Transcript Document
Computational Neural Modeling and Neuroengineering
The Hodgkin-Huxley Model for Action Potential Generation
Action Potential Propagation in Dendrites
Stochastic influences on dendritic computation
The Hodgkin-Huxley Model of Action Potential Generation
Motivations
Action Potentials (
A
) Giant squid axon at 16 C (
B
) Axonal spike from the node of Ranvier in a myelinated frog fiber at 22 C (
C
) Cat visual cortex at 37 C (
D
) Sheep heart Purkinje fiber at 10 C (
E
) Patch-clamp recording from a rabbit retinal ganglion cell at 37 C (
F
) Layer 5 pyramidal cell in the rate at room temperatures, simulataneuous recordings from the soma and apical trunk (
G
) A complex spike consisting of several large EPSPs superimposed on a slow dendritic calcium spike and several fast somatic spikes from a Purkinje cell body in the rat cerebellum at 36 C (
H
) Layer 5 pyramidal cell in the rat at room temperature - three dendritic voltage traces in response to three current steps of different amplitudes reveal the all-or none character of this slow event. Notice the fast superimposed spikes. (
I
) Cell body of a projection neuron in the antennal lobe of the locust at 23 C
Historical Background
Bernstein The membrane “breakdown” hypothesis Prior to 1940, the excitability of neurons was only known via extracellular electrodes A major mystery was the underlying mechanism By the turn of the 20th century it was known that 1) cell membranes separated solutions of different ionic concentrations 2) [K + ]
o <<
[K + ]
i
3) [Na + ]
o >>
[Na + ]
i
In 1902, Bernstein, reasoning that the cell membrane was semi-permeable to K + and should have a
V m
~ -75mV, proposed that neuronal activity (measured extracellularly) represented a “breakdown” of the cell membrane resistance to ionic flow and the resulting redistribution of ions would lead from -75mV to 0mV transmembrane potential (
V m
=0)
Historical Background
Cole
et al
.
The space clamp Marmont (1949) and Cole (1949) developed the
space clamp
technique to maintain a uniform spatial distribution of
V m
over a region of the cell where one tried to record currents This was accomplished by threading the squid axon with silver wires to provide a very low axial resistance and hence eliminating longitudinal voltage gradients The voltage clamp Cole and colleagues developed a method for maintaining
V m
desired voltage level at any Required monitoring voltage changes, feeding it through an amplifier to then drive current into or out of the cell to dynamically maintain the voltage while recording the current required to do so Schematic of the voltage clamp apparatus for the giant squid axon (reproduced from Hille, 1992)
Historical Background
Hodgkin and Katz The “sodium hypothesis” Hodgkin and Katz (1949) had demonstrated that both sodium and potassium make significant contributions to the ionic current underlying the action potential First to realize that, in contrast to Bernstein’s theory of increased permeability for all ions, the “overshoot” and “undershoot” of the AP could be explained by bounded changes in the permeabilites for a few different ions Hodgkin and Katz postulated that during the upstroke of the AP, Na + was the most permeable ion and so the voltage of
V m
moved towards its Nernst potential of ~ 60mV. They predicted and then demonstrated that the AP amplitude would therefore depend critically on the external concentration of Na +. They generalized the Nernst equation to predict the steady-state
V m
multiple permeable ions. for the case of
E rest
RT
ln
F P Na P Na
o i
P K P K
i o
Goldman-Hodgkin Katz Voltage Equation
Historical Background
Hodgkin and Huxley The mechanism of action potential generation Following Hodgkin and Katz (1949), the big remaining question was
how is the permeability of the membrane to specific ions linked to time and V
m
? This was not answered until the tour-de-force of physiology and modeling presented in four papers in 1952 by Hodgkin and Huxley. This work represents one of the highest-points in cellular biophysics and the quantitative model they developed forms the basis for understanding and modeling the excitable behavior of all neurons.
Hodgkin and Huxley realized that by manipulating the ionic concentrations, combined with the techniques of the space and voltage clamps, they could disentangle the temporal contributions of different ions assuming that they responded differently to changes in
V m .
• Removing Na + from the bathing medium,
I Na
becomes negligible so
I K
can be measured directly. Subtracting this current from the total current yielded
I Na
. Disentangling the ionic currents (reproduced from Hodgkin and Huxley, 1952a)
Historical Background
Neher and Sakmann Ion channels Following Hodgkin & Huxley’s results in the 1950’s two classes of transport mechanisms competed to explain their results:
carrier molecules
and
pores
- and there was no direct evidence for either. It was not until the 1970’s that the nicotinic ACh receptor and the Na + channel were chemically isolated, purified, and identified as proteins. The technical breakthrough of the
patch-clamp
techniques developed by Neher and Sakmann (1976) allowed them to report the first direct measurement of electrical current flowing through a single channel for which they received the 1991 Nobel prize.
Patch-clamp recording from a single ACh-activated channel on a cultured muscle cell with the patch clamped to -80mV. Openings of the channel (downward events) caused a unitary 3 nA current to flow, often interrupted by a brief closing. Notice the random openings and closing, characteristic of all ion channels. Fluctuations in the baseline are due to thermal noise. Reproduced from Sigworth FJ (1983)
An example of analysis
in
Single Channel Recording,
eds. Sakmann B, Neher E. Pp 301-321. Plenum Press.
The Hodgkin-Huxley Formalism
Basic Assumptions
Vm Im Iionic Cm Rm Em gNa ENa gK EK I m
I ionic
C m dV m dt I ionic
I Na
I K
I leak
Ohm’s law
The Hodgkin-Huxley Formalism
Ohmic Currents
V C I R E g E g E
Currents are linearly related to the driving potential
V m I Na
g Na
V
,
t
E Na
The Nernst potential, here for Na + , gives the reversal potential
E Na
or the
ionic battery
– it is a function of the intra- and extracellular concentrations of the ion
The Nernst Equation
E Na
RT zF
ln
o i
The Hodgkin-Huxley Formalism
Voltage-Dependence of Conductances Experimentally recorded (circles) and theoretically calculated (smooth curves) changes in
g Na
and
g K
in the squid giant axon at 6.3C C during depolarizing voltage steps away from the resting potential (here set to 0).
Inactivation
is demonstrated by the decay of
g Na
following its initial rise. Reproduced from Hodgkin AL (1958)
Ionic movements and electrical activity in giant nerve fibres
,
Proc R Soc Lond B
148:1-37
The Hodgkin-Huxley Formalism
Gating Particles
V C I R E g E g E g Na g K
g Na
m
3
h
g K
n
4
Gating particles
(m,h,n, etc.) were introduced to describe the dynamics of the conductances (time- and voltage-dependent) and scale a maximal conductance. They can be
activating
or
inactivating.
The values range from 0 to 1 and (knowing what we know today with respect to ion channels) can be thought of as the percentage of channels in the activated or inactivated state.
The Hodgkin-Huxley Formalism
Gating particles obey first order kinetics
p i
= probability (or fraction of) gate(s) (1-
p i
) = probability (or fraction of) gate(s)
i i
being in permissive state being in non-permissive state Steady state solution Time constant
dp i dt
i
(
V
)( 1
p i
)
i
(
V
)
p i p i
,
t
(
V
)
i
(
V
i
) (
V
)
i
(
V
)
i
(
V
)
i
(
V
) 1
i
(
V
)
Activation and Inactivation Kinetics
Potassium Current
I K
Non-inactivating current
I K
g K
n
4
V
E K
Activation particle n
dn dt
n
(
V
)( 1
n
)
n
(
V
)
n
i.e.
dn dt
n
n n
Time-dependent solution
n
n
n
n
0
e
n t
Hodgkin and Huxley’s Parameterization
n
g K
36 mS/cm
2 100 10
e
( 10
V V
) / 10 1
0 .
125
e
V
/ 80
Activation and Inactivation Kinetics
Sodium Current
I Na
Activating and inactivating current
I Na
g Na
m
3
h
V
E Na
Gating particles m and h
dm
dt m
m m
activation
dh
dt h
h h
inactivation
Hodgkin and Huxley’s Parameterization
m
m g Na
120 mS/cm
2 10
e
25 ( 25
V
V
) / 10 1
4
e
V
/ 18
h
h
0 .
07
e
V
/ 20 1
e
( 30
V
) / 10 1
n h m
Activation and Inactivation Kinetics
Graphical Representation
m
Time constants (upper plot) and steady-state activation and inactivation (lower plot) as a function of the relative membrane potential
V
for sodium activation
m
(solid line) and inactivation
h
(long dashed line) and potassium activation
n
(short dashed line).
m
h
n
Reproduced from Koch C (1999)
Biophysics of Computation
, Oxford University Press.
Generation of Action Potentials
The Complete Hodgkin-Huxley Model Computed action potential in response to a 0.5 ms current pulse of 0.4 nA amplitude (solid lines) compared to a subthreshold response following a 0.35 nA current pulse (dashed lines).
(A) Time course of the two ionic currents – note their large size relative to the stimulating current (B) Membrane potential in response to threshold and subthreshold stimuli (C) Dynamics of the gating particles – note that the Na + activation
m
changes much faster than
h
or
n
Reproduced from Koch C (1999)
Biophysics of Computation
, Oxford University Press.
Generation of Action Potentials
The Complete Hodgkin-Huxley Model Results of the complete model: 1) Action potential generation 2) 3) Threshold for spike initiation Refractory period For an overview on the Loligo’s axon (Giant squid acon) see http://www.mbl.edu/publications/Loligo/squid/science.html
Activation and Inactivation Kinetics
Temperature Dependence
Q 10
Kinetics of channels/currents (i.e. and are strongly dependent on temperature while the peak conductance remains unchanged – be very careful when reading the methods section of a neurophysiology paper!!! Hodgkin and Huxley recorded from the
Loligo
axon at 6.3
C and so the rate constants shown above are for that temperature To adjust for different temperature, and must be scaled by (
Q
10
T
T mea sured
) / 10 Where the
Q
10 measures the increase in the rate constant for every 10 C change from the temperature at which the kinetics were measured – this is typically between 2 and 4
The Hodgkin-Huxley Formalism
Summary 1) 2) 3) 4) 5) 6) 7) The Hodgkin-Huxley 1952 model of action potential generation and propagation is the single most successful quantitative model in neuroscience The model represents the cornerstone of quantitative models of neuronal excitability The heart of the model is a description of the time- and voltage-dependent conductances for Na + and K + in terms of their
gating particles
(
m, h,
and
n
) Gating particles can be of the
activation
or
inactivation
variety – activation implies its amplitude (from 0 to 1) increases with depolarization while the converse is true of inactivation Kinetics of gating are represented either by the rate constants activation/inactivation and time constant (e.g.
n
and
n
) and or the steady-state Without any
a priori
assumptions about action potentials, this model generates APs of appropriate shape, threshold and refractory periods (both absolute and relative) Temperature can have a dramatic effect on the kinetics of gating and, ideally, should be accounted for in a model by incorporation of the
Q
10 scaling factor – this is an experimentally-determined quantity