Transcript Here - Chris Elliott, University of York

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
 How does it develop?
Caspr (axon) +
cell adhesion molecule
Cell adhesion molecule
recruits 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)

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
Not all APs are equal
 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
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
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
Na current (subtraction)
K current blocked
Na and K current blocked
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
“Gating current”
Is it really gating current?
Na current
Summary point
 Macroscopic analysis shows:
Voltage sensitivity important in axons
 Physiological diversity to reflect anatomical
diversity
 Implies cellular diversity

Revision – patch clamp
 Use a small patch of
membrane


Fixed voltage
Measure current
Summated channels
 Summation of the
effects of individual
channels give the
macroscopic result
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
Molecular biology
 4 repeats of 6
transmembrane regions
 S4 mutations affect
opening
 S6 line the pore
Mutations for disease?
 Most mutations probably fatal before birth
Channel radiation
 Similar genes encode channels
with different ionic specificity
K
cyclic
Ca
Na
Opening and closing?
 Inactivation (closing)

Ball and chain mechanism
Activation (opening)
Helix screw model

Mutagenesis of +ve
charged aminoacids affects
voltage sensitivity
+ residues
New hypothesis
 Rotation of charged
residues in S4 may
affect S5 and S6 to
change diameter of the
pore
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