W7 MHS1101 Nervous System 3

Blood Supply to Brain & Spinal Cord

• Main Arteries
  • Vertebral arteries
  • Internal carotid arteries
  • Function: Deliver nutrients and oxygen to brain/spinal cord
• Blood Removal: Dural venous sinuses drain into internal jugular veins

Vertebral Arteries

• Branch from subclavian arteries
• Ascend through neck via foramina transversaria of cervical vertebrae
• Loop over atlas (C1), enter skull via foramen magnum

Internal Carotid Arteries

• Branch from common carotid arteries
• Ascend through neck in carotid sheath
• Enter cranial cavity via carotid canal

Spinal Cord Blood Supply

• Vertebral arteries fuse to form anterior & posterior spinal arteries
• Medullary branches from aorta join spinal arteries
  • Posterior spinal artery: Supplies ascending (sensory) pathways
  • Anterior spinal artery: Supplies descending (motor) pathways

Intracranial Course of Main Arteries

Vertebral Arteries

• Run along medulla, fuse to form basilar artery at ventral surface of pons
• Basilar artery branches to pons, cerebellum, and gives off labyrinthine artery to internal ear
• Terminates as posterior cerebral arteries

Internal Carotid Arteries

• Emerge on either side of pituitary fossa
• Branches:
  • Anterior cerebral
  • Middle cerebral
  • Ophthalmic (to optic nerve/orbit)
• Supplies pituitary gland, hypothalamus, amygdala, hippocampus, thalamus, midbrain

Circle of Willis

• Anastomotic ring connecting cerebral arteries on each side
• Completed anteriorly by anterior communicating artery
• Completed posteriorly by posterior communicating arteries (connects posterior cerebral arteries)
• Ensures collateral blood flow

Distribution of Cerebral Arteries

• Cortical branches: supply surface cortex of cerebral hemispheres
• Central branches: penetrate deeper tissues (end arteries, no anastomosis)
• Clinical significance: blockage = loss of blood flow to specific areas

Anterior Cerebral Artery

• Medial surface of hemisphere above corpus callosum
• Olfactory bulb/tract, parieto-occipital sulcus
• Narrow strip of lateral surface (motor/sensory of lower limb)
• Occlusion: contralateral lower limb sensory/motor deficits

Posterior Cerebral Artery

• Occipital lobe (calcarine branch) – visual cortex
• Temporal lobe, thalamus, hypothalamus
• Terminal branches to calcarine & parieto-occipital sulci

Middle Cerebral Artery

• Most of lateral/external surface of hemispheres
• Corpus striatum (motor nuclei), internal capsule (projection fibres)
• Occlusion: upper limb involvement, aphasia (Broca's, Wernicke's areas)
• Rupture in internal capsule: paralytic stroke

Stroke (Cerebral Vascular Accident)

• Death/damage of brain tissue due to vascular causes
• Occlusive stroke: vessel blockage (thrombus) → ischaemia
  • TIA: resolves in ~24h
  • Prolonged: infarction
• Haemorrhagic stroke: vessel rupture, blood leakage, haematomas
  • Epidural (dura-skull, middle meningeal artery)
  • Subdural (dura-arachnoid, bridging veins)
  • Subarachnoid (arachnoid-pia, aneurysm rupture)

Venous Drainage of Brain

• Superficial system: subarachnoid space veins
• Deep system: deep veins form internal cerebral vein
  • Internal cerebral veins unite to form great cerebral vein (of Galen)
• All veins drain into dural venous sinuses

Dural Venous Sinuses

• Dilated channels with walls: dura, periosteum, endothelium
• Terminate at jugular foramen → internal jugular vein

Arachnoid Granulations

• Outpouchings that allow CSF absorption into venous circulation

Ventricular System of Brain

• Embryonic neural tube forms neurocoele → expands into brain ventricles
• Lined with ependymal cells

Ventricles

• 2 large lateral ventricles (one per cerebral hemisphere), separated by septum pellucidum
• 3rd ventricle (diencephalon): connects to lateral ventricles via interventricular foramen
• 4th ventricle extends into medulla, joins central canal of spinal cord, connects to 3rd ventricle via cerebral aqueduct

Dural Folds & Cranial Venous Sinuses

• Meningeal cranial dura forms folds to stabilise/support brain; contain dural venous sinuses:
  • Falx cerebri: Between cerebral hemispheres, contains superior/inferior sagittal sinuses
  • Tentorium cerebelli: Separates cerebrum/cerebellum, contains transverse sinus
  • Falx cerebelli: Divides cerebellar hemispheres

Cerebrospinal Fluid (CSF)

• Surrounds all CNS surfaces
• Functions: supports brain, cushions neural tissue, transports nutrients, chemical messengers, wastes

CSF Production

• Choroid plexus (specialised ependymal cells/capillaries) secretes ~500 ml CSF/day into ventricles
• Removes wastes, adjusts composition

CSF Circulation

• From choroid plexus → ventricles → central canal of spinal cord → subarachnoid space (via lateral/median apertures in roof of 4th ventricle) → surrounds brain, spinal cord, cauda equina

CSF Absorption

• Arachnoid villi/granulations (outgrowths of arachnoid membrane into superior sagittal sinus) absorb CSF into venous system

Excess CSF

• Hydrocephalus (accumulation due to impaired absorption/drainage)

Blood-Brain Barrier (BBB)

• Isolates CNS from general circulation
• Formed by tight junctions between CNS capillary endothelial cells
• Only lipid-soluble substances (O2, CO2, steroids, small alcohols) can diffuse into CNS interstitial fluid
• Astrocytes release chemicals to regulate BBB permeability

Blood-CSF Barrier

• Tight junctions between specialised ependymal cells of choroid plexus capillaries
• Restricts transfer, allowing differences in blood and CSF composition

Breaks in the BBB

• Hypothalamus (hormone release to circulation)
• Posterior pituitary (ADH, oxytocin release)
• Pineal gland (pineal secretions)
• Choroid plexus (where ependymal cells form the blood-CSF barrier)