Brainstem 3

Midbrain

Dr Rania Gabr

Objectives• Identify the gross features of the brainstem.• Briefly describe the internal structure of the brainstems (ascending

and descending pathways, sensory and motor cranial nuclei, substantia nigra, red nucleus, olivary nucleus and reticular formation).

• Describe the main connections of the sensory cranial nuclei.• Describe the main connections of the motor cranial nuclei. • Review the blood supply of the brainstem.• Describe lesions in the brainstem such as medial medullary

syndrome and lateral medullary syndrome.• Describe the main connections of the substantia nigra and the red

nucleus.

Shortest part of the brain stem, not more than 2cm in length, lies in the posterior cranial Fossa.

For descriptive purposes, divided into:Dorsal tectum and right and left Cerebral Peduncles.

-Cerebral peduncles contains:1-Descending fibers that go to the cerebellum via the pons2-Descending pyramidal tracts-Running through the midbrain is the hollow cerebral aqueduct which connects the 3rd and 4th ventricles of the brain.

MidbrainCrus cerebri

Each cerebral peduncle divides further into ventral crus cerebri (massive fibrous mass) and a dorsal Tegmentum by a pigmented lamina“ Substantia nigra”

MID BRAIN – VENTRAL SURFACE Large column of

descending fibers (crus cerebri or basis pedunculi), on either side, separated by a depression called the interpeduncular fossa with posterior perforated substance.

Nerve emerging from Midbrain (one):

• Occulomotor (3rd): from medial aspect of crus cerebri.

Dorsal surface-External Features• On the posterior surface, we find:• Superior and inferior colliculi• Trochlear nerve emerges below the

inferior colliculus• Superior and inferior brachii• Superior medullary velum

• Superior brachium: a ridge of white matter carrying visual information from the superior colliculus to the optic tract

• Inferior brachium: a ridge of white matter passing anteriorly from each inferior colliculus to the medial geniculate body of the thalamus

MID BRAIN – DORSAL SURFACE Marked by 4 elevations:1. Two superior colliculi:

concerned with visual reflexes.2. Two inferior colliculi: forms

part of auditory pathway. Nerve emerging from

Midbrain (one):• Trochlear (4th): just caudal to

inferior colliculus (The only cranial nerve emerging from dorsal surface of the whole brain stem).

INFERIOR COLLICULUS Level• Inferior colliculus is a large

nucleus of gray matter that lies beneath a corresponding surface elevation.

• It is part of the auditory pathway.

• It receives fibers from the lateral lemniscus.

• Its efferent fibers pass to the thalamus

1. Trochlear nucleus: lies in the central gray matter close to the median plane just posterior to the medial longitudinal bundle. The fibers of the trochlear nerve decussate in the superior medullary velum.2. Decussation of the

superior cerebellar peduncles in the mid line.

INFERIOR COLLICULUS Level

3. Substantia nigra:• Occupies the most ventral

part of the tegmentum. • It consists of pigmented,

melanin containing neurones.

• It projects to the basal ganglia. Its degeneration is associated with Parkinson’s disease.

INFERIOR COLLICULUS Level

Mask Face

Pill-Rolling Tremors

Flexion of the Trunk

Slow Shuffling Feet movement

• Composed Of: – Spinal (Lateral &

anterior spinothalamic tracts)

– Trigeminal (Lateral & medial).

– Lateral lemniscus.– Medial lemniscus.

• Position: – Deeply placed lateral

to the medial longitudinal fasciculus.

ASCENDING LEMINISCI

CRUS CEREBRI It is a massive mass ventral

to the substantia nigra. It consists entirely of

descending cortical efferent fibers (Frontopontine, Corticospinal & corticobulbar and Temporopontine Fibres) to the motor cranial nerve nuclei and to anterior horn cells.

Involved in the coordination of movement.

SUPERIOR COLLICULUS Level A large nucleus of gray

matter that lies beneath corresponding elevation.

It forms part of the visual reflexes.

Its efferent fibers go to the anterior horn cells & to cranial nuclei 3, 4, 6, 7 & 11).

It is responsible for the reflex movements of the eyes, head and neck in response to visual stimuli, as in following a moving object or altering the direction of the gaze.

SUPERIOR COLLICULUS Level 1. Occulomotor nucleus: • Situated in the central

gray matter close to the median plane.

• The fibers of the occulomotor nerve passes anteriorly through the red nucleus to emerge on the medial side of the crus cerebri.

SUPERIOR COLLICULUS Level2. Red nucleus : • A rounded mass of gray

matter that lies between the substantia nigra and the cerebral aqueduct in the central portion of the tegmentum.

• Its red coloration is due to its vascularity and the presence of an iron containing pigment in the cytoplasm of its neurons.

• It is involved in motor control.

It is a complex matrix of nerve fibers & small groups of nerve cells that extends throughout the brain stem and projects to thalamic nuclei that influence large areas of the cerebral cortex.

RETICULAR FORMATION

RETICULAR FORMATION It has a number of

important functions i.e. Respiratory and Cardio- vascular centers are located in the medullary and caudal pontine reticular formation.

Some reticular neurons have long ascending and descending axons that allow profuse interaction with other neuronal systems.

RETICULAR TRACTS Reticulo spinal tracts:

Influence a muscle tone & posture

Reticular Activating system: RAS

Midbrain portion of RAS most likely is its center

Formed of some of the ascending fibers of the reticular formation.

They activate the cerebral cortex through the thalamus.

• RAS• Functions as a net or filter for sensory input.• Filters out repetitive stimuli. • Allows passage of infrequent or important

stimuli to reach the cerebral cortex.• Unless inhibited by other brain regions, it

activates the cerebral cortex – keeping it alert and awake.

RETICULAR NEURONES Raphe Nuclei:

Midline reticular nuclei.– Its ascending fibers to the

cerebral cortex are involved in the mechanisms of sleep.

– Its descending fibers to the spinal cord are involved in the modulation of Pain.

Locus Coeruleus: Pigmented neurons that lie in

the tegmentum of the caudal mid brain & rostral pons

It is the main noradrenergic cell group of the brain. The locus coeruleus is the principal site for brain synthesis of norepinephrine

Internal Features Midbrain at level of inferior colliculus shows the following:• Corticospinal fibers• Frontopontine fibers • Temporopontine fibers• Substantia nigra• Decussation of superior cerebellar peduncle• Medial longitudinal fasciculus• Medial lemniscus• Spinal lemniscus• Lateral lemniscus• Nucleus of trochlear nerve• Trigeminal lemniscus

Internal Features Midbrain at level of superior colliculus shows the following:• Corticospinal fibers• Frontopontine fibers • Temporopontine fibers• Substantia nigra• Red nucleus• Decussation of rubrospinal tracts• Medial longitudinal fasciculus• Medial lemniscus• Spinal lemniscus• Trigeminal lemniscus• Nucleus of oculomotor nerve

Clinical Notes

Raised intracranial pressure and Arnold –Chiari malformation leads to:• Herniation of medulla and tonsils of cerebellum• Traction of the lower 4 cranial nerves• Paralysis of the above mentioned nerves

Lateral Medullary Syndrome (Wallenberg)• Occlusion of posterior inferior

cerebellar artery• All structures supplied by this artery

will be affected:1-Nucleus ambiguous2-Nucleus of spinal tract of trigeminal N3-Vestibular nuclei4-Descending sympathetic fibers5-Inferior cerebellar peduncle

Medial Medullary Syndrome• Occlusion of medullary branch of

vertebral artery• All structures supplied by this artery

will be affected1-Pyramidal tract2-Medial leminiscus3-Hypoglossal nerve

Pontine Hemorrhage• Pons is supplied by basilar,

anterior inferior and superior cerebellar arteries

• Unilateral occlusion of one of the above vessels will affect the

1-Facial nerve2-Abducent nerve3-Vestibular nuclei4-Corticospinal tract5-Trigeminal nerve nuclei6-Cochlear nuclei7-Medial and spinal leminisci8-Middle cerebellar peduncle

Vascular Lesions in Midbrain• Weber’s syndrome • Occlusion of a branch of

posterior cerebral artery• Affects oculomotor nerve and crus cerebri• Benedikt’s syndrome• Affects red nucleus and medial

leminiscus

SUMMARYThe brain stem is composed (from above

downwards) of: midbrain, pons & medulla oblongata which are continuous with each other, with diencephalon above & with spinal cord below.

The brain stem is connected with cerebellum through three pair of cerebellar peduncles.

The brain stem is the site of cranial nuclei, the pathway of important ascending & descending tracts & the site of emergence of cranial nerves (from 3rd to 12th).

Cranial nerves (with the exception of 4th) emerge from ventral surface of brain stem.