Topography and Blood Supply of the Brainstem and Base of Brain

Question 1

“Posteriorly, the pons and medulla are separated from the cerebellum by the ____(a)____. The ventricle is continuous below with the ____(b)____, which traverses the lower part of the medulla and becomes continuous with the ____(b)____ of the spinal cord. Cranially, the ____(a)____ is continuous with the ____(c)____. The midbrain, pons and medulla are connected to the cerebellum posteriorly by the ____(d)____.”

Answer 1

(a) fourth ventricle
(b) central canal
(c) cerebral aqueduct (of Sylvius)
(d) superior, middle and inferior peduncles

Question 2

Identify the cranial nerves attached to each of the following stuctures:
(a) forebrain
(b) midbrain
(c) pons
(d) junction of pons and medulla
(e) medulla

Answer 2

(a) I and II
(b) III and IV
(c) V
(d) VI - VIII
(e) IX - XII

Question 3

State the functions of the brainstem.

Answer 3

1. Serves as a passage for the ascending tracts and descending tracts connecting the spinal cord to the different parts of the higher centres in the forebrain.
2. Contains the cranial nerve nuclei of CN III to XII. Spinal part of accessory nerve extends up to C5 segment of spinal cord.
3. Contains important reflex centres associated with the control of respiration and the cardiovascular system.
4. Controls consciousness (ascending reticular-activating system), regulates muscle tone (medial and lateral reticulospinal tracts), controls other vegetative functions (ANS), inhibits pain (periaqueductal and raphespinal tracts) through reticular formation of brainstem.

NOTE:
1. Ascending tracts, in general, pass through the dorsal part of the brainstem (tegmentum); while descending tracts through the ventral part (basilar part).
2. Pons contains the pneumotaxic and apneustic centres. This influences the dorsal and ventral respiratory groups of medulla. Cardiovascular centre lies in the medulla.

Question 4

State the external features of the midbrain on the ventral aspect. (4)

Answer 4

• crus cerebri (bulging white matter fibres): a part of the cerebral peduncle
• interpeduncular fossa: note that tuber cinereum of hypothalamus and mamillary bodies of hypothalamus are not part of the midbrain butare contained in that fossa
• posterior perforated substance
• oculomotor nerves

Question 5

State the external features on the dorsal aspect of the midbrain.

Answer 5

• quadrigeminal bodies (comprises of superior and inferior colliculi)/corpora quadrigemina. There’s a dorsal median sulcus separating the right and left colliculi.
• superior brachium: bulge connecting superior colliculus to the lateral geniculate body of the hypothalamus
• inferior brachium: bulge connecting inferior colliculus to the medial geniculate body of the hypothalamus
• trochlear nerves (unique cranial nerve for it’s arising from the dorsal aspect of the brainstem unlike other cranial nerves)

Question 6

State the internal features of the midbrain (you can see these features when you cut a sagittal section of the midbrain, without a microscope).

Answer 6

(a) quadrigeminal plate (tectum/tectal plate)
(b) cerebral peduncles, which have the following regions:
~ tegmentum (mesencephalic tegmentum)
~ substantia nigra
~ cerebral crus

[The cerebral aqueduct marks the junction between the tectum and the cerebral peduncles.]

(c) cerebral aqueduct [basically it connects 3rd and 4th ventricles and has CSF] and the periaqueductal grey matter

Question 7

Name ventral external features of the pons.

Answer 7

• the bulge of pons
• basilar sulcus [groove for basilar artery]
• middle cerebellar peduncle
• trigeminal nerve origin
• pontomedullary junction/sulcus [where abducens nerve arises]
• [Diagram]

Question 8

Name dorsal external features of the pons (7).

Answer 8

☛ floor of the fourth ventricle
☛ posterior median sulcus
☛ medial eminence [represents motor column of the brainstem]
☛ facial colliculus [represents the bulge of the abducens nucleus and facial nerve fibres]
☛ sulcus limitans [junction between the medial eminence and the sensory colums]
☛ vestibular area [sensory column within the pons]
☛ stria medullaris [junction between pons and medulla oblongata]
☛ [Diagram 1] [Diagram 2]

Question 9

Name ventral external features of the medulla oblongata.

Answer 9

• pontomedullary angle/sulcus [CN VI, VII and VIII arise from here]
• ventral median sulcus
• medullary pyramids [on either side of the ventral median sulcus. They contain motor fibres of the corticospinal and corticobulbar tracts.]
• pyramidal decussation [marks junction between brainstem and spinal cord]
• ventrolateral sulcus [just lateral to the pyramids, hypoglossal nerve rootlets arise from here]
• olive [just lateral to the ventrolateral sulcus]
• retro-olivary sulcus/posterolateral sulcus [posterior to the olive; CN IX, X and XI arise from here]
• cerebellopontine angle [origin of CN VII and VIII]
• tuberculum cinereum [posterior to olive], (represents spinal tract and nucleus of trigeminal)

Question 10

Name dorsal external features of the medulla oblongata.

Answer 10

• the upper (open) and lower (closed) medulla
• floor of the 4th ventricle
• vagal and hypoglossal trigones
• inferior cerebellar peduncles [connect medulla oblongata to cerebellum]
• cuneate and gracile tracts and tubercles

Notes:
- The hypoglossal trigone is a slight elevation in the floor of the inferior recess of the fourth ventricle, beneath which is the nucleus of origin of the hypoglossal nerve. The idea is the same for the vagal trigone.
- open and closed medulla: This distinction is made based on whether the CSF-containing cavities are surrounded by the medulla (closed medulla) or not (open medulla). The medulla becomes open when the central canal opens into the fourth ventricle.

Question 11

The brainstem reticular formation consists of phylogenetically ancient extensive fields of interconnected neuronal cell bodies (reticular nuclei) and nerve fibers, forming a continuous core traversing the whole brain stem, and is continuous with spinal cord and diencephalon. State its key functions.

Answer 11

• control the level of attentiveness
• part of pain control pathways
• regulate respiratory, cardiovascular and gastrointestinal activity
• modulate spinal motor functions
• houses the micturition centre
• control release of various hormones e.g. antidiuretic hormone, luteinizing hormone, growth hormone and adrenocorticotropic hormone

Question 12

Outline the arterial blood supply to the midbrain.

Answer 12

• supplied mainly by branches of the basilar artery: posterior cerebral, superior cerebellar arteries and direct branches from the basilar artery
• it also receives branches from the posterior communicating and anterior choroidal arteries
• the quadrigeminal artery branches off from the posterior cerebral artery and is the main source of blood to the colliculi

Question 13

Outline the arterial blood supply to the pons.

Answer 13

• the blood supply of the pons is formed by branches of the vertebrobasilar system
• most of the pons is supplied by the pontine arteries [Note: pontine arteries are usually divided into three categories: the paramedian branches, the short and long circumferential branches]
• a smaller part of its blood supply comes from the anterior inferior cerebellar artery and the superior cerebellar artery [Diagram: Circle of Willis]

Question 14

Outline the arterial blood supply of the medulla oblongata.

Answer 14

• the medulla is supplied by various branches of the vertebral arteries
• these include the anterior and posterior spinal arteries, posterior inferior cerebellar artery and small direct branches

Question 15

Which nerves pass between the Posterior Cerebral Artery and the Superior Cerebellar Artery?

Answer 15

Oculomotor and Trochlear nerves

Question 16

Which arteries pierce the anterior perforated substance?

Answer 16

Lenticulostriate arteries; perforating arteries of the anterior and middle cerebral arteries [which supply the internal capsule and structures of the basal ganglia]

Question 17

State the boundaries and contents (5) of the interpeduncular fossa.

Answer 17

Boundaries
- laterally on either side: by the crus cerebri of cerebral peduncles
- anteriorly: optic chiasma and optic tracts
- posteriorly: antero-superior surface of pons (basal pons)

Contents
- mammillary bodies
- tuber cinereum [a raised area of grey matter lying anterior to the mammillary bodies]
- infundibulum [a narrow stalk which connects the hypophysis cerebri with the tuber cinereum]
- posterior perforated substance [a layer of grey matter in the angle between the crus cerebri]
- the oculomotor nerve [emerges immediately dorsomedial to the corresponding crus]
- [Diagram]

Question 18

What arteries pierce the posterior perforated substance?

Answer 18

Central branches of the posterior cerebral arteries en route to the thalamus and basal ganglia.

Question 19

State the boundaries, contents and clinical relevance of the cerebellopontine angle.

Answer 19

Boundaries
1. superiorly: tentorium cerebelli
2. anteromedially: pons
3. posteromedially: anterior surface of cerebellum
4. laterally: posterior surface of the petrous temporal bone

Contents
1. facial nerve (CN VII)
2. vestibulocochlear nerve (CN VIII)
3. nervus intermedius/intermediate nerve (of Wrisberg)
4. cerebellar flocculus
5. lateral recess of 4th ventricle

[Diagram 1] [Diagram 2] [Diagram 3]

Clinical relevance
It is a common site for tumors. A tumor in this space produces characteristic symptoms:
1. pressure on the spinal nucleus of the trigeminal nerve leads to loss of sensations of pain and temperature over the face
2. pressure on fibres and nucleus of the facial nerve results in facial paralysis
3. pressure on the middle cerebellar peduncle leads to ataxia
4. pressure on fibres and nucleus of vestibulocochlear nerve results in vertigo, tinnitus, nystagmus and deafness

Notes:
- nervus intermedius: [aka. intermediate nerve of Wrisberg] this is a small branch of the facial nerve
- lateral recess of 4th ventricle: The lateral recess is a paired lateral outpouching of the fourth ventricle that extends laterally between the inferior cerebellar peduncle and the peduncle of the flocculus of the cerebellum
- ataxia: a lack of muscle coordination and control
- vertigo: a feeling of movement, spinning, tilting or swaying when one’s body is not actually moving
- tinnitus: the perception of sound that does not have an external source
- nystagmus: involuntary, rapid and repetitive movement of the eyes

Question 20

Describe the formation of the circle of Willis/circulus arteriosus. [you must know how to draw it in 3D]

Answer 20

The circle of Willis is formed:
(a) anteriorly, by the anterior communicating and the anterior cerebral arteries
(b) posteriorly, by the basilar artery dividing into two posterior cerebral arteries
(c) laterally on each side, the posterior communicating artery connecting the internal carotid artery with the posterior cerebral artery

[Follow along with an atlas! 😎]

Notes:
- The basilar artery is formed by the union of the verterbral arteries at the level of the lower border of the ventral pons.
- The basilar artery bifurcates into the paired posterior cerebral arteries at or near the pontomedullary junction.
- The terminal branches of the internal carotid artery is the anterior cerebral and middle cerebral arteries.

Question 21

Discuss locked-in syndrome. (Hints: thrombosis of which artery? which part of brainstem is affected? which fibres are affected? symptoms?)

Answer 21

• This is due to thrombosis of short-circumferential branches of basilar artery. [Diagram: Circle of Willis]
• It results in infarction of the basal part of pons.
• The corticospinal and the corticonuclear fibres (which are responsible for voluntary movements of the body) of both sides are affected.
• The patient will present with complete paralysis due to involvement of corticospinal tracts and aphonia (loss of voice) due to involvement of corticobulbar (corticonuclear) fibres.
• The person is conscious because of noninvolvement of reticular formation (which is periventricular). Since, the ascending fibres are unaffected, all general and special sensory inputs are normal.
• The only way the patient can communicate is by blinking and by vertical gaze since CN III and CN IV nerves arising from midbrain are intact.
• [Diagram: locked-in syndrome area of lesion]

Question 22

What is berry aneurysm?

Answer 22

It is a localized dilatation on one of the arteries of the circle of Willis due to congenital muscular weakness of the arterial walls. The most common sites are at the:
(a) junction between anterior cerebral and anterior communicating arteries
(b) bifurcation of the internal carotid artery into the two terminal branches
Rupture of berry aneurysm may cause life-threatening subarachnoid haemorrhage.
[Diagram]

Further notes:
If you wish to study berry aneurysms more you can watch this 8-minute video.

Question 23

State the functional importance of the arterial circle of Willis.

Answer 23

• equalises the pressure of the blood flow to the two sides of the brain, as it is the main collateral channel
• provides an alternative route through which blood entering the internal carotid artery or the basilar artery may be distributed to any part of the cerebral hemispheres

Question 24

Briefly discuss Wallenberg’s syndrome (aka. lateral medullary syndrome). (cause and symptoms)

Answer 24

Cause: This syndrome is caused by an acute ischemic infarct of the lateral medulla oblongata, most commonly due to occlusion of the intracranial portion of the vertebral artery or the posterior inferior cerebellar artery. [Diagram: the Circle of Willis]

Structure involved and corresponding clinical presentations:
1. Spinal tract and nucleus of V: Ipsilateral loss of pain and temperature in the areas of distribution of CN V.
2. Spinal lemniscus: Contralateral loss of pain and temperature sensibilities with diminished rather than lost touch (Why contralateral? Why diminished rather than lost? Hint: What type of touch is this lemniscus carrying? Is it carrying both types or no? What tract are we dealing with here?)
3. Nucleus ambiguus: Ipsilateral paralysis of the muscle of the palate in swallowing and phonation.
4. Hypothalamospinal tract: Horner’s syndrome characterized by a small pupil, ptosis (drooping of eyelids) and anhydrosis.
5. Inferior cerebellar peduncle and vestibular nuclei: If included, causes dizziness, cerebellar ataxia and nystagmus.
6. [4-minute video]

Question 25

Briefly discuss Inferior Alternating Hemiplegia aka. Medial Medullary Syndrome/Dejerine syndrome.

Answer 25

This syndrome is caused by occlusion of the anterior spinal artery or the paramedian branches of the vertebral arteries, leading to death of the ipsilateral medullary pyramid, the medial lemniscus, and the hypoglossal nerve fibers that pass through the medulla.

Ipsilateral signs and symptoms:
☑ Flaccid (lower motor neuron) paralysis and atrophy of one half of the tongue (hypoglossal nerve).
☑ Deviation of the tongue to the side of the infarct on attempted protrusion, caused by ipsilateral muscle weakness.

Contralateral signs and symptoms:
☑ Spastic (upper motor neuron) paralysis of trunk and limbs (contralateral corticospinal tract).
☑ Impaired tactile, proprioceptive, and vibration sense of trunk and limbs (contralateral medial lemniscus).
☑ Limb weakness (or hemiplegia, depending on severity), on the contralateral side of the infarct.
☑ Loss of discriminative touch, conscious proprioception, and vibration sense on the contralateral side of the infarct (body below head).

Further notes:
Sensation to the face is preserved, due to the sparing of the trigeminal nucleus. Sensation of pain and temperature is preserved, because the spinothalamic tract is located more laterally in the brainstem and is not supplied by the anterior spinal artery. The syndrome is said to be “alternating” because the lesion causes symptoms both contralaterally and ipsilaterally.

Question 26

Briefly discuss Weber’s syndrome aka. Superior Alternating Hemiplegia/midbrain stroke syndrome.

Answer 26

This syndrome is caused by lesion of the midbrain due to occlusion of a mesencephalic branch of the posterior cerebral artery. The area of infarction involves the pyramidal fibres leading to contralateral hemiparesis [hemiparesis: one-sided muscle weakness].
Involvement of the oculomotor nerve causes ipsilateral paralysis of the ocular muscles except the lateral rectus and superior oblique (since CN IV supplies superior oblique and CN VI supplies lateral rectus). Consequently, the patient shows a lateral strabismus, inability to raise the upper eyelid [due to paralysis of levator palpebrae superioris] and pupillary dilation.
[Diagram: area of lesion] [6-minute video]

Question 27

Briefly discuss Foville syndrome aka. Middle Alternating Hemiplegia.

Answer 27

This syndrome typically involves a lesion in the caudal and medial pons, affecting the abducens nerve root and corticospinal fibers. It results in ipsilateral palsy of the abducens nerve (ipsilateral medial strabismus) and contralateral hemiparesis. It may be caused by ischemic infarction due to occlusion of perforating branches of the basilar artery. Other causes include subarachnoid haemorrhages, infection form tuberculosis, neoplasm from cerebellar tumor etc.

Question 28

Briefly discuss Parinaud’s syndrome aka. Dorsal Midbrain Syndrome.

Answer 28

This refers to a group of abnormalities of eye movement and pupil dysfunction. It is caused by lesions in the upper brainstem [e.g. due to pineal gland tumors, hemorrhages, multiple sclerosis, hydrocephalus etc.], sepcifically in the area of the dorsal midbrain. Symptoms include:
~ Difficulty in upward gaze (upgaze palsy)
~ Pupils that constrict less to light but react better to accomodation [Image]
~ Eyelid retraction (Collier’s sign)
~ Convergence-retraction nystagmus
~ One-and-a-half syndrome [syndrome characterized by horizontal movement disorders of the eyeballs]
~ [Diagram]

Question 29

Briefly discuss Benedikt’s syndrome aka. Paramedian Midbrain Syndrome.

Answer 29

This is a rare type of posterior circulation stroke of the midbrian, cerebellum and other related structures. It may be caused by occlusion of the basilar artery or the posterior cerebral artery. Symptoms include:
~ Ipsilateral oculomotor nerve palsy
~ Contralateral tremor and hemiparesis
~ Involuntary movements (choreoathetotic movements)
~ Cerebellar ataxia
~ Contralateral loss of proprioception and vibration sensations
~ [Diagram: area of lesion]

Question 30

Briefly discuss Internuclear Ophthalmoplegia.

Answer 30

Internuclear Ophthalmoplegia is a neurological condition that affects eye movement. It is caused by a lesion in the medial longitudinal fasciculus [a bundle of nerve fibres connecting the CNN whose nerves innervate the extraocular muscles of the eye]. The disorder impairs the ability to perform conjugate lateral gaze [the affected eye shows impairment of adduction, or movement towards the nose, when the person attempts to gaze to the side]. It may be caused by multiple sclerosis or a minor stroke.

Question 31

State the boundaries of the anterior perforated substance.

Answer 31

olfactory trigone
optic chiasma
optic tract
temporal lobe
[Diagram]