Cerebral Blood Flow and CNS Homeostasis

Question 1

Circle of Willis Source

Answer 1

• Internal Carotid
• Basilar
• Vertebral

Question 2

Circle of Willis Circulation

Answer 2

• stratified blood flow to limit damage
• little mixing of blood from the 3 major arteries
• blockage of blood flow in Circle causes localized ischemia vs global ischemic event

Question 3

Sympathetic Innervation of Cerebral Circulation

Answer 3

• NE and NPY released from post-ganglionic pre-synaptic terminal
• alpha 1 adrenergic receptors
• vasoconstriction and increased blood pressure

Question 4

Parasympathetic Innervation of Cerebral Circulation

Answer 4

• ACH
• VIP and its PHM-27 pre-cursor
• vasodilation of larger blood vessels proximal to heart

Question 5

Nociceptors

Answer 5

provide sensory innervation of distal blood vessels

Question 6

Vasodilator Neurotransmitters

Answer 6

• substance P
• neurokinin A
• CGRP

Question 7

Role of Sensory Innervation of Cerebral Circulation

Answer 7

• regulate sensitivity of brain vessels to painful torsion/manipulation
• reduced CSF volume = heavier brain and pain with motion
• activation of afferents causes vasodilation and increased blood flow to restore normal intracranial volume

Question 8

Oxygen-Mediated Cerebral Circulation

Answer 8

• flow locally controlled by oxygen consumption

- more metabolically active areas = more O2 consumption and vasodilation for O2 delivery

Question 9

Autoregulation

Answer 9

brain controls own blood flow which is held constant over range of systemic/mean arterial BP

Question 10

Mean Arterial BP vs Cerebral Blood Flow

Answer 10

• as MABP increases, cerebral flow increases
• direct relationship until autoregulation
• when autoregulation range is exceeded, direct relationship resumes

Question 11

Sympathetic Autoregulation

Answer 11

• range of autoregulation is extended to a higher MABP to protect capillaries from damage, esp in BBB
• vasculature will vasoconstrict to protect capillaries in brain, although systemic vascular resistance increases

Question 12

Intercranial Pressure vs Cerebral Blood Flow

Answer 12

as intercranial pressure increases, venous outflow is obstructed and arterial flow is reduced

Question 13

Effects on Systemic Blood Pressure

Answer 13

decreased brain perfusion activates CV control in medulla to increase systemic BP drastically ( >200) to try to force veins open for blood entry

Question 14

Increased Intercranial Pressure

Answer 14

• hydrocephalus: increased CSF volume
• cerebral edema: swelling of brain tissue
• intracranial bleeding: blockage of CSF flow from space-occupying mass

Question 15

CSF Quantities

Answer 15

• 150 mL in a given moment
• 500 mL / day
• no recirculation

Question 16

Four Ventricles

Answer 16

• 2 lateral
• Third Ventricle
• Fourth Ventricle

Question 17

Ventricular System

Answer 17

• Intraventricular Foramen: lateral to third
• Cerebral Aqueduct: third to fourth
• Median Aperture: fourth to cisterna magna
• Lateral Aperture: fourth to arachnoid space

Question 18

Choroid Plexus

Answer 18

specialized tissue in lateral ventricles producing most CSF, remainder made by tissues lining ventricles and vessels

Question 19

Passive Filtration of Serum

Answer 19

• first step of CSF production
• dependent on hydrostatic and oncotic pressure
• oncotic pressures cancel
• blood pressure in capillaries > tissue hydrostatic pressure
• fluid moves from vessels to ventricles
• capillary pressure dominates and pushes fluid into ventricle

Question 20

Hydrostatic Pressure

Answer 20

• pressure in capillary > pressure in ventricle
• pressure in capillary (blood pressure) favors movement out of ventricle
• pressure around capillary (tissue hydrostatic pressure) favors movement into ventricle

Question 21

Oncotic Pressure

Answer 21

• osmotic pressure
• depends on solute concentration
• inside capillary = pulls fluid into capillary
• surrounds fluid = pulls fluid out of capillary
• equal and opposite at choroid plexus

Question 22

Modification of Ion Composition

Answer 22

• second step of CSF formation
• HCO3, Cl, and K concentrations controlled by channels in choroid plexus epithelia
• Aquaporin 1 permits water passage

Question 23

Capillary Hydrostatic Pressure vs Intercranial Pressure

Answer 23

capillary hydrostatic pressure is higher than intercranial pressure would ever reach which drives CSF production

Question 24

[X] CSF = [X] plasma

Answer 24

• Na (ranges overlap)

- HCO3

Question 25

[X] CSF > [X] plasma

Answer 25

- Mg: interacts with Ca, added by choroid plexus - Cl: added by choroid plexus - CO2: from neuronal metabolism

Question 26

[X] CSF < [X] plasma

Answer 26

- K: tightly controlled bc changes potential - Ca: tightly controlled because toxic in high quantities - Protein - Glucose: fuel for neurons

Question 27

CSF Flow

Answer 27

-via ventricular system unidirectionally -reabsorbed by arachnoid villi -bulk flow into venous sinuses driven by pressure inside villi returns to general circulation

Question 28

CSF Absorption vs Intracranial Pressure

Answer 28

- CSF absorption proportional to intracranial pressure - no absorption at 68 mm CSF - normal pressure at 112 mm CSF - pressure of CSF drives movement into sinuses - body prevents reabsorption of CSF at low pressures bc movement is not driven

Question 29

CSF Role

Answer 29

- brain protection and "floatation" | - reduces weight of lipid/myelin rich brain so brain is not subject to motion/shock

Question 30

Limited Exchange Mechanisms of BBB Capillaries

Answer 30

- tight junctions between endothelial cells | - glial endfeet

Question 31

Glial Endfeet

Answer 31

- close contact with blood vessels - cover capillaries - increase distance over which substance diffuses from blood to CSF to reduce diffusion

Question 32

Chemical Mechanisms of BBB

Answer 32

lipophilic bilayer membranes to prevent passage of hydrophilic materials in blood

Question 33

Passive Diffusion of Substances Across BBB

Answer 33

- H2O: via AQP4 - CO2: moves from CSF to capillary for removal - O2: leave capillaries to neurons - free steroid hormones

Question 34

Glut-1 Transporter

Answer 34

- non-insulin dependent transporter of glucose across BBB | - larger transporter on capillaries vs astroglia

Question 35

Glut-3 Transporter

Answer 35

non-insulin dependent transporter to move glucose into neurons

Question 36

Na/K/2Cl Transporter

Answer 36

- electroneutral movement of ions from CSF to blood unidirectionally to keep K in CSF low - expression regulated by endothelin 1 and 2

Question 37

P-Glycoprotein/MDR-1

Answer 37

- transports drugs that cross BBB back into blood | - binds wide variety of substances with diff structures

Question 38

Circumventricular Organs

Answer 38

- neural tissue not protected by BBB due to capillaries lacking tight junctions - Four: Posterior Pituitary, Area Postrema, Organum Vasculosum of Lamina Terminalis, and subfornical organ

Question 39

Circumventricular Organs: Posterior Pituitary

Answer 39

exposure to plasma blood for hormone release and control

Question 40

Circumventricular Organs: Area Postrema

Answer 40

- cluster of neurons at surface of brain where fourth ventricle opens - triggers vomiting in response to chemical detection

Question 41

Circumventricular Organs: Organum Vasculosum of Lamina Terminalis and Subfornical Organ

Answer 41

control water/thirst/blood volume to regulate blood osmolarity