• 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