Physiology of CSF

Question 1

how much CSF is found in the CNS

Answer 1

150ml

Question 2

where is CSF found in the body

Answer 2

CSF occupies the subarachnoid space and ventricular system around and inside brain and spinal cord.

Question 3

what are the 3 major functions of CSF

Answer 3

mechanical protection: shock-absorbing medium that portects brain tissue

homeostatic function: pH of CSF affects pulmonary ventilation and cerebral blood flow, also transports hormones

circulation: medium for minor exchange of nutrients and waste products between blood and brain tissue

Question 4

how can CSF be clinically analysed

Answer 4

lumbar puncture

Question 5

what can a lumbar puncture reveal

Answer 5

intra cranial pressure

Question 6

what does normal CSF look like

Answer 6

clear, colourless and little protein

Question 7

Embryonic Development of the Brain and Ventricular System

describe the formation of the ventricules and spinal cord central channel

Answer 7

neural tube is the first structure to form (neural progenitor cells form the neural plate, which forms neural groove and finally neural tube)

the hollow centre of the tube becomes the ventricules and spinal cord central channel

Question 8

Embryonic Development of the Brain and Ventricular System

describe the formation of the choroid plexus

Answer 8

develops from ependymal cells in the walls of the ventricules

developing arteries invaginate into the roof of the ventricle to form the choroid fissure. The involuted ependymal cells and vessels form the choroid plexus

Question 9

where is CSF produced from

Answer 9

Ventricles are lined by choroid plexus, which is composed of ependymal cells that secrete CSF. The main function of the ventricles is the production and distribution of CSF.

Question 10

how is CSF made and moved from choroid plexus into ventricles

Answer 10

active transport of sodium from blood to CSF creates an electrical gradient which pulls chlorine across. water follows by osmosis

Question 11

compare the constituency of CSF to that of blood

Answer 11

lower concentration of potassium, glucose and protein

higher concentrations of Sodium and Chlorine

Question 12

which structure in the choroid plexus prevents substances flowing freely into CSF

Answer 12

tight junctions between ependymal cells

Question 13

describe the ventricular system

Answer 13

lateral ventricles via Foramen of Munro to 3rd ventricle

to 4th ventricle via cerebral aqueduct (of Sylvius)

to subarachnoid space via median and lateral apertures

Question 14

what are the median and lateral apertures also called

and how many of them are there

Answer 14

3 apertures - one median and two lateral

median = foramen of Magendie

lateral = foramen of Luschka

Question 15

describe the CSF circulation in the ventricules

Answer 15

it then circulates the central canal of spinal cord in subarachnoid space

Question 16

how much CSF is produced and absorbed every day (continuous process)

Answer 16

500ml

Question 17

how much CSF is there in the body at any one time

Answer 17

125 ml

Question 18

describe the reabsorption of CSF

Answer 18

returns to venous blood by entering the dural venous sinuses through arachnoid granules into the superior sagittal sinus

Question 19

blood-CSF barrier

Answer 19

The blood-CSF barriers separate the peripheral and cerebral blood from the CSF

Composed of epithelial cells of the choroid plexus at the peripheral blood-CSF boundary and the arachnoid membrane at the cerebral blood-CSF boundary.

Question 20

BBB

Answer 20

Highly selective semi-permeable membrane that separates the circulating blood from the brain and extracellular fluid in the CNS.

Question 21

what is the BBB formed of

Answer 21

• Central component is tight junctions between endothelial cells that make up blood vessels. Restrict diffusion across.
• Astrocytes (star shaped glial cells – homeostasis) surround blood vessels and help form BBB. Involved in signalling.

Question 22

give some examples of parts of the brain that don’t have a BBB

Answer 22

• Circumventricular organs have no BBB as their functions require access to blood stream e.g. posterior pituitary releases hormones straight into blood stream
• circumventricular organs (linkage between CNS and peripheral blood flow)
• pineal gland (secretes melatonin directly into systemic circulation)

Question 23

what does the BBB allow across

Answer 23

BBB endothelial cells allow the diffusion of hydrophobic molecules (O₂, CO₂) into the CSF

Restrict the diffusion of microscopic, large or hydrophilic molecules.

Question 24

signs of raised ICP

Answer 24

• bilateral papilloedema
• morning headache, worse on sneezing/coughing
• vomiting without nausea
• altered level of consciousness

Question 25

Munro Kellie hypothesis

Answer 25

The Munro-Kellie hypothesis states that the cranial compartment is rigid incompressible and volume of brain, CSF and blood inside it is fixed. Therefore, a state of volume equilibrium exists. The main buffers to increased volume are CSF, and blood volume to a lesser extent.