Anatomy and physiology of central nervous system

from anesthetic point of view

1. ANATOMY AND PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM Dr. aparna jayara ( p.g . 1 st year)

2. Organs CNS: Brain Spinal Cord PNS: Nerves

3. INTRODUCTION Brain is a closed structure Most of it is brain tissue while some of it is blood and CSF Brain comprises 80% Cerebral blood volume: 12% CSF contribute to 8% of the space inside the skull vault. Any increase in 1 component must be offset by equivalent decrease in other to prevent rise in ICT Monro – Kellie doctrine

4. Brain It is one of the largest organs in the body, and coordinates most body activities. It is the center for all thought, memory, judgment, and emotion. Each part of the brain is responsible for controlling different body functions, such as temperature regulation and breathing .

5. Cerebrum It is the largest section of the brain It is located in the upper portion of the brain and is the area that processes thoughts, judgment, memory, problem solving, and language. The outer layer of the cerebrum is the cerebral cortex, which is composed of folds of gray matter. The cerebrum is subdivided into the left and right halves called cerebral hemispheres. Each hemisphere has 4 lobes.

6. Lobes of Cerebrum

7. Lobes of Cerebrum 1. Frontal lobe : Most anterior portion of the cerebrum, controls motor function, personality, and speech 2. Parietal lobe : The most superior portion of the cerebrum, receives and interprets nerve impulses from sensory receptors and interprets language. 3. Occipital lobe : The most posterior portion of the cerebrum, controls vision. 4. Temporal lobe : The left and right lateral portion of the cerebrum, controls hearing and smell

8. Cerebellum Second largest portion of the brain Located beneath the posterior part of the cerebrum Aids in coordinating voluntary body movements and maintaining balance and equilibrium Refines the muscular movement that is initiated in the cerebrum

9. Brain Stem Midbrain —acts as a pathway for impulses to be conducted between the brain and the spinal cord. Pons — means bridge—connects the cerebellum to the rest of the brain. Medulla oblongata —most inferior positioned portion of the brain; it connects the brain to the spinal cord.

10. Spinal Cord Runs through the vertebral canal Extends from foramen magnum to 2 nd lumbar vertebra Regions Cervical Thoracic Lumbar Sacral Coccygeal Gives rise to 31 pairs of spinal nerves - all are mixed nerves

11. Meninges Dura mater: outermost layer; continuous with epineurium of the spinal nerves Arachnoid mater: thin and wispy Pia mater: bound tightly to surface

12. Blood Supply To The Brain

13. Arteries supplying the brain

14. 2 sources of blood: ICA and VA

15. atlas axis Vertebro-basilar system CTA: CT angiography C6 laterally upward backward

16. Circle of Willis

17. Circle of Willis

18. Circle of Willis oculomotor n. abducens n. optic chiasm mamillary bodies pituitary stalk

19. Circle of Willis encloses the optic chiasm, pituitary stalk and mamillary bodies. 2. Oculomotor nerve exits between the post. cerebral and sup. cerebellar arteries. 3. Vertebral arteries of the two sides unite to form the basilar artery at the ponto-medullary junction. The root of the abducens nerve and initial segment of the ant. inf. cerebellar artery can also be found here.

20. A1 A2

21. anterior cerebral middle cerebral posterior cerebral

22. Venous drainage 3 set of veins drain from brain Superficial cortical vein Deep cortical veins Dural sinuses All ultimately drain into right and left IJV

23. Almost the total volume of venous blood collected from the brain leaves the skull through the jugular foramen and the internal jugular vein. If the jugular foramen and/or the internal jugular vein is occluded , blood may escape through the diploic and emissary veins connecting the dural sinuses with the veins of the scalp skin .

24. Blood-brain barrier (BBB) The extracellular fluid of the CNS is separated from the blood by the BBB ensuring strictly controlled and mainly carrier protein assisted transport of macromolecules. Is formed by endothelial cells attached to one other by tight junctions, basement membrane, astrocytic endfeet. Protects the CNS from possibly toxic agents.

25. the Circumventricular organs “Circumventricular” = around the ventricles Incomplete or missing BBB Highly capillarized structures Secretion of neurohormones or detection of hormones, glucose, ions, etc.

26. Subfornical organ sensory fluid regulation Organum vasculosum sensory , secretory detects peptides, fluid regulation Median eminence secretory regulates the anterior pituitary through the release of neurohormones N eurohypophysis secretory store and secretes the hormones oxytocin and ADH into the blood, but does not synthesize either hormone Subcommissural organ secretory secretes certain proteins into the c erebrospinal fluid, its specific function is as yet unknown. Pineal gland secretory stimulated by darkness to secrete melatonin and is associated with circadian rhythms Area postrema sensory the vomiting centre of the brain (can detect noxious substances in the blood and stimulate vomiting in order to rid the body of these toxic chemicals)

27. The cerebrospinal fluid (CSF) Provides mechanical protection for the brain and the spinal cord. When floating in the CSF brain weighs only 50g (!) according to the Archimedes’ principle.

28. Cerebrospinal fluid (CSF) is a clear fluid present in the ventricles of the brain, the central canal of the spinal cord, and the subarachnoid space. CSF is produced in the brain by modified  ependymal cells in the  choroid plexus  (approx. 50-70%), and the remainder is formed around blood vessels and along ventricular walls. 

29. Circulation of CSF Lateral ventricles interventricular foramen of Monroe third ventricle mesencephalic aqueduct (aqueduct of Sylvius ) fourth ventricle spinal cord central canal; also, out the lateral apertures to the subarachnoid space to the venous system

30. internal and external CSF spaces internal = ventricles external = subarachnoidal space

31. Site of CSF resorption: arachnoid granulations in the superior sagittal sinus and lateral lacunae.

32. 20 ml of fluid produced every hr in choroids plexus and reabsorbed by arachnoid villi 500ml/day, yet total csf volume is only about 150 ml

33. Brain Functions Vision Taste Cognition Emotion Speech Language Hearing Motor Cortex Sensory Cortex Autonomic Functions

34. Cerebellum The cerebellum is connected to the brainstem, and is the center for body movement and balance. Click image to play or pause video

35. Thalamus Thalamus means “inner room” in Greek, as it sits deep in the brain at the top of the brainstem. The thalamus is called the gateway to the cerebral cortex, as nearly all sensory inputs pass through it to the higher levels of the brain.

36. Hypothalamus The hypothalamus sits under the thalamus at the top of the brainstem. Although the hypothalamus is small, it controls many critical bodily functions: Controls autonomic nervous system Center for emotional response and behavior Regulates body temperature Regulates food intake Regulates water balance and thirst Controls sleep-wake cycles Controls endocrine system The hypothalamus is shaded blue. The pituitary gland extends from the hypothalamus.

37. Cerebral blood supply: Physiological considerations: Brain accounts for 2% of body weight yet requires 20% of resting oxygen consumption O 2 requirement of brain is 3 – 3.5 ml/100gm/min And in children it goes higher up to 5 ml/100gm/min Brain has high metabolic rate That’s why brain requires higher blood supply 55ml/100gm/min is the rate of blood supply requires more requires more substrate te substrate requires m lacks of storage of energy substrate

38. Cerebral blood supply Cortical grey matter 75 – 80 ml/100gm/min Subcortical white matter 20ml/100gm/min CMRO 2 : 5.5 ml/100gm/min Functional requirement is 3.3 ml/100gm/min Integrity required 0 2 is 2.2 ml/100gm/min 60% functional 40 % integrity

39. High consumption bt low reserves responsible for unconsciousness within 10 sec of interruption of blood supply ( o2 tension dropping below 30mmhg, if sustained for 3-8 min can cause irrev . Injury to brain ( hippocampmpus and cerebellum being most sensitive) Main energy from glucose ( 5mg/100gm/min), 90% of which metabolized aerobically.

40. Regulation of cerebral blood flow CEREBRAL PERFUSION PRESSURE CPP=MAP-ICP(CVP if it is greater than ICP) Normally its is 80-100 mmhg , icp is normally less than 10 mmhg , cpp is primarily dependent on MAP. CPP less tha 50 mmhg show slowing of EEG, those with CPP between 25 and 40 mmhg hav a flat EEG. Sustained <25 mmhg lead to irrev . Brain damage.

41. Regulation of cerebral blood flow

42. Autoregulation Cerebral vasculature rapidly (10-60s) adapts to change in CPP( decrease in cpp causes vasodilatation and vice versa) CBF remains const b/w MAP of 60-160 mmhg . Beyond these limits , blood flow becomes pressure dependent.

43. Factors regulating cerebral blood flow Hemodynamic autoregulation Metabolic mediators and chemoregulation Neural control Circulatory peptides

44. Cerebral blood flow regulation Arteriolar diameter as well as cerebral vascular resistance both vary with CPP but CBF remains constant in this range.

45. Cerebral blood flow regulation 2. Venous physiology: Venous system contains most of the cerebral blood volume Slight change in vessel diameter has profound effect on intracranial blood volume But evidence of their role is less Less smooth muscle content Less innervations than arterial system

46. Cerebral blood flow regulation Pulsatile perfusion: Fast and slow components of myogenic response bring a change in perfusion pressure Cardiac output: Cardiac output may be responsible for improved cerebral blood flow They are indirectly related via central venous pressure and large cerebral vessel tone.

47. Cerebral blood flow regulation Rheological factors: Related with blood viscosity. Hematocrit has main influence on blood viscosity. Flow is inversely related with hematocrit . In small vessels cells move faster than plasma. This reduces microvascular hematocrit and viscosity FAHRAEUS LINDQVIST EFFECT

48. Metabolic and chemical regulation Carbon dioxide coupler between flow and metabolism At normal conditions CBF has linear relationship with CO 2 between 20 – 80 mm Hg For every mm Hg change of PaCO 2 CBF changes by 2 – 4 %

49. Metabolic and chemical regulation Oxygen: Within physiological range PaO 2 has no effect on CBF Hypoxia is a potent stimulus for arteriolar dilatation At PaO 2 50 mmHg CBF starts to increase and at PaO 2 30 mm Hg it doubles

50. CO 2 and Oxygen

51. Metabolic and chemical regulation Temperature: Like other organs cerebral metabolism decreases with temperature For every 1˚C fall in core body temperature CMRO 2 decreases by 7 % At temperature < 18 ˚C EEG activity ceases

52. Temperature

53. Pharmacology and autoregulation Anaesthetic drugs can alter autoregulatory responses as seen with blood pressure and CO2

54. Pharmacology and autoregulation Inhalational agents affecting CBF

55. EFFECTS @ DIFFERENT MACs

56. Pharmacology and autoregulation Vasoactive agents: Drugs that do not cross blood brain barrier do not affect CBF

57. Pharmacology and autoregulation Dexmedetomidine : Causes 25% reduction in CBF primarily by reducing CMR ACE inhibitors, Angiotensin receptor antagonists, β blockers…….. Do not reduce CBF or alter autoregulation

58. Metabolic mediators and chemoregulation Control CBF by acting as local vasodilators Ions and chemicals H + , K + , adenosine and phospholipid metabolites Final common pathway is via NO

59. Neurogenic effects Neurogenic effects: sympathetic tone shift the curve to right

60. Circulatory peptides: Vasoactive peptides like angiotensin II do affect CBF. Reactive oxygen molecules Alteration to vasomotor function Vascular remodeling De silva et al: effects of angiotensin II on cerebral circulation: role of oxidative stress; review article – front physiology ; jan 2013

61. To summarize

62. Clinical considerations Hypertensive patients: Autoregulatory curve shifts to right Protection from breakthrough but at the cost of risk of ischemia May suffer cerebral ischaemia during hemorrhage, shock or hypotension

63. Clinical considerations Elderly patients: With age CBF decreases Younger people have increased blood flow in frontal areas…. Frontal hyperaemia But with age this increased flow reduces Flow in other areas are well maintained hence blood is more uniformly distributed Autoregulatory failure occurs in morel elderly

64. Auto regulatory failure For auto regulatory failure to occur vasomotor paralysis is the end point Acute ischemia Mass lesions all lead to Inflammation vasomotor Prematurity paralysis Neonatal asphyxia Diabetes mellitus

65. Autoregulatory failure Two stages before infarction: Penlucida at flow 18 – 23 ml/100gm/min brain becomes inactive but function can be restored at any time by reperfusion Penumbra at lower flow rates brain function can be restored by reperfusion but only within a time limit

66. Hemodynamic considerations Cerebral steal : it means blood is diverted from one area to another if pressure gradient exists between the two circulatory beds Vasodilatation in non ischemic brain takes blood from ischemic areas to normal areas causing more ischemia Vasoconstriction results in redistribution of blood from normal to ischemic areas leading to inverse steal or ROBIN HOOD EFFECT

67. Hemodynamic considerations Vessel length and viscosity At breakthrough point flow depends on vessel length and viscosity Autoregulation has failed and it behaves like fluid in a rigid tube Pressure gradient across the ends are now same so distal area have the lowest flow This makes watershed areas more vulnerable to ischemic changes

68. Considerations for ischemia Consideration relevant to global ischemia Prevent and treat hypotension as well as vasogenic & cytotoxic edema Induction of mild hypothermia for 24 hrs Consideration relevant to focal ischemia Barbiturate coma, volatile anesthetics (xenon), calcium channel antagonists PaCO 2 and temperature

69. Therapies for enhancing perfusion Induced hypertension Inverse steal Hypocapnea Hemodilution Pharmacological agents Barbiturates, propofol Intra arterial delivery of drugs. Like mannitol and vasodilators

70. Thank You !!!