第三节 细胞的生物电现象 chap. 2 The resting membrane potential 第三节 细胞的生物电现象 chap. 2 The resting membrane potential chap. 3 Action potential from Berne & Levy Principles of Physiology (4th ed) 2005

Extracellular recording Observations of Membrane Potentials Extracellular recording

Intracellular recording

Voltage clamp macroscopical current

Patch clamp

single channel current

1. IONIC EQUILIBRIA Electrical force Concentration 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 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

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 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.

The membrane potential is a weighted average of the equilibrium potentials of all the ions to which the membrane is permeable. The average is weighted by the ion’s conductance (determined by open channels).

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.

All the ions that the membrane is permeable to contribute to the establishment of the potential of the membrane at rest. The Na+,K+-ATPase contributes directly to generation of the resting membrane potential.

3. SUBTHRESHOLD RESPONSES

graded potential 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)

local response Subthreshold potentials decrease in strength as they spread from their point of origin, i.e. conducted with decrement. This passive spread of electrical signals with no changes in membrane property is known as electrotonic conduction.

spatial summation & temporal summation

membrane capacitance: Cm membrane 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

An action potential is triggered when the depolarization is sufficient for the membrane potential to reach a threshold. Rising phase (depolarization phase) At peak of action potential membrane potential reverses from negative to positive (overshoot). Repolarization phase During the hyperpolarizing afterpotential, the membrane potential actually becomes less negative than it is at rest.

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 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.

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. Action potentials differ in size and shape in different cells, but the fundamental mechanisms underlying the initiation of these potentials does not vary.

model of the voltage-dependent Na+ channel closed open inactivated

Properties of Action Potential All-or-None Response Either a stimulus fails to elicit an action potential or it produces a full-sized action potential. 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

Conduction velocity diameter myelination myelin sheath node of Ranvier