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Central Nervous System Physiology

Central Nervous System Physiology. Dr Tony Morrissey Speciality Doctor Anaesthetics 25/09/13. What will be covered. CNS CSF BBB Cerebral blood flow ICP Brain metabolism. CSF. Definition Function Appearance Production Circulation Absorption Normal constituents. CSF – Definition.

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Central Nervous System Physiology

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  1. Central Nervous SystemPhysiology Dr Tony Morrissey Speciality Doctor Anaesthetics 25/09/13

  2. What will be covered • CNS • CSF • BBB • Cerebral blood flow • ICP • Brain metabolism

  3. CSF • Definition • Function • Appearance • Production • Circulation • Absorption • Normal constituents

  4. CSF – Definition • The CSF is a specialized extracellular fluid in the ventricles and the subarachnoid space. • Power and Kam, Principles of physiology for the anaesthetist

  5. CSF – Function • Mechanical protection by buoyancy • Effective weight reduced from 1400 g to 47 g • Maintenance of constant ionic environment • Acid-base regulation • Esp. via control of respiration • Nutritional

  6. CSF – Appearance • Clear colourless liquid

  7. CSF – Production (1) • Formed: • 70% in choroid plexus • 30% around blood vessels and ventricular walls • Volume of CSF: 150 ml • Rate of formation • 550 ml/day • 0.4 ml/min • Turnover 4 times per day

  8. CSF – Production (2) • Fenestrated capillaries • ultrafiltrate • Transport • Hydrostatic pressure and bulk flow: H2O • Active transport: Na+, K+, H+ • Facilitated transport: glucose • Passage through junctions and vesicles: protein

  9. CSF – Circulation • From lateral ventricles to 3rd ventricle: • Foramina of Monro • From 3rd to 4th ventricles: • Aqueduct of Sylvius • Leaves 4th ventricle to cisterna magna: • Foreman of Magendie • Foramen of Luschka • From cisterna magna • Superiorly: subarachnoid space around cerebellum • Caudally: spinal subarachnoid space • Cephalad to basilar cisterns

  10. CSF – Absorption • Absorbed into venous blood • Arachnoidvilli: • Dural walls of sagittal and sigmoid sinuses (85-90 %) • Dural walls of dural sinusoids on dorsal nerve roots (10-15 %) • Mean CSF pressure: 15 cmH2O • Pressure in superior sagittal sinus: 9 cmH2O

  11. CSF – Normal constituents

  12. Blood-Brain Barrier • Definition • Morphology • Function • Factors affecting rate of transfer

  13. BBB – Definition • A physiological boundary between the bloodstream and central nervous system, preventing transfer of substances from plasma to brain. • Yentiset al, Anaesthesia and Intensive Care A-Z

  14. BBB – Morphology • Capillary endothelial cells have: • Tight junctions between adjacent cells • Absence of fenestrations • High content of mitochondria • Perivascular area of closely applied foot processes of astrocytes

  15. BBB – Function • Provides favourable environment for nervous tissue function • Protects brain from potentially toxic substances • Allows free access of metabolic substrates • Prevention of escape of NT into general circulation

  16. BBB – Factors affecting rate of transfer • Ion channels • Facilitated diffusion • Active transport • Pinocytosis • Factors influencing rate of diffusion: • Molecular size • Concentration gradient • Ionization • Lipid solubility • Protein binding

  17. Cerebral Blood Flow • What is it in ml/min, ml/100g/min, as % of CO • Measurement • What factors affect it? • Autoregulation

  18. CBF – Numbers • 14 % of cardiac output • 700 ml/min • 50 ml/100g brain tissue/min

  19. CBF – Measurement • Applying the Fick principle • Kety-Schmidt technique • N2O 10% breathed for 10-15 min • Jugular venous concentration assumed to be same as brain concentration • Detection of radioactive decay • Regional flow measured by Doppler

  20. CBF – Factors affecting • Arterial PCO2 • Arterial PO2 • MAP • Cerebral metabolic rate for oxygen • Drugs

  21. CBF – Arterial PCO2 http://www.trauma.org/archive/neuro/icpcontrol.html

  22. CBF – Arterial PO2

  23. CBF – Autoregulation • The phenomenon where CBF is kept constant over a MAP range of 50-150 mmHg. • Power and Kam, Principles of physiology for the anaesthetist • Thought to be through myogenic response in vascular smooth muscle. • As pressure rises smooth muscle constricts • Causing reduced flow • As pressure fall smooth muscle relaxes • Causing increased flow

  24. http://www.neuroicu.info/brainspinalcordbloodflow.htm

  25. CBF – coupled to metabolism • Regional CBF varies with local metabolic rates • Local metabolic factors: • H+ • K+ • Adenosine • Phospholipid metabolites • Glycolytic metabolites • NO

  26. Intracranial Pressure • Definition • Monro-Kellie doctrine

  27. ICP – Definition • Pressure exerted by the CSF in the frontal horns of the lateral ventricles of the brain. • Yentiset al, Anaesthesia and Intensive Care A-Z • Normally 7-17 mmHg supine. • CPP=MAP – ICP

  28. Monro-Kellie doctrine • Skull is a rigid closed container. • Its contents is incompressible. • Contents made up of: • Blood 50-70 ml (5-7 %) • CSF 50-120 ml (5-12 %) • Brain 1.4 kg (80-85 %) • ICP depends on volume of intracranial contents

  29. Monro-Kellie doctrine http://www.trauma.org/archive/neuro/icp.html

  30. Brain metabolism • Cerebral function is totally dependent on oxidative phosphorylation of glucose to provide ATP. • Uses 20% of resting total body O2 consumption (brain 2 % total body weight) • Sensitivity to hypoxia: • Lack of storage • High metabolic rate

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