extracellular environment of cns neurons & glia tony gardner-medwin, physiology room 331...
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Extracellular Environment of CNS Neurons & Glia
Tony Gardner-Medwin,
Physiology room 331
www.ucl.ac.uk/lapt/med Please use the Web Discussion Forum for problems/queries
From Neuron to Brain (Nicholls, Martin & Wallace)- Chapter on Neuroglia
CNS Extracellular Environment
Is there any e-c space?
- size, composition
The macro-environment: CSF, blood
Homeostasis, Disturbances
- (normal, pathological)
Role of glia in K+ homeostasis
Failures of regulation:
The war between + feedback and - feedback
Is there any extracellular space?
Photoreceptors (dark) and glial cells in the compound bee eye
Neurons & glia (shaded) in mammalian cerebellum
Measurement of Extracellular Space Volume1. 15-20nm gaps => 2% - 5%
but ? are cells swollen to occlude space?
2. Try e-c markers from blood: don’t show up ( ‘Blood-Brain Barrier’ )
? Is this lack of penetration due to zero e-c space?
3. Markers from CSF: 10-20% space, but v slow equilibration
4. Improved EM technique: rapid freezing -> 18 - 25%
Measurement of Extracellular Space Volume1. 15-20nm gaps => 2% - 5%
but ? are cells swollen to occlude space?
2. Try e-c markers from blood: don’t show up ( ‘Blood-Brain Barrier’ )
? Is this lack of penetration due to zero e-c space?
3. Markers from CSF: 10-20% space, but v slow equilibration
4. Improved EM technique: rapid freezing -> 18 - 25%
EM after rapid freezing, <30s after cessation of circulation
EM after rapid freezing, 8 min after cessation of circulation
Measurement of Extracellular Space Volume1. 15-20nm gaps => 2% - 5%
but ? are cells swollen to occlude space?
2. Try e-c markers from blood: don’t show up: ‘blood-brain barrier’
? Is the lack of penetration due to zero e-c space?
3. Markers from CSF: 10-20% space, but v slow equilibration
4. Improved EM technique: rapid freezing -> 18 - 25%
5. Release of e-c markers from electrodes, and measurement of concentration, with ion-selective electrodes -> 15 - 25%
Measurement of Extracellular Space Volume
Five methods ……
Conclusions~ 20% e-c space (similar to rest of body)
Cut off from blood (unlike elsewhere)
Free (but slow) diffusion exchange with CSF
….. but what about its composition?
Functions:
Mechanical: floating brain
↓ postural effects
lubrication / movement
pressure ≈ venous (10 mmHg)
Variable volume reservoir
(but only a few % of brain volume)
Clearance (like lymph)
Homeostasis:
regulated composition
How successful is K+ homeostasis?
Plasma [K+] CSF [K+]
Normal Diet: 4.2 2.8 mM
Low K diet: 1.6 2.7 mM
High K diet: 7.1 3.0 mM
Baseline e-c [K+]o measured with ion-selective micro-electrodes is similar to CSF (normally less than plasma, and well regulated)
Does neural activity alter e-c [K+] ?
Epileptic discharges in cortex -> transient increase of extracellular [K+] and depolarisation of astrocytes
Leech ganglion, showing neurons and glia
Electrical properties of glial cellsInitially studied in leech n.s.
1. Large resting potentials ( ~ -90 mV cf neurons -70 mV)
2. Inexcitable (no action potentials)
3. Electrically coupled (via GAP junctions)
4. Vm sensitive to [K+]o - follows Veq(K+)
- membranes very selectively permeable to K+
5. Slow, long depolarisations when adjacent neurons are stimulated
6. Can take up K+, GABA, glutamate from e-c space
Effect of light stimulation on K+-selective electrode concentration measurements in bee eye
[K+]o
[K+]i
[K+]i
(mM)
-60
-72
Glial membrane potential (mV)
0 10 20 30 40 50 s
Effect of visual stimulation with moving bars of light, on glial Vm in cat visual cortex
K+ ‘spatial buffer’ mechanism disperses potassium from regions of activity & build-up, into normal tissue and to surface fluid
Diagrammatic version of the coupled astrocyte network
REGULATION OF E-C [K+]
1. EXCHANGE WITH BLOOD: SLOW !!Accurate homeostasis but only regulation of average
concentration over many hours
2. DIFFUSION :Evens out local disturbances, reducing their maximum effect.
Effective only over short distances, and would be BETTER without the blood-brain barrier.
3. GLIAL UPTAKE and DISPERSAL via SPATIAL BUFFER mechanism. Assists dispersal by diffusion ( ~ 5x) Helps to reduce disturbances due to neural activity.
But why is regulation important ??
Wave of drastic, but transient disturbance of extra-cellular K+ and Ca++ concentrations spreading from local trauma in baboon cortex
‘Cortical Spreading Depression’
(probably part of the syndrome in migraine and stroke)
Positive feedback may lead to e-c instability in stroke, migraine and trauma
Transmitter Release
Raised PNa, PK, PCl
K+ release
Spatial Buffer currents
K+ uptake and dispersal
+ feedback- feedback
GLIA NEURONS
Raised [K+]o
Depolarisation
The End