Download 5 Action Potential.key

Brain Science Fundamentals
Christopher Fiorillo
BiS 328, Fall 2011
042 350 4326, [email protected]
Part 5: A Neuron’s Membrane Voltage: Active Properties
Reading: Bear, Connors, and Paradiso, Chapter 4.
Assistant: Sora Yun <[email protected]>
Summary of a Neuron’s Inputs and Output
• Output corresponds to voltage across the
neuron’s membrane
• Inputs correspond to current flowing through
ion channels
• Ion channels are small pores in the
membrane. They only allow current to flow
when they are open.
• The opening of ion channels depends on the
concentration of chemical neurotransmitters,
or the voltage across the membrane.
Passive versus Active Membrane Properties
• “Passive” refers to the flow of current and
changes in voltage that do not involve
voltage-dependent opening or closing of ion
channels
• “Active” refers to voltage changes that
depend on channels that open or close in
response to changes in membrane voltage
– This depends on a feedback process
• The membrane voltage in neurons depends
on both active and passive membrane
properties
The Action Potential
•
Graded changes in current cause graded changes in voltage.
•
If voltage is depolarized beyond a threshold (about -50 mV), an action
potential is triggered.
An Action Potential is “All-or-None”
•
At a brief moment in time (~1 ms), an action potential either occurs or it does not
•
The shape of an action potential is always the same (almost)
•
The magnitude of the current and depolarization that triggered the action
potential do not matter (but the depolarization must reach threshold)
Firing Rate Depends on Current Magnitude
• The frequency of action potentials (“firing rate”) depends on
the magnitude of depolarizing current (assuming that the
current lasts long enough).
Why do neurons have action potentials?
• Current leaks out of the
membrane
• Therefore, a large change in
voltage at one location will
produce only a small change
at another location
• An action potential is
regenerative. It does not
decline in its voltage
amplitude as it moves through
a neuron
Why Do Neurons Have
Action Potentials?
•
Current that enters the cell at one point will spread passively through
the cytosol of a dendrite.
•
Because of membrane permeability, current leaks out of the neuron as
it travels along a dendrite.
•
Thus information cannot be conveyed over long distances through
passive spread of current.
•
An action potential is “regenerative” and therefore it can reliably carry
information over long distances.
•
Trade-off: Digital (action potential) versus Analog (membrane voltage)
•
Most neurons have action potentials.
•
Many neurons that do not need to transmit information over long
distances do not have action potentials. These include:
– Most retinal neurons
– All neurons in C. elegans (a small worm)
Types of Electrophysioligical Recordings from Single Neurons
• Intracellular
– Records voltage or current across the
membrane
– Sharp microelectrodes penetrate inside the cell
– Patch electrodes are attached to the membrane
• Extracellular
– Record only the times of action potentials
– Small and slow changes in membrane
voltage cannot be measured
An experiment in my lab
with two glass pipettes.
One is for patch
recording of membrane
voltage. The other is to
deliver a
neurotransmitter.
Haram Kim, a student in
my lab, made this image,
and she named it
“genesis.”
A neuron can be seen in
the center, but it is out
of focus.
A photo of the stage of
the microscope. A
living brain slice from
a rat is under the
objective of the
microscope, between
the two pipettes.
A very nice painting
by Haram. When
Haram is not working
in the lab, she makes
art like this.
Recording Methods: Current and Voltage Clamp
• “Current Clamp”
– Measures voltage across cell’s membrane
– A constant current is injected through the electrode
• The current can be manually adjusted by the experimenter
• “Voltage Clamp”
– Measures current passing through the cell’s
membrane
– Clamps voltage across the cell’s membrane
• A feedback circuit calculates the amount of current that is
necessary to keep the voltage constant, and then injects
that amount of current through the electrode.
Current versus Voltage Clamp
• Advantages of current clamp
– More physiological. Voltage is never clamped under
physiological conditions
– Fast and accurate voltage clamp can be difficult to achieve
• Especially in the dendrites of a large neuron
• Advantages of voltage clamp
– Greater experimental control: it eliminates voltage as a
variable
• Keeps driving force constant
• Better for studying voltage-gated channels
• Better temporal resolution for fast channels, because it removes the
effect of membrane capacitance
• In general, voltage-clamp is better for studying the
properties of ion channels. Current clamp is better for
studying the properties of neurons.
The Patch Clamp Method
• Developed by Erwin Neher
• Very useful for electrophysiology
• Enables the recording of single channels
Structure of a Voltage-Gated Ion Channel
• The Voltage-Gated Sodium Channel
– Four subunits
•
•
•
•
Each has 6 transmembrane alpha-helices
Voltage sensor (positive charges) on S4
K+ channel
Selectivity filter
Gate
Na+ channel
An Ion Channel Has Many States (Conformations)
• States of a glutamate-gated ion
channel (4 subunits) are shown
• Open states require that
glutamate is bound
• There are many desensitized
(inactivated) states
• Gray states are seldom visited
• This is typical of many ion
channels, including voltagegated ion channels
• This is probably more simplistic
than reality
Recordings of Single Na+ Channels
• Channels exist in discrete
states: Open or closed
• The channels “behavior”
will not be the same, even
under identical conditions.
It is “stochastic.”
• Inactivation usually occurs
after the channel opens
• A large number of sodium
channels like this can
produce an action potential
Phases of the Action Potential
•
•
•
•
Rising phase
Overshoot
Falling phase
Undershoot (after-hyperpolarization, or AHP)
Properties of the Action Potential
•
Rising phase
– Increase in Na+ conductance
•
Overshoot
– Positive membrane voltage (no functional
significance)
•
Falling phase
– Inactivation of Na+ conductance
– Activation of K+ conductance
•
Undershoot (After-Hyperpolarization)
– K+ conductance greater than before start of
action potential
•
Absolute refractory period
– Na+ channel inactivation makes it impossible
to elicit another action potential
•
Relative refractory period
– High K+ conductance means that a large
depolarizing current is necessary to elicit
another action potential
The Threshold of the Action Potential
• The threshold is between -50 to -40 mV in most cells
• The threshold is caused by positive feedback between sodium
channels
– When a few sodium channels open, sodium flows into the cell and causes a small
depolarization
– The depolarization causes more sodium channels to open, which causes a larger
depolarization, and so on
• The threshold depends on the density of sodium channels, and their voltagedependence
• The threshold is a property of the population of ion channels
• Single channels do not have a threshold
Relationship between single channel and
cellular currents
• Macroscopic currents in the cell result from the
summation of many microscopic single channel
currents
• Although single channel currents are stochastic,
currents within the cell are not. They are highly
reproducible.
• Single sodium channels do not have a threshold
voltage at which they open
• The action potential threshold depends on
positive feedback between many sodium
channels
• Action potentials require a high density of
sodium channels
– A sufficient number of channels must be deinactivated
(ready to be opened)
•
•
•
•
•
The Hodgkin-Huxley Model
Hodgkin and Huxley described the ionic basis of the action potential
(1952)
This is considered the most important single achievement in cellular
neurophysiology.
Their approach: Experiments on the squid giant axon
–
Two electrode voltage clamp (1 mm diameter axon)
–
Measured the voltage-dependence and kinetics of sodium and potassium
currents
–
Ion substitution experiments
They derived a relatively simple but detailed mathematical and
biophysical model of the action potential
Their model is still the “textbook” model
•
The utility of their model extends beyond the action potential. It is
useful for understanding all voltage-dependent ion channels
•
•
Their model predicted some of the key properties of ion channels
It was about 30 years later (~1980) that scientists were able to
identify and record single ion channels
•
It was about 10 years after that (~1990) that people began to clone
ion channels (to discover their amino acid sequence)
It was about 10 years after that (~2000) that the 3-dimensional
structure and function of ion channels began to be understood
•
Three Key Properties of Voltagegated Ion Channels
• Ion selectivity
• Voltage-dependence
• Time-dependence (Kinetics)
Voltage-Dependence of Ion
Channels in HH model
• ‘n-infinity’ is the steady-state
probability that a K+ channel subunit
“gate” is in the “open” conformation
• The channel is only open when all four
subunit gates are in open conformation
– Thus, the steady-state probability that a
channel is open:
• Po = ninfinity4
• The Na+ channel has three activation
gates (m) and one inactivation gate (h)
– Thus, the steady-state probability that a
channel is open:
• Po = minfinity3h
Summary of Voltage-Dependence of Parameters in HH model
Kinetics
Steady-state VoltageDependence
Kinetics of Ion Channels Underlying the Action Potential
• Sodium channels activate and inactivate quickly
• Potassium channels activate more slowly
Initiation and propagation of action potentials
• Requires a high density of sodium channels
• High density is found in:
– Axon
– Nerve endings of primary somatosensory neurons
– Dendrites have lower density, but some dendrites can have action
potentials
• Dendritic action potentials are not always all-or-none
• Forward and backward propagation
Action Potential Conduction
• Propagation of the
action potential
– It is an active, regenerative
process, but it still relies
upon the passive spread of
current
– Orthodromic: Action
potential travels from
soma to terminal
– Antidromic: Backward
propagation (towards
soma)
• This is artificial and only
happens in experiments
Conduction Velocity
• Conduction velocity (0.5-80 m/s)
• Two means of increasing velocity
– Diameter of axon (or dendrite)
• Increases speed by decreasing axial
resistance
• Squid Giant Axon: 1 mm
– Insulation
• Myelination by glia
• Increases membrane resistance and decreases
membrane capacitance
• Some information needs to be
transmitted quickly, some does not
• Large axons and myelination are each
costly
• Some axons are small and unmyelinated
Conduction of Action Potentials through Myelinated Axons
• The myelinated part of the
axon has few or no channels
– Very little current leaks out
• The action potential’s voltage
amplitude gets smaller as it
spreads
– But it spreads over a long distance
with little decay, because of the low
conductance of the membrane
• At the node of Ranvier, there
is no myelin, and there is a
high density of sodium and
potassium channels
• Thus the A.P. is regenerated
at nodes of Ranvier
• This is called “saltatory”
conduction, which means that
the A.P. jumps from one node
to the next
Beyond the Action Potential
• We have focused on action potentials, but the HH model
can be extended (by changing parameter values) to
include many other types of voltage-gated ion channels
• Voltage-gated ion channels do not only mediate the
action potential. They also influence the pattern of
action potentials.
• Different neurons express different sets of voltageregulated ion channels
– Therefore, different neurons have different firing patterns in
response to the same excitatory input