Topic 5 - Homeostasis in the nervous tissue

Question 1

Why must the BECF (Brain extracellular fluid) composition be tightly regulated?

Answer 1

• because it can influence neurons and be influenced by neurons.
• Function - to protect neurons from fluctuations in concentrations of substances in the blood

Question 2

How can a change in BECF composition change neuronal activity?

Answer 2

• increased K+ conc in BECF = elevates resting potential bringing cell close to threshold for firing an action potential.
• increased neurotransmitter conc in BECF = can lead to unspecific receptor activation = unspecific neuronal activity.

Question 3

What are the 4 ways in which the BECF and neuronal microenvironment is regulated?

Answer 3

1. Blood brain barrier
2. Cerebrospinal fluid (CSF) in ventricular system
3. Neurons
4. Glial cells – focus on astrocytes

Question 4

What is the Blood brain barrier? (BBB)

How is it maintained?

Answer 4

• BBB can be seen with injection of dyes
• Dyes pass across leaky capillaries (No staining in brain)

BB is maintained with neurons and astrocytes…

• tight junctions between epithelial cells, thick basement membrane and astrocytic endfeet.
• molecules get through by facilitated transport, exchangers and co-transporters.
• small, lipid soluble molecules can pass BBB more easily.

Question 5

What are the leaky regions of the Blood brain barrier?

Why have these leaky areas?

Answer 5

1. ) Chroid plexus - ventricular system
2. ) Circumventricular organs

Leaky areas for hormone release, osmoreceptors and temp control and fever.

Question 6

What is the CSF - cerebrospinal fluid?

Answer 6

The ventricular system has cerebrospinal fluid (CSF):

● Secreted by choroid plexus
● Circulates around the ventricles and central canal
● Absorbed from the subarachnoid space to the venous blood system at the superior sagittal sinus

Question 7

How is cerebrospinal fluid secreted?

Answer 7

1. Ultrafiltration of plasma into ECF across normal ‘leaky’ capillaries
2. Selective absorption of substances into CSF across choroidal epithelial cells
3. Free movement of substances from CSF to BECF across ependymal cells

Question 8

What are the 3 types of meninges?

Answer 8

Leptomeninges:

• Pia mater
• Arachnoid mater
• Dura mater

Question 9

What happens if CSF cannot circulate properly?

Answer 9

Hydrocephalus

Question 10

How do neurons and astrocytes regulate the BECF?

Answer 10

• Astrocytes act as a syncytium to buffer extracellular K+ —-> Neurons and astrocytes remove K+ from the extracellular spac

Neurons and astrocytes terminate neurotransmission at the tripartite synapse
- Can recycle neurotransmitters to presynaptic terminals

Increases in extracellular K+ affect astrocyte function

• Increased glucose metabolism
• Increased K+ uptake
• Astrocyres enable neurovascualr coupling.

Question 11

How does astrocytic syncytium allow spatial buffering?

Answer 11

● Gap junctions create a syncytium
● Redistributes K+ to areas of decreased activity
● Can also transport sugars, amino acids, cAMP, Ca2+

Question 12

Name 2 types of Functional imaging techniques.

Answer 12

1) PET - Positron emission tomography - exploits glucose use

2) fMRI - Functional magnetic resonance imaging - exploits oxygen use
- is Blood Oxygenation Level Dependent (BOLD)
- fMRI uses changes in levels of oxygenated blood to identify active areas