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See Soon's notes

  • Brain needs about 15% of CO and about 20% of the oxygen
  • Lose supply = ischaemia within 4-5 minutes, lose consciousness in seconds
  • Two major blood supplies to the brain: internal carotid artery and vertebral artery (from subclavian)
    • Vertebral system = main blood supply to hindbrain
  • The internal carotid and vertebral system form an anastamosis and we divide the circulation into anterior and posterior circulations
    • Internal carotid forms by bifurcation of the common carotid at the bottom of the thyroid cartilage and has a swelling at the base at the carotid sinus
  • The internal carotid artery enters the skull through the carotid canal (in the petrous temporal bone); opening just above foramen lacerum, sitting beside the sphenoid bone. It runs along the side of the sphenoid in an S shape, pierces the dura, and divides into two terminal branches (middle cerebral and the anterior cerebral)
  • Carotid artery runs forward and then backward (carotid siphon). It is in a space called the cavernous sinus. It pierces the dura at the level of the anterior clinoid process.
    • Before dividing into its terminal branches it gives off the ophthalmic artery (the only thing that the ICA supplies outside the brain; eyes are an outgrowth of the CNS)

Introduction

  • The brain requires a rich blood supply
    • Uses 15% of cardiac output, 20% of available oxygen
      • Normal sized brains require 750ml of blood/minute
        • Nervous tissue is susceptible to ischaemia
    • 4 minute without oxygen causes irreversible damage
    • 5 seconds can cause a loss of consciousness
      • Thus, a well regulated blood flow is important – a precise system of autoregulation exists
        • Two sources of blood for the brain: internal carotid arteries and vertebral arteries
    • Internal carotid arteries supply the rostral parts of the brain – anterior circulation
    • Vertebral arteries supply the caudal regions – posterior circulation
    • At the base of the brain, these anastomose to form the Circle of Willis

Inferior view of brain

Circle of Willis en.svg

  • Internal carotid appears lateral to optic chiasm, and rapidly divides into middle cerebral (run around deep to the temporal lobe via the lateral fissure and then bifurcate into two branches that run parallel in the lateral fissure), and anterior cerebral arteries.
    • Branches of middle cerebral artery include perforating arteries (e.g. anterolateral central arteries; lateral striate)
  • The two anterior cerebral arteries are joined up (by the anterior communicating artery)
  • Anterior choroidal artery is a branch of the internal carotid artery and it supplies the choroid of the eye
  • Internal capsule is most common part of brain for stroke
  • See the area that is supplied by the MCA; comparing to yesterday's lecture, we can see which functions are lost in a stroke involving this artery
  • Anterior cerebral artery supplies medial structures, including the paracentral lobule; a stroke would destroy conscious feeling of lower leg and foot, and cause paralysis in that area

Anterior circulation

  • Common carotid artery bifurcates at the upper border of the thyroid cartilage
    • External carotid supplies the face and neck
    • Internal carotid ascends in the carotid sheath (with the internal jugular vein and vagus nerve)
      • Thus, it enters the carotid canal in the base of the temporal bone
      • It continues in the canal into the petrous part of the temporal bone
  • Next it enters the middle cranial fossa through the foramen lacerum
  • Here, it stays outside the dura in the cavernous sinus
    • It pierces the dura lateral to the optic chiasm and gives off 4 branches
  • Branches of the internal carotid
    • Ophthalmic artery – enters the orbit with the optic nerve supplying the eye and orbital contents
    • Anterior choroidal artery – follows the optic tract and supplies the optic tract, choroid plexus and deep structures of the brain
    • Anterior cerebral artery – passes dorsal to the optic nerve to the medial side of the brain
      • Sends branches to:
        • orbital gyri (inferior surface of the frontal lobe)
        • corpus callosum
        • cortex of the medial surface of the frontal and parietal lobes, including primary sensory and motor areas for the leg/foot
      • occlusion causes paralysis and sensory loss in the opposite leg/foot
    • anterior communicating artery – joins the two anterior cerebral arteries •
  • Middle cerebral – continuation of the internal carotid artery
    • Passes laterally into the lateral fissure
      • Gives off cortical branches supplying the lateral surface of the hemisphere
      • Thus supplies primary motor, somatosensory, auditory and speech areas
    • Occlusion causes paralysis and sensory loss in the contralateral upper limb and face and aphasia if the dominant hemisphere is involved
    • Near its commencement, perforating branches (lateral striate arteries) are given off
      • Supply the deep structures of the brain – ie: the internal capsule and basal ganglia
      • Vulnerable to rupture when blood pressure rises suddenly

Posterior circulation

  • Vertebral artery is the 1st branch of the subclavian artery (on the left, if can arise directly from the aorta)
    • Passes backwards and enters foramen transversarium –a hole in the transverse process of C2-C6
    • Ascends through the foramen transversaria of the vertebrae above this
      • At C1 it winds around the atlanto-occipital joint and enters the cranial cavity through foramen magnum (no transverse foramen in C1); if there is atherosclerosis in that artery, and the person turns their head to one side, and if their supply on the other side is dodgey as well, they'll feel faint
    • The two vertebral arteries pass up the front of the medulla and unite at the pontomedullary junction forming the basilar artery, which runs up the basilar surface of the pons (basilar groove in the middle) and branches at the top of the pons into the posterior cerebral artery (supplying the posterior part of the cerebral hemispheres and the midbrain via perforating branches) and the ____. The AICA comes off the basilar a at the level of the pons, which goes to supply the floculus (ventral lump on the cerebellum), and the superior cerebellar artery also comes off, to go above the tentorium cerebelli. The occulomotor nerve (CN3) comes out of the interpeduncular fossa near where the SCA comes out
      • Vertebral arteries branch into the posterior inferior cerebellar artery (PICA; which goes up and then back down).
    • Branches:
      • Anterior spinal artery – descends in the midline suppling the medial medulla (pyramids, medial lemniscus, CN12) and cervical spinal cord
      • Posterior spinal arteries – small branches that supply the dorsal medulla below the obex and the dorsal columns and dorsal horns of the cervical cord (spinothalamic tract, trigeminal nerve nuclei)
      • Posterior inferior cerebellar artery – has an S-shaped course over the olive and inferior cerebellar peduncle
        • Supplies the inferior cerebellum, the dorsolateral medulla and the choroid plexus
        • Occlusion results in lateral medullary (Wallenberg’s) syndrome
  • All these arteries have little perforating branches off them (e.g. pontine arteries off the basilar artery) to supply the immediate region around it
  • Basilar artery runs on the ventral surface of the pons in the basilar groove and divides into the posterior cerebral arteries
    • Branches:
      • Pontine arteries – paramedian and circumferential groups supplying the pons
      • Labyrinthine (internal acoustic artery) – enters the internal acoustic meatus supplying the inner ear
      • Anterior inferior cerebellar artery – supplies the anterior inferior surface of the cerebellum
      • Superior cerebellar artery – passes onto the superior surface of the cerebellum and supplies it
  • Posterior cerebral artery
    • Passes around the medial surface of the cerebral peduncle and thus supplies the inferior temporal lobe and medial and inferior occipital lobe
    • Thus supplies the primary visual and association cortices
      • Occlusion causes a loss in vision on the contralateral half of the visual field (hemianopia)
  • Circle of Willis (cerebral arterial circle)
    • Proximal parts of the anterior, middle and posterior cerebral arteries are joined by anterior and posterior communicating arteries
      • Thus an anastomotic ring is formed at the base of the brain – the circle of willis; connecting the anterior and posterior circulations. Unfortunately the posterior communicating arteries are too small to provide enough blood to the other side if there is an occlusion and very little mixing between the two circulations
    • Perforating branches from the circle of the proximal parts of the cerebral arteries supply the deep brain
      • Ie: the diencephalon, midbrain, internal capsule, basal ganglia
    • In cases of occlusion, the circle can provide collateral supply
      • Not very effective in cases of sudden occlusion
  • The circle of willis and its associated arteries occupy the subarachnoid space
    • Thus, rupture here is known as an subarachnoid haemorrhage
      • Rupture of perforating branches is a cerebral haemorrhage because blood accumulates in brain tissue
  • Aneurysms are common around the circle of Willis (typically at branches); if present in the interpeduncular fossa it can compress the occulomotor nerve
  • Note: Posterior cerebral artery supplies everything below the calcarine sulcus on the medial, lateral and inferior sides.
    • Stroke = lose vision in the visual field on the opposite side in both eyes
  • See the summary diagram; the watershed areas are where they overlap

Venous drainage

These veins all sit in the subarachnoid space, along with the arteries

  • Deep structures (walls of the lateral ventricle) drain into the internal cerebral veins (it comes out from the interventricular foramen, runs along the sides of the thalamus to form the great cerebral vein which is quite short)
    • These unite to form the great cerebral vein (of Galen)
  • Veins in the cortex drain to the surface forming superficial cerebral veins which drain into venous sinuses
  • Venous sinuses
    • Modified veins located between the dura and the skull
      • Follow the lines of attachment and folds of the dural reflections (falx cerebri and tentorium cerebelli)
    • Major sinuses
      • Superior sagittal sinus (sinus = vein with no valve) – along the falx cerebri attachment; most of the blood drains upwards towards this sinus. In the sinus, the blood runs backwards
      • Inferior sagittal sinus – along the lower free edge of the falx cerebri (down the bottom; its blood also drains backwards)
      • Straight sinus – at the attachment of the falx cerebri to the tentorium cerebelli
        • Formed by union of the inferior sagittal sinus and great cerebral vein
        • Meets at the back with the superior sagittal sinus
      • Transverse sinuses – at the attachment of the tentorium cerebelli to the occipital bone
        • Receive blood from the straight and superior sagittal sinuses at the confluence of the sinuses (where the transverse sinuses meet the straight sinus; it's near the internal occipital protuberance)
      • Sigmoid sinus – continuation of the transverse sinus, dives under the tentorium cerebelli
        • Runs in the groove between the petrous temporal and the occipital bones to the jugular foramen
        • Becomes the internal jugular vein below the foramen
      • Cavernous sinus – at the sides of the body of the sphenoid bone (NB: cavernous sinus is a space where the carotid artery enters, see above; this space is also a vein, so there is an artery running inside of a vein)
        • Receives blood from the pituitary gland and orbit
        • Communicates with blood vessels outside the cranial cavity
    • Superficial veins drain into the nearest venous sinus
      • Fragile and can be sheared off as they pass through the dura into the sinuses
        • If blood accumulates in this space between the dura and the arachnoid, a subdural haematoma forms
      • See also the petrosal sinuses