Download Academic Half-Day Neurophysiology 101

Academic Half-Day
Neurophysiology 101:
A humble review of
basic principles
Ruba Benini
Pediatric Neurology (PGY-2)
McGill University
October 6th, 2010
Preamble

The nervous system is a complex
organ with an intricate network of
excitable cells
 Using transient electrical signals
for transferring information rapidly
and over long distances

It is estimated that the human brain
contains about 1011 neurons, with as
many as 10,000 different types

Despite this complexity, the
mechanisms via which neurons
receive, process, generate and transmit
information is essentially the similar

Neuronal signaling occurs via both
electrical and chemical signals
OUTLINE

PART I : What makes nerve cells excitable?

Establishment of the membrane potential
 Ion channels
 Generation of action potential and saltatory conduction

PART II: How do nerve cells communicate with each other?

Synaptic transmission (central versus peripheral)
 Neuromuscular junction

PART III: Mechanisms of synaptic plasticity

Long-term potentiation (LTP)
 Long-term depression (LTD)
 Clinical relevance
OUTLINE

PART I : What makes nerve cells excitable?

Establishment of the membrane potential
 Ion channels
 Generation of action potential and saltatory conduction

PART II: How do nerve cells communicate with each other?

Synaptic transmission (central versus peripheral)
 Neuromuscular junction

PART III: Mechanisms of synaptic plasticity

Long-term potentiation (LTP)
 Long-term depression (LTD)
 Clinical relevance
PART I: What makes nerve cells excitable?
Cell membrane and ion gradients

Neurons, as other cells, have a cell membrane that consists of a hydrophobic lipid
bilayer that prevents/acts as a barrier to prevent diffusion of polarized molecules/ions
across it

By generating ionic concentrations across the lipid bilayer, cell membranes are able to
store potential energy in the form of electrochemical gradients

These electrochemical gradients are used by excitable cells such as neurons to convey
electrical signals
PART I: What makes nerve cells excitable?
Membrane potential

Refers to the potential difference across the neuronal cell membrane

Resting potential is usually between -60 to -70mV with net negative inside the
membrane

Membrane potential results from the separation of charge across the cell membrane
 Results from the unequal distribution of intracellular and extracellular ions
PART I: What makes nerve cells excitable?
Membrane potential

Factors contributing to the separation of charge (membrane potential) across
neuronal membranes include:
 Na-K ATPase pump: uses energy to set-up a concentration gradient
PART I: What makes nerve cells excitable?
Membrane potential

Factors contributing to the separation of charge (membrane potential) across
neuronal membranes include:
 Na-K ATPase : uses energy to set-up a concentration gradient
 K leak channels: which allow K to diffuse across the membrane along it’s
concentration gradient until it reaches equilibrium
PART I: What makes nerve cells excitable?
Equilibrium potential

The Nernst equation:
E = equilibrium potential,
R= gas constant
T= temperature in degrees K
z = charge of ion
EK: -75mV
ENa: +55mV
ECl: -60mV

The Goldman Equation: the greater the concentration and permeability of an ion, the
more likely it contributes to the membrane potential

Thus, at rest, the membrane potential ( usually between -60 to – 70mV) is largely due
to K+ currents
PART I: What makes nerve cells excitable?
Take home point 1

Neurons store potential energy in the form of electrochemical gradients

These gradients are instituted by active/energy consuming mechanisms that maintain
a high concentration of Na+ & Cl- outside the cell and high concentrations of K+
inside the cell

The voltage gradient at rest is largely due to K+ leak channels that allow K+ to flow
down it’s electrochemical gradient, leaving an negatively charged inner cell
membrane

Passive flux of ions (through ion channels) down their electrochemical gradient
(concentration gradient & voltage across the membrane) forms the basis of electrical
signaling
PART I: What makes nerve cells excitable?
Ion Channels

Permeability of the cell membrane to ions is made possible by specialized
transmembrane proteins called ion channels

Hydrophilic pores that allow ions to flow down their electrochemical gradient

Each channel allows > 1million ions to pass through per second thereby permitting fast
transport of charged molecules

Transport across channels occurs via passive transport
PART I: What makes nerve cells excitable?
Ion Channels

Conduct ions: generate a large flow of ionic current

Recognize and select among specific ions

Open and close in response to specific signals (electrical, mechanical or chemical
signals) – i.e. gating
PART I: What makes nerve cells excitable?
Ion Channels: Voltage gated

K+, Na+ and Ca2+ voltage-gated channels are structurally and genetically related
Inward current, depolarizes
membrane and generates AP
Outward current, hyperpolarizes
membrane
PART I: What makes nerve cells excitable?
Ion Channels: Voltage gated

Na+ channels have an open, inactivated and closed state
PART I: What makes nerve cells excitable?
Ion Channels: Transmitter gated
Acetylcholine receptor
Acetylcholine
Glutamate
Glutamatergic receptors
Serotonin
Dopamine
GABA(A) receptor
Glycine
GABA
PART I: What makes nerve cells excitable?
Ion Channels: Transmitter gated
GABA(B) receptor
GABA
Metabotropic Glutamate receptors (mGluR)
Glutamate
PART I: What makes nerve cells excitable?
Take home point 2

Ion channels are transmembrane proteins with hydrophilic pores that, when activated, allow for
selective ions to travel down their electrochemical gradient

In general, ligand gated channels have larger pores and allow for more than one type of ion to
pass through as compared to voltage-gated ion channels which are specific to single ions.

Voltage gated ion channels are involved in action potential generation

Excitatory neurotransmitters (glutamate, Ach) bind to channels that are selective for Na+, K+,
Ca2+  influx of cations results in depolarization of membrane  closer to threshold for action
potential generation

Inhibitory neurotransmitters (GABA, glycine) bind to channels that are selective for K+, Cl- 
outward currents result in hypperpolarization of membrane  further away from threshold for
action potential generation

Metabotropic receptors mediate slower neurotransmission with longer term consequences

Mutations in ion channels clinically relevant
 Channelopathies: epilepsy, migraine, periodic paralysis, etc
 Sites of action of anticonvulsants
PART I: What makes nerve cells excitable?
Generation of the Action potential
Fundamental task of the neuron is to receive, conduct and transmit signals

+
Axon Hillock
_
EPSPs (Excitatory Postsynaptic Potentials)
IPSPs (Inhibitory Postsynaptic Potentials)
-66mV

Each neuron is continuously being bombarded by synaptic input from other neurons
 Apical dendrites, proximal dendrites, dendritic shaft, cell body
 Inputs can be excitatory, inhibitory, weak or strong
PART I: What makes nerve cells excitable?
Generation of the Action potential

Voltage signal decreases in amplitude with distance from its site of initiation within a
neuron because
1. Small cross-sectional area of the cytoplasmic core of the dendrites offers
significant resistance to the longitudinal flow of ions
2. inhibitory inputs at cell body can dampen signal
PART I: What makes nerve cells excitable?
Generation of the Action potential

The action potential (AP) is an all
or nothing response

Neuronal integration is the
processes by which inputs
separated by time and space are
summated to reach the threshold
for voltage-gated Na channels to
open and thus for the AP to be
generated

Temporal summation
 The longer the time constant,
the greater the chance for
temporal summation

Spatial summation
 The longer the length
constant, the greater the
chance for spatial summation
PART I: What makes nerve cells excitable?
Generation of the Action potential

The action potential (AP) is an all or nothing
response generated at the axon hillock
 Due to the high proportion of voltage
gated Na channels in this segment, the
threshold needed to reach action
potential firing is lower in this region
(10mV as compared to 30mV at cell
body)

Influx of Na further depolarizes the
membrane thereby opening more channels
which admit more Na and cause further
depolarization of the membrane

This results in a rapid shift of the potential
from -70mV to close to the equilibrium
potential of Na of about +50mV

At this point, the net electrochemical driving
force of Na+ is zero
PART I: What makes nerve cells excitable?
Generation of the Action potential
1.
The Na+ channels open and Na+ is forced
into the cell by the electrochemical gradient
causing the neuron to depolarizes
 The K+ channels open slowly and K+ is
forced out of the cell by its
electrochemical gradient.
2.
The Na+ channels inactivate at the peak of
the action potential.
3.
The neuron starts to repolarize.
4.
The K+ channels close, but they close
slowly and K+ leaks out.
5.
The resting potential is overshot and the
neuron falls to a -90mV (hyperpolarize)
6.
After hyperpolarization the Na-K ATPase
pump brings the cell membrane back to the
resting potential
http://bcs.whfreeman.com/thelifewire/content/chp44/4402s.swf
PART I: What makes nerve cells excitable?
Generation of the Action potential
Absolute refractory period
Relative refractory period
PART I: What makes nerve cells excitable?
Propagation of the Action potential – Saltatory conduction

The passive spread of the action potential down the axon occurs by electrotonic
conduction
 This depolarization is spread by a “local circuit” current flow resulting from the
potential difference between the active and inactive regions of the axon
membrane.
http://www.blackwellpublishing.com/matthews/actionp.html
PART I: What makes nerve cells excitable?
Propagation of the Action potential – Saltatory conduction

The velocity of the action potential propagation is made faster by 3 main
mechanisms:

Large axon diameter
 The larger the diameter, the lower the resistance to ionic flow

Myelination of the axons
 Results in a functional increase in the thickness of the axonal membrane by
as much as 100 times
 Acts as an insulator (↓ resistance & ↓capacitance)

Interruption of myelin:
 In order to boost up the signal, the axon is interrupted every 1-2mm by
nodes of Ranvier (bare patches of membrane) about 2um in length, where
there is a high density of voltage-gated Na channels that can boost the
amplitude of the AP and prevent it from dying out.
→ Consequently, the AP moves down the axon as though it is jumping from
node to node. This is known as saltatory conduction.
PART I: What makes nerve cells excitable?
Propagation of the Action potential – Saltatory conduction

The velocity of the action potential propagation is made faster by 3 main
mechanisms:
 Large axon diameter
 Myelination of the axons
 Interruption of myelin (Nodes of Ranvier): Saltatory conduction
http://www.blackwellpublishing.com/matthews/actionp.html
PART I: What makes nerve cells excitable?
Clinical Relevance: Demyelinating diseases
In diseases such as Guillame
Barre syndrome, Multiple
sclerosis, peripheral axonal
neuropathies, loss of the
insulating myelin sheath
results in slowing of the AP
conduction or complete
conduction block.
Waxman 1998
PART I: What makes nerve cells excitable?
Take home point 3

The action potential is an all or none response generated at the axon hillock when neuronal
integration of synaptic inputs for the cell summate to depolarize the membrane to the threshold of
firing

Refractoriness of the membrane to firing immediately after an AP ensures that the signal is
propagated in an anterograde fashion

The neuronal axon is enveloped by myelin sheaths from Schwann cells that are interrupted by
bare segments called nodes of Ranvier where a high density of voltage gated Na channels
amplifies the signal and ensures propagation of the AP towards the end of the axon terminal.

This manner of propagation of the AP along the axon is called saltatory conduction.
OUTLINE

PART I : What makes nerve cells excitable?

Establishment of the membrane potential
 Ion channels
 Generation of action potential and saltatory conduction

PART II: How do nerve cells communicate with each other?

Synaptic transmission (central versus peripheral)
 Neuromuscular junction

PART III: Mechanisms of synaptic plasticity

Long-term potentiation (LTP)
 Long-term depression (LTD)
 Clinical relevance
PART II: How do neurons communicate with each other?
Synaptic Transmission

Rapid and precise communication between neurons is made possible by 2 main
signaling mechanisms:
 Fast axonal conduction
 Synaptic transmission

Synaptic transmission
 Electrical – gap junctions
 Chemical

Synapse refers to the specialized zone of contact between neurons
 Presynaptic and post-synaptic cell

In electrical synapses, the presynaptic and post-synaptic neurons are bridged by
gap-junction channels made of Connexin proteins that conduct flow of ionic current
and thus mediate electrical transmission
PART II: How do neurons communicate with each other?
Synaptic Transmission

In chemical synapses, the presynaptic and postsynaptic neurons
are separated by a synaptic cleft
 Transmission not as fast as electrical synapses (0.3ms to
several ms)
 Advantage of amplifying signal

The receptors for neurotransmitters fall into two main categories
 Directly ligand-gated receptors/channels:
 Fast synaptic actions lasting milliseconds
 Role in neural circuitry that produces behavior

Metabotropic/G-protein coupled receptors:
ligand binds, activates GTP-binding protein which in term
activates a channel via phosphorylation.
 Slower synaptic potentials lasting seconds or
minutes
 Involved in strengthening synaptic connections of
basic neural circuitry
 Role in modulating synaptic pathways such as
those involved in learning,
PART II: How do neurons communicate with each other?
Neuromuscular Junction

Motor unit consists of an α-Motor neuron and all the muscle fibers that it innervates
 One muscle fiber is innervated by only one motor neuron

The NMJ is an example of a chemical synapse where synaptic transmission is
mediated by ion channels that are directly gated by Ach
PART II: How do neurons communicate with each other?
Neuromuscular Junction

The motor endplate refers to the specialized
region of the muscle membrane that is innervated
by the motor neuron’s axon

As the motor axon approaches the end plate, it
loses it myelin sheath and splits into several fine
branches. A fine branch is approximately 2um thick

Each branch forms at its end multiple grape-like
varicosities called synaptic boutons where the
transmitter is released

At the site where the synaptic boutons lie, the
surface of the muscle fiber is depressed and forms
deep junctional folds lined by the basal lamina

Ach receptors are clustered at the crests of the
junctional folds (10,000 receptors per um2)

Voltage gated Na channels are also clustered at
the motor end-plate
PART II: How do neurons communicate with each other?
Neuromuscular Junction: Synaptic transmission

Stimulation of the motor neuron results in an excitatory postsynaptic potential in the
muscle membrane called an endplate potential (~70mV in amplitude)
PART II: How do neurons communicate with each other?
Neuromuscular Junction: Synaptic transmission

AP generated in the sarcolema
travels down T-Tubules to activate
voltage gated Ca2+ channels that
are directly linked to Ca-channels in
the sarcoplasmic reticulum

Release of Ca2+ from SR → Ca2+
bind to troponin → allows myosinactin interaction and subsequent
muscle contraction
PART II: How do neurons communicate with each other?
Clinical Relevance: NMJ disorders
Lambert-Eaton
Botulism
Congenital Myasthenia Gravis
Myasthenia Gravis
OUTLINE

PART I : What makes nerve cells excitable?

Establishment of the membrane potential
 Ion channels
 Generation of action potential and saltatory conduction

PART II: How do nerve cells communicate with each other?

Synaptic transmission (central versus peripheral)
 Neuromuscular junction

PART III: Mechanisms of synaptic plasticity

Long-term potentiation (LTP)
 Long-term depression (LTD)
 Clinical relevance
PART III: Mechanisms of synaptic plasticity
Synaptic Plasticity: Learning and Memory

Learning: process of acquiring new knowledge

Memory: retention or storage of that knowledge
Reflexive memory
•Automatic or reflexive quality
•Not dependent on awareness, consciousness, or cognitive
processes
•Occurs via slow accumulation through repetition over many trials
Declarative memory
•Depends on conscious reflection for its
acquisition and recall
•Relies on cognitive processes such as
evaluation, comparison and inference
•Ex. Perceptual and motor skills; learning of procedures or rules

Generalizations about the neural basis of memory:
 Memory has stages and is continually changing (shortterm vs longterm memory)
 Longterm memory may involve physical/plastic changes in the brain
 Physical changes coding memory are localized to multiple regions throughout the CNS
 Reflexive and declarative memory may involve different neural systems
 Reflexive → Cerebellum
 Declarative → Limbic structures (hippocampus)
PART III: Mechanisms of synaptic plasticity
Synaptic Plasticity

In 1949, Hebb postulated that when firing in one
neuron repeatedly produces firing in another
neuron connected to it, changes occur in one or
both of the neurons so as to strengthen the
synaptic connection between them. This he
postulated was the mechanism underlying
learning.

Longterm Potentiation (LTP): refers to the
longterm plasticity in the form of facilitated
synaptic transmission induced by correlated preand postsynaptic activity

Longterm Depression (LTD): refers to longlasting decrease in synaptic efficacy
PART III: Mechanisms of synaptic plasticity
Longterm Potentiation

Longterm Potentiation (LTP): implicated in learning and memory formation

Hippocampus implicative in declarative memory

Majority of work looking at mechanisms for LTP have been studied in the horizontal
hippocampal slice preparations in vitro
 400-500um thick slices with conserved connections between dentate gyrus,
CA3/CA1 layers
 Slices maintained in artificial CSF
 Field (extracellular) and intracellular recordings can be done
 Stimulating electrodes
Schaffer collaterals
EC
Mossy fibers
Perforant pathway
Cooke and Bliss (2006)
PART III: Mechanisms of synaptic plasticity
Longterm Potentiation

Delivering high-frequency trains of electrical stimuli (tetani) to Schaffer collaterals from
CA3 to CA1 results in potentiation of postsynaptic response that can last for hours in vitro
and for days/weeks in the intact animal

Similarly pairing a single stimulus with depolarization of the postsynaptic membrane gives
a similar potentiating effect
 Indicating that induction of LTP is dependent on both pre- and postsynaptic activity
Schaffer collaterals
EC
Nicoll et al. (1988)
PART III: Mechanisms of synaptic plasticity
Longterm Potentiation

Studies showed that LTP in the CA1 region of the hippocampus has 3 properties:
 Co-operativity (more than one fiber must be activated to obtain LTP)
 Associativity (the contributing fibers and the postsynaptic cell must be active together)
 Specificity (LTP is specific to the active pathway)
Schaffer collaterals
EC
Nicoll et al. (1988)
PART III: Mechanisms of synaptic plasticity
Longterm Potentiation: Molecular basis

NMDA receptors are ligand gated glutamatergic receptors that play a role in LTP induction
in the hippocampus
Cooke and Bliss (2006)
PART III: Mechanisms of synaptic plasticity
Longterm Potentiation: Molecular basis

Activation of NMDA receptors results in influx of calcium activates Ca-signaling pathways
that result in LTP induction by:
 Modification of postsynaptic receptors (AMPA)
 Activation of transcription factors that alter gene expression
 Changes to the number and structure of synapses
 Presynaptic increase in neurotransmitter release
Cooke and Bliss (2006)
PART III: Mechanisms of synaptic plasticity
Longterm Potentiation: Learning and Memory

Extensive evidence from in vitro and in vivo models that synaptic plasticity in the form of
LTP underlies learning and memory




NMDA-R antagonism in rats impairs spatial learning
Mice with mutations in NMDA receptor subunit in CA1 cells do not exhibit LTP at these synapses
and these animals have specific learning and memory deficits characteristic of hippocamal
dysfunction
Evidence that cAMP-dependent pathways are need for maintenance of LTP
LTP induction and maintenance varies from synapse to synapse



Mossy fiber-CA3: NMDA receptors not required, nor Calcium influx, LTP mostly mediated by
enhancement of presynaptic transmitter release
Perforant pathway-dentate granule cells: NMDA receptors needed, mostly postsynaptic
Schaffer collaterals-CA1: same
Schaffer collaterals
EC
Perforant pathway
Mossy fibers

This however is a simplification of the story – other mechanisms are sure to play a role in
learning and memory
PART III: Mechanisms of synaptic plasticity
Longterm Depression (LTD)

LTD refers to activity dependent decreases in synaptic efficacy that last hours or more
 Occurs in several brain regions, via differing mechanisms
 Best studied in cerebellar cortex and hippocampus

LTD in cerebellum
 Induced by pairing low-frequency (1Hz) stimulation of parallel fibers (PFs) and
climbing fibers (CFs) or by pairing PF activity with direct hyperpolarization of Purkinje
cells
Parallel fibers
Climbing fibers
Ito (2001)
PART III: Mechanisms of synaptic plasticity
Longterm Depression (LTD)- molecular mechanisms



Parallel fiber terminals → glutamate → activates
AMPA and metabotropic glutamate receptors in
the postsynaptic Purkinje cell.
Activation of climbing fibers → calcium enters
the postsynaptic cell through voltage-gated ion
channels→ ↑ intracellular calcium levels.
↑ intracellular calcium levels + DAG → activates
PKC → internalization of AMPA receptors →
weakening of synapse
Purves et al. (2001)
PART III: Mechanisms of synaptic plasticity
Longterm Depression (LTD)


LTD occurs in several brain regions, via differing mechanisms

Cerebellar LTD → adaptation of the vestibulo-ocular reflex

Cerebellar LTD → motor learning

Hippocampal LTD → clearing of old memories

Acute stress facilitates hippocampal LTD → stress-induced memory impairment

Visual cortex: LTD mechanisms underlie the reduced responsiveness of cortical
neurons from inputs of the deprived eye
And more …
PART III: Mechanisms of synaptic plasticity
Summary

Despite the complexity of the nervous system, the mechanisms underlying neuronal
signaling are essentially similar across various neuronal subtypes

Neuronal signaling occurs via electrical (changes in membrane potential, action potential
generation) and chemical signals (neurotransmitters)

Ion channels form the basis of these signaling pathways

Synapses are plastic – changes are activity dependent

LTP and LTD occur in various brain regions via differing mechanisms

Despite this, in essence:
 LTP: enhancement in synaptic transmission involves facilitation of presynaptic
neurotransmitter release as well as postsynaptic changes (increase in receptors,
structure and # of synapses)
 LTD: decrease in synaptic transmission involves reduction in presynaptic release of
neurotransmitters as well as postsynaptic changes (decrease in receptors, etc)
References

Kandel ER, Schwartz JH and Jessell TM (1991) Principles of Neural Science. Third
Edition. Chapters 5-11, 65.

Alberts B, Bray D, Lewis J, Raff M, Roberts K and Watson JD (1994) Molecular
Biology of the cell. Third Edition. Chapter 11.

Cooke SF and Bliss TVP (2006) Plasticity in the human central nervous system. Brain
129, 1659–1673.

Waxman SG (1998) Demyelinating diseases--new pathological insights, new
therapeutic targets. N Engl J Med. 29;338(5):323-5.

Ito M (2001) Cerebellar Long-Term Depression: Characterization, Signal
Transduction, and Functional Roles. Physiological Reviews, Vol. 81, No. 3, July 2001,
pp. 1143-1195.

Nicoll RA, Kauer JA, Malenka RC (1998) The current excitement in long-term
potentiatio. Neuron. 1(2):97-103.

Purves D, Augustine GJ, Fitzpatrick D, Katz LC, LaMantia A, McNamara JO, and
Williams SM (2001) Neuroscience. Second Edition. Chapter 25.
Seizing hold of seizures
Gregory L Holmes & Yezekiel Ben-Ari
Explanation of Paroxysmal Depolarizing shift (PDS)
Figure 1. Focal seizures result from a limited group of neurons that fire abnormally because of intrinsic
or extrinsic factors.
(a) In this simplified diagram, II and III represent epileptic neurons. Because of extensive cell-to-cell connections,
termed 'recurrent collaterals', aberrant activity in cells II and III can fire synchronously, resulting in a prolonged
depolarization of the neurons. (b) This intense depolarization of epileptic neurons is termed the
paroxysmal depolarization shift. The prolonged depolarization results in action potentials and propagation of
electrical discharges to other cells. The paroxysmal depolarization shift is largely dependent on glutamate excitation
and activation of voltage-gated calcium and sodium channels. After the depolarization, the cell is hyperpolarized by
activation of GABA receptors as well as voltage-gated potassium channels. Axons from the abnormal neurons also
activate GABAergic inhibitory neurons (green) which reduce the activity in cells II and III in addition to blocking the
firing of cells outside the seizure focus (cells I and IV). An electroencephalogram (EEG) recorded during this time
would show a spike and a subsequent slow wave. When the balance of excitation and inhibition is further disturbed,
there will be a breakdown in containment of the epileptic focus and a seizure will occur. (c) A sustained
depolarization without repolarization occurs in many cells during the seizure. An EEG would show repetitive spikes
during the seizure. By inducing cells to release galanin, an endogenous anticonvulsant that reduces glutamate
release, Haberman et al. successfully increased inhibition and thereby reduced seizure susceptibility.