Anatomy and physiology of Central Nervous System

Anatomy and physiology of Central Nervous System

1. Dr Himanshu Jangid Anatomy and Physiology of CNS

2. The Brain : embryonic development Develops from neural tube Brain subdivides into Forebrain Midbrain Hindbrain These further divide, each with a fluid filled region: ventricle, aqueduct or canal Spinal cord also has a canal Two major bends, or flexures, occur (midbrain and cervical)

3. Brain development forebrain , midbrain and hindbrain

4. Space restrictions force cerebral hemispheres to grow posteriorly over rest of brain, enveloping it Cerebral hemispheres grow into horseshoe shape (b and c) Continued growth causes creases, folds and wrinkles

5. Anatomical classification Cerebral hemispheres Diencephalon Thalamus Hypothalamus Brain stem Midbrain Pons Medulla Cerebellum Spinal cord

6. Parts of Brain Cerebrum Diencephalon Brainstem Cerebellum

7. Cerebral Hemispheres (Cerebrum) Slide 7.28a Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Paired (left and right) superior parts of the brain Include more than half of the brain mass Figure 7.13a

8. Cerebral Hemispheres (Cerebrum) Slide 7.28b Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings The surface is made of ridges ( gyri ) and grooves ( sulci ) Figure 7.13a

9. Lobes of the Cerebrum Slide 7.29a Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Fissures (deep grooves) divide the cerebrum into lobes Surface lobes of the cerebrum Frontal lobe Parietal lobe Occipital lobe Temporal lobe

10. Lobes of the Cerebrum Slide 7.29b Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 7.15a

11. Specialized Areas of the Cerebrum Slide 7.30 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Somatic sensory area – receives impulses from the body’s sensory receptors Primary motor area – sends impulses to skeletal muscles Broca’s area – involved in our ability to speak

12. Sensory and Motor Areas of the Cerebral Cortex Slide 7.31 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 7.14

13. Specialized Area of the Cerebrum Slide 7.32a Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Cerebral areas involved in special senses Gustatory area (taste) Visual area Auditory area Olfactory area

14. Specialized Area of the Cerebrum Slide 7.32b Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Interpretation areas of the cerebrum Speech/language region Language comprehension region General interpretation area

15. Specialized Area of the Cerebrum Slide 7.32c Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 7.13c

16. Cerebral cortex Executive functioning capability Gray matter: of neuron cell bodies, dendrites, short unmyelinated axons 100 billion neurons with average of 10,000 contacts each No fiber tracts (would be white) 2-4 mm thick (about 1/8 inch) Brodmann areas (historical: 52 structurally different areas given #s) Neuroimaging : functional organization (example later)

17. Prenatal life: genes are responsible for creating the architecture of the brain Cortex is the last to develop and very immature at birth Birth: excess of neurons but not inter-connected 1 st month of life: a million synapses/sec are made; this is genetic 1 st 3 years of life: synaptic overgrowth (connections) After this the density remains constant though some grow, some die Preadolescence: another increase in synaptic formation Adolescence until 25: brain becomes a reconstruction site Connections important for self-regulation (in prefrontal cortex) are being remodeled: important for a sense of wholeness Causes personal turbulence Susceptible to stress and toxins (like alcohol and drugs) during these years; affects the rest of one’s life adapted from Dr. Daniel Siegel, UCLA

18. Cerebral cortex The mind changes the brain (throughout life) Where brain activation occurs, synapses happen When pay attention & focus mind, neural firing occurs and brain structure changes (synapses are formed) Human connections impact neural connections (ongoing experiences and learning include the interpersonal ones)

19. Diencephalon (part of forebrain) Contains dozens of nuclei of gray matter Thalamus Hypothalamus Epithalamus (mainly pineal)

20. Thalamus (egg shaped; means inner room) Two large lobes of gray matter (over a dozen nuclei) Laterally enclose the 3 rd ventricle Gateway to cerebral cortex: every part of brain that communicates with cerebral cortex relays signals through a nucleus in the thalamus (e.g. certain nucleus for info from retina, another from ears, etc.) Processing (editing) occurs also in thalamus Coronal section

21. Hypothalamus Forms inferolateral walls of 3 rd ventricle Many named nuclei Coronal section

22. Diencephalon – surface anatomy Hypothalamus is between optic chiasma to and including mamillary bodies (from Ch 14: cranial nerve diagram)

23. Cranial Nerve names Identify as many as you can when looking at model and sheep brain (they will be more fully discussed in Chapter 14)

24. Hypothalamus “Below thalamus” Main visceral control center Autonomic nervous system (peripheral motor neurons controlling smooth and cardiac muscle and gland secretions): heart rate, blood pressure, gastrointestinal tract, sweat and salivary glands, etc. Emotional responses (pleasure, rage, sex drive, fear) Body temp, hunger, thirst sensations Some behaviors Regulation of sleep-wake centers: circadian rhythm (receives info on light/dark cycles from optic nerve) Control of endocrine system through pituitary gland Involved, with other sites, in formation of memory

25. Hypothalamus ( one example of its functioning) Control of endocrine system through pituitary gland

26. Epithalamus Third and most dorsal part of diencephalon Part of roof of 3 rd ventricle Pineal gland or body (unpaired): produces melatonin signaling nighttime sleep Also a tiny group of nuclei Coronal section

27. Brain Stem Midbrain Pons Medulla oblongata Rigidly programmed automatic behavior necessary for survival Passageway for fiber tracts running between cerebrum and spinal cord Heavily involved with innvervation of face and head (10 of the12 cranial nerves attach to it)

28. __ Cerebral peduncles ____ Contain pyramidal motor tracts Corpora quadrigemina: X Visual reflexes X Auditory reflexes Midbrain ______ Substantia nigra (degeneration causes Parkingson’s disease) _______ Periaqueductal gray (flight/flight; nausea with visceral pain; some cranial nerve nuclei)

29. __Middle cerebellar peduncles _ Pons 3 cerebellar peduncles__ Also contains several CN and other nuclei (one to each of the three parts of the brain stem) Dorsal view

30. Medulla oblongata Relays sensory info to cerebral cortex and cerebellum Contains many CN and other nuclei Autonomic centers controlling heart rate, respiratory rhythm, blood pressure; involuntary centers of vomiting, swallowing, etc. Dorsal view _______Pyramids ____pyramidal decussation “Pyramidal”=corticospinal tracts; these are motor tracts which cross over in the decussation. They are named pyramids because they supposedly look like them, and also they originate from “pyramidal” neurons in the motor cortex. The tracts have the name of origin 1 st , therefore “corticospinal” tells you they go from the cortex (“cortico-”) to the spinal cord (“-spinal”) see later slides

31. Brain Stem in mid-sagittal plane Note cerebral aqueduct and fourth ventricle * * *

32. Cerebellum Two major hemispheres: three lobes each Anterior Posterior Floculonodular Vermis: midline lobe connecting hemispheres Outer cortex of gray Inner branching white matter, called “arbor vitae” Separated from brain stem by 4th ventricle

33. Functions of cerebellum Smooths, coordinates & fine tunes bodily movements Helps maintain body posture Helps maintain equilibrium How? Gets info from cerebrum re: movements being planned Gets info from inner ear re: equilibrium Gets info from proprioceptors (sensory receptors informing where the parts of the body actually are) Using feedback, adjustments are made Also some role in cognition Damage: ataxia, incoordination, wide-based gait, overshooting, proprioception problems

34. Functional brain systems (as opposed to anatomical ones) Networks of distant neurons that function together Limbic system Reticular formation

35. Limbic system (not a discrete structure - includes many brain areas) Most important parts: Hipocampus Amygdala Cingulate gyrus Orbitofrontal cortex (not labeled; is behind eyes - part of the prefrontal cortex but connects closely)

36. Limbic system continued Called the “emotional” brain Is essential for flexible, stable, adaptive functioning Links different areas so integration can occur Integration: separate things are brought together as a whole Processes emotions and allocates attentional resources Necessary for emotional balance, adaptation to environmental demands (including fearful situations, etc.), for creating meaningful connections with others (e.g. ability to interpret facial expressions and respond appropriately), and more…

37. Reticular formation Runs through central core of medulla, pons and midbrain Reticular activating system (RAS): keeps the cerebral cortex alert and conscious Some motor control

38. Ventricles Central cavities expanded Filled with CSF (cerebrospinal fluid) Lined by ependymal cells (these cells lining the choroid plexus make the CSF: see later slides) Continuous with each other and central canal of spinal cord

39. Lateral ventricles Paired, horseshoe shape In cerebral hemispheres Anterior are close, separated only by thin Septum pellucidum

40. Third ventricle In diencephalon Connections Interventricular foramen Cerebral aqueduct

41. Fourth ventricle In the brainstem Dorsal to pons and top of medulla Holes connect it with subarachnoid space

42. Subarachnoid space Aqua blue in this pic Under thick coverings of brain Filled with CSF Red: choroid plexus ________

43. Brain protection 1 . Meninges 2. Cerebrospinal fluid 3. Blood brain barrier

44. Meninges Dura mater : 2 layers of fibrous connective tissue, fused except for dural sinuses Periosteal layer attached to bone Meningeal layer - proper brain covering Arachnoid mater Pia mater Note superior sagittal sinus

45. Dura mater - dural partitions Subdivide cranial cavity & limit movement of brain Falx cerebri In longitudinal fissure; attaches to crista galli of ethmoid bone Falx cerebelli Runs vertically along vermis of cerebellum Tentorium cerebelli Sheet in transverse fissure between cerebrum & cerebellum

46. Arachnoid mater Between dura and arachnoid : subdural space Dura and arachnoid cover brain loosely Deep to arachnoid is subarachnoid space Filled with CSF Lots of vessels run through (susceptible to tearing) Superiorly, forms arachnoid villi : CSF valves Allow draining into dural blood sinuses Pia mater Delicate, clings to brain following convolutions

47. Cerebrospinal Fluid CSF Made in choroid plexuses (roofs of ventricles) Filtration of plasma from capillaries through ependymal cells (electrolytes, glucose) 500 ml/d; total volume 100-160 ml (1/2 c) Cushions and nourishes brain Assayed in diagnosing meningitis, bleeds, MS Hydrocephalus: excessive accumulation

48. CSF circulation: through ventricles, median and lateral apertures, subarachnoid space, arachnoid villi, and into the blood of the superior sagittal sinus CSF: - Made in choroid plexus - Drained through arachnoid villus

49. Blood-Brain Barrier Tight junctions between endothelial cells of brain capillaries, instead of the usual permeability Highly selective transport mechanisms Allows nutrients, O2, CO2 Not a barrier against uncharged and lipid soluble molecules; allows alcohol, nicotine, and some drugs including anesthetics

50. The Spinal Cord Foramen magnum to L1 or L2 Runs through the vertebral canal of the vertebral column Functions Sensory and motor innervation of entire body inferior to the head through the spinal nerves Two-way conduction pathway between the body and the brain Major center for reflexes

51. Fetal 3 rd month: ends at coccyx Birth: ends at L3 Adult position at approx L1-2 during childhood End: conus medullaris This tapers into filum terminale of connective tissue, tethered to coccyx Spinal cord segments are superior to where their corresponding spinal nerves emerge through intervetebral foramina (see also fig 17.5, p 288) Denticulate ligaments : lateral shelves of pia mater anchoring to dura (meninges: more later) Spinal cord http://www.apparelyzed.com/spinalcord.html

52. Spinal nerves Part of the peripheral nervous system 31 pairs attach through dorsal and ventral nerve roots Lie in intervertebral foramina

53. Spinal nerves continued Divided based on vertebral locations 8 cervical 12 thoracic 5 lumbar 5 sacral 1 coccygeal Cauda equina (“horse’s tail”): collection of nerve roots at inferior end of vertebral canal

54. Spinal nerves continued Note: cervical spinal nerves exit from above the respective vertebra Spinal nerve root 1 from above C1 Spinal nerve root 2 from between C1 and C2, etc. Clinically, for example when referring to disc impingement, both levels of vertebra mentioned, e.g. C6-7 disc impinging on root 7 Symptoms usually indicate which level More about spinal nerves in the peripheral nervous system lecture

55. Protection: Bone Meninges CSF (cerebrospinal fluid) 3 meninges: dura mater (outer) arachnoid mater (middle) pia mater (inner) 3 potential spaces epidural: outside dura subdural: between dura & arachnoid subarachnoid: deep to arachnoid

56. Dura mater Arachnoid mater Pia mater Spinal cord coverings and spaces http://www.eorthopod.com/images/ContentImages/pm/pm_general_esi/pmp_general_esi_epidural_space.jpg

57. LP (lumbar puncure) = spinal tap ( needle introduced into subdural space to collect CSF) Lumbar spine needs to be flexed so can go between spinous processes Epidural space is external to dura Anesthestics are often injected into epidural space Injection into correct space is vital; mistakes can be lethal Originally thought to be a narrow fluid -filled interval between the dural and arachnoid ; now known to be an artificial space created by the separation of the arachnoid from the dura as the result of trauma or some ongoing pathologic process ; in the healthy state , the arachnoid is attached to the dura and a naturally occurring subdural space is not present . http://cancerweb.ncl.ac.uk/cgi-bin/omd?subdural+space

58. Spinal cord anatomy Posterior median sulcus (“p”) Anterior median fissure (“a”) White matter (yellow here) Gray matter (brown here) “p” “a”

59. Gray/White in spinal cord Hollow central cavity (“central canal”) Gray matter surrounds cavity White matter surrounds gray matter (white: ascending and descending tracts of axons) “H” shaped on cross section Dorsal half of “H”: cell bodies of interneurons Ventral half of “H”: cell bodies of motor neurons No cortex (as in brain) Dorsal (posterior) white gray Ventral (anterior) Central canal______

60. Spinal cord anatomy Gray commissure with central canal Columns of gray running the length of the spinal cord Posterior (dorsal) horns (cell bodies of interneurons) Anterior (ventral) horns (cell bodies of motor neurons) Lateral horns in thoracic and superior lumbar cord * * * *

61. White matter of the spinal cord (myelinated and unmyelinated axons) Ascending fibers: sensory information from sensory neurons of body up to brain Descending fibers: motor instructions from brain to spinal cord Stimulates contraction of body’s muscles Stimumulates secretion from body’s glands Commissural fibers: white-matter fibers crossing from one side of cord to the other Most pathways cross (or decussate ) at some point Most synapse two or three times along the way, e.g. in brain stem, thalamus or other

62. Cerebral Physiology

63. Cerebral Metabolism 20% of total body oxygen. The cerebral metabolic rate (CMR) is usually expressed in terms of oxygen consumption (CMRO 2 ) = 3–3.8 mL /100 g/min (50 mL /min) in adults. CMRO 2 is greatest in the gray matter of the cerebral cortex relatively high oxygen consumption and the absence of significant oxygen reserves, interruption of cerebral perfusion usually results in unconsciousness within 10 s as oxygen tension rapidly drops below 30 mm Hg. If blood flow is not reestablished within 3–8 min irreversible cellular injury begins to occur. The hippocampus and cerebellum appear to be most sensitive to hypoxic injury.

64. Cerebral Metabolism Neuronal cells =primary energy source glucose consumption is 5 mg/100 g/min, of which over 90% is metabolized aerobically. CMRO 2 therefore normally parallels glucose consumption. During starvation, ketone bodies become major energy substrates. Acute sustained hypoglycemia is equally as devastating as hypoxia. Paradoxically, hyperglycemia can exacerbate global and focal hypoxic brain injury by accelerating cerebral acidosis and cellular injury.

65. Cerebral Blood Flow Cerebral blood flow (CBF) varies with metabolic activity. measured with a -emitting isotope such as xenon ( 133 Xe). positron emission tomography (PET) in conjunction with short-lived isotopes such as 11 C and 15 O also allow measurement of CMR (for glucose and oxygen, respectively). Regional CBF parallels metabolic activity Total CBF =50 mL /100 g/min, Gray matter =80 mL /100 g/min, White matter =20 mL /100 g/min. Total CBF in adults averages 750 mL /min (15–20% of cardiac output). Flow rates below 20–25 mL /100 g/min =cerebral impairment CBF rates between 15 and 20 mL /100 g/min =a flat ( isoelectric ) EEG, below 10 mL /100 g/min =irreversible brain damage.

66. Regulation of Cerebral Blood Flow Cerebral Perfusion Pressure Cerebral perfusion pressure (CPP) is the difference between mean arterial pressure (MAP) and intracranial pressure (ICP) (or central venous pressure [CVP], whichever is greater). MAP – ICP (or CVP) = CPP. CPP is normally 80–100 mm Hg. Moreover, because ICP is normally less than 10 mm Hg, CPP is primarily dependent on MAP. Moderate to severe increases in ICP (> 30 mm Hg) can significantly compromise CPP and CBF even in the presence of a normal MAP. CPP values less than 50 mm Hg slowing on the EEG, CPP between 25 and 40 mm Hg flat EEG. Less than 25 mm Hg irreversible brain damage.

67. Autoregulation Brain normally tolerates wide swings in blood pressure with little change in blood flow. The cerebral vasculature rapidly (10–60 s) adapts to changes in CPP, but abrupt changes in MAP will lead to transient changes in CBF even when autoregulation is intact. Decreases in CPP cerebral vasodilation Elevations vasoconstriction. In normal individuals, CBF remains nearly constant between MAPs of about 60 and 160 mm Hg. Beyond these limits, blood flow becomes pressure dependent. Pressures above 150–160 mm Hg can disrupt the blood–brain barrier and may result in cerebral edema and hemorrhage.

68. Autoregulation

69. Autoregulation Both myogenic and metabolic mechanisms may explain cerebral autoregulation . Myogenic mechanisms involve an intrinsic response of smooth muscle cells in cerebral arterioles to changes in MAP. Metabolic mechanisms indicate that cerebral metabolic demands determine arteriolar tone. Thus, when tissue demand exceeds blood flow, the release of tissue metabolites causes vasodilation and increases flow. Nitric oxide, adenosine, prostaglandins, and perhaps ionic (electrolyte) concentration gradients.

70. The most important extrinsic influences on CBF are respiratory gas tensions—particularly PaCO 2 . CBF is directly proportionate to PaCO 2 between tensions of 20 and 80 mm Hg. Blood flow changes approximately 1–2 mL /100 g/min per mm Hg change in PaCO 2 . This effect is almost immediate and is thought to be secondary to changes in the pH of CSF and cerebral tissue. Respiratory Gas Tensions

71. Respiratory Gas Tensions Because ions do not readily cross the blood–brain but CO 2 does, acute changes in PaCO 2 but not HCO 3 – affect CBF. Thus, acute metabolic acidosis has little effect on CBF because hydrogen ions (H + ) cannot readily cross the blood–brain barrier. After 24–48 h, CSF HCO 3 – concentration adjusts to compensate for the change in PaCO 2 , so that the effects of hypocapnia and hypercapnia are diminished. Marked hyperventilation (PaCO 2 < 20 mm Hg) shifts the oxygen–hemoglobin dissociation curve to the left and, with changes in CBF, may result in EEG changes suggestive of cerebral impairment even in normal individuals.

72. Temperature CBF changes 5–7% per 1°C change in temperature. Hypothermia decreases both CMR and CBF, whereas pyrexia has the reverse effect. Between 17°C and 37°C for every 10° increase in temperature, the CMR doubles. CMR decreases by 50% if the temperature of the brain falls by 10°C, eg , from 37°C to 27°C, and another 50% if the temperature decreases from 27°C to 17°C. At 20°C, the EEG is isoelectric , but further decreases in temperature continue to reduce CMR throughout the brain. Above 42°C, oxygen activity begins to decrease and may reflect cell damage.

73. Viscosity Normally, changes in blood viscosity do not appreciably alter CBF. The most important determinant of blood viscosity is hematocrit . A decrease in hematocrit decreases viscosity and can improve CBF; unfortunately, a reduction in hematocrit also decreases the oxygen-carrying capacity and thus can potentially impair oxygen delivery. Elevated hematocrits , as may be seen with marked polycythemia , increase blood viscosity and can reduce CBF. Some studies suggest that optimal cerebral oxygen delivery may occur at hematocrits of approximately 30%.

74. Autonomic Influences Intracranial vessels sympathetic ( vasoconstrictive ), parasympathetic ( vasodilatory ), and noncholinergic nonadrenergic fibers; serotonin and vasoactive intestinal peptide appear to be the neurotransmitters for the latter. Normal physiological function uncertain, But important role some pathological states. Intense sympathetic stimulation induces marked vasoconstriction in these vessels, which can limit CBF. Autonomic innervation may also play an important role in cerebral vasospasm following brain injury and stroke.

75. Blood–brain Barrier Cerebral blood vessels are unique in that the junctions between vascular endothelial cells are nearly fused. The paucity of pores is responsible for what is termed the blood–brain barrier. This lipid barrier allows the passage of lipid-soluble substances but restricts the movement of those that are ionized or have large molecular weights. Movement of a given substance across the blood–brain barrier its size, charge, lipid solubility, and degree of protein binding in blood. Carbon dioxide, oxygen, and lipid-soluble substances (such as most anesthetics) freely enter the brain. Most ions, proteins, and large substances such as mannitol penetrate poorly.

76. Blood–brain Barrier Water moves freely across the blood–brain barrier. BUT Movement of even small ions is impeded to some extent. As a result, rapid changes in plasma electrolyte concentrations (and, secondarily, osmolality ) produce a transient osmotic gradient between plasma and the brain. Acute hypertonicity of plasma results in net movement of water out of the brain, whereas acute hypotonicity causes a net movement of water into the brain. These effects are short-lived, as equilibration eventually occurs, but, when marked, they can cause rapid fluid shifts in the brain. Thus, marked abnormalities in serum sodium or glucose concentrations should generally be corrected slowly.

77. Blood–brain Barrier Mannitol , an osmotically active substance that does not normally cross the blood–brain barrier, causes a sustained decrease in brain water content and is often used to decrease brain volume. The blood–brain barrier may be disrupted by severe hypertension, tumors, trauma, strokes, infection, marked hypercapnia , hypoxia, and sustained seizure activity. Under these conditions, fluid movement across the blood–brain barrier becomes dependent on hydrostatic pressure rather than osmotic gradients.

78. Cerebrospinal Fluid CSF is found in the cerebral ventricles and cisterns and in the subarachnoid space surrounding the brain and spinal cord. Its major function is to protect the CNS against trauma. Most of the CSF is formed by the choroid plexuses of the cerebral (mainly lateral) ventricles. Smaller amounts are formed directly by the ventricles' ependymal cell linings and yet smaller quantities from fluid leaking into the perivascular spaces surrounding cerebral vessels (blood–brain barrier leakage). In adults, normal total CSF production is about 21 mL /h (500 mL /d), yet total CSF volume is only about 150 mL.

79. Cerebrospinal Fluid CSF flow Lateral ventricles Intraventricular foramina (of Monro ) Third ventricle, Cerebral aqueduct (of Sylvius ) Fourth ventricle, Median aperture of the fourth ventricle (foramen of Magendie ) and the lateral aperture of the fourth ventricle (foramina of Luschka ) cerebellomedullary cistern ( cisterna magna)

80. Cerebrospinal Fluid From the cerebellomedullary cistern CSF enters the subarachnoid space circulating around the brain and spinal cord absorbed in arachnoid granulations over the cerebral hemispheres.

81. Cerebrospinal Fluid CSF formation involves active secretion of sodium in the choroid plexuses. The resulting fluid is isotonic with plasma despite lower potassium, bicarbonate, and glucose concentrations. Its protein content is limited to the very small amounts that leak into perivascular fluid. Carbonic anhydrase inhibitors ( acetazolamide ), corticosteroids, spironolactone , furosemide , isoflurane , and vasoconstrictors decrease CSF production.

82. Cerebrospinal Fluid Absorption appears to be directly proportionate to ICP and inversely proportionate to cerebral venous pressure. Because the brain and spinal cord lack lymphatics , absorption of CSF is also the principal means by which perivascular and interstitial protein is returned to blood.

83. Intracranial Pressure The cranial vault is a rigid structure with a fixed total volume, consisting of brain (80%), blood (12%), and CSF (8%). Any increase in one component must be offset by an equivalent decrease in another to prevent a rise in ICP. ICP by convention means supratentorial CSF pressure measured in the lateral ventricles or over the cerebral cortex and is normally 10 mm Hg or less. Minor variations may occur depending on the site measured, but, in the lateral recumbent position, lumbar CSF pressure normally approximates supratentorial pressure.

84. Intracranial Pressure Intracranial compliance is determined by measuring the change in ICP in response to a change in intracranial volume. Normally, increases in volume are initially well compensated. A point is eventually reached, however, at which further increases produce precipitous rises in ICP. Major compensatory mechanisms include (1) an initial displacement of CSF from the cranial to the spinal compartment, (2) an increase in CSF absorption, (3) a decrease in CSF production, and (4) a decrease in total cerebral blood volume (primarily venous).

85. Intracranial Pressure The concept of total intracranial compliance is useful clinically even though compliance probably varies in the different compartments of the brain and is affected by arterial blood pressure and PaCO 2 . Increases in blood pressure can reduce cerebral blood volume because autoregulation induces vasoconstriction in order to maintain CBF. In contrast, hypotension can increase cerebral blood volume as cerebral vessels dilate to maintain blood flow. Cerebral blood volume is estimated to increase 0.05 mL /100 g of brain per 1 mm Hg increase in PaCO 2 .

86. Intracranial Pressure Compliance can be determined in patients with intraventricular catheters by injecting sterile saline. An increase in ICP greater than 4 mm Hg following injection of 1 mL of saline indicates poor compliance. At that point, compensatory mechanisms have been exhausted and CBF is progressively compromised as ICP rises further. Sustained elevations in ICP can lead to catastrophic herniation of the brain.

87. Intracranial Pressure Herniation may occur at one of four sites (1) the cingulate gyrus under the falx cerebri (2) the uncinate gyrus through the tentorium cerebelli (3) the cerebellar tonsils through the foramen magnum (4) any area beneath a defect in the skull ( transcalvarial ).