CH 11 Membrane Potential

Exam 3 Review: Chapter 11: Membrane Potential

membrane potential - An electrical voltage (a charge differential with the potential to do work) across the cell membrane, due to ionic disequilibria (unequal distribution of ions), primarily the differences in sodium and potassium ion concentrations inside and outside the cell; this ionic disequilibrium is maintained by the action of the sodium-potassium pump and consequently, by the expenditure of much ATP energy.

resting membrane potential- The membrane potential of a nonconducting neuron, due to ionic disequilibria, primarily the differences in sodium and potassium concentrations inside and outside the cell because of the action of the Na⁺/K⁺ ATPase pump; a nonconducting neuron is charged positively on the outside and negatively on the inside with a typical value of -70mV.

Na⁺/K⁺ ATPase - An integral membrane protein active transport molecule which has the capacity to bind and hydrolyze ATP and use the energy from the ATP hydrolysis to move 3 Na⁺ions from the cytoplasm to the exterior of the cell while simultaneously moving 2 K⁺ions from the exterior of the cell into the cytoplasm; the action of this ion pump is the main factor in establishing a resting membrane potential for the cell.

chemical gradient = concentration gradient - A (usually graduated) difference in mass/unit volume (concentration) of a substance, a solute, through a solution or between two locations; such a difference in solute strength promotes diffusion or osmosis or both.

charge gradient - A (usually graduated) difference in net charge due to the sum of all charges of all ions present between two locations such as two fluid compartments on opposite sides of a semi-permeable membrane; such a gradient creates a membrane voltage potential and promotes diffusion or osmosis or both.

graded potential - A small deviation from the resting membrane potential on an excitable cell, which may make the membrane more polarized (hyperpolarization) or less polarized (depolarization) and, therefore, contributing to whether or not an action potential will be generated at that moment; graded potentials occur most often in the dendrites and cell body of a neuron; the changes in potentials vary in amplitude (size) depending on the strength of the stimulus and localized to small areas of the membrane where presynaptic neurons have axon terminals attached to the cell membrane forming synapes.

hyperpolarization - A negative change in a cell's resting potential (which is normally negative), thus making the negative charge numerically larger; the membrane becomes more polarized; when an excitable cell becomes more polarized, it becomes less likely to depolarize and generate an action potential.

List:

8. Two types of polarization possible in graded potentials.

1) depolarization; a decrease in the measured voltage potential between the inside and the outside of the neurolemma, i.e., the cell cytoplasm becomes less negative than the resting potential
2) hyperpolarization; an increase in the measured voltage potential between the inside and the outside of the neurolemma, i.e., the cell cytoplasm becomes more negative than the resting potential

11. Six differences between graded potentials and action potentials.

Graded Potentials

Action Potentials

1. neurolemma voltage potential change variable; change proportional to stimulus strength

2. no minimum threshold stimulus required

3. neurolemma voltage potential change may be depolarizing or hyperpolarizing

4. observed in dendrites or on the neuron cell body = soma

5. brief duration

6. local phenomenon, i.e., not communicated to a next cell in a pathway

1. neurolemma voltage potential change always the same; independent of stimulus strength, the "all-or-nothing" response

2. a minimum threshold stimulus required

3. neurolemma voltage potential change always depolarizing

4. observed in an axon hillock or in the axon

5. greater duration

6. not local, i.e., may be communicated to a next cell in a pathway

Explain:

2. Two important factors responsible for the creation of resting membrane potential.

The differences in sodium and potassium (and, to a lesser extent, chloride) concentrations inside and outside the cell create the resting membrane potential, approximately -70 mV.

(1) primarily due to the continuous action of the Na⁺/K⁺ ATPase pumps in the neurolemma

(2) also due to the fact that the various chemically-gated and voltage-gated Na⁺, K⁺, and Cl^- pumps are closed when the cell is in the resting state

3. How a neuron maintains its resting potential. What resources are required to do this?

                 a) all voltage-gated and chemically-gated ion channels are closed
                 b) the Na⁺ K⁺ ATPase pumps are operating (3 Na⁺out/2 K⁺ in per ATP hydrolysis)
                 c) the source of ATPs is aerobic respiration in the mitochondria
                 d) aerobic respiration in the mitochondria requires a continuous supply of oxygen and glucose from the blood stream

4. How hyperpolarization during the refractory period prevents action potentials from being generated and moving backwards along the axon toward the soma.

The recently depolarized membrane still has an excess of potassium ions outside the cell during hyperpolarization which affects the gradients for ion flow.

More importantly, during hyperpolarization, all local voltage-gated and chemically-gated ion channels are still in transition back to their resting ready state.

Therefore, when the adjacent membrane is depolarized, the recently depolarized membrane's voltage-gated Na⁺ion channels cannot respond. By the time they are ready to respond, a few milliseconds later, the adjacent membrane has repolarized. See the figures below.