Lecture 10 - homeostasis in the Nervous Tissue

Question 1

What is the neuronal micro environment composed of?

Answer 1

• glia
• capillaries
• other neurons
• extracellular space (extracellular matrix & brain extracellular fluid - BECF)

Question 2

What is in the extracellular matrix?

Answer 2

structural molecules that help with keeping shape

Question 3

What is brain extracellular fluid (BECF)?

Answer 3

BECF can influence neurons & be influenced by neurons

• change in BECF composition leads to increased activity in a neuron
• this will then change the composition of the BECF

Question 3

What can lead to uncontrolled activity in the neuron?

Answer 3

• neuronal dysfunction
• neuronal death if too much activity

As a result, there must be mechanisms to regulate the BECF composition because of this

Question 4

How could neuronal activity change the composition of the BECF?

Answer 4

• decrease sodium
• increase in potassium
• changes in calcium
• changes in oxygen, glucose & CO2 concentration
• increased Hydrogen, thus acidification
• neurotransmitter concentration

Question 5

How could a change in BECF composition change neuronal activity?

Answer 5

• increased K+ concentration in the BECF could elevate resting potential bringing the cell closer to threshold for firing an action potential
• increase in neurotransmitter release, increased neurotransmitter concentrations could lead to unspecific receptor activation and thus unspecific neuronal activity

Question 6

How do we regulate the neuronal micro environment, including BECF?

Answer 6

1. Blood brain barrier
2. Cerebrospinal fluid (CSF) in the ventricular system

Question 7

What is the purpose & structure of the blood brain barrier?

Answer 7

• revealed by the intravenous injection of dyes
• dyes pass across leaky capillaries
• stain soft tissues
• but not in the brain - no staining

Question 8

Describe the function of the blood brain barrier

Answer 8

• function - to protect neurons from fluctuation in concentration of substances in the blood e.g.
• increased amino acid concentrations after a meal
• increased in K+ & H+ concentration after exercise
• circulating hormones
• inflammatory mediators
• toxins

Question 9

What can happen to amino acids?

Answer 9

can be turned into neurotransmitters & affect the brain function

Question 10

How is the Brain Blood Barrier maintained?

Answer 10

• tight junctions (between endothelial cells)
• thick basement membrane
• astrocytic endfeet (glial cells) - surround capillaries within the brain (providing extra barrier between blood and BECF)

Question 11

How do important molecules get through the blood brain barrier?

Answer 11

• facilitated transport
• exchangers
• co-transporters
• increased numbers of mitochondria = active transport
• small, uncharged and/or lipid soluble can pass the BBB more easily - e.g. CO2, O2, nicotine, heroin, caffeine

Question 12

Describe the leaky regions of the blood brain barrier

Answer 12

• choroid plexuses - ventricular system
• circumventricular organs

In these areas ependymal cells beneath have tight junctions

Question 13

Why does the blood brain barrier have leaky areas?

Answer 13

• hormone release e.g. hypothalamus & pituitary glands
• osmoreceptors e.g. OVLT & SFO, hypothalamus
• temperature control centres & fever - cytokines e.g. OVLT

Question 14

What is present in the cerebrospinal ventricular system?

Answer 14

• cavity
• cerebrospinal fluid
• provides physical protection - buffer
• maintains appropriate levels of ions
• removes waste products

Question 15

What is exchanged between CSF & BECF?

Answer 15

CSF to BECF:
- macronutrients - e.g. glucose
- micronutrients - e.g. vitamins
- ions - e.g. HCO3-

BECF to CSF:
- metabolic waste products e.g. CO2
- neurotransmitters

Question 15

What is in the CSF (cerebrospinal fluid)?

Answer 15

• lower in potassium (we want o keep it low)
• low in amino acids (don’t want them to act as neurotransmitter)
• low in proteins

Question 16

Describe the flow of cerebrospinal fluid

Answer 16

• secreted by choroid plexus
• circulates around the ventricles and central canal (down the spinal cord)
• absorbed from the subarachnoid space to the venous blood system at the superior sagittal sinus

Question 17

How many mls of CSF is produced each day?

Answer 17

500mls

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