SESSION OUTLINE
• Presentation (~60mins)
- Very Brief Introduction to Fundamental concept in neuronal
signaling (Membrane potential and Action Potential)
- Part 1 (Electrophysiological properties of neurons and Signal
processing)
- Part 2 (Intrinsic plasticity in learning, aging and CNS disorder)
• Scenario facts exploration (~40mins)
Membrane Potential
Refer to separation of opposite charges across the cell membrane
(unequal distribution of charges establish by active protein pumps present on membrane)
Key Ions : Na+ , Cl-, Ca2+ (concentrated outside)
K+, anionic proteins (concentrated inside)
Membrane Potential
All cells possess membrane potential, however neurons and few other cell types could
actively manipulate changes in their membrane potential for signaling purpose.
Each specify ion possess an equilibrium
potential that is determined by their
distribution.
In general,
When Na+ or Ca2+ channel opens, Na+ /Ca2+ rush into the cell producing depolarisation.
When K+ channel opens, K+ rush out of the cell producing hyperpolarisation
*Changes in membrane potential is brought about by changing the cell
permeability to selective ions.
Anatomy of an Action Potential
*Changes in membrane potential is brought about by changing the cell
permeability to selective ions. (Brought about by Neurotransmitter binding resulting in
opening of specific ligand gated channels).
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Part 1
(Electrophysiological properties of neurons
and Signal processing)
Neurons exhibit widely varying electrophysiological properties.
•Differential
patterns
of
electrical activity are due to the
presence of distinct mixture
and distribution of different
types of voltage gated ion
channels with varying kinetics
and regulatory properties.
•Firing pattern are also affected
by input received by neurons
(Duration, magnitude and phase
of input).
Voltage gated ion channel superfamily
Important characteristics/parameter of a voltage gated channel:
•Kinetics (mode of activation/inactivation, duration of activation…)
•Voltage sensitivity/threshold for activation
•Selectivity (Na+,K+,Ca2+..)
•Regulatory properties
•Types of agonist and antagonist the channel is sensitive to
Current trace obtained when
membrane depolarised from
-100 to -10 mV
Voltage dependence and kinetics of different ionic currents mediated by different voltage
gated ion channel.
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l
General distribution of voltage-gated
channels in a typical hippocampus CA1
neuron.
Note that different neuronal
compartments express characteristic
combinations of ion channel subunits.
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Dendritic A-type Current
Key Properties:
•Voltage dependent K+ current mediated by Kv4.2 subunits
•Activated by depolarisation (activates at subthreshold voltage)
•Rapid inactivation at maintained depolarisation
Functions:
•Activate by EPSP and increase EPSP attenuation
•Attenuates back-propagating action potential (More on this later)
• Important regulator of dendritic excitibility (filter off many weak signal)
Dendritic H Current (Ih)
Key Properties:
•Voltage dependent non-specific monovalent cationic current (both K+ and Na+)
mediated by HCN subunits.
•Activated by hyperpolarisation (tonically active at resting potential - *effects mainly
depolarisation)
• Range of inactivation kinetics when depolarised (depending on types of subunits)
•Ligand gated by cyclic nucleotide
Functions:
•Increase EPSP attenuation and reduced summation
•Influence resting membrane potential
• Pacemaker activity (affecting the rate of neuronal activity oscillation)
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Rhythmic pattern during sleep (Thalamic spindles)
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The thalamic neuron spontaneously generates rhythmic bursts of action potentials due to the
interaction of the Ca2+ current IT and the inward “pacemaker” current Ih.
Depolarization of the neuron changes the firing mode from rhythmic burst firing to tonic
action potential generation in which spikes are generated one at a time.
Removal of this depolarization during sleep reinstates rhythmic burst firing.
Processing and regulation of electrical signal at various stages along a neuron
1) Synaptic Input
Synaptic weight can undergo bidirectional
plasticity in the form of LTP and LTD modulated by
AMPA, NMDA
2) Propagation of Synaptic Input to Cell Body
Temporal and spatial summation of EPSP and IPSP
generated by synaptic input can be modulated by
6) APpresence
Backpropagation
of voltage gated channel at dendrites e.g
In addition,
AP canofactively
into dendritic region
attenuation
EPSP bypropagated
e.g. A and back
H Current
Magnitude and travel distance can be subjected to modulation
3) Initiation
of threshold
Action Potential
by A current
and high
activated Ca2+ channel (Ca2+ L
Once
the
signal
reaches
Axon
hillocks, initiation of AP can
current)
be affected by factors such as AP threshold level, resting
membrane potential… Density of Na+ Channel at axon
hillocks determine its excitability.
Apical
Dendrites
Axon
4) Action
5) Firing
Potential
Properties
Properties
Pyramidal neurons
Properties
Profile of
ofAP
APinlike
turn
duration
determine
and amplitude
the overallcan
firing
bepattern
affectedof
by neurons.
variety ofE.g.
voltage
bust gated
firing channel
due to excess
with different
charges delivered
kinetics by
andpersistent
regulation.
Na+
E.g.
current.
Persistent
Firing
Na+
Properties
current, M
cancurrent….
show
frequency adaptation due to calcium sensitive K+ Channel
Back-Propagating Action Potential
•Even though dendrites are not as excitable as axons, they
contain voltage-gated channels that can support action
potential propagation.
•Properties and mechanism governing dendritic spike is
fundamentally different from the spikes propagated along axon.
bp-AP primarily supported by relatively high threshold Ca2+
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ion channels (L-type Ca2+) as well as Na+ channels.
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Regulation
Degree of attenuation depends :
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•Density and types of voltage channels. E.g. Kv4.2 (A type current)
and HCN attenuates bp-AP, Na+ and Ca2+ channel supports it.
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•Amount of Branching (Surface area for current leakage).
Proposed function
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•Increased intracellular [Ca2+], which could support plasticity role
•Important for Spike-Time Dependent Plasticity?
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Individual Neuron are highly complex processing unit!
VS
Take Home Messages
1) Different types of neurons have distinct electrophysiological properties that are determined
by expression and distribution of different types of voltage gated channels.
2) The processing of electrical signal by neurons are subjected to dynamic modulation.
3) Action Potential once initiated propagate along axon towards axon terminal. Concurrently,
the wave of depolarization can actively and passively propagate towards dendrites
Part 2
(Intrinsic plasticity in learning, aging and
CNS disorder)
Learning
Neuroplasticity as the substrate for learning and memory establishment
Plasticity of brain at cellular level Changes which modulates efficacy in which electrical signal is
transmitted among neurons in the neural circuits.
Synaptics connectivity
(signal passing from one neurons to another)
Intrinsic Properties
(Integration and transmission of signal)
Learning – Intrinsic Plasticity
•In a nutshell, Intrinsic plasticity involves changes to the electrophysiological properties of
neuron.
•Plasticity of this nature directly affects excitability of neurons and efficacy of spatial
temporal summation….
Both share common induction mechanisms
E.g. what induced synaptic potentiation
also induce increased in excitability of neurons
Synaptic and Intrinsic plasticity operates in functional synergistic manner that
enhances reliability of signal transmission.
Investigative approach in studying intrinsic plasticity
•Measuring number of action potential (frequency) that can
are elicited by certain depolarizing current injected
•Measuring threshold of current magnitude needed to elicit
action potential
*Coupling between EPSP and Action potential
Intrinsic Plasticity can be Local or Global in scale
Localized changes – confine to small portion of dendrites.
E.g. Downregulation of Kv4.2 that mediate A current
-> Increased dendritic excitability
Global – functional changes affecting soma,
axon or many dendritic regions.
E.g. Increased density of Na+ channel at axon hillock
-> Lower threshold for AP
*Neural Homeostasis – Mechanisms in which each neurons maintain certain balances
of excitatory or inhibitory inputs/activites and minimum number of active output in
order to survive.
Aging
After–Hyperpolarisation (AHP)
•Mediated by efflux of K+
•Different type of K+ channel
can mediate AHP : Primary by Ik
but other channels also play
Important role e.g. Ca2+ sensitive
K+ channel, M current, apamin
sensitive K+ channel….
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Enhanced
After-Hyperpolarisation current observed in aged mice hippocampus
d
Kkjj
neurons
as compared to young mice
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ad
A number of studies have indicated
aberrant function of K+ channels as
well as intracellular Ca2+
homeostasis that contributes to
aged related cognitive decline due
to changes in electrophysiological
properties of neurons
Neurons in aged mice has less capacity in sustaining increased
neuronal activity at high frequency
CNS Disorder – Epilepsy
•In chronic epilepsy, a number of changes in voltage-gated ion channels might act together
to cause output mode changes (E.g. regular firing to busting)
•Aberrant downregulation of K+ current and enhance persistent Na+ current, leading to
increase intrinsic excitability of neurons.
CNS Disorder – Alzheimer's Disease
Chronic neurodegeneration disease characterised by significant loss of neurons and synapses
in the cerebral cortex and certain subcortical regions.
Impairment in cognitive ability  Profound loss of learning and memory capacity
*Changes in electrophysiological properties
of neurons in AD patient can be observed.
In other neurodegeneration diseased,
marked changes in the electrophysiological
properties of neurons are also observed
In AD, there is a significant loss of forebrain cholinergic projection neurons. Degeneration of
Septal nuclei resulted in reduced cholinergic input to hippocampus neurons.
In experimental model, cholinergic receptor antagonists impair learning and memory.
Normal Cholinergic input to hippocampus
1) Acetylcholine activates muscarinic receptor  2) Activates G proteins leading to chemical
Signaling cascade  3) Closure of M current (reduced K+ efflux)  4) Reduced After
Hyperpolarisation  5) Busting firing mode that could sustain memory consolidation and LTP
Reduced Cholinergic input to hippocampus in AD
1) Increased M current activation at subthreshold voltage depolarisation  2) Enhanced
After- hypolarisation following Action potential  3) Fidelity of AP fails at high frequency
 4) Suppression of Bust firing  5) Reduced capacity for memory encoding and consolidation
Take Home Messages
1) Intrinsic plasticity involves changes to the electrophysiological properties of neuron, and can
interact synergistically with synaptic plasticity for producing changes in efficacy of neuronal
signal transmission.
2) Intrinsic plasticity can be local or global in scale, and is relevant in maintaining neural
homeostasis.
3) Profound changes in intrinsic neuronal properties are observed in many CNS disorders.
These changes reflect alterations in functional properties of voltage and chemical messenger
sensitive ion channels.
Reference
1. Squire B, et al. Fundamental Neuroscience Fourth Edition
2. Zhang W, Linden DJ. The other side of the engra: experience-driven changes in neuronal
intrinsic excitability.
3. Heinz B & Yoel Y. Plasticity of intrinsic neuronal properties in CNS disorders.
4. Hausser M, Spruston N & Stuart, G. J. Diversity and dynamics of dendritic signaling.
5. Campanac E & Debanne D. Spike timing-dependent plasticity: a learning rule for dendritic
integration in rat CA1 pyramidal neurons.
6. Wang Z., Xu NI, Wu CP, Duan S & Poo MM. Bidirectional changes in spatial dendritic integration
accompanying long-term synaptic modifications.
7. Levy WB & Steward O. Temporal contiguity requirements for long-term associative potentiation
/ depression in the hippocampus.
8. Markram H, Lübke J, Frotscher M & Sakmann, B. Regulation of synaptic efficacy by coincidence
of postsynaptic APs and EPSPs.
Scenario Facts Exploration
1. The Glial Revolution (more than just glue of the brain)
Glia cells outnumber neurons by about 10-1 (depending on source of information)
Glia cells especially astrocytes are versatile and highly diverse in function:
Remove neurotransmitter, form BBB, couple blood flow with neuronal activity,
Immune system of the brain, form myelin sheath, participate in neuronal communication…..
Gliotransmission – Tripartite Synapse
Astrocytes can sense and integrate synaptic activities.
Through release of gliotransmitter, astrocytes can exert
feedback signal onto neurons and modulate their activities.
1) Neurons release Glutamate into synaptic cleft  2) Glutamate binds to mGluR 
3) second messenger activate chemical cascade that opens opens Na+ ion channels
 4) Membrane depolarised (graded potential)  5) Open T type voltage Ca2+ channels
leading to increased Ca2+ concentration  6) Release of gliotransmitters
2. Light Activated Mouse.
Neural network is highly complex.
In order to understand it better, tool is needed to dissect complexity of brain at high level of
specificity – cellular level.
2. Light Activated Mouse.
Channelrhodosin
Gene identified and
Express on neurons
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Light OFF Light ON Light OFF
Chlamydomonas reinhardtii
(Unicellular Green Algae)
Light control of aggression in mouse
*Channelrhodosin 2 – non selective cation
channel that opens within microsecond
following brief exposure to light
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-Gene
encoding Channelrhodopsin
was introduced into subpopulation
of hypothalamus neurons. When light
turned on, mouse turned aggressive
3. The Magnetized Brain - Magnetoreception
How does some animals sense and use Earth magnetic field to perceive direction
and location for orientation and navigation?
Magentosensing neurons in C.elegans
NATURE REVIEWS NEUROSCIENCE 16, 444 (2015)
Using genetic screening procedure, the AFD neuron (previously known to sense CO2 and
temperature) was crucial in magnetic field sensing.
Similar Magnetosensitive neurons were soon found in migratory pigeons and trouts
Ion channel complex that respond to magnetic field found
in AFD neurons!
Complicated ion channel complex that are associated with oxidase enzyme that catalyst
oxidation of Iron to Iron (III) oxide.
1) Iron oxide particle associated with high affinity iron oxide protein domain  2) In response to
spatial magnetic field strength protein complex are deform to varying degree  3) open ion
channel, that produce depolarisation of graded strength  4) Spatial coordination map of earth
magnetic field.
4. Death by Fugu
Danger!
Tetrodotoxin- potent neurotoxin (20x more toxic than cyanide)
4. Death by Fugu
Tetrodotoxin- potent neurotoxin
-Binds with high affinity to Na+ channel
-Block propagation of action potential leading
to failed nerve transmission and respiratory failure
Why is Pufferfish resistant to their own toxin?
A single amino acid residue substitution in Puffer fish Na+ channel makes them 10000 fold less
sensitive to tetradotoxin than Na+ channel in humans.
False
1) Neurons release Glutamate into synaptic cleft  2) Glutamate binds to mGluR 
3) G proteins activates phospholipase that generate IP3  4) IP3 leads to release of Ca2+
from ER  5) Increased Ca2+ concentration  6) Release of gliotransmitters
True
The field of optogenetics has revolutionized the study of neural circuits
-Offers excellent spatial and temporal control.
-Frequency of light pulse can control frequency of action potentials.
-Can channelrhodopsin can be selectively expressed in neuronal subpopulation.
-Complement with synthetic biology produced chimeric proteins from multiple
Rhodopsins, each with different kinetics, sensitivity, conductance…
Light Activated locomotion of mouse
False
No such ion channel complex that can respond directly to magnetic field exist!
However, scientist do prove the existence of neurons that are selectively activated by
induced magnetism
Cellular mechanism of magnetoreception still not completely understood; several
model has been proposed.
Voltage induction model
Magnetic oxide crystal
Can we design a ion channel that can respond to magnetic
field just as nature has designed a channel that can respond
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to light?
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Enormous potential
Unlike in optogenetic, application of alternating magnetic field is non-invasive and can
offer much higher degree of spatial resolution
Brain control helmet?
True
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