26-09-23 - Meninges, venous drainage of the brain and CSF Flashcards
Learning outcomes
- Describe the structure and organization of the meninges, their features in relation to the brain and venous sinuses
- Describe the spaces between meninges, structures found in these spaces and name the subarachnoid cisterns.
- Describe the venous drainage of the brain.
- Describe the ventricles of the brain
- Explain the structure of the choroid plexus and describe how CSF is produced and circulated
- Describe the causes and consequences of hydrocephalus
- Explain the structure of blood-brain barrier and its clinical importance
- Explain the causes and locations of subarachnoid, extradural and subdural haemorrhages and identify them on medical images
What do the meninges surround?
What are the 3 layers of meninges?
- Meninges Surround the central nervous system
- 3 layers of meninges:
1) Dura mater
2) Arachnoid mater
3) Pia mater
What is the outermost meningeal layer?
What is it composed of?
What are the 2 layers of the dura?
Where are these layers fused/not fused?
Where does the dura end?
- The dura is the outermost meningeal layer
- It is a tough layer of fibrous tissue
- 2 layers of the dura:
1) Outer endosteal layer (inner periosteum of the skull)
2) Inner meningeal layer - The two layers are fused with each other except where the dural venous sinuses are
- The dura ends at the level of S2 around the spinal cord
What is the flax cerebri? Where is it found?
What is it attached to anteriorly and posteriorly?
- The falx cerebri is a sickle-shaped extension of the dura mater, lying between the cerebral hemispheres (longitudinal fissure).
Describe how the falx cerebri encloses the superior sagittal sinus.
Describe how the straight sinus is formed
- The falx cerebri goes Along the vault of the skull, upper convex margin, to layers of dura separate to enclose the superior sagittal sinus
- The free inferior border contains the inferior sagittal sinus which unites with the great cerebral vein of Galen to form the straight sinus
What is the tentorium cerebelli?
What structures does it arch over?
What is it lifted up by in the midline?
What structures do it support?
What is the tentorial notch the only connection between?
- The tentorium cerebelli is a fold of dura mater that arches like a tent above the posterior cranial fossa, covering the cerebellum
- The tentorium cerebelli is lifted up (kept in place) by the falx cerebri in the midline
- It supports the occipital lobes of the cerebral hemispheres
- The tentorial notch is the only connection between supra- and infratentorial compartments – allows the brainstem to move down
What does the posterior margin of the tentorium cerebelli enclose?
Where are the superior pretrial sinuses found?
- The posterior margin of the tentorium cerebelli encloses the transverse sinuses on the inner surface of the occipital bone
- Superior petrosal sinuses are found where the tentorium cerebelli is attached to the upper border of the petrous temporal bone
How does the tentorium cerebelli divide the cranial cavity?
How do these structures communicate to each other?
- Tentorium cerebelli divides the cranial cavity into a supratentorial and infratentorial compartments, which communicate with each other through tentorial notch
What is an epidural haemorrhage caused by?
What does the middle meningeal artery supply?
Where are its branches located?
How does the blood accumulate in epidural haemorrhages?
What is required for treatment?
- Epidural haemorrhages are due to rupture of meningeal artery, blood accumulates between skull and dura
- The middle meningeal artery supplies meninges and diploe space (filled with CSF)
- Branches of middle meningeal artery is found in the endosteal layer of dura
- In epidural haemorrhages there is a biconvex accumulation of blood that does not cross the suture lines
- Emergency neurosurgery is required to stop bleeding and prevent herniation
How can the uncus be affected in epidural haemorrhages?
What can this lead to the compression of?
What effects can this have? Why is this?
- During epidural haemorrhages, displacement of the uncus of the temporal lobe into the tentorial notch can occur (uncal herniation)
- Compression of the ipsilateral crus cerebri by the uncus may give rise to contralateral motor weakness
- This is because the descending tracts are compressed
- This is before they have crossed over, so compressing the right side will cause motor weakness on the left side
How is the Diaphragma sella formed?
Where does this occur?
What structure does it cover?
- The tentorium cerebelli forms Diaphragma sella on the roof of the sella turcica
- This covers the hypophysis, with a central aperture for the hypophyseal stalk (infundibulum)
How is the lateral wall of the cavernous sinus formed?
What are the 4 nerves/nervous divisions in the lateral wall of the carvernous sinus?
What 3 nervous/arterial structures are found within the cavernous sinus?
- The lateral wall of the cavernous sinus is formed from the dura falling into the middle cranial fossae
- 4 nerves/nervous divisions in the lateral wall of the carvernous sinus:
1) CN3 (oculomotor nerve)
2) CN4 (trochlear nerve)
3) V1 (ophthalmic) division of trigeminal (CN5)
4) V2 (maxillary) division of trigeminal (CN5) - 3 nervous/arterial structures are found within the cavernous sinus:
1) CN6 (abducens nerve)
2) Internal carotid artery
3) Caverns that contain venous blood, with CSF between each cavern
What modalities is the Dura mater sensitive to?
What is the innervation of the dura mater lining the supratentorial compartment?
What 3 structures are supplied by the ophthalmic (V1) division of the trigeminal nerve (CN5)?
What 2 structures are supplied by the recurrent meningeal branch of the maxillary division (V2) division of the trigeminal nerve (CN5)?
What is the innervation of the dura mater lining the infratentoria compartment?
What is acute meningitis involving posterior cranial fossa meninges associated with?
- The dura mater is sensitive to pressure and stretching but not to touch
- The dura mater lining the supratentorial compartment receives sensory innervation from the trigeminal nerve (CN5):
- 3 structures are supplied by the ophthalmic (V1) division of the trigeminal nerve (CN5):
1) Anterior cranial fossa
2) Anterior part of the falx cerebri
3) Tentorium cerebelli - 2 structures are supplied by the recurrent meningeal branch of the maxillary division (V2) division of the trigeminal nerve (CN5):
1) Middle cranial fossa
2) Midregion of the vault - The dura mater lining the infratentorial compartment is supplied by branches of the upper three cervical spinal nerves entering the foramen magnum
- Acute meningitis involving posterior cranial fossa meninges is associated with neck rigidity and often with head retraction brought about by reflex contraction of the posterior nuchal muscles, which are supplied by cervical nerves
What is the middle layer of the meninges?
Where is the arachnoid mater found?
What are arachnoid granulations?
Where are arachnoid granulations found?
What is their role?
- The arachnoid mater is the middle layer of the meninges
- The arachnoid mater is found in large fissures where dural partitions are also found
- Arachnoid granulations are extensions of the arachnoid membrane that pierce the dura and enter foveola granulares
- Arachnoid granulations absorb CSF and return it to the venous system via superior sagittal sinus (circulate CSF)
What is the subdural space?
Where is the subdural space located?
What structure is it close to?
What veins transverse the subdural space?
What level does the subdural space end?
Where is the subarachnoid space located?
What does it contain?
What vascular structures run in this space?
- The subdural space is the potential space between dura and arachnoid
- It is in apposition with (near) the meningeal dura in living
- Bridging veins transverse the subdural space
- The subdural space ends at the level of S2 in the vertebral canal
- The subarachnoid space is the space between arachnoid and pia
- The subarachnoid space contains abundant CSF
- The cerebral arteries run in this space
What is subdural haemorrhage caused by?
What is the spread of blood limited by?
Where is blood located in relation to the brain in subdural haemorrhage?
When is neurosurgery required?
- Subdural haemorrhage is due to rupture of bridging veins and blood accumulating between dura and arachnoid
- The spread of blood is limited by the arachnoid mater
- In subdural haemorrhage, blood is on the surface of the brain and may cross suture lines
- Neurosurgery is required if the patient is symptomatic (may regress)
What is subarachnoid haemorrhage most commonly caused by?
What is a major sign of this?
- Subarachnoid haemorrhage is most commonly caused by a ruptured Berry aneurysm (85%) on Circle of Willis
- A major sign of subarachnoid haemorrhage is the ‘Worst ever headache’ of very sudden onset, typically in the occipital region (back of head)
- These are often referred to as “thunderclap” headaches
What is the innermost layer of the meninges?
What does it rap around?
Is it vascular or avascular?
What structures does it enter?
What are perivascular spaces?
Where are they located?
What condition can they contribute to?
- The pia mater is the innermost layer of the meninges
- It raps around the brain and spinal cord
- The pia mater is highly vascular
- The pia mater enters all sulci and extends into the brain tissue around the vessels
- Perivascular spaces (PVSs), also known as Virchow-Robin spaces, are pia-lined, fluid-filled structures found in characteristic locations throughout the brain.
- They can become abnormally enlarged or dilated and in rare cases can cause hydrocephalus
What are denticulate ligaments formed by?
Where are they found?
Where does the pia mater close?
What does it continue as?
- Denticulate ligaments are formed by condensed pia mater
- Denticulate ligaments are found between roots of spinal nerves
- Pia mater closes at the level of L1/2
- It continues as filum terminale to S2
What does the ventricular system consist of?
What is the total volume of the ventricles?
Where are the lateral ventricles located?
What is the role of the interventricular foramen?
Where is the 3rd ventricle located?
Where is the 4th ventricles located What is the role of the mesencephalic aqueduct (cerebral aqueduct)?
What does the 4th ventricle also communicate with?
- The ventricular system consists of four cavities, filled with CSF, connected to one another and to the central canal of the spinal cord
- It is a continuous series of sacs and collecting ducts
- Total volume of the ventricles ~25 mL
- Lateral ventricles are located in the cerebral hemispheres, inferior to the corpus callosum
- The interventricular foramen connects the two lateral ventricles and third ventricle
- The 3rd ventricle is in the diencephalon, between the left and right thalamus
- The 4th ventricle is between the pons/medulla and cerebellum.
- The mesencephalic aqueduct (cerebral aqueduct) connects the 3rd ventricle to the 4th ventricle
- The 4th ventricle also communicates with the central canal of the spinal cord
Ventricular system diagram (in picture)
What is CSF?
What volume of CSF do we have?
How much is produced daily?
What are 4 roles of CSF?
What structure is CSF produced by?
What 3 structures does the chordoid plexus consist of?
- CSF is Clear, colourless, odourless fluid
- Volume: 125 mL
- Daily production: 500-750 mL
- 4 roles of CSF:
1) Protects brain from traumas
2) Removes metabolic end-products
3) Helps maintain ion balance
4) Hormones produced in the hypothalamus is carried to median eminence on the floor of the 3rd ventricle by CSF - CSF is produced by choroid plexus in the cerebral ventricles
- 3 structures the chordoid plexus consists of:
1) Pia mater
2) Ependymal cells
3) Choroidal vessels
Where does CSF circulate?
How does CSF pass through the 4th ventricle?
- CSF circulates in cerebral ventricles, subarachnoid space, subarachnoid cisterns and central canal of the spinal cord
- CSF passes through x2 lateral and x1 median apertures of the 4th ventricle, into the subarachnoid space (cisterna magna – a localised wide space between the pia and arachnoid)
How does CSF flow?
What is CSF absorbed by?
- CSF Flows against gravity by the pulsation of the choroid plexus and pressure of itself
- CSF is absorbed by arachnoid granulations into the superior sagittal sinus
What is hydrocephalus?
How does this affect the ventricular system?
What are the 2 types of hydrocephalus?
How does hydrocephalus affect adults and children differently?
- Hydrocephalus is an abnormal buildup of cerebrospinal fluid in the ventricles
- This causes the ventricles widen, putting pressure on the periventricular tissues
- 2 types of hydrocephalus:
1) Communicating hydrocephalus: Flow of CSF is blocked after it exits the ventricles or reduced absorption of CSF (still flow between ventricles)
2) Non-communicating hydrocephalus: Flow of CSF is blocked
- Since the sutures of the head aren’t fused in children, hydrocephalus will cause the head to swell
- If its an adult, it will start to kill axons/cells around the ventricles, with symptoms varying depending on what cells are killed
What are subarachnoid cisterns?
Where are they particularly common?
What is a suboccipital puncture?
Which subarachnoid cistern is often used?
- Subarachnoid cisterns are cavities formed by the wide separation of the pia and arachnoid
- They are particularly common on the basal aspect of the brain
- A suboccipital puncture is injecting viral mater into subarachachnoid cisterns, as it has been proven to be more effective than injecting into veins
- Often the cisterna magna (largest subarachnoid cistern) is tapped
What does the CNS require to function?
How is this achieved?
What is the BBB formed by?
- The CNS requires a very stable environment to function normally
- This stability is provided by isolating the nervous system from the blood through the:
1) Blood-brain barrier (BBB)
2) Blood-cerebrospinal fluid barrier - The BBB is formed by basal lamina of the endothelial cells and the tight junctions between them (and foot processes of astrocytes)
Describe the sequence of venous drainage of the brain
- Sequence of venous drainage of the brain:
1) Networks of small venous channels
2) Larger veins
3) Dural venous sinuses
* Emissary veins, diploic veins, and bridging veins all drain into the dural venous sinuses
* The bridging veins are veins between the cerebral veins and dural sinuses
4) Internal jugular veins
Venous drainage of the brain summary (in picture)
Where are dural venous sinuses located?
What structures do they lack?
What can this cause?
- Dural sinuses (equivalence of veins) are between the meningeal and periosteal layers of the dura
- They lack valves and their walls are devoid of muscular tissue, meaning increased pressure can reverse circulation
