Neuroscience Chapter on Action Potentials

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Questions and Answers

What distinguishes an action potential from a graded potential?

Action potentials can only occur in neurons and not in muscle cells.

Action potentials decrease in amplitude over distance, while graded potentials do not.

Action potentials are always triggered by external stimuli.

Action potentials reverse the membrane potential, whereas graded potentials do not. (correct)

Which statement about action potentials is true?

Their amplitude is typically less than 50 mV.

They can occur in muscle, neural, and some endocrine cells. (correct)

They are short-distance signals that are not regenerative.

They always decrease in amplitude as they propagate.

What is the minimum requirement for a graded potential to generate an action potential?

The graded potential must be of a decremental nature.

The graded potential must last longer than 1 second.

The graded potential must exist in isolation from other signals.

The graded potential must exceed a certain voltage threshold. (correct)

What is true about the propagation of action potentials?

Action potentials travel over long distances without losing amplitude. (D) Signup and view all the answers

In which types of cells can action potentials be generated?

In neural, muscle, and some immune cells. (A) Signup and view all the answers

What occurs during the absolute refractory period of an action potential?

Voltage-gated Na+ channels are open or inactivated. (A) Signup and view all the answers

Which of the following statements best explains the significance of the refractory period in neuronal signaling?

It limits the number of action potentials that can be generated. (C) Signup and view all the answers

What is the effect of a strong stimulus during the absolute refractory period?

It has no effect on action potential generation. (D) Signup and view all the answers

During which phase of the action potential are sodium channels inactivated?

During the absolute refractory period. (B) Signup and view all the answers

What primarily determines the frequency of action potentials in a neuron?

The length of the refractory period. (A) Signup and view all the answers

What is responsible for the existence of membrane potential?

Unequal distribution of positive and negative charges on both sides (A) Signup and view all the answers

Which situation has a higher magnitude of potential?

A situation with 16 net charges (C) Signup and view all the answers

How does the net difference in charges relate to the ability to perform work?

More charges lead to an increased ability to perform work (A) Signup and view all the answers

Which of the following statements is true about resting membrane potential?

It is constant for both excitable and non-excitable cells (B) Signup and view all the answers

What is indicated by a higher net charge across a membrane?

An increase in membrane polarization (C) Signup and view all the answers

What does the term 'net magnitude of potential' refer to?

The combined effect of all positive and negative charges (B) Signup and view all the answers

Which of the following cells is likely to have a resting membrane potential?

All of the above (D) Signup and view all the answers

What happens to the membrane potential if there is equal distribution of charges?

The potential remains unchanged at zero (B) Signup and view all the answers

Which factor does NOT contribute to membrane potential?

Equal ion distribution (A) Signup and view all the answers

What is primarily affected by the net difference of charges across the membrane?

Cell communication (D) Signup and view all the answers

What occurs during the relative refractory period?

A second action potential can occur with a stronger stimulus. (D) Signup and view all the answers

What prevents the backward propagation of action potentials?

The absolute refractory period. (C) Signup and view all the answers

What happens after an action potential is completed?

Resting membrane potential is restored through leak channels. (A) Signup and view all the answers

How quickly does the restoration of concentration gradients occur?

It occurs at the same speed as the depolarization period. (B) Signup and view all the answers

What is true about the absolute refractory period?

It prevents any response to stimuli, even if voltage changes. (A) Signup and view all the answers

Which phase describes the potential to generate an action potential with a stronger stimulus?

Relative refractory period. (C) Signup and view all the answers

What effect does hyperpolarization have on action potential generation?

It can allow for a stronger stimulus to generate a second action potential. (C) Signup and view all the answers

What cannot occur if the channel is closed during an action potential?

Generation of an action potential. (C) Signup and view all the answers

In what scenario can an action potential not be generated?

If the stimulus is not strong enough. (B) Signup and view all the answers

What is a characteristic feature of the leak channels after an action potential?

They help to restore the resting membrane potential. (B) Signup and view all the answers

What process is primarily responsible for maintaining concentration gradients of Na+ and K+ in a cell?

Primary active transport (D) Signup and view all the answers

How many Na+ ions are pumped out of the cell for every cycle of the ATPase pump?

3 Na+ ions (A) Signup and view all the answers

What is the net charge transfer when 2 K+ ions are pumped into the cell and 3 Na+ ions are pumped out?

1 positive charge out (C) Signup and view all the answers

What is the role of ATP in the process of ion pumping by the ATPase pump?

To provide energy for conformational change (D) Signup and view all the answers

Which ions are primarily involved in the concentration gradient maintained by the ATPase pump?

Na+ and K+ (D) Signup and view all the answers

What does the Na+/K+ ATPase pump primarily address?

Restoring the original concentrations of Na+ and K+ (B) Signup and view all the answers

What is the main role of the Sodium-Potassium pump in the context of ion balance?

It counterbalances the rate of passive leakage of ions. (D) Signup and view all the answers

Which ions does the Na+/K+ ATPase pump move in their respective directions?

3 Na+ out and 2 K+ in (D) Signup and view all the answers

What is the effect of the pump on the resting membrane potential (RMP)?

It stabilizes RMP by maintaining ion concentrations. (C) Signup and view all the answers

What is a characteristic of leak channels mentioned in the content?

They are constantly open. (C) Signup and view all the answers

Which statement best describes the consequence of a malfunctioning Na+/K+ ATPase pump?

It could cause a significant shift in ion concentrations, disrupting RMP. (A) Signup and view all the answers

What happens to the concentrations of Na+ and K+ when the pump operates?

Na+ is removed from the cell and K+ is brought in. (A) Signup and view all the answers

What role do passive leakage channels have in relation to the Sodium-Potassium pump?

They work to balance the effect of the pump. (D) Signup and view all the answers

How does the Na+/K+ ATPase pump contribute to maintaining cell potential?

By actively transporting Na+ and K+ against their gradients. (D) Signup and view all the answers

Which of the following statements accurately reflects the purpose of the Na+/K+ ATPase pump?

To maintain the necessary gradients of Na+ and K+ ions. (A) Signup and view all the answers

Study Notes

Module 3 - Neural Physiology

Module 3.1: Intercellular Communication Focus on Neural Communication
- Covers the principles of neural communication between cells.
- Discusses membrane potential, ion permeability, and concentration differences.
- Explains graded potentials, action potentials, signal propagation along nerve fibers, synapses, and neuronal integration.
- Key factors regarding membrane potential: separation of charge across a membrane; difference in the relative number/concentration of cations (+ ions) and anions (- ions) in the intracellular fluid (ICF) and extracellular fluid (ECF); difference in permeability of key ions.
Module 3.2: The Peripheral Nervous System Focus on the Somatic System
- Focuses on the somatic system in the Peripheral Nervous System.
- Textbook references: Human Physiology, Nelson 4th edition, Chapter 2, pages 54-83, Chapter 3, page 92, Chapter 4, pages 141-142 and 187-203 (Specific page numbers may vary depending on the editions.)
- Nelson 5th edition reference: Chapters 2-6, find the relevant sections to lecture content.

Module 3.1 - Learning Objectives

Principles of neural communication between cells, including the concept of membrane potential, with separation of charges, ion permeability, and ion concentrations.
Graded potentials, action potentials, signal propagation along nerve fibers, synapses and neuronal integration.

Module 3.1 - Membrane Resting Potential

The plasma membranes of all living cells are electrically polarized.
This separation of charge creates a membrane potential, which is the potential for ion movement across the membrane.
Factors affecting membrane potential: difference in permeability of key ions and differences in concentrations of cations and anions inside and outside of the cell.

Module 3.1 - Ion Movement

The movement of ions across a membrane is determined by concentration gradients (ions move from high to low concentration) and electrical gradients (opposites attract, similar charges repel).
The combined effects of these gradients are termed the electrochemical gradient.
Membrane permeability (restrictions to what passes across the membrane) determines the flow of ions across the cell membrane.

Module 3.1 - Example 1

Membrane (maximal permeability), equal charges on both sides of membrane, no potential.

Module 3.1 - Example 2

Membrane (maximal permeability), unequal charges on both sides of the membrane, a potential is generated.

Module 3.1 - Key Ions

Sodium (Na+), potassium (K+), and negatively charged intracellular proteins (A-) are key ions regulating membrane potential in nerve and muscle cells.

Module 3.1 - Ion Concentrations in Resting Nerve Cells

Important Considerations
- The concentrations of extracellular and intracellular fluids of these ions.
- Differences of the ions' relative permeability.
- Sodium (Na+), potassium (K+), and intracellular proteins (A-).

Module 3.1 - How do K+ and Na+ cross?

Ions like Na+ and K+ cannot cross the cell membrane, since they're water-soluble, therefore they must pass through protein channels.
These channels are always open and very specific.

Module 3.1 - Action Potentials

Very Different from Graded Potential
- The membrane potential reverses during an action potential.
- The inside becomes more positive than the outside, which is different from graded potential.
- Action potential signaling is not decremental: signal is propagated throughout the entire membrane.

Module 3.1 - Action Potential Timeline

Triggering Event: Initiates depolarization.
Depolarization: Membrane potential becomes less negative, approaching 0.
Threshold Potential: Reached when enough depolarization occurs to trigger an action potential.
Rapid Depolarization: Inside of the cell becomes more positive (+30mV at peak).
Repolarization: Inside of the cell returns to resting potential (less positive).
Hyperpolarization: Inside of the cell becomes more negatively charged than resting potential .

Module 3.1 - Action potential generation

Generating an action potential requires a sufficient magnitude of a graded potential that is large enough to reach the threshold potential.
The magnitude and duration of the graded potential depend on the strength and duration of the triggering event such as a stimulus

Module 3.1 - What if initial depolarizing is lower than threshold

Subthreshold: The positive feedback cycle does not start, so no action potential will occur.

Module 3.1 - What if initial depolarizing reaches the threshold

Action potential: A rapid change in the membrane potential that results in an action potential.

Module 3.1 - All or None Law

All action potentials have similar amplitude regardless of how long it took to reach the threshold.
Once the threshold is met, it fires completely.

Module 3.1 - How do we integrate the magnitude of stimulus

The strength of a stimulus is encoded by the frequency at which action potentials occur. A stronger stimulus produces action potentials at a higher frequency

Module 3.1 - Refractory Period and Action Potential

Absolute refractory period: There cannot be another action potential generated during this time. During this period, the Na+ channels are inactive.
Relative refractory period: There can be a second action potential generated, but only if the stimulus is larger than usual.

Module 3.1 - Restoration of Concentration Gradients

After an action potential is completed, the resting membrane potential is restored through leak channels.
However, the concentration of Na+ and K+ is altered.
Restoring the original concentration of the ions occurs through the Na+/K+ ATPase pump.

Module 3.1 - The Sodium-Potassium Pump

The pump counterbalances the rate of passive leakage of ions against the concentration gradient.
When 2K+ are pumped back into the cell, 3Na+ are pumped out.
This is a primary active transport process that requires ATP.
This active transport of ions results in the transfer of 1 positive charge out of the cell for each ATP molecule that's hydrolyzed.

Module 3.1 - Synaptic Integration

thousands of inputs contribute.

Module 3.1 - Two Key Events in Synaptic Integration

Temporal summation: Multiple EPSPs occurring close together in time sum together.
Spatial summation: Postsynaptic potentials occurring at different locations on the membrane sum together.
Spatiotemporal summation: A combination of temporal and spatial summation.

Module 3.1 - Importance of Postsynaptic Integration

The level of activity between presynaptic axons and post synaptic neurons can be influenced by other neurons and circuits.

Module 3.2 - The Central and Peripheral Nervous System

The central nervous system (CNS) consists of the brain and spinal cord.
The peripheral nervous system (PNS) connects the CNS to the rest of the body.

Module 3.2 - Nervous System Divisions

Afferent division: Carries information from sensory receptors to the CNS.
Efferent division: Carries information from the CNS to muscles/glands (effectors).
somatic nervous system (voluntary)
autonomic nervous system (involuntary; sympathetic; parasympathetic)

Module 3.2 - Autonomic Nervous System; Key Structure; Purpose

Ganglion: A cluster of neuronal cell bodies outside the CNS.
Purpose: Coordinates and relays signals from various neurons.

Module 3.2 - Pathways

Each autonomic neural pathway consists of a two-neuron chain.

Module 3.2 - Varicosity

In autonomic pathways, postganglionic autonomic fibers end in terminal branches with numerous swellings called varicosities.
These release neurotransmitters over large areas of target organs rather than single cells.

Module 3.2 - Sympathetic and Parasympathetic Nervous Systems

Sympathetic system (thoracic and lumbar regions): short preganglionic fibers; long postganglionic fibers.
Parasympathetic system (brain and sacral/lower spinal cord): long preganglionic fibers, short postganglionic fibers; ganglia in or near effector organs.

Module 3.2 - The Adrenal Medulla

Modified sympathetic ganglion; no axons.
Releases secretions (epinephrine and norepinephrine) into the blood.

Module 3.2 - Cholinergic Receptors

Found on postganglionic cell bodies in all autonomic ganglia.
Nicotinic receptors: ion channels that cause depolarization.
Muscarinic receptors: metabotropic receptors with diverse effects.

Module 3.2 - Adrenergic Receptors

Coupled to G-proteins, have second messengers such as cyclic AMP and Ca2+.
Beta receptors: usually have excitatory effects (e.g., heart stimulation, smooth muscle relaxation).
Alpha receptors: can either have excitatory or inhibitory effects.

Module 3.2 - Drugs as Agonists or Antagonists

Drugs can be agonists or antagonists to affect autonomic responses.
Agonists mimic neurotransmitters; antagonists block neurotransmitters.

Module 3.2 - Comparison between Sympathetic and Parasympathetic Systems

Similarities: Both use acetylcholine (ACh) at preganglionic synapses and have nicotinic receptors on the postganglionic cell bodies.
Differences: Sympathetic neurons are generally thoracic and lumbar; parasympathetic originate in brain and sacral.
Sympathetic has short preganglionic and long postganglionic fibers; parasympathetic has long preganglionic, short postganglionic
Sympathetic has ganglia closer to the spinal cord; parasympathetic has ganglia near or on the effector organs..
Neurotransmitters: Sympathetic postganglionic: norepinephrine or epinephrine; Parasympathetic postganglionic: acetylcholine

Module 3.2 - Somatic Nervous System

Consists of motor neurons that innervate skeletal muscles.
Signaling is continuous, one neuron from CNS directly to target organ; the neurotransmitter released is always acetylcholine (ACh).
Neuromuscular junction: The axon terminal is enlarged (terminal button) and contacts a specialized portion on skeletal muscle (Motor end plate.)

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Test your understanding of action potentials and graded potentials in this neuroscience quiz. Explore key concepts such as the refractory period, propagation, and the conditions required for generating action potentials in neurons.