BLOOD SUPPLY TO THE BRAIN

what are the cranial meninges

three layers of thin tissue that cover and protect the brain and spinal cord

how many layers does the cranial meninges consist of

three layers

name the layers of the cranial meninges from superficial to deep

• dura mater

• arachnoid mater

• pia mater

which meningeal layer is the toughest and which is the thinnest

• dura mater: toughest layer

• pia mater: thinnest and most delicate of the three layers

what structure is the cranial meninges continuous with

the cranial meninges is continuous with the spinal meninges

how many layers is the dura mater composed of

two layers

what are the two layers of the dura mater and outline them

DURA MATER

outer layer

• endosteal layer - tightly bound to the cranium

inner layer

• meningeal layer - in contact with the arachnoid mater

outline the structure and function of dural folds

• inward foldings/ projections/ reflections of the meningeal layer of the dura mater to form double layered folds

• these folds help stabilise and protect the brain

what are the main dural folds

• falx cerebri

• tentorium cerebelli

• falx cerebelli

diagram of the dural folds

what does the falx cerebri separate

falx cerebri

• separates the two cerebral hemispheres

what does the tentorium cerebelli separate

tentorium cerebelli

• separates the occipital and temporal lobes

what does the falx cerebelli separate

falx cerebelli

• separates the right and left cerebellar hemispheres (cerebellum)

• beneath the tentorium cerebelli

outline the arachnoid mater

ARACHNOID MATER

• beneath the dura mater

• covers the brain surface

• does not follow the brain’s underlying folds

what is the subarachnoid space

• extends between the arachnoid and pia mater

• fibres extend from the arachnoid to the pia mater to bridge the gap

outline the pia mater

PIA MATER

• thin layer tightly adhered to the brain

• extends into every fold of the brain

• loosely connected to the arachnoid mater via tiny fibres

• not distinguishable from the surface of the brain using the naked eye - can be seen histologically

image of the arachnoid and pia mater layers

what is a potential space

a space that is not usually present in the brain of a healthy individual but can appear in disease/ trauma

what is a real space

a space in the brain that is normally present in a healthy individual

what type of space is the extradural space

the extradural space is a potential space between the dura mater and cranium

outline how the extradural space can appear

• a potential space

• between the dura mater and cranium

—

• the meningeal arteries are embedded in the endosteal dura mater

• rupture of these vessels results in bleeding into the extradural space

• this causes rapid localised increase in intracranial pressure

• could lead to coning

what type of space is the subarachnoid space

a real space between the arachnoid and pia mater

outline the subarachnoid space

• contains cerebrospinal fluid, arteries to the brain, veins from the brain

• large space, therefore a subarachnoid haemorrhage takes time to spread - symptoms are usually 24-48h after injury

• increased pressure will eventually cause coning

what is a function of the CSF other than cushioning the brain

• the CSF reduces the apparent weight of the brain

  • 1400g in air compared to 50g in CSF

where are venous sinuses located

venous sinuses

• spaces between reflections of the meningeal layer of the dura mater

outline venous sinuses

• veins containing deoxygenated blood moving away from the brain empty into venous sinuses

• venous sinuses drain into the internal jugular veins

what is the function of emissary veins

emissary veins

• emissary veins passing through the skull connect venous sinuses with extracranial blood vessels

what does the course of emissary veins increase the risk of

potential spread of infection

label the venous sinuses

which venous sinuses are paired and which are unpaired

paired

• transverse sinus

• sigmoid sinus

• cavernous sinus

unpaired

• superior sagittal sinus

• inferior sagittal sinus

• straight sinus

what is the ultimate function of venous sinuses

venous drainage - the system of veins that collect deoxygenated blood from tissues and organs and returns it to the heart

through which structures do venous sinuses connect with superficial and deep veins

• the orbit

• the pterygoid venous plexus - drains into maxillary vein then heart

• the pharyngeal venous plexus - drains into internal jugular vein then heart

• emissary veins - pass through the cranium bone

diagram of location of pterygoid and pharyngeal venous plexuses

the brain is highly _____________ active but has no _________ reserves

the brain is highly metabolically active but has no metabolic reserves

which ONE nutrient is the brain dependent on

glucose

what is the term for a lack of glucose

hypoglycaemia

what other element is the brain dependent on

oxygen

what does the figure ‘≈ 2 minutes’ refer to

within how long oxygen deprivation to the brain can cause irreversible damage

where does the brain receive its arterial supply from

• internal carotid arteries - enters brain via carotid canals

• vertebral arteries - branches of the subclavian arteries

what is the collective term given to the arteries supplying the brain i.e. the internal carotid and vertebral and what does it mean

true end arteries: one artery supplies a specific area and there is no alternative blood supply

• if the artery is damaged or diseased the area will be deprived of nutrients and oxygen and will die

carotid and vertebral artery anatomy diagram

outline the vertebral arteries

vertebral arteries

• run either side of the medulla and join at the level of the pons to form a single basilar artery

  • vertebrobasilar system

• as they travel across the pons the basilar artery gives off short medullary and pontine arteries

where do the cerebellar arteries arise from and what do they supply

cerebellar arteries

• arise from the vertebrobasilar system

• supply the cerebellum and lateral aspects of the brainstem

how many pairs of cerebellar arteries are there

there are three pairs of cerebellar arteries

outline damage to the cerebellar arteries

• parts of the brainstem supplied by the cerebellar arteries contain the cranial nerve nuclei

• damage to the cerebellar arteries therefore results in:

  • cerebellar ataxia

  • cranial nerve signs and symptoms depending on the level of brainstem that is affected

what is lateral medullary syndrome

loss of blood supply to the lateral part of the medulla

how does lateral medullary syndrome come about

• manifestation of blockage of posterior inferior cerebellar artery (PICA) or the vertebral artery

  • can result in cerebellar ataxia

which cranial nerve nuclei can be damaged as a result of lateral medullary syndrome and what are the consequences

damage to nuclei of CN IX (glossopharyngeal)

• no sensation to pharynx

• no swallowing reflex » dysphagia

damage to nuclei of CN X (vagus)

• no sensation to larynx - no cough reflex

• no motor innervation to pharyngeal constrictor muscles » dysphagia

• to motor innervation to larynx » dysphonia

which arteries supply the cerebral hemispheres and how many pairs are there

cerebral arteries - there are three pairs

which artery supplies the majority of the cerebral hemispheres

the internal carotid arteries

outline the internal carotid arteries

internal carotid arteries

• paired

• when they enter the cranium through the base of the skull they almost instantly divide into:

  • anterior cerebral arteries (paired)

  • middle cerebral arteries (paired)

where do the posterior cerebral arteries arise from

• the vertebrobasilar system

• the basilar artery eventually bifurcates and gives rise to the posterior cerebral arteries

diagram of the cerebral arteries

diagram showing the supply of cerebral arteries to the corresponding area of the cerebral hemisphere

functional areas of the cortex supplied by the anterior cerebral artery

• leg area of motor and sensory cortex

  • loss of blood flow would affect the contralateral side of the body

functional areas of the brain supplied by the middle cerebral artery

• Broca’s area - Broca’s aphasia if blood flow is lost

• Wernicke’s area - Wernicke’s aphasia if blood flow is lost

• sensory cortex

• motor cortex

• auditory cortex

functional areas of the brain functional areas of the brain supplied by the posterior cerebral artery

• visual cortex

  • loss of blood flow would affect vision

what is the medical term for a stroke

cerebrovascular accident (CVA)

what are the types of stroke and outline them

ischemic stroke

• due to a blood clot that blocks the flow of blood

haemorrhagic stroke

• due to a burst blood vessel (aneurysm)

• can also be due to trauma (tearing)

consequences of an ischemic stroke

• brain function may be lost in areas where blood supply is permanently absent

  • not much resolution

consequences of a haemorrhagic stroke

• short term effects are worse than long term

• as the mass of the hematoma decreases, function may return to the affected area

brain scan showing damage

what is the Circle of Willis (CoW)

Circle of Willis

• a ring shaped network of arteries at the base of the brain

• connects the internal carotid arteries with the vertebrobasilar system

function of the CoW

can act as an alternative blood supply if major vessels are damaged

in what % of people is the CoW complete in

≈ 60% of people

label the coloured boxes

• blue rectangle: anterior communicating artery (unpaired)

• purple rectangle: anterior cerebral arteries

• green rectangle: posterior communicating arteries

complete CoW diagram