1.4 Membrane and Action Potentials
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Questions and Answers

What is the primary factor that establishes a membrane potential?

  • The absolute number of ions on either side of the membrane.
  • The presence of proteins in the membrane.
  • The temperature of the surrounding environment.
  • The concentration difference and selective permeability of ions across the membrane. (correct)

    • Which ion's movement contributes to a negative charge inside the cell during the establishment of the resting membrane potential?

  • Chloride ions diffusing into the cell.
  • Potassium ions diffusing out of the cell. (correct)
  • Sodium ions diffusing into the cell.
  • Calcium ions diffusing into the cell.

    • What does the Nernst potential represent?

  • The total membrane potential of a cell.
  • The equilibrium potential when no ions are moving.
  • The electrical potential due to a single specific ion. (correct)
  • The diffusion potential of multiple ions present at the same time.

    • According to the Nernst equation, what factor has the MOST influence on an ion's Nernst potential?

    <p>The ratio of the ion concentrations on either side of the membrane. (A)</p> Signup and view all the answers

    What is the relationship between ion concentration gradient and the electrical charge needed to stop diffusion?

    <p>A higher concentration gradient requires a greater electrical charge to stop diffusion. (A)</p> Signup and view all the answers

    Which of the following does the Goldman equation take into consideration, that the Nernst equation does not?

    <p>The effect of multiple types of ions diffusing. (B)</p> Signup and view all the answers

    Which three ions are identified as the most important in establishing the membrane potential in nerve and muscle fibers according to the text?

    <p>Sodium, potassium and chloride. (D)</p> Signup and view all the answers

    If potassium ions diffuse out of a cell, but sodium ions are also diffusing into the same cell at the same time, what situation will this cause?

    <p>The membrane potential will be determined by relative permeability of the membrane to each ion. (B)</p> Signup and view all the answers

    What is the primary mechanism responsible for the rapid depolarization phase of an action potential?

    <p>Sodium influx through voltage-gated channels (D)</p> Signup and view all the answers

    What is the approximate threshold membrane potential that is typically required to initiate the positive feedback loop of an action potential in a large nerve fiber?

    <p>-55 mV (B)</p> Signup and view all the answers

    According to the provided content, what is meant by ‘all-or-nothing’ principle concerning action potentials?

    <p>Once initiated, an action potential spreads throughout the entire membrane if conditions are right, or not at all (B)</p> Signup and view all the answers

    What role does the sodium-potassium pump play in maintaining the excitability of the nerve membrane?

    <p>It maintains the concentration gradients necessary for action potentials (C)</p> Signup and view all the answers

    What is the major function of the myelin sheath in nerve fibers?

    <p>To act as an excellent electrical insulator (C)</p> Signup and view all the answers

    Which of the following is NOT a typical cause for initiation of the positive feedback loop during action potential?

    <p>Increased potassium concentration outside the membrane (D)</p> Signup and view all the answers

    What is the primary function of the nodes of Ranvier in myelinated nerve fibers?

    <p>To allow ion flow and action potential generation (D)</p> Signup and view all the answers

    What is the term used to describe the ‘jumping’ of action potential between nodes of Ranvier?

    <p>Saltatory conduction (A)</p> Signup and view all the answers

    What event is responsible for the increased negative charge of the nerve membrane, leading to hyperpolarization, following an action potential?

    <p>Potassium outflow (D)</p> Signup and view all the answers

    Which of the following scenarios is the most likely to induce a repetitive self-induced discharge of action potentials?

    <p>A membrane with adequate permeability to sodium ions (C)</p> Signup and view all the answers

    What is the primary responsibility of the rapid changes in sodium and potassium channel permeability?

    <p>Signaling transmissions in neurons. (C)</p> Signup and view all the answers

    Which of the following best describes the state of membrane potential in cardiac pacemaker cells?

    <p>Membrane potential is continuously changing. (C)</p> Signup and view all the answers

    What is the approximate resting membrane potential of large nerve fibers?

    <p>-70 millivolts (D)</p> Signup and view all the answers

    Why is the sodium-potassium pump considered an electrogenic pump?

    <p>It pumps three sodium ions outside for every two potassium ions inside. (D)</p> Signup and view all the answers

    What is the key factor in determining the normal resting membrane potential?

    <p>The difference in permeability between sodium and potassium channels. (D)</p> Signup and view all the answers

    What is the resting membrane potential primarily determined by?

    <p>Potassium diffusion potential, sodium diffusion potential, and the sodium-potassium pump. (A)</p> Signup and view all the answers

    During the resting stage of an action potential, how is the membrane described?

    <p>Polarized (C)</p> Signup and view all the answers

    Which event marks the end of the depolarization stage during an action potential?

    <p>Sodium channels begin to close and potassium channels open. (A)</p> Signup and view all the answers

    What is the role of the voltage-gated sodium channels in action potentials?

    <p>They are necessary for both depolarization and repolarization. (B)</p> Signup and view all the answers

    How does the inactivation gate of voltage-gated sodium channel respond to membrane potential change?

    <p>It closes after a slight delay following the activation gate opening. (C)</p> Signup and view all the answers

    During the repolarization, what is the relative conductance of potassium ions compared to sodium ions?

    <p>Potassium conductance is much higher. (D)</p> Signup and view all the answers

    What leads to the positive feedback cycle that opens the sodium channels?

    <p>A slight change in the membrane potential. (B)</p> Signup and view all the answers

    What is the resting stage of an action potential characterized by?

    <p>A negative membrane potential of close to -70mV. (A)</p> Signup and view all the answers

    What is the main contributor to the negative membrane potential?

    <p>Potassium efflux. (C)</p> Signup and view all the answers

    What immediately follows the sodium channel activation during an action potential?

    <p>Sodium channels undergo a rapid, but transient, inactivation. (A)</p> Signup and view all the answers

    Flashcards

    Membrane potential

    The electrical difference across a cell membrane due to the uneven distribution of ions.

    Ion diffusion

    The movement of ions across a cell membrane, creating an electrical potential.

    Nernst potential

    The potential difference across a membrane that would completely oppose the movement of a specific ion down its concentration gradient.

    Nernst equation

    The equation used to calculate the Nernst potential for a specific ion.

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    Goldman equation

    The potential difference across a membrane that is permeable to multiple ions. It takes into account the permeability of the membrane to each ion and the concentration gradient of each ion.

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    Concentration gradient

    The concentration gradient of an ion across a membrane.

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    Membrane permeability

    The ability of a membrane to allow specific ions to pass through.

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    Membrane potential stability

    The ability of a membrane to maintain a potential difference across it.

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    Sodium Influx Feedback Loop

    The rapid influx of sodium ions into a neuron, triggered by a change in membrane potential, leading to the opening of voltage-gated sodium channels. This influx further depolarizes the membrane, causing more sodium channels to open, creating a positive feedback loop.

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    Action Potential Threshold

    The minimum change in membrane potential required to initiate an action potential. Once this threshold is reached, the positive feedback loop of sodium influx takes over.

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    Action Potential Propagation

    The spread of an action potential along the membrane of a neuron, caused by local circuit currents flowing from the depolarized area to adjacent, resting areas.

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    All-or-None Principle

    The principle that an action potential either occurs fully or not at all, regardless of the strength of the initial stimulus. Once the threshold is reached, the action potential will be the same size and shape.

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    Nodes of Ranvier

    The gaps between myelin sheaths on a neuron, where ion flow can occur and action potentials regenerate during saltatory conduction.

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    Saltatory Conduction

    The mechanism by which action potentials jump between the Nodes of Ranvier on myelinated axons, increasing the speed of nerve impulse transmission.

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    Repetitive Self-Induced Discharge

    The ability of a neuron to generate action potentials repeatedly, often occurring spontaneously in the heart, smooth muscle, and the central nervous system.

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    Repolarization and Hyperpolarization

    The process by which a neuron returns to its resting membrane potential after an action potential, often involving an increased permeability to potassium ions, leading to hyperpolarization.

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    Myelin Sheath

    The fatty substance that wraps around axons of some neurons, providing electrical insulation and increasing the speed of nerve impulse conduction.

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    Action Potential Initiators

    Factors that can initiate an action potential by causing sodium ions to flow into the neuron, such as mechanical disturbances, chemical changes, or electrical stimuli.

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    Resting Membrane Potential

    The state of a neuron when it is not actively transmitting a signal. It is characterized by a negative membrane potential, typically around -70 millivolts.

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    Potassium Diffusion Potential

    The difference in electrical potential across the membrane due to the movement of potassium ions from the inside to the outside of the cell.

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    Sodium Diffusion Potential

    The difference in electrical potential across the membrane due to the movement of sodium ions from the outside to the inside of the cell.

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    Sodium-Potassium Pump

    A specialized protein embedded in the cell membrane that pumps three sodium ions out of the cell for every two potassium ions pumped inside. This creates a concentration gradient and contributes to the negative resting membrane potential.

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    Action Potential

    The rapid changes in membrane potential that travel along a nerve fiber, enabling the transmission of signals throughout the nervous system.

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    Depolarization

    The initial stage of an action potential, where the membrane potential becomes less negative, moving towards zero.

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    Repolarization

    The stage of an action potential where the membrane potential returns to its negative resting state.

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    Voltage-Gated Sodium Channel

    A protein channel embedded in the cell membrane that is specifically permeable to sodium ions. It opens and closes in response to changes in membrane potential, playing a crucial role in depolarization.

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    Voltage-Gated Potassium Channel

    A protein channel embedded in the cell membrane that is specifically permeable to potassium ions. It opens and closes in response to changes in membrane potential, playing a crucial role in repolarization.

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    Refractory Period

    The period during which a neuron cannot generate another action potential, regardless of the strength of the stimulus.

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    Positive Feedback Cycle

    The process by which the opening of voltage-gated sodium channels triggers a cascade of further channel openings, amplifying the depolarization signal and ensuring a strong action potential.

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    Conduction Velocity

    The phenomenon where the speed of an action potential increases as it travels along a nerve fiber, due to factors such as myelination and the diameter of the fiber.

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    Myelin

    A fatty sheath that wraps around certain nerve fibers, acting as an insulator and increasing the speed of signal transmission.

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    Synapse

    The junction between two neurons where a signal is transmitted from one neuron to another.

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    Study Notes

    Membrane Potentials and Action Potentials

    • Membrane Potential: A difference in ion concentration across a selectively permeable membrane creates a membrane potential. This potential difference arises from the unequal distribution of ions (e.g. potassium, sodium) inside and outside the cell.

    • Potassium Ions: The internal concentration of potassium is high, and its external concentration is low. This creates a diffusion gradient pushing potassium ions outwards, carrying their positive charge. This builds a positive charge outside the membrane and a negative charge inside. The electrical difference quickly reaches a level that halts further potassium diffusion, resulting in a membrane potential of approximately -94 millivolts.

    • Sodium Ions: Sodium ions have a high concentration outside and a low concentration inside a nerve fiber. This gradient causes sodium ions to diffuse inwards, creating a positive inside charge and a negative outside charge. Like potassium, this process stops when the potential difference negates further diffusion. This results in a membrane potential of approximately +61 millivolts.

    • Nernst Potential: The electrical potential required to exactly oppose the diffusion of a particular ion across a membrane is its Nernst potential. The magnitude of this potential is determined by the ratio of ion concentrations across the membrane: larger concentration differences lead to a larger potential. The Nernst equation calculates this potential.

    • Goldman Equation: This equation calculates membrane potential when the membrane is permeable to multiple ions. It considers the permeability of each ion and its respective concentrations to predict the overall membrane potential. Sodium, potassium, and chloride are the most important ions in determining nerve and muscle fiber membrane potential. The quantitative significance of each ion is proportional to the membrane's permeability for that ion.

    • Resting Membrane Potential: The resting membrane potential of large nerve fibers is approximately -70 millivolts. The sodium-potassium pump contributes to maintaining this potential by actively pumping three sodium ions out and two potassium ions in, creating a concentration gradient. The membrane is significantly more permeable to potassium, driving the resting membrane potential towards the potassium equilibrium potential.

    • Action Potential: Rapid changes in membrane potential—action potentials—are how nerve signals are transmitted. These signals begin with a sudden shift from the resting membrane potential to a positive value and return just as rapidly to a negative resting potential. The stages involved are polarization, depolarization, and repolarization.

    • Depolarization: During this stage, the membrane becomes more permeable to sodium ions which rush inside. This rapid inward flow of positive charge causes the membrane potential to rise to a positive value.

    • Repolarization: During this stage, sodium channels close, and potassium channels open, allowing potassium to rapidly diffuse outwards, restoring the negative membrane potential.

    • Voltage-Gated Channels: Voltage-gated sodium channels are crucial for both depolarization and repolarization. They have activation gates (near the outside) and inactivation gates (near the inside). These gates open and close in response to changes in membrane potential. The timing of gate opening and closing is precise, allowing the rapid up-and-down flow of sodium and potassium.

    • All-or-None Principle: Once an action potential is initiated, it propagates across the entire membrane if conditions are suitable. If not, it doesn't occur. This is due to the positive feedback loop involving the opening of sodium channels and the subsequent generation of more positive charge.

    • Action Potential Propagation: The action potential travels along the nerve fiber by influencing adjacent areas due to current flow. In myelinated fibers, the action potential "jumps" between nodes of Ranvier, significantly increasing conduction velocity. This process is called Saltatory conduction.

    • Re-establishment of Concentration Gradients: The sodium-potassium pump works relentlessly to re-establish the ion concentration gradients disturbed by action potentials, preserving the capacity for future impulses.

    • Cardiac Pacemaker Cells: These cells continuously change their membrane potentials, never resting fully. These ongoing changes are governed by the availability of sodium and calcium. The positive feedback loop for these cells starts close to the end of the action potential and are responsible for the rhythms of the heart.

    • Factors Influencing Membrane Potential: Mechanical or electrical stimuli, as well as various chemical messengers, can cause nerve impulses by influencing ion diffusion through the nerve membrane.

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    Description

    This quiz explores the concepts of membrane potential and action potentials in cells. Understand how ion concentrations of potassium and sodium contribute to electrical differences across membranes. Test your knowledge on the dynamics of ion movement and their physiological significance.

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