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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
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
Neurons
  • Why are neurons so interesting ?
    • Fast signalling
    • Specific connections
    • Long distances
  • Key features:
    • Need glia
    • Ion channels
    • Synaptic transmission
slide3
Glia
  • About 100 times more glial cells than neurons
  • Support neurons
revision cell shape
Revision – cell shape
  • Axon
  • Dendrites
  • Soma
channel distribution
Channel distribution
  • Non-uniform
    • Different in cell body and axon/dendrites
    • Myelinated axons – Na channels at node of Ranvier

K orange; Na red

node of ranvier
Node of Ranvier

Caspr (axon) + cell adhesion molecule

  • How does it develop?

Cell adhesion molecule recruits ankyrin

node of ranvier1
Node of Ranvier
  • How does it develop?

Cam x 3

Caspr in axon,

linked to cell adhesion molecule in Schwann

summary so far
Summary so far
  • Neuronal organisation is complex
    • Cell geometry
    • Channel distribution
  • Signalling by cell-cell interaction important for organistion
revision electrics
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)
revision voltage clamp
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
not all aps are equal
Action potentials in

Myelinated

Unmyelinated

Cell bodies

Dendrites

Snails

Note differences in time scale!

Not all APs are equal
not all aps are equal1
Action potentials in

Myelinated

Unmyelinated

Cell bodies

Dendrites

Snails

Mammals are different to amphibians

Not all APs are equal
not all aps are equal2
Mammals have many less K channels

AP depends on inactivation of Na current to end

Not all APs are equal
many types of channels
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
VC- refractory period
  • Two pulse experiment
    • K-current blocked
    • Na current only
vc gating current
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
Is it really gating current?
  • Two pulse experiment
    • K-current blocked
    • Na current only

Plot initial Na vs gating current

is it really gating current1
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
Summary point
  • Macroscopic analysis shows:
    • Voltage sensitivity important in axons
    • Physiological diversity to reflect anatomical diversity
    • Implies cellular diversity
properties of channels
Properties of channels
  • Obey Ohm’s law
  • Ions flow freely through open channels
  • Channels selective for particular ions
channels vs transporters
Channels flow freely

Transporters need energy

ATP

ion gradient

Channels vs transporters
molecular biology
4 repeats of 6 transmembrane regions

S4 mutations affect opening

S6 line the pore

Molecular biology
rna editing
RNA Editing
  • ADARs (adenosine deaminases that act on RNA) A → I (treated as G)
how often in ion channels
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
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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