Brainstem and Cerebellum Flashcards
What are the 3 broad functions of the brainstem?
- Conduit: Allows passage of long pathways between the brain, brainstem, thalamus, cerebellum and spinal cord
-
Integrative:
- Reticular formation: control of consciousness, cardiovascular and, respiratory functions, perception of pain.
- Influences motor patterns (e.g. balance, posture, muscle tone)
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Cranial nerves:
- Contains the nuclei of Cranial Nerves III to XII
- Head’s equivalent to spinal nerves plus sight, hearing, equilibrium and gustation
- Reflex centres
What is the reticular formation?
- A set of interconnected nuclei that are located throughout the brainstem. It is not anatomically well defined, because it includes neurons located in different parts of the brain.
- The neurons of the reticular formation make up a complex set of networks in the core of the brainstem that extend from the upper part of the midbrain to the lower part of the medulla oblongata.
What composes the brainstem?
- Midbrain
- Pons
- Medulla oblongata
What is the brainstem continuous with rostrally?
The midbrain is continuous with the diencephalon (D)
What is the brainstem continuous with caudally?
Medulla is continuous with the spinal cord (SC) at level of the foramen magnum
What is the brainstem continuous with ventrally?
The clivus of the occipital bone
What is the brainstem continuous with dorsally?
The cerebellum
Running through the brainstem are components of the ventricular system. What are these components?
- IVth Ventricle (IVth) - between the pons/medulla anteriorly and cerebellum posteriorly
- Cerebral aquaduct (Caq) - at the level of the midbrain
How many cranial nerves arise from the brainstem?
10 out of 12 cranial nerves - these are part of the PNS and have motor and sensory parts (somatic and autonomic)
Revision of development from neural tube
At any level of brainstem, what 3 areas can be identified?
- Tectum (blue)
- Tegmentum (purple)
- Basal area (red)
Where is the tectum found?
The most dorsal area (roof) –> posterior to the ventricular system
Where is the tegmentum found?
Anterior to ventricular system
Where is the basal area found?
Most ventral part of brainstem
At the level of the midbrain, describe the locatio of the tectum, tegmentum and basal area in relation to cerebral aqueduct
- Tectum – located posterior to the cerebral aqueduct
- Tegmentum - anterior to cerebral aqueduct
- Basal - most ventral
Which area of the brainstem has most significant tectum?
Midbrain
What is found in the tegmentum?
- The tegmentum is the location of several cranial nerve (CN) nuclei
- Some pathways that pass through the brainstem
What is the basal area important for?
Descending motor fibres - fibres start in cortex and bring motor info down through the brainstem either to spinal cord, brainstem or cerebellum
What does locked in syndrome typically occur from? Why does it result in this syndrome?
- Results from stroke at level of pons
- Descending motor pathways are affected (in basal area)
- Paralysis of everything except eye movements
Why are most eye movements still intact in locked in syndrome?
Nerves that are responsible for eye movement arise above the pons
In locked in syndrome, why are people still able to breathe and heart is still functioning?
The tegmentum where we find these centres isn’t affected
What is the junction between the pons and the medulla called?
Pontomedullary junction (PMJ)
There are several structures visible on the anterior surface of the medulla. What are these?
- the three fissures/sulci
- the pyramids
- the olives
- five cranial nerves.
What fissure is found in the midline of the medulla?
Anterior median fissure - continuous along the length of the spinal cord.
What is the anterior median fissure temporarily interrupted by?
By the decussation of the pyramids
What is the pyramidal decussation?
marks the border between the spinal cord and the medulla oblongata.
What are the pyramids?
- Paired swellings of the medulla found between the anterior median fissure and the ventrolateral sulci
- Located on one either side of anterior median fissure
What is found under the pyramids?
These bumps exist as there is something underneath them - collections of white matter motor fibres coming from the cortex and descending down the CNS
What are descending tracts of the CNS?
The descending tracts are the pathways by which motor signals are sent from the brain to lower motor neurones. The lower motor neurones then directly innervate muscles to produce movement.
The motor tracts can be functionally divided into which two major groups?
- Pyramidal tracts
- Extrapyramidal tracts
Where do the pyramidal tracts originate? Where do they carry motor fibres to? What are they responsible for?
- These tracts originate in the cerebral cortex
- Carry motor fibres to the spinal cord and brain stem.
- They are responsible for the voluntary control of the musculature of the body and face.
What does the right hemisphere control?
The muscles on the left side of the body
What does the left hemisphere control?
Muscles on the right side of the body
As each hemisphere controls the opposite side of the body, the fibres coming from the right hemisphere need to cross the midline of the CNS. Where does this happen?
At the decussation of the pyramids
What is found lateral to the pyramids?
Another bump - the olives (paired)
What sulcus is located between the pyramids and the olives?
Antero-lateral sulcus
Which cranial nerve extends out of the antero-lateral sulcus?
The hypoglossal nerve (CN XII)
Where does the hypoglossal nerve (CN XII) emerge?
At the medulla between the pyramids and the olives (antero-lateral sulcus)
What 2 sulci are found on the anterior surface of the medulla?
- Antero-lateral sulcus
- Postero-lateral sulcus
Where is the postero-lateral sulcus located?
Lateral to the olives
What cranial nerves are found at the postero-lateral sulcus?
CN IX - glossopharyngeal nerve
CN X - vagus nerve
CN XI - spinal accessory nerve
Unlike the anterior surface of the medulla, the posterior surface is largely obstructed from view and is relatively devoid of features. In order to appreciate the posterior surface, what must be removed?
The cerebellum
The floor of the fourth ventricle can be seen in the dorsal surface of the medulla. What is this? What is it also known as?
- Also known as the rhomboid fossa
- This is a rhombus-shaped depression that is the anterior part of the fourth ventricle.
- Its anterior wall, formed by the back of the pons and the medulla oblongata, constitutes the floor of the fourth ventricle.
Similar to the anterior surface, the posterior surface has a midline structure. What is this?
The posterior median fissure
What is found lateral to the posterior median fissure?
The fasciculus gracilis / the gracile fasciculus
What does the gracile fasciculus contain?
White matter fibres travelling from the spinal cord and up towards the brainstem
What is found lateral to the gracile fasciculus?
The cuneate fasciculus
What does the cuneate fasciculus contain?
White matter fibres
The cuneate and gracile fasciculus swell at the top. What are these swellings called?
The cuneate and gracile tubercle
What causes the formation of the cuneate and gracile tubercles?
Under the white matter is a collection of nuclei (cell bodies) –> causes fasciculus to swell
The 4 structures: cuneate & gracile tubercles and cuneate & gracile fasciculi, form part of which pathway?
Form part of ascending sensory pathway bringing sensory info from the body up towards the brain
What do the tubercles in the medulla contain? What do the fascicles contain?
Fascicles –> axons
Tubercles –> cell bodies
The medulla can be subdivided into which 2 portions? What makes this distinction?
- The most rostral part of the medulla
- The most caudal part of the medulla
This distinction is made based on whether the CSF-containing cavities (ventricles) are surrounded by the medulla (closed medulla) or not (open medulla).
What is the rostral part of the medulla also referred to as? Why?
The open medulla –> as behind we are at the level of the 4th ventricle (seen in red pen)
What is the caudal part of the medulla also referred to as? Why?
Way below the 4th ventricle (4th ventricle has ended)
What is the point where the 4th ventricle ends called?
The obex –> the point in the human brain at which the fourth ventricle narrows to become the central canal of the spinal cord
Cross-section through closed (caudal) medulla
Cross section through open (rostral) medulla
The internal structures of the medulla must be viewed in cross section to understand the layout. Three levels of the medulla are typically discussed (inferior to superior). What are these?
- Level of decussation of the pyramids
- Level of decussation of the medial lemnisci
- Level of the olives
What features can be seen in all 3 cross sections of the medulla?
- Anteriorly we can see the paired lumps representing the pyramids which are separated by the anterior median fissure.
- Centrally, the central canal can be seen as it rises to form the fourth ventricle in the final cross section.
What occurs at the level of the decussation of the pyramids?
This is the major decussation point of the descending motor fibres. Roughly 75% of motor fibres housed within the pyramids cross diagonally and posteriorly, and continue down the spinal column as the lateral corticospinal tracts.
Explanation of white and grey matter at the level of the decussation of the pyramids:
- The central portion of the medulla contains gray matter
- The outer portions consist of white matter.
- The posterior white matter contains the gracile fasiculus and the more lateral cuneate fasiculus
- Corresponding portions of gray matter extend to these regions and are the gracile and cuneate tubercles
What is the significant change at the level of the olives?
The central canal has now expanded into the fourth ventricle and as such makes this region the open medulla.
The inferior cerebellar peduncles come into view at the level of the olives. What are these?
Paired structures containing important white matter fibre tracts which connect the cerebellum to the medulla.
Summary of ventral view of medulla oblongata:
-
Two pyramids
- Separated by the anterior median fissure (Interrupted at the decussation of pyramids)
- Contain descending motor fibres
-
Anterolateral sulcus separates pyramids from olives
- Hypoglossal nerves emerge here
-
Posterolateral sulcus emerges dorsal to olive
- Vagus and glossopharyngeal nerves emerge here
Summary of dorsal view of medulla oblongata
- Viewed after removal of the cerebellum
- Reveals the IVth ventricle
- Central canal opens into IVth ventricle at obex
- Open medulla (rostral) – IVth ventricle
- Closed medulla (caudal) – central canal
- Floor of the IVth ventricle – Rhomboid fossa
- Either side of midline - gracile fascicle (GF)
- Forms a swelling rostralling - gracile tubercle
- Lateral to gracile fascicle - cuneate fascicle
- Forms a swelling rostrally - cuneate tubercle
Gracile and cuneate fascicle and tubercles form part of dorsal column pathway - sensory information (from body to brain)
What is the junction between the pons and the medulla?
Pontomedullary junction (PMJ)
What is the junction between the pons and the midbrain?
Ponomesencephalic junction
Ventral and dorsal view of pons
What does the pons bridge?
Transversely orientated structure that bridges between the two cerebellar hemispheres
Ventral view of pons. What structures can be seen?
- Basilar groove
- Transverse fibres
- CN V emerges at level of pons
- Motor and sensory root
- Sensory root bigger as trigeminal nerve mainly sensory
- Motor and sensory root
- CN XI emerges just lateral to midline
- Cerebellopontine angle
- CN VII and CN VIII emerges here
What is the basilar groove? Where is it found?
- Found on ventral surface of pons
- Groove for basilar artery after it has been formed from the 2 vertebral arteries
What are the transverse fibres seen on the ventral aspect of the pons?
Fibres from cerebellum that pass from either side
What is the cerebellopontine angle? Which cranial nerves emerge here?
Shown in dotted rectangular area:
- CN VII and CN VIII emerge here
Which cranial nerves emerge from the ventral surface of the pons?
- CN V (trigeminal)
- Motor and sensory roots (sensory bigger)
- Originates from the lateral aspect of mid pons
- CN VI (abducens)
- originates from the pontomedullary junction, close to the midline
- CN VII (facial) and CN VIII (vestibulocochlear)
- Both emerge from cerebellopontine angle
Dorsal view of pons. What structures can be seen here?
- Floor of 4th ventricle
- Facial colliculus
- Cerebellar peduncles
What are cerebellar peduncles?
A cerebellar peduncle is a nerve tract (white matter) that permits communication between the cerebellum and the other parts of the central nervous system.
How many pairs of cerebellar peduncles are there?
3: superior, middle and inferior
What does the superior cerebellar peduncle attach?
Attaches the cerebellum to the midbrain
What does the middle cerebellar peduncle attach?
Attaches the cerebellum to the pons
What does the inferior cerebellar peduncle attach?
The cerebellum to the medulla
What is the facial colliculus? How is it formed?
- Visible in the dorsal view, at the floor of the 4th ventricle in the rhomboid fossa
- Is a bump formed by fibres of the facial nerve
When taking a crosssection through the caudal pons, what structures can be seen?
- 4th ventricle posteriorly
- Groove for basilar artery ventrally
- Transverse pontine fibres
- Tegmentum of pons
- Middle cerebellar peduncle
- Cranial nerve nuclei
- Descending motor fibres
Describe course of facial nerve after they emerge from pons
- Loop around the nuclei of abducens nerve to emerge at cerebellopontine angle –> forms the facial colliculus
Cross-section of rostral pons
- Cerebral aqueduct dorsally
- Transverse pontine fibres (very prominent)
- Slightly interrupt path of descending motor fibres
- Groove for basilar artery
Ventral view summary of pons:
- Cerebellopontine angle (where medulla and pons join cerebellum laterally)
- Facial and vestibulocochlear emerge here
- Abducens from pontomedullary junction
- Trigeminal nerve enters at mid-pontine level
- Midline groove for basilar artery
Dorsal view summary of pons
- Main feature on dorsal view – cerebellar peduncles
- Superior: connects midbrain to cerebellum
- Middle: connects pons to cerebellum
- Inferior: connects medulla to cerebellum
- Superior cerebellar peduncle forms roof of IVth ventricle
View of SCP forming roof of 4th ventricle
What junction is found between the pons and the midbrain?
Ponto-mesencephalic junction (PmcJ)
Ventral view of midbrain.
- Mamillary bodies
- Pituitary stalk
- Optic chiasm
- Optic tract
- Cerebral peduncles
- Interpeduncular fossa
- CN III emerges here (occulomotor)
- CN IV (trochlear) also emerges from midbrain
- Actually emerges from dorsal aspect but makes it way round ventrally
The mamillary bodies, pituitary stalk, optic chiasm and optic tract are all seen on the ventral view of the midbrain. Where are these structures actually from?
The diencephalon
Ventral view of midbrain (highlighted in green). What is this ventral bit of midbrain called? What are these structures?
- Cerebral peduncles (crus cerebri)
- Are a collection of descending motor fibres coming from the cortex
What is the space between the two cerebral peduncles called?
The interpeduncular fossa
Which cranial nerve emerges out of the interpeduncular fossa?
CN III (occulomotor)
Dorsal view of midbrain
- 4 paired bumps –> colliculi
- 2 superior colliculi
- 2 inferior colliculi
- CN IV (trochlear)
What are the superior and inferior colliculi important for?
- Superior –> Visual system
- Inferior –> Auditory system
What is the ‘brachium’ of the superior and inferior colliculi?
A connecting arm between each colliculi and the thalamus - ‘brachium’ means ‘arm’
Midbrain cross-section (level of inferior colliculi)
Which part of the brainstem is the only region with a signficant tectum?
The midbrain
Anterior view of midbrain summary:
- Main feature –> cerebral peduncle (basis pedunculi)
- Separated by interpenduncular fossa
- Oculomotor nerve emerges
Dorsal view of midbrain summary:
- Main features: Paired superior and inferior colliculi
- Four bumps:
- Superior – associated with visual pathway
- Inferior – associated with auditory pathway
- Four bumps:
- Inferior to inferior colliculi and at junction with pons
- Trochlear nerve
- Only nerve to emerge from dorsal surface
- Has the cerebral aquaduct (Caq) running through it
What is the reticular formation?
- A complex, multisynaptic network of neurons within the tegmentum of the brainstem
- Spans length of brainstem (and a little bit into the spinal cord)
- Multiple functions
What does the reticular formation include?
-
Reticular nuclei (many cell bodies)
- Give rise to a descending motor pathway (descends to spinal cord) –> reticulospinal tract
-
Vital autonomic centres
- Respiratory, cardiovascular control
-
Ascending reticular activating system (ARAS)
- Series of neurones that pass to the cortex and involved in consciousness and arousal
- Damage = coma
- Series of neurones that pass to the cortex and involved in consciousness and arousal
- Pain modulation
- Sleep wake cycle
- Arousal
Blood supply to brainstem:
- What is CN III sandwiched between?
- Over which CN is AICA seen?
- CN III (occulomotor) –> sandwiched between the superior cerebellar artery and the posterior cerebral artery
- AICA –> just over the top of CN VI (abducens)
Territories of brainstem supplied by each vessel
Which artery is the most anterior aspect of the medulla supplied by?
Anterior spinal arteries
Review of cranial nerve location:
- CN III occulomotor –> between cerebral peduncles of midbrain
- CN V trigeminal –> middle of pons
- CN VI abducens –> pontomedullary junction
- CN VII facial –> cerebellopontine angle
- CN VIII vestibulocochlear –> cerebellopontine angle between cerebellum and pons
- CN IX glossopharyngeal –> emerges lateral to olives of medulla
- CN X vagus –> emerges lateral to olives of medulla
- CN XI accessory –> emerges lateral to olives of medulla
- CN XII hypoglossal –> emerges between the pyramids and the olives of the medulla
Which cranial nerves are missing? Where do these originate from?
- CN I (olfactory) and CN II (optic nerve) are missing
- These originate from the forebrain and are part of the CNS
What is the cerebellum primarily involved in?
Primarily involved in coordination of movement, maintenance of balance and posture
Injuries to the cerebellum will result in what?
- Loss of coordination of movement, loss of balance and loss of posture
- tremors, gait problems, shaking
- Loss in muscle tone
Where is the cerebellum found?
Under the occipital lobes of the cerebral hemispheres and dorsal to the brainstem
How is the cerebellum attached to the brainstem?
Via 3 paired cerebellar peduncles:
- Superior cerebellar peduncle
- Connects cerebellum to midbrain
- Middle
- Connects cerebellum to pons
- Inferior
- Connects cerebellum to medulla
Dorsal view of brainstem and ventral view of cerebellum –> cerebellar peduncles
Views and surfaces of the cerebellum
- What is the superior view also known as?
- Where is the ventral view from?
- Superior / dorsal view
- Ventral view from the pons and through the IVth ventricle
Superior view of cerebellum:
- What structure separates the anterior and posterior lobe?
- What structure separates the 2 hemispheres of the cerebellum
-
Primary fissure represented by dotted line
- Separates the anterior lobe from the posterior lobe
-
Vermis in the middle
- Separates 2 hemispheres
Inferior view of cerebellum and ventral view
- Inferior view: brainstem attached
- Hemisphere each side with vermis in middle
- Ventral view: brainstem removed and cerebellar peduncles exposed
- Nodulus is an extension of the vermis that projects into the ventral view
- Tonsils (2 bumps) just posterior to medulla
- Flocculonodular lobe
What is the floccoulonodular lobe?
- Flocculus + nodule = Flocculonodular lobe
- A lobe of the cerebellum consisting of the nodule and the 2 flocculi
What can happen to tonsils of cerebellum during increased intracranial pressure?
Tonsils can herniate through the foramen magnum and compress the medulla –> a tonsilar herniation
