Physiology Chapter 5

Questions and Answers

What primarily influences the establishment of the normal resting membrane potential?

Concentration gradient of only potassium ions

Differential permeability of sodium and potassium ions (correct)

Pump action of the Na+-K+ pump only

Diffusion of sodium ions alone

What is the ratio of Na+ concentration inside to outside the cell?

0.1 (correct)

10.0

0.5

1.0

Which ion has a higher concentration inside the nerve cell?

Potassium ions (correct)

Calcium ions

Sodium ions

Chloride ions

What is the permeability ratio of potassium to sodium ions in the cell membrane?

Which factor contributes to the resting membrane potential besides ion concentration?

Potassium diffusion potential Signup and view all the answers

Which of the following describes the movement of sodium ions across the membrane?

Leaked slightly and primarily involves potassium channels Signup and view all the answers

What happens when both sodium and potassium ions are pumped by the Na+-K+ pump?

It stabilizes the membrane potential Signup and view all the answers

What is the ratio of K+ concentration inside to outside the cell?

35.0 Signup and view all the answers

What is the primary reason for the resting membrane potential being negative?

Higher concentration of K+ inside compared to Na+ Signup and view all the answers

Why is the permeability for potassium ions greater than that for sodium ions?

Because of the structure of ion channels Signup and view all the answers

What primarily occurs during the depolarization of a nerve fiber?

Sodium ions diffuse inside the membrane. Signup and view all the answers

What contributes to the recharging process of a nerve fiber?

Concentration differences of sodium and potassium ions. Signup and view all the answers

During a plateau in action potentials, what typically happens?

A prolonged phase occurs after peak potential. Signup and view all the answers

What effect does the diffusion of potassium ions have during the recharging phase?

It helps in repolarizing the nerve membrane. Signup and view all the answers

What initiates the rapid motion of the nerve fiber's recharging process?

Significant differences in sodium and potassium ion concentrations. Signup and view all the answers

What role does sodium ion diffusion play in the context of nerve impulses?

It initiates and contributes to depolarization. Signup and view all the answers

What condition is necessary for a nerve fiber to recharge quickly?

Significant intra-membrane sodium and potassium differences. Signup and view all the answers

Which of the following statements about action potentials is true?

Some action potentials display a plateau before repolarization. Signup and view all the answers

What is the primary reason sodium ions diffuse into the nerve fiber during depolarization?

To contribute to the action potential spike. Signup and view all the answers

What is the effect of saltatory conduction on energy expenditure in neural transmission?

It allows for significant energy conservation. Signup and view all the answers

Which statement best describes the role of myelin in nerve fibers?

Myelin provides insulation that aids in the speed of neural conduction. Signup and view all the answers

What mechanisms are primarily involved in transmitting signals between neurons?

Mechanical pressure and chemical neurotransmitters. Signup and view all the answers

How does partial wrapping of Schwann cells affect nerve fibers?

It provides insulation for both myelinated and unmyelinated fibers. Signup and view all the answers

What is one of the consequences of weak insulation in nerve fibers?

Higher ionic loss and inefficient signal transmission. Signup and view all the answers

What effect does the increased outflow of potassium ions have on excitable tissues?

It allows for a lower threshold for stimulation, leading to repetitive discharges. Signup and view all the answers

During what time frame does the increased outflow of potassium ions continue to affect the membrane potential after an action potential?

For almost 1 second. Signup and view all the answers

What is most likely to occur in large nerve fibers when the threshold for stimulation is lowered?

They discharge repetitively. Signup and view all the answers

What happens to the membrane potential as potassium ions flow out?

The membrane potential becomes more negative. Signup and view all the answers

What kind of tissue is most affected by reduced stimulation thresholds due to potassium ion outflow?

Excitable tissues, including nerve and muscle fibers. Signup and view all the answers

What mechanism allows for rhythmic discharges in excitable tissues like the heart?

Sustained outflow of potassium ions following action potentials. Signup and view all the answers

How does the action potential affect the conductance of potassium ions?

It increases potassium conductance immediately. Signup and view all the answers

What physiological change occurs as a result of the internal negativity in excitable cells due to potassium outflow?

Greater susceptibility to subsequent action potentials. Signup and view all the answers

What role does the rhythmic control center of the heart play in action potentials?

It regulates the frequency of rhythmic action potentials. Signup and view all the answers

What phenomenon does the Nernst equation primarily describe?

Diffusion potential generation in membranes Signup and view all the answers

When using the Nernst equation, how is the electromotive force (EMF) related to the concentration of ions?

EMF is proportional to the logarithm of the concentration ratio Signup and view all the answers

In the context of the Nernst equation, what does the variable 'z' represent?

The charge of the ion Signup and view all the answers

What is the expected sign of the Nernst potential when a negatively charged ion diffuses from inside to outside?

Positive (+) Signup and view all the answers

Under normal physiological conditions, what is the average potential difference for a mammalian nerve fiber inside the membrane?

94 millivolts Signup and view all the answers

How does the permeability of the membrane affect ion diffusion according to the Nernst equation?

Increased permeability enhances ion diffusion rates Signup and view all the answers

What does the concentration gradient of potassium ions typically suggest about their diffusion?

High internal concentration favors outward movement Signup and view all the answers

What would likely occur if the concentration of sodium ions outside the membrane exceeds that inside?

Net diffusion of sodium ions would occur into the nerve fiber Signup and view all the answers

What creates a membrane potential of opposite polarity?

Concentration difference of ions across a membrane Signup and view all the answers

What is the Nernst potential?

The potential across a membrane that exactly opposes the net diﬀusion of a particular ion. Signup and view all the answers

What is the electromotive force (EMF) used to calculate?

The Nernst potential for any univalent ion. Signup and view all the answers

Which ion is used in the Nernst equation?

Potassium the main ion. Signup and view all the answers

The resting potential for neurons is approximately _____ mV.

-60 to -70 Signup and view all the answers

What happens during the diffusion of sodium ions into a cell?

Negatively charged anions remain inside Signup and view all the answers

The Goldman equation considers only one type of ion.

False Signup and view all the answers

What causes an electrochemical driving force (Vdf) for ions?

The difference between the membrane potential (Vm) and the equilibrium potential (Veq) of the ion. Signup and view all the answers

Match the ion to its role in membrane potential:

Sodium (Na+) = Contributes to depolarization Potassium (K+) = Contributes to repolarization Chloride (Cl-) = Influences negativity inside the cell Signup and view all the answers

What is the resting membrane potential before the action potential begins?

-70 millivolts Signup and view all the answers

Which ions significantly determine the membrane potential during the resting stage?

Potassium ions (K+) Signup and view all the answers

During depolarization, the membrane potential overshoots beyond zero.

True Signup and view all the answers

What is the term used for the process of returning the membrane potential to its resting state?

Repolarization Signup and view all the answers

The voltage-gated sodium channel closes through a process called __________.

inactivation Signup and view all the answers

What determines the activation of voltage-gated potassium channels?

Membrane potential Signup and view all the answers

What is the typical state of the sodium channel when the membrane potential is -70 millivolts?

Closed Signup and view all the answers

Potassium channels open at the same rate as sodium channels during an action potential.

False Signup and view all the answers

Who were the scientists awarded the Nobel Prize for their research on sodium and potassium channels?

Hodgkin and Huxley Signup and view all the answers

Voltage-gated channels allow the flow of ions such as Na+ and K+ in response to __________ changes.

voltage Signup and view all the answers

What happens to the activity of the pump as the internal sodium concentration rises from 10 to 20 mEq/L?

The activity of the pump increases about eightfold. Signup and view all the answers

During depolarization, which ions diffuse to the inside of the membrane?

Sodium ions. Signup and view all the answers

Which type of action potential occurs in heart muscle fibers?

Plateau action potential. Signup and view all the answers

What causes the plateau in heart muscle action potentials?

Prolonged opening of L-type calcium channels Signup and view all the answers

The resting membrane potential in the rhythmical control center of the heart is only -60 to -70 millivolts.

True Signup and view all the answers

The small uninformed area between successive Schwann cells along the axon is called the ______.

node of Ranvier. Signup and view all the answers

What types of channels contribute to the depolarization process in heart muscle?

Fast sodium channels Signup and view all the answers

What is the average speed of conduction in small unmyelinated fibers?

0.25 m/sec. Signup and view all the answers

Saltatory conduction occurs in which type of nerve fibers?

Myelinated fibers Signup and view all the answers

What is the membrane potential when Vm = Veq?

no net movement of the ion into or out of the cell Signup and view all the answers

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

-70 millivolts Signup and view all the answers

Which ions are primarily involved in establishing the resting membrane potential?

sodium (Na+) and potassium (K+) Signup and view all the answers

The Na+-K+ pump transports more sodium ions into the cell than potassium ions.

False Signup and view all the answers

What occurs when the membrane potential is at -70 millivolts?

the potential is steady but can reverse to zero when passing through the membrane. Signup and view all the answers

The potential inside the nerve fiber is ______ millivolts more negative than the potential in the extracellular fluid.

70 Signup and view all the answers

What is the Nernst potential for potassium considering its intracellular/extracellular ratio?

-94 millivolts Signup and view all the answers

What is the function of the Na+-K+ pump?

to transport sodium ions out and potassium ions into the cell. Signup and view all the answers

The membrane potential can reverse from -70 millivolts to as much as +35 millivolts rapidly.

True Signup and view all the answers

Which of the following describes the state of the ion channels related to potassium ions?

They allow potassium ions to leak out. Signup and view all the answers

What is the ratio of potassium conductance to sodium conductance during the resting state?

50:100 Signup and view all the answers

Sodium channels close slowly during the action potential.

False Signup and view all the answers

What causes a sudden rise in membrane potential leading to an action potential?

Increased influx of sodium ions Signup and view all the answers

Calcium ions create a diﬀusion gradient that is more than _____ times greater in the extracellular fluid than in the intracellular fluid.

10,000 Signup and view all the answers

What initiates the action potential?

Positive-feedback cycle opening sodium channels Signup and view all the answers

What must occur for an action potential to be generated?

The threshold potential must be reached. Signup and view all the answers

What is responsible for the negative charge inside a nerve fiber?

Impermeant negatively charged ions Signup and view all the answers

Calcium channels open quickly compared to sodium channels.

False Signup and view all the answers

The movement of sodium ions allows for the propagation of a _____ along the nerve fiber.

nerve impulse Signup and view all the answers

What is the safety factor for propagation?

The ratio of action potential to threshold for excitation must be greater than 1. Signup and view all the answers

Match the following ions to their roles during an action potential:

Sodium = Rapid influx causes depolarization Potassium = Rapid efflux causes repolarization Calcium = Sustained depolarization in some cells Impermeant Anions = Contribute to negative charge inside the fiber Signup and view all the answers

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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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.