Lecture 5: The Brain Stem and Cerebellum

Question 1

What are the functions of the brainstem?

Answer 1

• Acts as a conduit for many ascending and descending pathways that pass through the brainstem.
• Integration - the reticular formation
• Influences motor patterns e.g. balance, posture and muscle.
• Origin of cranial nerves which are associated with cranial nerve nuclei found in the brainstem.

Question 2

What are the anatomical relations of the brainstem?

Answer 2

Rostral -The midbrain is the most rostral structure which is continuous with the diencephalon.
Caudal - The most caudal structure is the medulla oblongata which is continuous with the spinal cord.
Ventrally - Brainstem is in contact with the the circle of the occipital bone.
Dorsal - Cerebellum

Running through the brainstem is IV Ventricle between the pons medulla anteriorly and the cerebrum posteriorly. The cerebral aqueduct is found at the level often the midbrain.

Question 3

Which cranial nerves arise from the brainstem? Which system are they apart of?

Answer 3

CN III - CN XII These are apart of the PNS.

Question 4

How is the brainstem divided?

Answer 4

Tectum - Most dorsal region found posterior to the ventricular system.
Tegmentum - Anterior to the ventricular system. This is the site of cranial nerve nuclei and some of the pathways pass through here.
Basal Area - The most ventral surface. This is the site of descending motor fibres.

Question 5

How is the ventral medulla organised? Which cranial nerves arise from here?

Answer 5

The ponto-medullary junction separates the pons from the medulla.

There is a midline fissure called the anterior median fissure. Beside this are bumps known as pyramids. Lateral to the pyramids is another fissure called the anterior-lateral sulcus. Cranial Nerve XII emerges from here. Beneath the pyramids is white matter fibres or nuclei travelling down from the cortex.

Interrupting the anterior median fissure is the descussation of the pyramids where these fibres cross over.

Lateral to the anterior lateral sulcus is the olives. Lateral to the olives is the posterolateral sulus. From here emerges CN XI, CN X and CN IX.

Question 6

How is the dorsal medulla organised?

Answer 6

The rhomboid area is floor of the 4th ventricle/rhomboid fossa. In the midline is the posterior median fissure; lateral to this a bump called the Gracile fasciculus. This contains many white fibres travelling from the spinal cord and up to the brain stem. Lateral to this is the Cuneate fasciculus again containing white matter fibres. These fascicles swell at the top forming tubercles (Cuneate tubercle and Gracile tubercle). Underlying the white matter fibres here are nuclei - Gracile nucleus and a Cuneate nucleus. These 4 structures form parts of the ascending sensory pathway bringing sensory information from the body up towards the brain. The pass here at the level of the medulla.

Question 7

What does the closed medulla refer to?

Answer 7

The closed medulla is at the level where the IV ventricle has closed. This is in the caudal part. The open medulla is in the postal area. The obex is the part where the IVth ventricle closes. Teh obex is continuous with the central canal.

Question 8

How is the ventral pons organised? Which cranial nerves arise form here?

Answer 8

The ponto-mesencephalic junction separates the pons from the midbrain.

In the medial aspect is the Basillar groove. The Basilar artery sits here. Transverse fibres travel here from each side of the cerebellar hemispheres.

On the middle of the pons is CN5. It has a motor root and a bigger sensory root. CN6 emerges lateral to the midline.

	The cerebella pontine angle is where we see CN 7 and 8.

Question 9

How is the dorsal pons organised?

Answer 9

Behind the pons is the floor of the IV ventricle.
In the Tegmentum (dorsal) - we see the peduncles and cranial nerve nuclei. The most prominent aspect is the cerebella peddles - superior, middle and inferior. They formed the attachment to the cerebellum. The SCP - attached the cerebellum to the midbrain. The MCP - attached the cerebellum to the pons and the ICP attached the cerebellum to the medulla.

The facial colliculus is formed due to fibres of the facial nerve.

Question 10

How is the ventral midbrain organised? Which cranial nerves arise from here?

Answer 10

The midbrain continues rostrally with the diencephalon. The cerebral peduncles are found here - there are a collection of descending motor (mainly) fibres from the cortex. They are the anterior part of the midbrain that connects the remainder of the brainstem to the thalami.

Between the two peduncles is the interpeduncular fossa. Emerging from this fossa is CN III in the pontomesencephalic junction.

Question 11

How is the dorsal midbrain organised? Which cranial nerve arises from here?

Answer 11

The most prominent feature of the dorsal view 4 paired pumps Colliclui. The 2 superior colliculi are important for visual system and the inferior colliculi are important for the auditory system. They have a projection extending form the colliculi to connect to the thalamus - these projections are called the brachium.

CN IV arises from the dorsal surface and wraps around closely the midbrain as it makes its way to the anterior aspect.

Question 12

What does a section of a midbrain often compared to?

Answer 12

Mickey Mouse
The ears are made of the cerebral peduncles with the interpeduncular fossa between them.
The head is the dorsal region made of the colliculi and the cerebral aqueduct between them. There is grey matter posterior to the cerebral peduncles where we find many dopamine containing neurones - substantia nigra.

Question 13

What is the reticular formation?

Answer 13

A complex, multisynaptic network of neurons within the tegmentum of the brainstem. It spans the length of the brainstem and even into the spinal cord. It has many functions:
Includes:
• Reticular nuclei – give rise to a descending motor pathway (reticulospinal tract)
• Vital autonomic centres – respiratory, cardiovascular
• Ascending Reticular Activating System (ARAS) – pass to the cortex and involved in consciousness (damage = coma). It is also involved in:
• Pain modulation
• Sleep-wake cycle
• Arousal

Question 14

What is the blood supply to the brainstem?

Answer 14

PCA supplies the midbrain.

The Basillar artery and its poutine branches supply the pons.

The most anterior portions of the medulla are supplied by branches of the ASA, laterally is the branches of the vertebral artery.
The medulla is supplied by the vertebral artery and its branches such as PICA in the posterolateral portion.

Question 15

What is the function of the cerebellum?

Answer 15

The cerebellum, or “little brain”, is primarily involved in coordination of movement, maintenance of balance and posture. Lesions cause loss of coordination, problems with balance and posture and loss of muscle tone.

Question 16

What are the relations of the cerebellum?

Answer 16

Found under the occipital lobes of the cerebral hemispheres and dorsal to the brainstem. Attached to the brainstem by three paired cerebellar peduncles .

Question 17

How is the cerebellum organised?

Answer 17

The cerebellum also has lobes and hemispheres. The primary fissure divides the anterior lobe form the posterior lobe. The vermis separates the two hemispheres.

	The nodulus is an extension of the vermis that extends into the ventral view.
	The tonsils are two bumps on the inferior view lateral to the vermis. They are just posterior to the medulla. Increased intracranial pressure may cause the tonsils to herniate into the foramen magnum, compressing the medulla oblongata. 

	The flocculi are found on either side tucked under the peduncles. 

In the ventral surface is the flocullarnodule node formed by the two flocculi and the nodes. This is found at the level of the horizontal fissure which separates the lobes.

Question 18

Which brain stem structure contains the most tectum?

Answer 18

The midbrain

Question 19

At what level is the lesion in locked-in syndrome?

Answer 19

Locked in syndrome typically is due to a injury at the level of the pons. The basal area of the pons has many descending pathways. Any injuries affecting this area interfere with the motor information going to the parts of our body. Patient are tetraplegic but eye movements are intact (these nerves arise above the pons). Patients can breathe and their heart is functional as the tegmentum where we find these centres are unaffected.

Question 20

What is the discussion of the pyramids?

Answer 20

Interrupting the anterior median fissure of the medulla is the decussation of the pyramids. Decussation means crossing over of the fibres.

	Normally our cerebrum starts a motor instruction and sends it through the brains stem, to the SC and to the part of the body we want to move. It is the opposite cerebral hemisphere that controls the contralateral part of the body. This crossing over occurs at the decussation  of the pyramids.

Question 21

What is the inferior cerebellar peduncle?

Answer 21

The inferior cerebellar peduncle is a white matter collection of axons that connect the medulla to the cerebellum. They are found dorsally at the rostral medulla.

Question 22

Give some relations between the cranial nerves and blood supply of the brain.

Answer 22

Overlying AICA is CN VI

Between SCA and PCA is CN III.