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?

Cell adhesion moleculerecruits ankyrin

Caspr (axon) + cell adhesion molecule

Node of Ranvier How does it develop?

Caspr in axon, linked to cell adhesion molecule in Schwann

Cam x 3

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

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

Is it really 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

Na current

“Gat

ing

curr

ent”

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

Na

Ca

cyclic

K

Opening and closing? Inactivation (closing)

Ball and chain mechanism

Activation (opening)

Mutagenesis of +ve charged amino-acids affects voltage sensitivity

+ residues

Helix screw model

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