Welcome to 725
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Welcome to 725. Cellular and Molecular Neuroscience Chris Elliott & Sean Sweeney Aim: describe the cellular workings of the CNS in health and disease Neurons Glia Blood vessels See http://biolpc22.york.ac.uk/725. Neurons. Why are neurons so interesting ? Fast signalling

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Welcome to 725

  • Cellular and Molecular Neuroscience

  • Chris Elliott & Sean Sweeney

  • Aim: describe the cellular workings of the CNS

    • in health and disease

      • Neurons

      • Glia

      • Blood vessels

  • See http://biolpc22.york.ac.uk/725


Neurons

  • Why are neurons so interesting ?

    • Fast signalling

    • Specific connections

    • Long distances

  • Key features:

    • Need glia

    • Ion channels

    • Synaptic transmission


Glia

  • About 100 times more glial cells than neurons

  • Support neurons


Human CNS


Revision – cell shape

  • Axon

  • Dendrites

  • Soma


Channel distribution

  • Non-uniform

    • Different in cell body and axon/dendrites

    • Myelinated axons – Na channels at node of Ranvier

K orange; Na red


Na channel is anchored


Node of Ranvier

Caspr (axon) + cell adhesion molecule

  • How does it develop?

Cell adhesion moleculerecruits ankyrin


Node of Ranvier

  • How does it develop?

Cam x 3

Caspr in axon,

linked to cell adhesion molecule in Schwann


Summary so far

  • Neuronal organisation is complex

    • Cell geometry

    • Channel distribution

  • Signalling by cell-cell interaction important for organistion


Revision - electrics

  • Current is rate at which ions flow

    • Measure in ions/sec or Amps

  • Voltage is driving force

  • Resistance = V/I

  • Conductance = I/V

    • More current flowing means a bigger hole to flow through

    • Measure in Siemens S (pS)


Aim: to separate capacitance current (IC) from ionic current

IC only flows when the voltage is changing

Use ion substitution or pharmacological blockers to identify ionic currents

Revision – voltage clamp


Action potentials in

Myelinated

Unmyelinated

Cell bodies

Dendrites

Snails

Note differences in time scale!

Not all APs are equal


Action potentials in

Myelinated

Unmyelinated

Cell bodies

Dendrites

Snails

Mammals are different to amphibians

Not all APs are equal


Mammals have many less K channels

AP depends on inactivation of Na current to end

Not all APs are equal


Ion channels for Na, K, Ca, Cl, etc

Subtypes for each ion may have different characteristics

Here 3 K channels

Maintained

Transient

Off transient

Many types of channels


VC- refractory period

  • Two pulse experiment

    • K-current blocked

    • Na current only


If Na channels are opened by voltage, then they need a voltage sensor

Measure the current when Na and K are blocked

VC- gating current

K current blocked

Na and K current blocked

Na current (subtraction)


Is it really gating current?

  • Two pulse experiment

    • K-current blocked

    • Na current only

Plot initial Na vs gating current


Mostly ?

Corresponds to movement of about 3 ionic charges

Also measure using asymmetry of positive and negative pulses, so may be called asymmetry current

Is it really gating current?

“Gating current”

Na current


Summary point

  • Macroscopic analysis shows:

    • Voltage sensitivity important in axons

    • Physiological diversity to reflect anatomical diversity

    • Implies cellular diversity


Use a small patch of membrane

Fixed voltage

Measure current

Revision – patch clamp


Summation of the effects of individual channels give the macroscopic result

Summated channels


Properties of channels

  • Obey Ohm’s law

  • Ions flow freely through open channels

  • Channels selective for particular ions


Channels flow freely

Transporters need energy

ATP

ion gradient

Channels vs transporters


4 repeats of 6 transmembrane regions

S4 mutations affect opening

S6 line the pore

Molecular biology


Most mutations probably fatal before birth

Mutations for disease?


Similar genes encode channels with different ionic specificity

Channel radiation

K

cyclic

Ca

Na


Inactivation (closing)

Ball and chain mechanism

Opening and closing?


Mutagenesis of +ve charged amino-acids affects voltage sensitivity

Activation (opening)

  • Helix screw model

+ residues


Rotation of charged residues in S4 may affect S5 and S6 to change diameter of the pore

New hypothesis


Alternative splicing


RNA Editing

  • ADARs (adenosine deaminases that act on RNA) A → I (treated as G)


How often in ion channels?

  • Multiple genes in mammals (9)

  • Much alternative splicing

  • Many RNAi editing sites

    • Glu ion channels

    • Serotonin receptor

    • Potassium voltage gated channels

  • In flies,

    • one Na channel gene

    • > 3 alternative spices

    • 10 RNAi editing sites


Conclusion

  • Microscopic physiology and molecular studies contribute together to our understanding of channels

  • Mechanism of opening and of closing relates to channel morphology and sequence

  • Evolutionary diversity and adaptation to different functions

  • References


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