NB2-3 - Transport Within the CNS and DLA

Question 1

What are the four functions of the CSF?

Answer 1

1. Providing buoyancy of the brain and spinal cord
2. Decrease traction of nerves and blood vessels
3. Provide cushioning in the case of moderate trauma
4. Provide a medium that is a stable ionic environment, a source of electrolytes, a pathway for neuroactive products, and place to deposit metabolites for removal

Question 2

How does the pH of CSF compare to plasma?

Answer 2

CSF pH is less than that of plasma

Question 3

Which ion concentrations are increased in CSF relative to plasma?

Answer 3

Cl-, Mg++, and Na+ (although [Na] are almost equal)

Question 4

What is a key difference between the transport mechanisms used by capillary endothelial cells in the CNS and the rest of the body? Why does this difference exist?

Answer 4

Normally both paracellular and transcellular transport occurs, however, within the CNS only transcellular transport occurs. This is important to maintaining the BBB.

Question 5

T/F - no substances can pass the BBB via simple diffusion

Answer 5

False, small uncharged hydrophobic substances can

Question 6

It is important to note that which two molecules can pass the BBB via facilitated diffusion?

Answer 6

Glucose and L-DOPA

Question 7

Answer 7

L-Dopa

Dopamine cannot cross the BBB so L-Dopa must be used

Question 8

Answer 8

D

Question 9

At what point is CSF pressure considered to be high? What symptom(s) is typically seen when CSF pressure is high?

Answer 9

CSF pressure >15 mmH2O is considered high

Nausea is often seen with high CSF pressure

Question 10

T/F - releasing CSF is a good way to lower intracranial pressure (ICP)

Answer 10

False

While this will lower intracranial pressure it will also likely cause the brain to herniate through the foramen magnum

Question 11

What are the major pathological factors that can lead to a weakening of the BBB?

Answer 11

• Hypertension
• Hyperosmolarity (sometimes done on purpose to permit delivery of lipid insoluble drugs to the CNS)
• Trauma
• Ischemia
• Inflammation
• Pressure
• Infection

Question 12

What are the physiological causes of weaknesses in the BBB and where are they located?

Answer 12

• Circumventricular Organs (most around the 3rd ventricle)
  
  • Area Postrema (superior medulla and 4th ventricle)
  • Median Eminence (hypothalamus and hypophyseal portal system)
  • Neurohypophysis (posterior pituitary)
  • Organum Vasculosum (lamina terminalis and 3rd ventricle)
  • Pineal Body (3rd ventricle)
  • Subcommissural Organ (3rd ventricle)
  • Subfornical Organ (3rd ventricle)

Question 13

Answer 13

Question 14

Why does an increase in CSF pressure cause nausea?

Answer 14

The area postrema is the vomitting center of the brain. An increase in CSF pressure strectches the receptors of the area postrema

Question 15

What are the key differences between ependymal cells and choroid plexus epithelial cells?

Answer 15

• Ependymal cells line ventricles while the other line the choroid plexus
• Choroid plexus epithelium secretes mostly CSF while ependymal cells mostly regulate CSF composition
• Ependymal cells are connected by gap junctions while choroid plexus epithelial cells are connected by tight junctions.

Question 16

How do ions and water enter the CSF?

Answer 16

Ions are moved via transporters and pumps while water passively follows the ions through aquaporins

Question 17

Describe the route of CSF circulation

Answer 17

1. CSF is produced in choroid plexuses located in all the brain ventricles
2. Using the various foramina, the CSF circulates from ventricle to ventricle (including central canal)
3. Eventually the CSF enters the subarachnoid space from the 4th ventricle via the Foramen of Magendie
4. CSF within the subarachnoid space travels up to the superior sagittal sinus and enters the venous system via the arachnoid villi (granulations)

Question 18

List the foramina important to CSF circulation and what they connect. Also include their AKAs

Answer 18

• Interventricular foramina (aka foramen of Monro) connect lateral ventricles to third ventricles
• Cerebral Aqueduct (aka mesencephalic duct or sylvian aqueduct) connects the 3rd and 4th ventricles
• Foramina of Luschka (lateral apertures of 4th ventricle) connect 4th ventricle to the superior aspect of the subarachnoid space surrounding the cerebellum
• Foramen of Magendie (aka median aperture) connects 4th ventricle to the inferior aspect of the subarachnoid space surrounding the cerebellum

Question 19

Answer 19

Question 20

Answer 20

Question 21

Answer 21

Question 22

Answer 22

H

Question 23

What is hydrocephalus? List the two forms of hydrocephalus and describe their various causes.

Answer 23

Hydrocephalus is an abnormal accumulation of CSF within the brain

• Non-Communicating (obstructive) - caused by a blockage within the ventricular system - congenital or by cellular debris
• Communicating - caused by CSF overproduction (ie - choroidal tumor), insufficient CSF drainage (ie - malfunction of arachnoid villi), or parenchymal brain tissue loss resulting in compensatory ventricle enlargement (termend hydrocephalus ex vacuo)

Question 24

Answer 24

B

Question 25

What is brain edema? List the two types of brain edema and their various causes.

Answer 25

Brain edema is an excess accumulation of water in the intra- and/or extracellular spaces of both the grey and white matter.

1. Vasogenic edema is caused by a disruption of the BBB
2. Cytotoxic edema may arrise with tissue damage and/or Na/K pump failure in neurons and glia (usually due to inadequate blood supply) causing ionic gradient dissipation and cellular swelling

Question 26

Which cells take the longest to get damaged from brain edema? Why

Answer 26

Astrocytes

They can self-regulate cellular volume at the initial onset of edema

Question 27

Answer 27

F

Question 28

Answer 28

C