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 Where E X equilibrium potential of X + Rideal gas constant Tabsolute temperature zcharge number of the ion FFaraday ’ s number natural logarithm of concentration ration of X + on the two sides of the membrane

At any membrane potential other than the E x, there will be an electrochemical driving force for the movement of X + across the membrane, which tend to pull the membrane potential toward its E X. The greater the difference between the membrane potential and the E X 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 where E m membrane potential E s equilibrium potentials of the ion s g s conductance of the membrane to the ion s. the more permeable, the greater the conductance.

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: C m membrane resistance: R m membrane conductance: g m

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

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 g Na causes the membrane potential to move toward E Na. The rapid return of the action potential toward the resting potential is caused by the rapid decrease in g Na and the continued increase in g K.

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, g Na returns to baseline levels, but g K 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