers of the lower limb,
6- tempo-parieto-occipto-pontine tract.
( 2+ 3+4+5)= represent the pyramidal tract which contains one million
Fibers & occupy 3/5 of the crus cerebri area.
While ( 1 + 6)= (corticopontine tract ) contains 20 Million Fibers &
form 2/5 of crus cerebri area.
Although pyramidal Fibers are less in number they occupy most of the
area because they are thickly myelinated & the corticopontine Fibers
are less myelinated so they occupy a smaller area.
There is another demonstration of the crus cerebri in some books
showing the whole area is occupied by the corticopontine Fibers & only
a small area is left for the pyramidal Fibers.
Both demonstrations the old &the new are acceptable.
Superior cerebellar peduncle:
output :comes from the dentate nucleus (mainly) , interpositus &
fastigial nuclei of the cerebellum, the fibers of these cerebellar nuclei
decussate in the most caudal part of Mid Brain ( at level of inferior
colliculus ) then part of these fibers synapse with red nucleus of the
contalateral side , others go around it forming what is called the
capsule of the red nucleus then both fibers continue to gather to
synapse with the thalamic nuclei ( VA , VL (mainly ) ). Then ascend
upward to motor & pre motor area of the cortex.
Because of the superior cerebellar peduncle decussation & pyramidal
decussation each cerebellar hemisphere controls the ipsilateral muscles
of the body.( Ya3ni the left cerebellar hemisphere fibers ascend to the
right cerebral hemisphere which controls muscles of the left side due to
- This is only applied to the cerebellar hemispheres, on the other hand
the vermis controls Muscles of both sides.
Substantia Nigra is formed of 2 parts:-
1- Substantia Nigra reticulata= which is similar to the internal
segment of globus pallidus in shape & function.
2- Substantia Nigra compata = that is responsible for dopamine
Substantia Nigra sends some Fibers to striatum that stimulate the
direct path way & Inhibit the indirect pathway.
Note : MLF appears all through the brainstem.
- The ventral trigeminothalamic tract &the dorsal trigeminothalamic
tract( which ascends close to the central tegmental tract) form the
- In the picture G represents the 2ry ascending taste fibers that arise
from the solitary nucleus, but we don’t know exactly the part of the
cortex that receives the taste sensation (some say its close to v1,2)
The nuclei of the 4th CN locate inside the central gray substance
The trochlear nerve is peculiar for 2 reasons:
1- It's the only CN which emerges from the dorsal Aspect of the Brain
2- It's the only CN that decussates before it exits.
As a consequence of decussation, a damage to the nerve after it
emerges causes ipsilateral weakness to superior oblique muscles(the
only muscle supplied by it), while damage to the nucleus itself will
cause contralateral weakness.
Actions of superior oblique muscle.( The action differs according to
the original state of the eye):
1 If the eye was abducted superior oblique muscle causes intorsion
(medial rotation ) of the eye .
2 If the eye was in neutral position or adduction the muscle causes
depression of the eye.
Therefore, the patient who has damage in the trochlear nerve is
unable to depress his eye, When looking down (e.g. going
downstairs), one eye will be depressed, and the other which is
supplied by the damaged nerve will be elevated , and this
unconjugate movement causes diplopia ( double vision ) because the
image in both eyes did not fall at the same focus on the retina.
Recall that there is a connection between the nuclei of the third,
fourth, and sixth, nerves called MLF -medial longitudinal fasciculus-
which presents all through the brain stem connecting the nuclei of
the three aforementioned nerves to achieve conjugated movement
of the eyes. Recall also that when the head rotates, the vestibular
nerve is simulated by semi circular canals , the fibers come out of the
semicircular canals to vestibular nucleus then go to the pontine gaze
center stimulating the abducent of the same side &the oculomotor
nerve of the other side ),so that when you rotate your head your eyes
will move to the opposite side in order to keep the focus of the eyes.
The trochlear nucleus receives fibers from:
1- bilateral corticobulbar fibers but mainely contralateral
2- vestibular fibers through the MLF (to coordinate
- In addition to the trochlear nuclei the central gray substance
contains the mesencephalic nuclei.
*Inferior colliculus ( part of the auditory pathway).
- In put : from the lateral lemniscus which ends at this level, unlike
the other 3 lemnisci which will continue in the higher sections.
- Out put: MGB.
The output of the inferior colliculus passes through the Brachium of
the inferior colliculus to reach the medial geniculate body of
The Mid Brain at the level of Superior colliculus = at the red nucleus
=at the oculomotor nuclei.
Input = from the cerebral cortex ( the corticorubrospinal tract )+
Cerebellum ( dentatorubrothalamic tract ).
Output: rubrospinal tract
Superior colliculus receives input from the frontal eye field
Fibers of the superior colliculus decussate in the dorsal tegmentum
(dorsal tegmental decusssation) forming the tectobulbar tract which
goes to the paramedian pontine reticular formation controlling the
abducent of the same side of the RF &the oculomotor of the other
Oculomotor nerve has a nucleus formed from 2 part:.
1- Lateral Somatic part : sends Fibers to 5 extraoccular muscles.
2- Medial Parasympathetic part : ( Edinger- westphal nucleus )
which sends Fibers to cilliary ganglia where they synape &
continue to supply 2 intraoccular muscles( cilliary muscle &
constrictor pupillae) .
- So the accommodation process that occurs by the action of cilliary
muscles needs parasympathetic stimulation.
- The nerve Fibers emerge at the interpeduncular Fossa. (which lies
in the mid line between two crus cerebri ).
Causes of oculomotor injury:
1- Herniation of midbrain.
2- Aneurysm which is the most common cause of oculomotor injury..
3- Peripheral neuropathy this is also applied to facial nerve injury .
3rd, 4th CNs emerge between the posterior cerebral Artery & the
superior cerebellar artery so in patient with Berry aneurysm the 1st
sign to appear is the previous two CN injuries.
* 3rd. CN injury will appear as:-
1- Down ward & out ward deviation of the eye Ball caused by lateral
rectus &superior oblique muscles.
2- Marked ptosis by affecting levator palpebrae superioris
( 90% of the levator palpebrae superioris is composed of skeletal
muscle fibers supplied by oculomotor , & only 10% are smooth
muscle Fibers supplied by sympathetic) .
So Marked ptosis caused by injury of the 3rd CN & Mild ptosis
caused by damaging the sympathetic (Horner syndrome )
3- Dilated pupil unresponsive to light ( we need parasympathetic
Fibers in the light reflex ).
4- Double vision when looking up ward & medially.
Mid Brain receives Blood supply from the posterior cerebellar
If we had an upper motor neuron lesion due to left sided stroke
involving the midbrain (the crus cerebri and what is behind it,
oculomotor nucleus ) leading to right spastic hemiplegia (due to
extra pyramidal damage), and ipsilateral oculomotor nerve injury
causing what we call crossed or alternating hemiplegia
(hemiplegia at one side, and oculomotor involvement at the other
اني أختٌ انغانَة رشا ...... وجودك فٌ هذا انعانم ٍمنحه مستوى اخز مه انزقٌ و
............ انجمال ......كم عام وانكون بوجودك به أسهي
ٌشكز خاص ل.. اٍمان عبدانغنٌ ،ربي انعساف ،اٍمان انعنتزً ،بشزى انطباخ
،مَدٍا........................ وكم حدا