Homeostasis in the CNS 1 & 2

Question 1

what is in a neural microenvirnomennt

Answer 1

glia
neurons
capillaries

Question 2

extracellular space

Answer 2

ECM

BECF - interstitial fluid

Question 3

changes in BECF composition

Answer 3

can both influence or be influenced by other neurons
increased activity in neuron leads to change in BECF which increases neuronal activity
if the cycle becomes uncontrolled = neuronal dysfunction and death
so BECF composition must be tightly regulated

Question 4

how do we regulate neuronal microenvironment, including BECF?

Answer 4

blood brain barrier
CSF in ventricular system
neurons
glial cells and astrocytes

Question 5

how could BECF composition be affected by increased neuronal activity?

Answer 5

increased K, changes is Ca, O2, CO2 and glucose concentration and different concs of neurotransmitters

Question 6

how could a change in BECF change neuronal activity?

Answer 6

increase in K in BECF changes resting potential bringing cell closer to threshold for firing action potential
inc neurot release could lead to unspecific neural activity

Question 7

blood brain barrier

Answer 7

revealed by IV injection of dyes
dye passes through leaky capillaries
stain soft tissues and all organs (inc spinal cord) apart from brain
so brain must have something to stop things leaking into and out of it
function = protect neurons from fluctuations in concentration of substances in blood
e.g. amino acid conc increases after a meal - dont want in brain as could act as unspecific neurot
same for increases in K and H after exercise

Question 8

maintaining blood brain barrier

Answer 8

tight junctions between endothelial cells
thick basment membrane - additional layer that molecules have to get through
astrocytic endfeet
brain capillaries dont have intracellular clefts or fenestra
all form barriers to stop molecules getting into BECF

Question 9

how do important molecules get through

Answer 9

facilitated transport
exchangers
co-transporters
means that treating dieseases in brain is difficult as most things cant get through
small, uncharged and/or lipid soluble molecules can pass through more easly such as CO2, nicotine, heroine, caffine

Question 10

leaky regions

Answer 10

choroid plexuses - ventricular system
circumventricular organs
in these ares ependymal cells beneath have tight junctions to protect rest of brain

Question 11

why have leaky regions?

Answer 11

in brain there are osmoreceptors and temp control centres that need to send hormones from pituitary into circulation

Question 12

CSF in ventricular system

Answer 12

ventircular system = fluid filled cavity for physical protection, maintaining ion levels and removing waste
secreted by choroid plexuses - constant production, some in lateral, 3rd and 4th ventricle
circulates around ventricles and central canal, moves around brain and absorbed by subarachnoid space to return to venous vlood system by superior sagittal sinus
foramens allows CSF out of central canal and back up to sinus

Question 13

sinus

Answer 13

something that drains into venous system

Question 14

secretion of CSF

Answer 14

500mls/day
3 mechanisms:
ultrafiltration of plasma into ECF across normal ‘leaky’ capillaries = free movement
selective absorption of substances into CSF across choroidal epithelial cells = regulated movement
free movement of substances from CSF to BECF across ependymal cells = free movement

Question 15

structure of choroid plexus

Answer 15

outer epithelium separates brain tissue from ventricular system, made of leaky ependymal cells so CSF can move freely into brain
choroidal epithelium contains pockets of fluid with tight junctions to separate it from CSF

Question 16

what is in CSF?

Answer 16

K
amino acids
proteins
much higher plasma than CSF

Question 17

outwards bumps on cerebral cortex

Answer 17

giri

Question 18

inward bumps on cerebral cortex

Answer 18

sulci

Question 19

the meninges

Answer 19

membranes around brain and spinal cord
leptomeninges:
pia mater
arachnoid mater

dura mater

Question 20

pia mater

Answer 20

innermost
thin
permeable
allows diffusion

Question 21

arachnoid mater

Answer 21

separated by CSF in subarachnoid space, has tight junctions

Question 22

dura mater

Answer 22

can split into 2 layers, one clings to bumps on cerebral cortex, other is more circular

Question 23

absorption of CSF

Answer 23

evaginations of arachnoid membrane:
arachnoid granulations
arachnoid villi
both do same thing and project through duramater into sinus space
single layer of cells joined by tight junctions
an increase in pressure sue to constant production of CSF leads to the membrane pinching CSF and transporting back into circulatory system
so inc absorption with inc intercranial pressure

Question 24

exchange between CSF and BECF?

Answer 24

exchange occurs from ventricles across ependymal cells and from subarachnoid space across pia mater - doesnt have tight junctions

Question 25

CSF —-> BECF

Answer 25

macronutrients e.g. glucose
micronutrients e.g. vitamins
ions e.g. HCO3

Question 26

BECF —-> CSF

Answer 26

metabolic waste products e.g. CO2

neurotransmitters

Question 27

what if CSF cannot circulate properly?

Answer 27

leads to hydrocephalus
due to blockage in ventricular system, usally in cerebral aqueduct as its so thin
constant production of CSF leads to major swelling of brain
- dilation of ventricular system
can also be caused by interrupted CSF absorption but a blockage is more likely
- inc intercranial pressure
- loss of cells in brain
- loss of brainstem reflexes - swelling pushes on brainstem = cant regulate heart rate etc

Question 28

tripartite synapse

Answer 28

neurons, glial cells and astrocytes
neurons and astrocytes work together to clear neurot from BECF an terminate neurotransmission
e.g. glutamate needs to be removed from cleft or it will continuously signal
EAAT3 - recycles glutamate into pre syn terminal
EAAT1/2 - moves neurot into astrocytes - breaks dwon into glutamine to recycle in pre syn terminal

Question 29

neurons and astrocytes removing K from extracellular space

Answer 29

goes back into cell to maintain low extracell K
inc extracell K affects astrocyte function
inc K uptake means astrocyte moves more K into cell which drives ic in glucose metabolism = more ATP so ATPase pump works faster

Question 30

why do neurons ahve -65 resting potential but glia have -85 when their eqm potenital for K is around the same?

Answer 30

neuronal membranes are more permeable to Na than astrocyte membranes, astros have higher K sensitivity = more selective = more extra cell K with influence Vm

Question 31

astrocytic syncytium

Answer 31

allows spatial buffering
gap junctions create syncytium = lots of astrocytes joined to each other with electrical synapses that couple them together
redistributes K to areas of decreased activity
can transport sugars, aa, cAMP and Ca

Question 32

neurovascular coupling

Answer 32

inc neuron firing rate —- astrocyte Ca inc —- release of vasoactive substances from astrocyte —- blood vessel diameter increase
joining activity of neurons to vasculature

Question 33

functional imaging techniques

Answer 33

MRI allows 3D structural images of brain
how to measure activity:
- active neruons need more glucose and O2
- more blood directed to those areas
- 2 techniques detect changes in blood flow

Question 34

positron emission tomography (PET)

Answer 34

exploits glucose use in brain

Question 35

fMRI

Answer 35

exploits O2 use in brain
uses electromagnetic wave to disrupt H atom state
Blood Oxygen Level Dependant - BOLD
oxyhemoglobin and deoxy distort the magnetic resonance properties if H atoms differently