chap. 2 the resting membrane potential chap. 3 action potential 第三节 细胞的生物电现象...
TRANSCRIPT
• chap. 2 The resting
membrane potential
• chap. 3 Action potential
第三节 细胞的生物电现象
from Berne & Levy Principles of
Physiology
(4th ed)
2005
Observations of Membrane Potentials
• Extracellular recording
• Intracellular recording
• Voltage clamp
macroscopical current
• Patch clamp
single channel current
1. IONIC EQUILIBRIA
Concentration force Electrical force
Electrochemical Equilibrium
• When the force caused by the concentration
difference and the force caused by the
electrical potential difference are equal and
opposite, no net movement of the ion occurs,
and the ion is said to be in electrochemical
equilibrium across the membrane.• When an ion is in electrochemical
equilibrium, the electrochemical potential
difference is called as equilibrium potential or
Nernst potential.
The Nernst Equation
B
ABAX X
X
zF
RTEEE
][
][ln
Where
EX equilibrium potential of X+
R ideal gas constant
T absolute temperature
z charge number of the ion
F Faraday’s number
natural logarithm of concentration ration
of X+on the two sides of the membrane
B
A
X
X
][
][ln
• At any membrane potential other than the
Ex , there will be an electrochemical driving
force for the movement of X+ across the
membrane, which tend to pull the membrane
potential toward its EX.• The greater the difference between the
membrane potential and the EX will result in
a greater driving force for net movement of
ions.• Movement can only happen if there are open
channels!
Distribution of Ions Across Plasma Membranes
The Chord Conductance Equation
ClCl
NaNa
KK
m Eg
gE
g
gE
g
gE
where
Em membrane potential
Es equilibrium potentials of the ion s
gs conductance of the membrane to the
ion s. the more permeable, the greater
the conductance.
ClNaK gggg
• The average is weighted by the ion’s
conductance (determined by open
channels).
• The membrane potential is a weighted
average of the equilibrium potentials of
all the ions to which the membrane is
permeable.
2. RESTING MEMBRANE POTENTIALS
The cytoplasm is usually electrically
negative relative to the extracellular
fluid. This electrical potential difference
across the plasma membrane in a
resting cell is called the resting
membrane potential.
• The Na+,K+-ATPase contributes directly to generation of the resting membrane potential.
• All the ions that the membrane is
permeable to contribute to the
establishment of the potential of the
membrane at rest.
3. SUBTHRESHOLD RESPONSES
• The size (amplitude) of the subthreshold
potential is directly proportional to the
strength of the triggering event.
• A subthreshold potential can be either
hyperpolarizing (make membrane potential
more negative) or depolarizing (make
membrane potential more positive)
graded potential
• This passive spread of electrical
signals with no changes in membrane
property is known as electrotonic
conduction.
• Subthreshold potentials decrease in
strength as they spread from their point
of origin, i.e. conducted with decrement.
local response
spatial summation & temporal summation
membrane capacitance: Cmmembrane resistance: Rm
membrane conductance: gm
4. ACTION PONTIELS
An action potential is a rapid change in the
membrane potential followed by a return to the
resting membrane potential.
waveform of action potential
• At peak of action potential membrane
potential reverses from negative to positive
(overshoot).
• During the hyperpolarizing afterpotential,
the membrane potential actually becomes
less negative than it is at rest.
• Rising phase (depolarization phase)
• Repolarization phase
• An action potential is triggered when the
depolarization is sufficient for the
membrane potential to reach a threshold.
Ionic Mechanisms of Action Potential
)( KmKK EEgI
)( NamNaNa EEgI
changes of ion conductance during action potential
• Action potentials arise as a result of brief
alterations in the electrical properties of the
membrane.
• During the early part of the action
potential, the rapid increase in gNa causes
the membrane potential to move toward
ENa. • The rapid return of the action potential
toward the resting potential is caused by
the rapid decrease in gNa and the continued
increase in gK.
• Action potentials differ in size and shape
in different cells, but the fundamental
mechanisms underlying the initiation of
these potentials does not vary.
• During the hyperpolarizing
afterpotential, when the membrane
potential is actually more negative than
the resting potential, gNa returns to
baseline levels, but gK remains elevated
above resting levels.
model of the voltage-dependent Na+ channel
closed open inactivated
• Either a stimulus fails to elicit an action
potential or it produces a full-sized action
potential.
Properties of Action Potential
All-or-None Response
• The size and shape of an action potential
remain the same as the potential travels
along the cell.• The intensity of a stimulus is encoded by the
frequency of action potentials.
Refractory Period
Conduction of Action Potential
Local circuit current Self-reinforcing
• myelination
myelin sheath
node of Ranvier
Conduction velocity
• diameter