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Questions and Answers
What happens to the frequency of action potentials (APs) when the intensity of a stimulus increases?
What happens to the frequency of action potentials (APs) when the intensity of a stimulus increases?
Which characteristic is true regarding the size of action potentials (APs)?
Which characteristic is true regarding the size of action potentials (APs)?
What triggers the initiation of action potentials at the axon hillock?
What triggers the initiation of action potentials at the axon hillock?
How does myelination affect the conduction velocity of action potentials?
How does myelination affect the conduction velocity of action potentials?
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Which statement about fiber diameter and conduction speed is correct?
Which statement about fiber diameter and conduction speed is correct?
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What is the primary reason for the resting membrane potential (RMP) in cells?
What is the primary reason for the resting membrane potential (RMP) in cells?
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Which of the following statements accurately describes the Na⁺-K⁺ pump?
Which of the following statements accurately describes the Na⁺-K⁺ pump?
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What is the typical range of resting membrane potential (RMP) in most cells?
What is the typical range of resting membrane potential (RMP) in most cells?
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Which ion's movement primarily contributes to the negative charge inside a resting cell?
Which ion's movement primarily contributes to the negative charge inside a resting cell?
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What two gradients play a role in determining the equilibrium potential for an ion?
What two gradients play a role in determining the equilibrium potential for an ion?
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What is the primary reason for the slower conduction speed in contiguous conduction compared to saltatory conduction?
What is the primary reason for the slower conduction speed in contiguous conduction compared to saltatory conduction?
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How does myelin enhance the propagation of action potentials?
How does myelin enhance the propagation of action potentials?
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Which of the following statements about action potentials (APs) is true?
Which of the following statements about action potentials (APs) is true?
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What is the implication of demyelination in multiple sclerosis?
What is the implication of demyelination in multiple sclerosis?
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Which physiological change occurs due to higher temperatures affecting conduction velocity?
Which physiological change occurs due to higher temperatures affecting conduction velocity?
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What triggers a change in membrane potential in excitable tissues?
What triggers a change in membrane potential in excitable tissues?
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Which of the following best describes a graded potential?
Which of the following best describes a graded potential?
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Which type of ion channel is primarily involved in initiating graded potentials?
Which type of ion channel is primarily involved in initiating graded potentials?
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What occurs when there is a net inward flow of positively charged ions?
What occurs when there is a net inward flow of positively charged ions?
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What primarily generates the electrical gradient across the plasma membrane?
What primarily generates the electrical gradient across the plasma membrane?
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Which of the following examples is NOT a type of graded potential?
Which of the following examples is NOT a type of graded potential?
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How is the magnitude of the membrane potential determined?
How is the magnitude of the membrane potential determined?
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What effect does a longer duration of triggering events have on graded potentials?
What effect does a longer duration of triggering events have on graded potentials?
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What does a resting membrane potential represent?
What does a resting membrane potential represent?
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Which of the following best describes the role of charges in generating membrane potential?
Which of the following best describes the role of charges in generating membrane potential?
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Which statement correctly describes action potentials?
Which statement correctly describes action potentials?
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What is the GHK equation primarily used to measure?
What is the GHK equation primarily used to measure?
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In which units is membrane potential measured?
In which units is membrane potential measured?
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What happens when positive charges accumulate on one side of the membrane?
What happens when positive charges accumulate on one side of the membrane?
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Which statement is true regarding the separation of charges across the membrane?
Which statement is true regarding the separation of charges across the membrane?
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What is required to separate opposite charges and create potential energy?
What is required to separate opposite charges and create potential energy?
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What is the primary result of local currents during a graded potential?
What is the primary result of local currents during a graded potential?
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How does the magnitude of a graded potential change as it moves away from the initial active area?
How does the magnitude of a graded potential change as it moves away from the initial active area?
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Which of the following accurately describes an action potential?
Which of the following accurately describes an action potential?
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What is the typical threshold potential for triggering an action potential?
What is the typical threshold potential for triggering an action potential?
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What happens during the repolarisation phase of an action potential?
What happens during the repolarisation phase of an action potential?
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What initiates the explosive depolarisation phase of an action potential?
What initiates the explosive depolarisation phase of an action potential?
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What is the duration of an action potential in excitable cells?
What is the duration of an action potential in excitable cells?
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During the peak of an action potential, which of the following actions occur?
During the peak of an action potential, which of the following actions occur?
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Study Notes
Membrane and Action Potential
- Membrane potential defines the difference in charge across the plasma membrane
- It's measured in millivolts (mV)
- The difference arises from the unequal distribution of ions (cations & anions) inside and outside the cell.
- Opposite charges attract, while similar charges repel
- Work is needed to separate opposite charges to create potential energy.
- The magnitude of the membrane potential depends on the number of charges separated.
Learning Objectives
- Understand the generation of resting membrane potential.
- Explain the equilibrium potentials of important ions.
- Define local responses and firing thresholds.
- Describe action potential formation, characteristics, and factors impacting it.
- Explain the conduction of impulses along an axon.
- Define a compound action potential.
Membrane Potential
- Separation of opposite charges across the plasma membrane
- Represents the difference in the relative number of cations (+) and anions (-) in the intracellular fluid (ICF) and extracellular fluid (ECF).
- Basic principle: opposite charges attract, and similar charges repel.
- Work is required to separate opposite charges which generates potential energy.
- Measured in millivolts (mV).
- Only charges along the inner and outer surfaces of the membrane contribute to the potential.
- Movement of positive charges to one side creates an excess of positive charge on one side and negative charge on the other side, generating an electrical gradient (membrane potential).
- Magnitude depends on the number of charges separated.
Resting Membrane Potential (RMP)
- Constant membrane potential in non-excitable and excitable cells at rest.
- Caused by the unequal distribution of ions across the plasma membrane and their selective permeability.
- Primary ions involved: Na+, K+, and anionic proteins (A⁻).
- Na+ concentration is higher in the extracellular fluid (ECF), K+ is higher in the intracellular fluid (ICF), and A⁻ is mostly inside the cell.
- The membrane is more permeable to K+ than Na+.
- Mechanisms:
- Na⁺-K⁺ pump actively transports 3 Na⁺ out and 2 K⁺ in, creating and maintaining concentration gradients.
- Passive ion movement: K⁺ leaks out of the cell more readily than Na⁺ enters.
- Net effect: inside the cell becomes slightly more negative than the outside.
- RMP typically ranges between -70 mV to -90 mV (depending on the cell type).
Equilibrium Potential
- Membrane potential where the electrical gradient exactly counterbalances the concentration gradient for an ion, resulting in no net ion movement.
- Factors affecting:
- Ion concentration gradient (drives ion movement)
- Electrical gradient (opposes ion movement)
- Calculated using the Nernst equation.
Equilibrium Potential for K+
- Concentration gradient: high K⁺ inside the cell, low K⁺ outside. K⁺ moves outward.
- Electrical gradient: K⁺ efflux leaves negative charge inside the cell, and inside becomes increasingly negative. An opposing electrical gradient develops, pulling K⁺ back into the cell.
- Equilibrium point: no net movement of K⁺ occurs when concentration and electrical gradients are balanced.
- EK+ = -90 mV (inside negative relative to outside).
Equilibrium Potential for Na+
- Concentration gradient: high Na⁺ outside the cell, low Na⁺ inside. Na⁺ moves inward.
- Electrical gradient: Na⁺ influx creates positive charge inside the cell. The outside becomes more negative due to unbalanced negative ions (mostly Cl⁻).
- Equilibrium point: no net movement of Na⁺ occurs when concentration and electrical gradients are balanced.
- ENa+ = +60 mV (inside positive relative to outside).
Generation of RMP
- The Na⁺–K⁺ pump actively transports Na⁺ out of and K⁺ into the cell, maintaining high extracellular Na⁺ and intracellular K⁺ concentrations.
- The resting membrane is far more permeable to K⁺ than Na⁺.
- K⁺ diffuses out of the cell down its concentration gradient, creating a negative intracellular potential.
- The small inward movement of Na⁺ opposes the outward movement of K⁺, creating resting potential around -70 mV.
Nerst Equation
- Calculates the equilibrium potential for a specific ion across a membrane based on ion concentration gradient and charge.
- Determines the membrane voltage needed to counteract ion diffusion.
- Shows the relationship between electrical and concentration gradients.
- Formula: Ex = -61 x log ([Xin]/[Xout]).
Graded Potentials
- Local changes in membrane potential, varying in magnitude or strength.
- Triggered by gated ion channels opening in a specific region of the membrane (often in response to a stimulus).
- Usually caused by Na⁺ entry, leading to depolarization.
- Magnitude diminishes as it spreads from the origin.
Action Potentials (APs)
- Brief rapid large changes in membrane potential (~100 mV)
- Temporarily reverses the membrane potential, making the inside more positive than the outside.
- Propagated non-decrementally, maintaining full strength.
- Triggered when threshold potential (-50 to -55 mV) is reached.
- Duration is consistent for a given excitable cell (~1 ms).
- Phases: resting, depolarization, repolarization, and hyperpolarization.
- Generation: -Resting potential: all voltage-gated channels (Na+ and K+) are closed. -Threshold: Na+ activation gate opens and PNa+ increases. -depolarization: Na+ enters the cell, causing explosive depolarization to +30 mV -peak: Na+ inactivation gate closes, and PNa+ falls, Na+ stops moving in. At the same time, K+ activation gate opens and PK+ rises. -repolarization: K+ leaves the cell, causing repolarization to resting potential. -hyperpolarization: K+ channels remain open after the potential reaches resting level.
Propagation of Action Potentials
- Propagated from the axon hillock to the axon terminals as non-decremental signals.
- Local current flow between active and adjacent inactive areas depolarizes inactive areas to threshold, initiating APs sequentially.
Factors affecting conduction velocity of APs
- Myelination
- Fiber diameter
- Temperature
Contiguous Conduction (unmyelinated axons)
- APs propagated by sequential depolarization of each patch of membrane along the axon.
- Na⁺ influx depolarizes the adjacent region by triggering a new AP.
- Slower conduction speed due to continuous regeneration of APs in each segment.
- Amplitude remains consistent regardless of distance.
Saltatory Conduction (myelinated axons)
- Myelin, a lipid insulator, prevents ion flow across myelinated regions to conserve current flow. APs are generated only at the nodes of Ranvier, causing current to “jump” from one node to the next.
- Faster propagation compared to contiguous conduction.
- Metabolically more efficient.
Multiple Sclerosis
- Chronic autoimmune disease of the CNS (brain and spinal cord)
- Characterized by immune-mediated damage to the myelin sheath (demyelination).
All-or-none Law
- APs follow the all-or-none law: an excitable membrane either completely responds to a threshold stimulus with a maximal AP or does not respond at all.
- The magnitude of an AP is independent of the triggering event's strength.
How Stimulus Intensity is Coded
- Increasing stimulus intensity leads to more APs being fired and an increase in the frequency of firing, not in the size of the AP.
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Description
Test your understanding of membrane potentials and action potentials in neurons. This quiz covers key concepts such as resting membrane potential, equilibrium potentials, and factors affecting action potential formation. Gain insights into the mechanisms that drive nerve impulses and their conduction along axons.
