Questions and Answers
What primarily influences the establishment of the normal resting membrane potential?
What is the ratio of Na+ concentration inside to outside the cell?
Which ion has a higher concentration inside the nerve cell?
What is the permeability ratio of potassium to sodium ions in the cell membrane?
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Which factor contributes to the resting membrane potential besides ion concentration?
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Which of the following describes the movement of sodium ions across the membrane?
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What happens when both sodium and potassium ions are pumped by the Na+-K+ pump?
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What is the ratio of K+ concentration inside to outside the cell?
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What is the primary reason for the resting membrane potential being negative?
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Why is the permeability for potassium ions greater than that for sodium ions?
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What primarily occurs during the depolarization of a nerve fiber?
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What contributes to the recharging process of a nerve fiber?
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During a plateau in action potentials, what typically happens?
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What effect does the diffusion of potassium ions have during the recharging phase?
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What initiates the rapid motion of the nerve fiber's recharging process?
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What role does sodium ion diffusion play in the context of nerve impulses?
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What condition is necessary for a nerve fiber to recharge quickly?
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Which of the following statements about action potentials is true?
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What is the primary reason sodium ions diffuse into the nerve fiber during depolarization?
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What is the effect of saltatory conduction on energy expenditure in neural transmission?
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Which statement best describes the role of myelin in nerve fibers?
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What mechanisms are primarily involved in transmitting signals between neurons?
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How does partial wrapping of Schwann cells affect nerve fibers?
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What is one of the consequences of weak insulation in nerve fibers?
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What effect does the increased outflow of potassium ions have on excitable tissues?
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During what time frame does the increased outflow of potassium ions continue to affect the membrane potential after an action potential?
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What is most likely to occur in large nerve fibers when the threshold for stimulation is lowered?
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What happens to the membrane potential as potassium ions flow out?
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What kind of tissue is most affected by reduced stimulation thresholds due to potassium ion outflow?
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What mechanism allows for rhythmic discharges in excitable tissues like the heart?
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How does the action potential affect the conductance of potassium ions?
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What physiological change occurs as a result of the internal negativity in excitable cells due to potassium outflow?
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What role does the rhythmic control center of the heart play in action potentials?
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What phenomenon does the Nernst equation primarily describe?
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When using the Nernst equation, how is the electromotive force (EMF) related to the concentration of ions?
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In the context of the Nernst equation, what does the variable 'z' represent?
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What is the expected sign of the Nernst potential when a negatively charged ion diffuses from inside to outside?
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Under normal physiological conditions, what is the average potential difference for a mammalian nerve fiber inside the membrane?
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How does the permeability of the membrane affect ion diffusion according to the Nernst equation?
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What does the concentration gradient of potassium ions typically suggest about their diffusion?
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What would likely occur if the concentration of sodium ions outside the membrane exceeds that inside?
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What creates a membrane potential of opposite polarity?
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What is the Nernst potential?
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What is the electromotive force (EMF) used to calculate?
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Which ion is used in the Nernst equation?
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The resting potential for neurons is approximately _____ mV.
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What happens during the diffusion of sodium ions into a cell?
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The Goldman equation considers only one type of ion.
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What causes an electrochemical driving force (Vdf) for ions?
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Match the ion to its role in membrane potential:
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What is the resting membrane potential before the action potential begins?
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Which ions significantly determine the membrane potential during the resting stage?
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During depolarization, the membrane potential overshoots beyond zero.
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What is the term used for the process of returning the membrane potential to its resting state?
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The voltage-gated sodium channel closes through a process called __________.
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What determines the activation of voltage-gated potassium channels?
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What is the typical state of the sodium channel when the membrane potential is -70 millivolts?
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Potassium channels open at the same rate as sodium channels during an action potential.
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Who were the scientists awarded the Nobel Prize for their research on sodium and potassium channels?
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Voltage-gated channels allow the flow of ions such as Na+ and K+ in response to __________ changes.
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What happens to the activity of the pump as the internal sodium concentration rises from 10 to 20 mEq/L?
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During depolarization, which ions diffuse to the inside of the membrane?
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Which type of action potential occurs in heart muscle fibers?
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What causes the plateau in heart muscle action potentials?
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The resting membrane potential in the rhythmical control center of the heart is only -60 to -70 millivolts.
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The small uninformed area between successive Schwann cells along the axon is called the ______.
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What types of channels contribute to the depolarization process in heart muscle?
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What is the average speed of conduction in small unmyelinated fibers?
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Saltatory conduction occurs in which type of nerve fibers?
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What is the membrane potential when Vm = Veq?
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What is the typical resting membrane potential of large nerve fibers?
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Which ions are primarily involved in establishing the resting membrane potential?
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The Na+-K+ pump transports more sodium ions into the cell than potassium ions.
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What occurs when the membrane potential is at -70 millivolts?
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The potential inside the nerve fiber is ______ millivolts more negative than the potential in the extracellular fluid.
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What is the Nernst potential for potassium considering its intracellular/extracellular ratio?
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What is the function of the Na+-K+ pump?
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The membrane potential can reverse from -70 millivolts to as much as +35 millivolts rapidly.
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Which of the following describes the state of the ion channels related to potassium ions?
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What is the ratio of potassium conductance to sodium conductance during the resting state?
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Sodium channels close slowly during the action potential.
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What causes a sudden rise in membrane potential leading to an action potential?
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Calcium ions create a diffusion gradient that is more than _____ times greater in the extracellular fluid than in the intracellular fluid.
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What initiates the action potential?
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What must occur for an action potential to be generated?
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What is responsible for the negative charge inside a nerve fiber?
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Calcium channels open quickly compared to sodium channels.
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The movement of sodium ions allows for the propagation of a _____ along the nerve fiber.
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What is the safety factor for propagation?
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Match the following ions to their roles during an action potential:
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Study Notes
Nernst Equation and Membrane Potential
- The Nernst equation calculates the potential difference (Nernst potential) across a membrane for any univalent ion at normal body temperature (98.6°F or 37°C).
- The electromagnetic force (EMF) is determined using the formula:
EMF (millivolts) = ± (61/z) × log (Concentration inside / Concentration outside), where z is the electrical charge of the ion. - A potential difference of about 94 millivolts is achieved within 1 millisecond due to potassium ion diffusion, creating electronegativity inside the membrane and electropositivity outside.
Ion Concentration and Permeability
- Normal concentrations in mammalian nerve fibers be as follows:
- K+ inside: 140 mEq/L; K+ outside: 4 mEq/L
- Na+ inside: 14 mEq/L; Na+ outside: 140 mEq/L
- The ratio of ion concentrations affects membrane potential:
- Na+ inside / Na+ outside = 0.1
- K+ inside / K+ outside = 35.0
- Ion channels are highly permeable to potassium ions compared to sodium ions, approximately 100 times more permeable.
Resting Membrane Potential
- The normal resting membrane potential is influenced by the differential permeability of sodium and potassium ions.
- Potassium diffusion potential is a significant factor in establishing and maintaining the resting membrane potential.
- Leakage of potassium ions through channels slightly reduces concentration differences of sodium and potassium across the membrane.
Action Potentials and Depolarization
- In some instances, membranes do not repolarize immediately after depolarization, leading to a plateau phase in action potentials.
- This plateau is characterized by a continued outflow of potassium ions after the initial action potential, affecting membrane potential and intracellular negativity.
Saltatory Conduction
- Myelin insulation allows for saltatory conduction, preventing loss of ions during action potentials as only the nodes depolarize.
- Saltatory conduction is energy-efficient, requiring significantly less energy for re-establishing ion concentration gradients after nerve impulses.
Sensory and Neural Excitation
- Nerve fibers can be excited by various stimuli:
- Mechanical pressure for sensory nerve endings in the skin.
- Chemical neurotransmitters for signal transmission between neurons.
- Electrical currents for communication between muscle cells in the heart and intestine.
Membrane Potentials and Action Potentials
- Electrical potentials are present across the membranes of nearly all body cells, crucial for the function of nerve and muscle cells.
- Nerve and muscle cells can generate rapid electrochemical impulses, transmitting signals effectively.
- Non-excitable cells, like glandular and macrophage cells, experience local changes in membrane potential that activate functions.
Basic Mechanisms of Membrane Potentials
- Resting and action potentials arise through ion diffusion across selectively permeable membranes.
- The Nernst equation describes diffusion potential opposing net ion diffusion, specifically for potassium (K+) and sodium (Na+).
Nernst Equation
- Defines the potential difference (Nernst potential) for ions due to concentration gradients.
- The equation:
EMF (mV) = ±61 × log (Concentration inside / Concentration outside) - Example: In a nerve fiber, potassium concentration is high inside and low outside, leading to a potential difference.
Diffusion Potentials
- Potassium ions diffusing from inside to outside a cell create a negative potential inside the membrane.
- Sodium ions, when diffusing into the nerve fiber, demonstrate a positive potential inside due to a higher concentration outside.
Goldman Equation
- Calculates membrane potential when a membrane is permeable to multiple ions (Na+, K+, Cl-).
- Incorporates ion concentrations and membrane permeability to predict membrane voltage.
Resting Membrane Potential Variability
- Resting membrane potentials differ among cell types, e.g.,
- Neurons: -60 to -70 mV
- Skeletal muscle: -85 to -95 mV
- Cardiac muscle: -80 to -90 mV
- Dynamic changes, especially in excitable cells, can occur from stimuli affecting ion transporters and channels.
Electrochemical Driving Force
- The difference between membrane potential (Vm) and equilibrium potential (Veq) creates a driving force for ions.
- The driving force predicts ion movement direction:
- Positive for cations (e.g., Na+, K+) means movement out of the cell.
- Negative for anions (e.g., Cl-) means movement into the cell.
Measurement of Membrane Potential
- Membrane potential is measured using microelectrodes; the method is conceptually simple but practically challenging due to cell size.
- A typical resting membrane potential for a nerve fiber is around -70 mV, established by the diffusion of a minuscule number of ions across the membrane.
- Shows the significance of minute ionic movements in maintaining overall membrane potential.### Measurement of Membrane Potential
- Membrane potential is measured using a sophisticated voltmeter that can detect small voltages.
- Potential difference inside and outside nerve fibers can shift from −70 mV (resting) to +35 mV (depolarized) in approximately 1/10,000 of a second.
- Changes in membrane potential are essential for the transmission of nerve signals.
Resting Membrane Potential
- The resting membrane potential of large nerve fibers is about −70 mV, indicating the inside is more negative than the outside.
- Concentration gradients for sodium and potassium across the membrane are critical; sodium outside: 142 mEq/L, sodium inside: 14 mEq/L, potassium outside: 4 mEq/L, and potassium inside: 140 mEq/L.
- Na+-K+ pump is electrogenic, transporting three Na+ ions out for every two K+ ions in, contributing to the negative interior of the cell.
Ion Concentrations and Potentials
- The high permeability of the membrane to K+ compared to Na+ (100 times more) influences resting potential.
- Nernst potential for potassium ions based on concentration gradient is −94 mV.
- The resting potential, considering all factors, is about −90 mV due to diffusion of ions and action of the Na+-K+ pump and chloride ions.
Contribution of Sodium and Potassium Channels
- K+ "leak" channels allow potassium ions to diffuse out, maintaining the resting potential.
- Any slight sodium permeability also influences the membrane potential but to a lesser extent.
- Goldman equation helps predict the membrane potential balance between sodium and potassium.
Action Potential Stages
- Action potentials are rapid changes in membrane potential crucial for nerve signaling.
- Stages include:
- Resting Stage: Membrane is polarized at about −70 mV.
- Depolarization Stage: Sodium permeability increases significantly, causing rapid influx of Na+ and overshooting to positive potential.
- Repolarization Stage: Sodium channels close, potassium channels open, leading to the outflow of K+ restoring the negative resting potential.
Voltage-Gated Channels
- Voltage-gated sodium channels undergo activation that allows massive influx of Na+ ions during depolarization.
- Inactivation of sodium channels occurs shortly after activation, ceasing the influx of Na+.
- Voltage-gated potassium channels open during repolarization, facilitating K+ efflux, which helps return the membrane to its resting state.
Summary of Membrane Potentials
- Membrane potential is determined by ion concentrations, permeability, and active transport mechanisms.
- Resting membrane potential is largely governed by K+ diffusion, while action potentials rely on rapid shifts in Na+ and K+ permeability through voltage-gated channels.### Voltage-Gated Sodium Channels
- Two gates: activation gate (outside) and inactivation gate (inside).
- Activation gate opens at approximately -55 millivolts, allowing sodium ions to enter.
- Inactivation gate closes after a few microseconds, preventing further sodium influx.
- Membrane potential begins to repolarize as sodium entry decreases and potassium exits.
Activation and Inactivation Dynamics
- Activation gate responds quickly to changes in voltage, while inactivation gate closure is slower.
- Sodium channels remain inactivated until the membrane potential returns to resting state (~-70 millivolts).
- This delay in reopening ensures consistent action potential signaling.
Voltage-Gated Potassium Channels
- Potassium channels remain closed during resting state and open when membrane potential rises.
- Channels open more slowly than sodium channels but stay open longer, facilitating sustained potassium efflux.
- The delayed opening contributes to the rapid repolarization of the membrane potential.
Measurement Techniques: Voltage Clamp Method
- Developed by Hodgkin and Huxley, essential for understanding ion channel dynamics.
- Uses two electrodes: one to measure membrane voltage, another to inject current to maintain voltage.
- Graphs illustrate the dynamic changes in sodium and potassium conductance during action potentials.
Action Potential Sequence
- Onset of action potential sees over 1000-fold increase in sodium conductance.
- Initial rise in membrane potential (positive) due to rapid sodium entry exceeds potassium efflux.
- As sodium channels inactivate, potassium channels open, reversing the charge back toward resting state.
Conductance Changes and Ion Roles
- Conductance ratio between sodium and potassium is pivotal for action potential dynamics.
- During resting state, potassium conductance is significantly higher than sodium.
- Impermeant negatively charged anions contribute to the overall negative internal membrane potential.
Role of Calcium Ions
- Calcium ions also play a vital role in action potentials, depending on cell type.
- Calcium gradients are maintained by calcium pumps, creating significant concentration differences across membranes.
Summary of Ionic Movement
- Rapid opening of sodium channels leads to depolarization; subsequent potassium channel activation promotes repolarization.
- Sodium conductance spikes facilitate initial depolarization, while potassium conductance helps restore negative membrane potential.
- The intricate timing of ion channel opening and closing underpins the action potential's propagation along nerve fibers.
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Description
Test your knowledge on the Nernst equation and how it relates to membrane potential in biological systems. This quiz covers key concepts related to ion concentrations and their impact on electrical potential differences across membranes. Explore the role of potassium and sodium ions in nerve fibers.