Extracellular Environment of CNS Neurons & Glia

Tony Gardner-Medwin,

Physiology room 331

ucgbarg@ucl.ac.uk

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