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)
SlideCopyright © 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)
SlideCopyright © 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
SlideCopyright © 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
SlideCopyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 7.15a

11. Specialized Areas of the Cerebrum
Slide 7.30Copyright © 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

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

14. Specialized Area of the Cerebrum
SlideCopyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
 Cerebral areas involved in special senses
 Gustatory area (taste)
 Visual area
 Auditory area
 Olfactory area

15. Specialized Area of the Cerebrum
SlideCopyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
 Interpretation areas of the cerebrum
 Speech/language region
 Language comprehension region
 General interpretation area

16. Specialized Area of the Cerebrum
SlideCopyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 7.13c

17. 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)

18.  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
 1st month of life: a million synapses/sec are made; this is
genetic
 1st 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 lifeadapted from Dr. Daniel Siegel, UCLA

19. 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)

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

21. Thalamus (egg shaped; means inner room)
 Two large lobes of gray matter (over a dozen nuclei)
 Laterally enclose the 3rd 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

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

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

24. 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)

25. 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

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

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

28. 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)

29. __Cerebral peduncles____
Contain pyramidal motor tracts
Corpora quadrigemina:
XVisual reflexes
XAuditory reflexes
Midbrain
______Substantia nigra
(degeneration causes Parkingson’s disease)
_______Periaqueductal gray
(flight/flight; nausea with visceral pain; some
cranial nerve nuclei)

30. __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

31. 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 1st, therefore
“corticospinal” tells you they go from the cortex (“cortico-”)
to the spinal cord (“-spinal”)
see later slides

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

33. 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

34. 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

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

36. 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)

37. 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…

38. 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

39. 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

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

41.  Third ventricle
 In diencephalon
 Connections
 Interventricular foramen
 Cerebral aqueduct

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

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

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

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

46. 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

47.  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

48. 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

49. 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

50. 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

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

52.  Fetal 3rd 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

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

54. 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

55. 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

56. 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

57. Spinal cord
coverings
and spaces
http://www.eorthopod.com/images/ContentImages/pm/pm_general_esi/pmp_g
eneral_esi_epidural_space.jpg

58. 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

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

60. 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______

61. 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
*
*
*
*

62. 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

63. Cerebral
Physiology

64. Cerebral Metabolism
 20% of total body oxygen.
 The cerebral metabolic rate (CMR) is usually expressed in
terms of oxygen consumption (CMRO2)
 = 3–3.8 mL/100 g/min (50 mL/min) in adults.
 CMRO2 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.

65. Cerebral Metabolism
 Neuronal cells =primary energy source
glucose
 consumption is 5 mg/100 g/min, of which over 90% is
metabolized aerobically.
 CMRO2 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.

66. Cerebral Blood Flow
 Cerebral blood flow (CBF) varies with metabolic activity.
 measured with a -emitting isotope such as xenon (133Xe).
 positron emission tomography (PET) in conjunction with
short-lived isotopes such as 11C and 15O 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.

67. 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.

68. 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

69. Autoregulation

70. 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.

71.  The most important
extrinsic influences on
CBF are respiratory gas
tensions—particularly
PaCO2.
 CBF is directly
proportionate to PaCO2
between tensions of 20
and 80 mm Hg.
 Blood flow changes
approximately 1–2
mL/100 g/min per mm Hg
change in PaCO2.
 This effect is almost
immediate and is thought
to be secondary to
changes in the pH of CSF
and cerebral tissue.
Respiratory Gas
Tensions

72. Respiratory Gas Tensions
 Because ions do not readily cross the blood–brain but CO2
does, acute changes in PaCO2 but not HCO3
– 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 HCO3
– concentration adjusts to
compensate for the change in PaCO2, so that the effects of
hypocapnia and hypercapnia are diminished.
 Marked hyperventilation (PaCO2 < 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.

73. 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.

74. 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%.

75. 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.

76. 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.

77. 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

78. 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.

79. 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.

80. 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)

81. 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.

82. 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.

83. 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.

84. 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.

85. 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).

86. 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 PaCO2.
 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 PaCO2.

87. 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.

88. 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).