Neurophysiology: Key Concepts to
Understand
• Resting potential
– All cells in our body maintain a “transmembrane potential” at rest
referred to the resting potential. All neural activity begins with a
change in the resting potential. (http://bcs.whfreeman.com/thelifewire/content/chp44/4401s.swf)
• Graded Potentials
– neurons receive chemical signals from other cells and translate the
chemical message into electrical signals that have variable
amplitudes and sign (depolarizing and hyperpolarizing)
• Action Potential
– large graded potentials trigger action potentials that get sent from
the dendrites to the synaptic terminals. Action potentials are all-ornone (have the same shape and size once triggered).
• Neurotransmission
– Action potentials gets translated into chemical signals that are sent
from one cell to the next
• Neurotransmitters: many
• Circuits: create behaviors and sensations
1
Resting Potential is due to 2 factors: concentration
gradients and selective permeability of the membrane
to ions
Ion
IN (mM)
OUT(mM)
Na+
5
145
K+
100
5
Ca + +
.0000001
2-5
Mg + +
30
1-2
Cl -
4
110
Big Anions
High
Low
2
Figure 3.15
Resting Membrane Potential
?
3
Figure 11.7
Observation: All cells have maintain a membrane potential with the
inside negative relative to the outside. This is measured by placing
a measuring device (electrode) inside the cell and one outside
connected to a display meter.
http://www.st-andrews.ac.uk/~wjh/neurotut/mempot.html
4
Observation: Model cell with 2 compartments. Both filled with
water to start. Left side “inside cell”, right side outside cell.
Membrane barrier separates the 2 compartments. Since the 2
compartments are isolated, there is no difference in potential
between the 2 and the voltmeter reads 0 mV.
5
Observation: Add high concentration of KCl to the “inside” (100
mM) and low concentration to the outside (10 mM). KCL is salt and
dissociates in water. Concentration gradient (diffusive force)
favors K+ and Cl- leaving from In to Out but there is no pathway.
There is no potential difference between In and Out.
6
Observation: There is no path for Cl- to move across
the membrane, but there is a K+ channel in the
membrane so K+ is free to move down its concentration
gradient.
7
What happens? K+ diffuses down its concentration gradient out
of the cell and leaves an excess of negative charge behind (Cl-) .
Eventually the concentration gradient favoring K+ exit from the cell
is exactly balanced by the electrical gradient (Cl-) pulling K+ back
in to the cell. When these 2 forces (the chemical and electrical
gradients) exactly balance, the cell is at the resting potential.
8
Chemical and Electrical gradients determine the
resting potential just like in model
1. Chemical (concentration) gradient: caused by different concentrations of
a single ion on either side of the plasma membrane
– e.g. high potassium concentration inside the cell tends to make K+
ions leave the cell (movement down concentration gradient).
2. Electrical gradient: caused by different total numbers of positively and
negatively charged particles on either side of membrane
– Because the membrane is permeable to K + at rest, as K + leaves the
cell moving down its concentration gradient, excess negative charge
accumulates inside cells. Note that K + is still more concentrated
inside than outside, but it is the TOTAL balance of charges that
matters (there are more total + charges outside than inside).
*An important point: Only a few ions need to move across the membrane to
generate the membrane potential so the concentration gradient does not change
with K ion flow by very much (you can calculate how many – about .0001% of K +
ions move to generate resting potential).
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Membrane Potential
In real cells, the membrane is
selectively permeable to K+ ions
at rest – and no other ions*.
Therefore the concentration
gradient for K+ sets the resting
potential of the neuron – just like in
the model cell.
K+ diffuses down its concentration
gradient through K+ channels,
leaving behind a slight excess
negative charge inside, generating
an electrical gradient that
eventually opposes K exit from the
cell
*(this is a slightly simplified perspective: In real cells,
there is a small permeability to Na at rest (a few
leaky Na channels are open). This means over time,
Na enters, K exits and after along time, the
concentration gradients would change. The Na/K
pump maintains the gradients moving 3 Na out and 2 K
back in. The pump has little effect on membrane
potential – other than to maintain the important ion
gradients.
10
Review: Electrical and Chemical
Forces
3. Enough K+ leaves
to create charge
separation equal and
opposite to
concentration.
gradient
1. K+ starts to
leave
Cell
2. Opposite
electrical force
develops due to
charge separation
Red = chemical gradient force
Electrochemical
Equilibrium
Black = electrical gradient force
Vm= -60mV
11
Why -60 mV?
The membrane potential of the cell will exactly balance
the force due to the concentration gradient. If you
know the concentration gradient of the permeant ion you
can determine the cell resting potential (the electrical
force that exactly balances the concentration force).
The Nernst equation expresses the energy of the
concentration gradient in terms of the electrical gradient:
“For any monovalent ion (like K+), a 10 fold concentration
gradient contains exactly the same energy as a 60mV
voltage difference (at 37C).”
– This means that if [K] is 10 mM out and 100 mM in, the
membrane potential of the cell is -60 mV. In this case
the energy of the concentration gradient is exactly
balanced by -60 mV (electrical gradient) due to
separation of charge. The resting potential of the cell =
-60 mV.
12
Why do we care?
At rest, leaky K channels are open and set resting
potential. But cells have other ion channels in their
membranes that open and close in response to
stimulation. When these channels open, they shift
resting potential.
EX: What would happen to the membrane potential of our
model cell if a stimulus opened a leaky Na+ channel (e.g.
acetylcholine at the neuromuscular junction)?
13
Experiment: Close the leaky K channel, open a leaky Na channel with
acetylcholine.
Questions to ask:
What is the chemical (concentration) gradient for permeant ion?
Favors Na entry
What is the electrical gradient for permeant ion?
Favors Na entry
Does the cell get more + or more – when the Na channel opens?
Positive
By how much?
Simplified situation
Ion
IN (mM)
OUT(mM)
Na+
1
100
K+
100
10
Ca + +
.0000001
1
IN (cytoplasm)
OUT (blood, interstitial
fluid)
[Na+] = low
[K+] = high
[Ca2+] = very low
[Cl-] = low
[Na+] = high
[K+] = low
[Ca2+] = low
[Cl-] = high
14
Nernst Equation can be used to
predict equilibrium potential
The Nernst Equation (to right) can be used to
predict the voltage or electrical gradient
that exactly equals balances a given
concentration gradient. It is a mathematical
expression for equating these two forces and
tells you what the resting potential (the
equilibrium potential) will be in the case of a
single permeant ion (only leaky Na channels
open in the cell membrane).
EX:
[Na+]
[Na+]
– If
in = 1mM and
out = 100mM, E=
120mV since 60log(10) = 60(2) = 120 mV.
– This means that if the inside of the cell is 120
mV with respect to the outside of the cell,
there is no net movement of Na+ into the cell,
even though there is an open Na+ channel.
– The cell resting potential in this case would be
+120 mV.
– Can you draw this on a plot of membrane
potential vs. time?
Simplified equation: where
z=valence of ion (charge: +1, 1, +2), Erest = membrane
potential, 60 = simplification
of constants (R – gas constant,
T – temperature, F – Faraday’s
constant), ln = 2.303 Log10 x)
Erest
60
[ion]out
log 10
z
[ion]in
Log10 1 = 0
Log10 10 = 1
Log10 100 = 2
Log10 .1 = -1
Log10 .01 = -2
15
FYI : Recording membrane potential patch clamp technique
http://www.science-display.com/new/datenbank/detail.php?action=edit&id=23
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Why are these equilibrium potential values
slightly different than our model cell?
?
K+
?
Na+
17
Real Ion Concentrations for mammalian
cell solutions
Mammalian
Intracellular and Extracellular Solution compositions
Ion
IN (mM)
OUT(mM)
Na+
5
145
K+
100
5
Ca + +
.0000001
2-5
Mg + +
30
1-2
Cl -
4
110
DNA/RNA
High
Low/0
Big Anions
High
Low
IN (cytoplasm)
[Na+] = low
[K+] = high
[Ca2+] = very low
[Cl-] = low
OUT (blood, interstitia
fluid)
[Na+] = high
[K+] = low
[Ca2+] = low
[Cl-] = high
Nernst Potential practice and great tutorial on membrane potential: Heitler website
18
19
Key Concept
All cells have negative resting membrane potentials
defined by two factors:
– Permeability of the cell membrane to ions (what ion
channel is open)
– Concentration gradient for the permeant ion.
Any change in membrane potential, reflects a change
in one of these two factors.
In real cells, concentration gradients change very
little. Therefore, changes in cell membrane
potential (e.g. the action potential) are caused
by changes in membrane permeability to ions
(opening and closing of ion channels).
Review: http://bcs.whfreeman.com/thelifewire/content/chp44/4401s.swf
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FYI: Electricity Definitions
Voltage (V) – measure of potential energy generated by
charge separation
Potential difference – voltage measured between two
points (e.g. resting membrane potential)
Current (I) – the flow of electrical charge between two
points (the movement of ions through ion channels)
Resistance (R) – hindrance to charge flow (ion channels,
membrane, cytoplasm)
Insulator – substance with high electrical resistance to
current flow (the cell membrane, myelin sheath)
Conductor – substance with low electrical resistance to
current flow (the cytoplasm or interstitial fluid)
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Ion Channels
Functional Classification
– Passive or leak channels: non-gated channels that are
always open (e.g. K leak channel that sets rest)
– Active or “gated” channels that open and close to
mediate shifts in membrane potential (e.g. action
potentials)
• Chemically-gated channels that open or close in response to chemical
binding (receptors like the acetylcholine receptor)
• Voltage-gated channels that open/close in response to changes in
membrane potential (e.g. voltage-gated Na channel of the action
potential)
• Mechanically-gated channels that open/close in response to mechanical
stimulation (touch receptors, base of hair shaft)
Permeant ion classification: what ion can fit through the
channel (e.g. Na+ vs. K + vs. Ca + +)
22
“Active” or Gated Channels
Figure2312–10
Membrane Potential in most neurons is dynamic
24
Changes in Membrane Potential
Membrane potential can shift in response to changes in
membrane permeability caused by opening or closing of
gated ion channels
Changes in membrane potential are used by neurons for
communication - receiving, integrating, and sending
information.
Two types of shifts in membrane potential
– graded potentials
– action potentials
25
Graded potentials are caused by the opening of gated channels usually
in response to chemical stimulus (transmitter binding).
Response shifts membrane potential positive or negative to rest
– Depolarization –membrane potential becomes less negative (shifts
towards 0mV) if Na channel opens
– Hyperpolarization – membrane potential becomes more negative than the
resting potential if Cl channel opens
– Repolarization – the membrane returns to its resting membrane potential
(as chemically gated channel closes)
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Graded Synaptic Potentials
EX: Resting membrane exposed to stimulus (ligand binding to receptors,
Mechanical stimulation, Temperature changes, Spontaneous change
in permeability)
• Gated sodium channel opens
• Sodium ions move into cell through channel down electrochemical
gradient
• Cell depolarizes>local current spreads and depolarizes nearby
regions of cell membrane (graded potential)>signal decays with
distance (leak across membrane and resistance of cytoplasm)
• Change in potential is proportional to the stimulus
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Graded Potentials (aka local
potentials) - Summary
– Short-lived, local changes in membrane
potential
– Amplitude varies directly with the strength
of the stimulus therefore amplitude is a
code for stimulus strength
– Can be depolarizing (excitatory) or
hyperpolarizing (inhibitory)
– Can summate (add)
– Decrease in amplitude with distance from
source (not good for long distance transfer
of info)
– Sufficiently strong graded potentials can
initiate action potentials (threshold)
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11-29
Action Potentials
Action potentials, or nerve impulses,
are the underlying functional feature
of the nervous system.
– Regenerative change in cell
membrane potential
– Propagated along the length of the
axon.
– All or none: always the same
amplitude, the same shape
regardless of stimulus
– Travel from cell body down axon
to terminal
–
Tutorials:
http://bcs.whfreeman.com/thelifewire/content/chp44/4402s.
swf and
http://www.sumanasinc.com/webcontent/animations/conten
t/action_potential.html and here
http://www.dnatube.com/video/1364/Action-Potential-Epilepsy
Movie: How Neurons Work – GOOD GENERAL SUMMARY
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Initiating Action Potential
Action potentials are only generated by excitable cells (e.g. muscle cells,
neurons), cells that contain voltage-gated channels (even oocytes have
voltage-gated channels).
All or None: If a stimulus exceeds threshold, the action potential is the
same shape and amplitude no matter how large the stimulus for a given
cell. The AP is NOT proportional to the stimulus. The action potential is
either triggered, or not
– Weak (sub-threshold) stimuli are not relayed into action potentials
– Strong (threshold) stimuli are relayed into action potentials
Threshold = voltage that, if reached will trigger an action potential where
enough voltage-gated sodium channels open to cause net inward
current to be greater than outward current and a regenerative cycle
to* begin. Na+ coming in through gated channels depolarizes the
membrane causing more voltage-gated Na+ channels to open.
Since the Axon Hillock has the highest concentration of voltage-gated
sodium channels it is the site where threshold is lowest and
therefore where APs are initiated
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Action Potential
AP sequence:
1 – resting state
2 – depolarization phase – regenerative opening of Na channels (Why does this cause
depolarization?)
3 – repolarization phase – Na channels close to inactivated state, K channels open
slowly increasing permeability to K (Why does this cause repolarization?)
4 – hyperpolarization – undershoot, big permeability to K (leak channels plus voltagegated K channels). (Would you see a hyperpolarization in a cell with rest set by K
channels?)
1 - Return to rest
32
33
AP Details
1. REST: Voltage-gated Na+ and K+ channels are
closed, K leak channels open at rest and set
resting potential
V-gated Na+ channel
– Activation gates – closed in the resting state
– Inactivation gates – open in the resting state
2 . Depolarizing Phase: v-gated Na+ activation gates
are opened; v-gated K+ activation gates are closed
– Threshold – a critical level of depolarization
(-55 to -50 mV) where depolarization becomes
self-generating. Na entry depolarizes
cell>opens more v-gated Na channels.
– Na+ permeability increases; membrane
potential is more positive than 0 mV (thus
reverses)
3. Repolarizing Phase:
– V-gated Na inactivation gates close – thus
channel closes and membrane permeability to
Na+ declines to resting levels
– V-gated K+ gates open>K leaves cell down
electrochemical gradient>cell repolarizes
4. Repolarization/Hyperpolarization: K+ exits the cell
through leak and v-gated K channels. K equilibrium
potential is reached = rest. V-gated K channels
close with repolarization.
34
Action
Potential
A different view of AP
channels – same story
35
Figure 12–13 (Navigator)
Details: Refractory Periods or how frequently
can APs be fired?
Absolute Refractory Period: No AP can be
fired if cell is stimulated again. Defined as
the time from the opening of the Na+
activation gates until the closing of
inactivation gates. There are not enough
available Na channels to generate
depolarization to threshold. The absolute
refractory period:
Ensures that each action potential is
separate
Enforces one-way transmission of nerve
impulses (Na channels are closed to
inactivated state and cannot be reopened during this phase).
Relative Refractory Period: A stronger-thanthreshold stimulus can initiate another
action potential. The cell is hyperpolarized.
Some of the inactivation gates of the Na
channel are open but activation gates are
closed. Many voltage-gated K channels open
– keeping the membrane near Ek. A larger
stimulus is required to reach threshold.
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11-37
2 Methods of Propagating Action Potentials
Continuous propagation:
–
unmyelinated axons
–
Slow (1 m/s)
–
Local currents depolarize each patch of membrane to
threshold
–
After firing AP, membrane is in refractory period
therefore AP travels in 1 direction
Saltatory propagation:
–
myelinated axons
–
Fast (up to 150 m/s)
–
Local currents depolarize each patch of “unwrapped”
membrane called a node of Ranvier to threshold. The local
currents travel fast between nodes down the center of the
axon and there is no leak across the membrane due to the
glial cell insulation (high resistance). This step speeds up AP
propagation.
–
After firing AP, membrane is in refractory period
therefore AP travels in 1 direction
–
Myelinated axons are only found in vertebrates
Tutorial on AP propagation: http://www.blackwellpublishing.com/matthews/actionp.html
38
Slow AP Propagation: Continuous
Properties:
•An new action potential
occurs in each little patch
of membrane
•Speed: 1 meter/second
39
Faster AP Propagation: Saltatory
Speed = 15 - 120 meters/second
40
41
How Fast do Axons Conduct APs?
Factors that determine speed:
– Diameter - the larger the diameter, the faster the AP
travels
– Myelination - myelination dramatically increases impulse
speed
Classification of Axons:
– Group A: Fast, large diameter myelinated motor axons
(Group A) can conduct up to 150 meters/sec (300 mph)
– Group B (Intermediate): 15 m/s (40 mph)
– Group C (small, unmyelinated): 1 m/s (2 mph): pain fibers
42
Information Coding: What Pathway is firing
Aps? How Frequent are the Aps?
All action potentials are alike in terms of shape.
To interpret the APs, the nervous system pays
attention to 2 variables:
Pathway/Wiring: Pathways are wired to provide
specific information to parts of the brain for
interpretation
EX: Info from the eyes travels to different parts of the brain
than auditory info from ear.
AP frequency: The intensity of the stimulus is
coded by AP frequency. Strong stimuli can
generate an action potential more often than
weaker stimuli therefore the frequency of
impulse transmission codes the intensity of
the stimulus.
Example: Boiling hot water poured on your hand
stimulates a temperature and pain receptors in your
hand to trigger more action potentials than a warm
water wash of your hands. The information coming
into your brain as action potentials would be
integrated so that you respond appropriately.
– Warm water? Boiling water?
43
Key Concept: Coding for Stimulus Intensity – AP
Frequency
44
Multiple Sclerosis (MS) effects AP
propagation:
• MS is an autoimmune disease
• Glial cells that form myelin in the CNS and PNS are attacked by the
immune system and killed
• AP propagation is disrupted > many APs fail to reach axon terminal
therefore communication is disrupted
• Symptoms: Vision and hearing problems, muscle weakness, and urinary
incontinence
•
•
Treatments: Drugs that block immune system (like interferon beta-1a and -1b,
Avonex, Betaseran, and Copazone)
Web Info:
–
–
–
–
–
Multiple Sclerosis: http://www.youtube.com/watch?v=qgySDmRRzxY
Montel Williams on Oprah:
http://www.youtube.com/watch?v=rgS4H-hTo0Q&feature=related
NYT Patient Stories: http://www.nytimes.com/interactive/2008/12/03/health/healthguide/TE_MULTIPLESCLEROSIS.html
http://www.nationalmssociety.org/about-multiple-sclerosis/index.aspx
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