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The midbrain is comprised of what structures?

The tectum, tegmentum and crus cerebri


What are the structures found in the midbrain at the level of the superior colliculus?

1. Superior colliculus
2. Oculomotor nucleus

3. Red nucleus

4. Superior cerebellar peduncle

5. Substantia nigra


Unilateral damage to the superior colliculus produces:

Contralateral visual field neglect.
Impaired tracking, but with no deficit in eye movements.


Stimulation of the superior colliculus produces what?

Contralateral conjugate deviation

1. Stimulates rostral interstitial nucleus of the MLF to excite the ipsilateral 3rd cranial nerve.

2. Contralateral CN 6 via the PPRF


Input to the superior colliculus

1. Contralateral retina
2. Cortex - frontal, parietal, temporal and occipital

3. Brainstem nuclei

4. Spinal cord


Output of the superior colliculus

Parabigeminal nucleus



RiMLF - rostral interstitial nucleus of the MLF

Reticular formation

Spinal cord


What are the three portions of the oculomotor nucleus. 

1. Lateral part
2. Medial part

3. Central part


Lateral portion of the CN3 nucleus

Ipsilateral cell columns for inferior rectus, inferior oblique, and medial rectus muscles


Medial part of the CN 3 nucleus

Contralateral cell columns for the superior rectus muscle


Central part of the CN 3 nucleus

Cell columns for the levator palpebrae superioris and the Edinger-Westphal nucleus. 


Input to the CN3 nucleus

Nucleus of Cajal
Contralateral abducens nucleus

Perihypoglossal nucleus


Pretectal olivary nucleus

Flocculus via the nucleus prepositus to control vertical eye movements.


The PPRF projects to 

The abducens nucleus (conjugate horizontal gaze center) and the RiMLF (conjugate vertical gaze center)


The RiMLF input and output

Input: Superior vestibular nucleus and PPRF

Output: Oculomotor complex/inferior rectus portion.


Role of the interstitial nucleus of Cajal

Function: vertical eye movements, pursuit, head movements and posture.

Input: Superior and medial vestibular nuclei, pretectum, FEF and fastigial nuclei.

Output: Ipsilateral medial vestibular nuclei and spinal cord. Trochlear nuclei. Contralateral oculomotor nuclei.


What are the subcommissural organs

Modified ependymal cells in the aqueduct below the posterior commissure which lack a BBB.


What is the posterior commissure

White matter tract which lies posterior to the aqueduct at the junction of the midbrain/diencephalon. It is involved with the light reflex. Contains fibers from the pretectal nuclei, nucleus of the posterior commissure, interstitial nuclei and Darkshevich's nucleus. 


Nuclei in the pretectal region are involved with..

Pupillary light reflexes. 


Role of the inferior colliculus

The inferior colliculus is responsible for the tonotopic organization of auditory information and projects via the brachium of the inferior colliculus to the MGB. 


Key structures in the midbrain at the level of the inferior colliculus include

1. Inferior colliculus
2. Parabigeminal area

3. Periaqueductal gray

4. Interpeduncular nucleus


Role of the parabigeminal area

Connects to the visual system via the superior colliculus.

Located ventrolateral to the inferior colliculus.


Components of the periaqueductal gray

1. Mesencephalic nucleus of 5.
2. Locus ceruleus

3. Ventral and dorsal tegmental nuclei

4. Dorsal nucleus of the raphe

5. Medial nucleus of the raphe


Function and connections of the periaqueductal gray

Function: Central analgesia, vocalization, control of reproductive behavior, aggressive behavior and upward gaze.

Connections: Hypothalamus, spinal cord, locus ceruleus, raphe nucleus, reticular formation.


Features of the interpeduncular nucleus

1. Located just dorsal to the interpeduncular fossa.
2. Input: habenular nucleus via the fasciculus retroflexus

3. Output: diffuse cholinergic fibers to the CNS.


What is the midbrain tegmentum

This is the ventral surface of the midbrain, containing the reticular formation and tracts, periaqueductal gray(surrounding the cerebral aqueduct), substantia nigra, red nucleus, CN3 and CN4 nuclei. 


Where is the red nucleus located and what fibers run through it?

It is located in the reticular formation.
Fibers of CN3 and the superior cerebellar peduncle run through it.


Input to the red nucleus

1. Deep cerebellar nuclei: rostral 1/3 from the dentate and cadual 2/3 from the interposed. These fibers exit via the SCP and cross to the contralateral side in the midbrain to get to the red nucleus.

2. Motor Cortex: Pre-central, pre-motor, supplementary motor and motor cortex


Output of the red nucleus

1. Contralateral cervical and lumbar spine via the rubrospinal tract
2. Contralateral nucleus interpositus, facial nucleus and medulla via the ventral tegmental tract.

3. Ipsilateral inferior olivary nucleus via uncrossed central tegmental tract


Role of the red nucleus

Maintains flexor muscle tone


Key components of the pons

1. Reticular formation
2. Isthmus rhobencephali

3. Locus ceruleus


Role, input and output of the pedunculopontine nuclei. 

Located in the lateral tegmentum ventral to the inferior colliculus. Controls locomotion and walking movements.

Input from the cortex, mGP, and SNpr.

Output to the thalamus and SNpc (ACH output)