electric
current
Observation
measuring
electrodes
membrán
extracellular spece
Alteration of resting membrane
potential
1. “passive” electric properties of the membrane
intracellular space
Inward current
Depolarization of
the membrane
What is it like?
Interpretation with equvivalent circuit model:
Charge and discharge of RC-circuit
I ion + I c = I m = 0
Im
UC=I0R(1-e-t/τ)
100%
UC=U0e-t/τ
INa
++++
----
gNa
ENa
IK
gK
EK
gNa (Um-ENa)= INa
ICl
gCl
gion (Um-E)= Iion
Um
ECl
Cm
63%
ΔU m ΔU m − E
+
− I stimulus = 0
Δt
Rm
Time from the
beginning of stimulus
37%
τ=RC
t
−
⎡
⎤
Rm C m
U m (t ) = U t ⎢1 − e
⎥
⎣⎢
⎦⎥
Membrane potentiel
after t
Saturation value of
membrane potential
inward current
Capacitance of the
membrane
Resistance of the
membrane
τ = Cm Rm
t
−
⎡
⎤
Rm C m
U m (t ) = U t ⎢1 − e
⎥
⎢⎣
⎥⎦
outward current
depolarization
τ : time constant of membrane
hyperpolarization
-the time required for the membrane potential to reach 63%
Ut is proportional to the stimulating current
of its saturation value
The rate of the change depends on Ut
-during which the membrane potential decreases to the e-th
of its original value
Local changes of membrane potential
The local changes are not isolated from the neighbourhood
Observation
obligate
graded
magnitude varies directly
with the strength of the stimulus
direction varies
with the direction of the stimulus..
„localised”
Membrane
potential change
at the site of
stimulation
100%
ΔUx=ΔU0e-x/λ
ΔU
37%
λ
x
Decrease in amplitude with distance due to leaky membranes
λ: space constant of the membrane:
distance in which the maximal value of induced
membrane potential change decreses to its e-th value
Local changes of resting membrane potential can be
induced
- by electric current pulses
100%
Membrane
potential change
at the site of
stimulation
ΔUx=ΔU0e-x/λ
ΔU
37%
- by neurotransmitters at postsynaptic membrane
λ
λ~
Rm
Ri
- by adequate stimulus at receptor cells
x
-excitatory inhibitory postsynaptic potential - depolarization
- inhibitory postsynaptic potential - hyperpolarization
Resistance of intracellular space
Significance of the local changes of resting
membrane potential
Alteration of resting membrane
potential
Sensory function
Impulse conduction
Signal transduction
2. “active” electric properties of the membrane in
excited state
Observation
Phases and landmark of the action potential
inward current
+20mV
0mV
outward current
Threshold
potential
critical
membrane
potential level at
which an action
potential can
occur
depolarization
hyperpolarization
Action potential
Peak potential
Repolarizing phase
depolarization
afterpotential
-70mV
stimuli
facultative
“All-or-none” amplitude
conducted with constant amplitude
Hodgkin-Katz hypothesis of action potential
Hodgkin-Katz hypothesis of action potential
generation
Voltage-Gated, potential sensitive ion channels
ϕe − ϕi = −
RT Σpk+ cke+ + Σpk− cki−
ln + +
F
Σpk cki + Σpk− cke−
depolarization
inward Na+ current
pNa ( gNa ) increases
sequence
Um (mV)
Measurement of separated ionic currents
Na+-current
Inhibited K+channels
Alan Loyd Hodgkin
(1914-1998)
The Nobel Prize in Physiology or Medicine
1963
t (ms)
Im (μA)
Andrew Fielding Huxley
(1917- )
Inhibited Na+channels
t (ms)
“for their discoveries concerning the ionic
mechanisms involved in excitation and
inhibition in the peripheral and central
portions of the nerve cell membrane"
Voltage-Gated Na+ and K+ Channels
States of voltage-gated sodium channels
at depolarization threshold
K+-current
t (ms)
Condictivities during action potential
Propagation of action potential (1)
Factors Influencing Conduction Direction and
Velocity
helyi áramok
The evolutionary need for the fast and efficient
transduction of electrical signals
based on local current flow and depolarization of
adjacent membrane area
Propagation of action potential (2)
Generation of the next peak potential
Where?
ΔU
λ
x
The greater the space constant, the more rapidly distant regions will be
brought to threshold and the more rapid will be the propagation velocity
When?
The smaller the time constant, the more rapidly a depolarization will
affect the adjacent region.
Speed and distance of propagation?
How are the time constant and the space constant related to
propagation velocity of action potentials
Velocity is the function of passive properties − τ and λ − of membranes
τ = Cm Rm
Effect of axon diameter:
λ~
r↑
Ri↓ (~1/r2)
τ↓
Rm↓ (~1/r)
λ↑
Rm
Ri
Myelination!
Rm – very high
big space constant
Cm – very small
small time constant
human nerve cell
Squid giant axon
r= 10 μm
v~ 100 m/s
r=250μm
v=25m/s
human nerve cell
r= 10 μm
v≠0.5m/s ?
Saltatory conduction – quick, energy saving
Node of Ranvier Myelin sheath
Na+
Na+
Myelin prevents ions from entering or leaving the axon
along myelinated segments.
Damage of Myelin sheath
Na+
Leak of current
Na+
Effect of axon diameter and Myelination
Signal transmission in synapses
postynaptic terminal
presynaptic terminal
Effect of passive electric properties on signal
transduction in synapses
Action
potential
How can neurons transmit information from presynaptic to
neurotransmitter
Temporal Summation : combined effects of neurotransmitter
release from the same sites over time
postsynaptic cells if most synaptic effects are subthreshold?
Postynaptic signal
Spatial Summation : combined influences at the same cell at
a particular moment in time
-40 mV
τ=10 ms
Postsynaptic signal
-40 mV
τ=1 ms
Um
Um
-60 mV
-60 mV
Presynaptic signal
Presynaptic signal
Temporal Summation : combined effects of neurotransmitter
release from the same sites over time
idő
idő
2 ms
2 ms
Temporal and spatial summation