Bio 12.2 Neuroscience: Action Potentials
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Questions and Answers

What initiates an action potential in a neuron?

  • Increased calcium ion concentration
  • Membrane potential becoming more positive than -55 mV (correct)
  • Hyperpolarization of the membrane
  • Increased potassium ion concentration

    • Which phase describes the period when the membrane potential becomes more positive during an action potential?

  • Repolarization
  • Resting phase
  • Depolarization (correct)
  • Hyperpolarization

    • During an action potential, what is the role of the trigger zone?

  • To assist in enzymatic reactions
  • To inhibit action potentials from occurring
  • To generate a regenerative wave of depolarization (correct)
  • To maintain resting membrane potential

    • How do action potentials travel along the axon?

    <p>Via regeneration of depolarization waves at multiple locations</p> Signup and view all the answers

    What does the term 'action potential' encompass?

    <p>Both the electrical impulse and local changes in membrane potential</p> Signup and view all the answers

    What is the primary change that occurs during the repolarization phase of an action potential?

    <p>Restoration of the resting membrane potential</p> Signup and view all the answers

    What can be inferred about the action potential's regenerative nature?

    <p>It consists of isolated, sequential local membrane events</p> Signup and view all the answers

    What does the analogy of a wave in a stadium represent in relation to an action potential?

    <p>It depicts the sequential and localized nature of membrane potential changes</p> Signup and view all the answers

    What primarily distinguishes electrical synapses from chemical synapses?

    <p>Electrical impulses are transmitted via gap junctions.</p> Signup and view all the answers

    What is the role of neurotransmitters at chemical synapses?

    <p>They bind to receptors and influence membrane potential.</p> Signup and view all the answers

    Which ion's entry is typically associated with excitatory neurotransmitters?

    <p>Ca2+</p> Signup and view all the answers

    How can the concentration of neurotransmitters in the synaptic cleft decrease?

    <p>Via diffusion or reuptake into the presynaptic neuron.</p> Signup and view all the answers

    What is a key characteristic of excitatory neurotransmitters?

    <p>They cause an influx of positive ions, depolarizing the membrane.</p> Signup and view all the answers

    What is the synaptic cleft?

    <p>The gap between the presynaptic axon terminal and postsynaptic cell.</p> Signup and view all the answers

    What consequence can occur if neurotransmitter levels are not adequately reduced?

    <p>Continuous stimulation of the postsynaptic neuron.</p> Signup and view all the answers

    What are neurotransmitters classified based on?

    <p>Whether they are excitatory or inhibitory.</p> Signup and view all the answers

    What is the peak membrane potential during the action potential?

    <p>+40 mV</p> Signup and view all the answers

    What occurs during the falling phase of the action potential?

    <p>Na+ channels are inactivated, and K+ channels are opened</p> Signup and view all the answers

    What is referred to as the 'undershoot' during an action potential?

    <p>When membrane potential falls below the resting membrane potential</p> Signup and view all the answers

    During the restoring phase, which gates return to their closed and open positions?

    <p>Na+ activation gates close and Na+ inactivation gates open</p> Signup and view all the answers

    What role do Na+/K+ pumps play during the action potential's restoring phase?

    <p>They help to maintain the resting membrane potential</p> Signup and view all the answers

    What is the approximate membrane potential threshold for initiating an action potential in a postsynaptic neuron?

    <p>−55 mV</p> Signup and view all the answers

    What causes the rapid rise in membrane potential during the action potential?

    <p>Influx of Na+ ions</p> Signup and view all the answers

    Which neurotransmitter is considered the primary excitatory neurotransmitter in the central nervous system?

    <p>Glutamate (Glu)</p> Signup and view all the answers

    Which statement correctly describes the condition of voltage-gated K+ channels during the peak of the action potential?

    <p>K+ channels are beginning to open</p> Signup and view all the answers

    How do inhibitory neurotransmitters primarily affect the postsynaptic neuron?

    <p>They cause hyperpolarization of the membrane potential.</p> Signup and view all the answers

    What is the primary consequence of the closing of Na+ channel inactivation gates?

    <p>Termination of inward flow of Na+</p> Signup and view all the answers

    Which neurotransmitter is primarily involved in parasympathetic signaling in the autonomic nervous system?

    <p>Acetylcholine (ACh)</p> Signup and view all the answers

    What effect do inhibitory neurotransmitters like GABA and glycine have on the postsynaptic membrane potential?

    <p>They hyperpolarize the membrane.</p> Signup and view all the answers

    Which of the following functions is primarily associated with serotonin (5-HT)?

    <p>Involvement in sleep, appetite, and mood</p> Signup and view all the answers

    What happens to the action potential once it is triggered in a neuron?

    <p>It remains constant in strength as it travels.</p> Signup and view all the answers

    Which neurotransmitter is involved in both cognition and reward mechanisms in the brain?

    <p>Dopamine (DA)</p> Signup and view all the answers

    What must happen for an action potential to be triggered in a neuron?

    <p>The soma must be depolarized past the threshold potential.</p> Signup and view all the answers

    Which type of potential responds incrementally to stimulus strength in dendrites and cell bodies?

    <p>Graded potential</p> Signup and view all the answers

    What happens to graded potentials as they travel from a synapse through the cell body?

    <p>They dissipate.</p> Signup and view all the answers

    What do we call the integration of multiple inputs from presynaptic neurons?

    <p>Summation</p> Signup and view all the answers

    What is the difference between spatial summation and temporal summation?

    <p>Spatial summation integrates signals from multiple presynaptic neurons; temporal summation integrates signals from one presynaptic neuron over time.</p> Signup and view all the answers

    What are neurons called if they can release more than one type of neurotransmitter?

    <p>Multi-transmitter neurons</p> Signup and view all the answers

    What characterizes the effect of an excitatory synapse?

    <p>It depolarizes the postsynaptic neuron.</p> Signup and view all the answers

    Which phenomenon refers to the release of more than one neurotransmitter from distinct vesicles?

    <p>Co-transmission</p> Signup and view all the answers

    What is the term for the membrane potential at which an ion's concentration and electrical gradients cancel each other?

    <p>Equilibrium potential</p> Signup and view all the answers

    Which factors lead to the establishment of the resting membrane potential (RMP) in a cell?

    <p>Ion concentration gradients and selective ion permeability</p> Signup and view all the answers

    Which ion predominantly influences the resting membrane potential, making it closer to its equilibrium potential than that of other ions?

    <p>K+</p> Signup and view all the answers

    What is the combined influence of electrical and concentration gradients on ion movement called?

    <p>Electrochemical gradient</p> Signup and view all the answers

    How does the Na+/K+ ATPase contribute to the maintenance of resting membrane potential?

    <p>By pumping K+ ions into the cell and Na+ ions out against their gradients</p> Signup and view all the answers

    What characterizes the rising phase of an action potential?

    <p>Opening of Na+ channel activation gates</p> Signup and view all the answers

    Which of the following describes the role of the inactivation gate in the voltage-gated Na+ channel?

    <p>It closes in response to depolarization</p> Signup and view all the answers

    What is the all-or-none principle in relation to action potentials?

    <p>An action potential either occurs completely or not at all</p> Signup and view all the answers

    What happens during the falling phase of an action potential?

    <p>Na+ permeability decreases and K+ permeability increases</p> Signup and view all the answers

    Which statement accurately describes the threshold potential?

    <p>It is around -55 mV and initiates action potentials</p> Signup and view all the answers

    What primarily contributes to the resting membrane potential in neurons?

    <p>Selective permeability to potassium ions</p> Signup and view all the answers

    How does the movement of ions across the membrane contribute to generating an action potential?

    <p>Changes in ion concentrations lead to depolarization and repolarization phases.</p> Signup and view all the answers

    What is the significance of the membrane potential measured in neurons?

    <p>It reflects the potential energy available for cellular processes.</p> Signup and view all the answers

    What occurs to the charge inside a neuron during the resting membrane potential?

    <p>The intracellular fluid is typically more negative than the extracellular fluid.</p> Signup and view all the answers

    Which statement best describes the role of membrane transport proteins in neurons?

    <p>They actively transport ions against their concentration gradients.</p> Signup and view all the answers

    Study Notes

    Action Potentials

    • Action potentials are brief, regenerative electric impulses that propagate away from their origin within excitable cells like neurons and muscle fibers.
    • They initiate in the trigger zone, encompassing the axon hillock and initial axon segment, when the membrane potential surpasses a threshold of approximately -55 mV.
    • Action potentials consist of depolarization (membrane potential becoming more positive) followed by repolarization (membrane potential returning to its baseline).

    Action Potential Phases

    • Rising Phase: Sodium (Na+) influx into the cell due to opening of voltage-gated Na+ channels causes the membrane potential to become more positive. This phase can reach a peak membrane potential of around +40 mV, referred to as overshoot.
    • Falling Phase: Inactivation gates of voltage-gated Na+ channels close, halting Na+ influx. Voltage-gated Potassium (K+) channels open, driving K+ out and repolarizing the membrane. This phase can exceed the resting potential, resulting in undershoot.
    • Restoring Phase: Voltage-gated Na+ and K+ channels reset to their resting states. Na+ channel activation gates close and inactivation gates open, while K+ channels close. Na+ and K+ leak channels, alongside the Na+/K+ pump, contribute to restoring the resting membrane potential (approximately -70 mV).

    Synaptic Transmission

    • Electrical Synapses: Electrical impulses transmit directly between neurons or between neurons and other cell types via gap junctions.
    • Chemical Synapses: Neurotransmitters, signaling molecules released from the presynaptic neuron, are released into the synaptic cleft and bind to postsynaptic receptors, influencing the postsynaptic membrane potential.

    Neurotransmitters

    • Excitatory Neurotransmitters: Depolarize the postsynaptic membrane, often by allowing positive ions like Ca2+ to enter the cell. They can trigger action potentials if they exceed the postsynaptic neuron's threshold potential.
    • Inhibitory Neurotransmitters: Hyperpolarize the postsynaptic membrane, either through influx of negative ions (e.g., Cl−) or efflux of positive ions (e.g., K+). They inhibit action potential initiation.
    • Examples of Neurotransmitters:
      • Glutamate (Glu): Primary excitatory neurotransmitter in the central nervous system.
      • Gamma-aminobutyric acid (GABA): Primary inhibitory neurotransmitter in the brain.
      • Glycine (Gly): Primary inhibitory neurotransmitter in the spinal cord.

    Summation of Postsynaptic Potentials

    • Graded Potential: Postsynaptic neuron membrane potential changes incrementally, dependent on stimulus strength from the presynaptic cell.
    • Excitatory Graded Potential: Depolarizes the postsynaptic neuron, bringing it closer to the threshold potential.
    • Inhibitory Graded Potential: Hyperpolarizes the postsynaptic neuron, moving it further away from the threshold potential.
    • Summation: Integrates multiple inputs from one or more presynaptic neurons.
      • Spatial Summation: Integrates multiple input signals from multiple presynaptic neurons.
      • Temporal Summation: Integrates multiple input signals from a single neuron over time.

    Multi-transmitter Neurons

    • Multi-transmitter Neurons: Can release more than one type of neurotransmitter.
      • Co-Transmission: Release different neurotransmitters from distinct vesicles, either from the same axon terminal or different branches.
      • Co-Release: Release two or more neurotransmitters from the same vesicle.
    • Summation of all neurotransmitter effects on the postsynaptic neuron determines its overall response – either excitatory (depolarizing) or inhibitory (hyperpolarizing), influencing whether an action potential is triggered in the postsynaptic neuron.

    Ion Movement Across Membranes

    • Equilibrium potential: The electrical gradient that exactly opposes a concentration gradient, preventing net movement of an ion across a membrane.
    • Electrochemical gradient: The combined influence of electrical and concentration gradients on ion movement across a membrane.
    • Ions diffuse across membranes down their electrochemical gradient.
    • Resting membrane potential (RMP): The electrical gradient across a cell's membrane under resting conditions.
    • The RMP is primarily determined by the movement of Na+ and K+ ions.
    • The plasma membrane is more permeable to K+ than Na+, which influences the RMP.

    Action Potential

    • Voltage-gated channels: Open or close in response to changes in membrane potential.
    • Activation gate: Opens to allow ion flow through voltage-gated channels in response to depolarization.
    • Inactivation gate: Closes in response to depolarization, specific to voltage-gated Na+ channels.
    • An action potential has three phases:
      • Rising phase: Membrane potential becomes more positive due to increased Na+ permeability.
      • Falling phase: Membrane potential becomes more negative, eventually becoming hyperpolarized.
      • Restoring phase: Membrane potential returns to the resting level from a hyperpolarized state.
    • Threshold potential: Around −55 mV; depolarization to this potential initiates a positive feedback cycle of depolarization-induced Na+ channel opening.
    • Action potentials are "all-or-none" events.

    Synapses

    • Synapse: The junction between two neurons.
    • Presynaptic neuron: The neuron sending the signal.
    • Postsynaptic neuron: The neuron receiving the signal.
    • Electrical synapse: Electrical impulses are transmitted directly via gap junctions.
    • Chemical synapse: Neurotransmitters are released from the presynaptic neuron into the synaptic cleft.

    Neurotransmitters

    • Neurotransmitter: A signaling molecule released from presynaptic neurons at chemical synapses that can elicit various responses in postsynaptic cells.
    • Excitatory neurotransmitters: Depolarize the postsynaptic membrane, often by allowing positive ions like Ca2+ to enter.
    • Inhibitory neurotransmitters: Hyperpolarize the postsynaptic membrane, often by allowing negative ions (like Cl-) in, or positive ions (like K+) out.

    Summation of Postsynaptic Potentials

    • Graded potential: A proportional response to the strength of a stimulus from the presynaptic cell.
    • Excitatory graded potential: Depolarizes the postsynaptic neuron.
    • Inhibitory graded potential: Hyperpolarizes the postsynaptic neuron.
    • Summation: The integration of multiple inputs from one or more presynaptic neurons.
      • Spatial summation: Integrated effect of multiple inputs from multiple presynaptic neurons.
      • Temporal summation: Integrated effect of multiple inputs from a single neuron.
    • Multi-transmitter neuron: Can release more than one type of neurotransmitter.
    • Co-transmission: Different neurotransmitters released from distinct vesicles.
    • Co-release: Two or more neurotransmitters released from the same vesicle.
    • The overall effect of neurotransmitters at a synapse determines whether it is excitatory or inhibitory.
    • Summation determines whether an action potential is triggered.

    Neural Communication

    • Neurons communicate with other cells using action potentials, electrical impulses that temporarily reverse the charge gradient at their axon terminals.
    • Action potentials are generated and elicit responses in target cells.

    Resting Membrane Potential

    • The resting membrane potential (RMP) is the electrical gradient across a cell's membrane under unstimulated conditions.
    • It is typically around -70 mV, with the intracellular fluid being more negative than the extracellular fluid.
    • The RMP is influenced by the electrochemical gradients of multiple ions, with ions with greater permeability exerting a greater influence.
    • Na+ and K+ play a major role in establishing the RMP.
      • Leak channels allow a small, continual stream of Na+ and K+ across the plasma membrane.
      • Na+/K+ ATPase pumps transport the ions back across the plasma membrane against their concentration gradients.
    • The plasma membrane is approximately 40 times more permeable to K+ than Na+, making the RMP closer to K+'s equilibrium potential (-90 mV) than Na+ (+60 mV).

    Action Potential

    • An action potential is a rapid depolarization and repolarization of the membrane potential that travels down an axon.
    • It is initiated when the membrane potential reaches the threshold potential, typically around -55 mV.
    • Voltage-gated ion channels are crucial for the action potential.
      • Sodium (Na+) channels open when the membrane potential depolarizes, causing Na+ to rush into the cell and further depolarize the membrane.
      • Potassium (K+) channels open when the membrane potential is positive, allowing K+ to flow out of the cell and repolarize the membrane.
    • Refractory periods exist to ensure unidirectional flow of the action potential:
      • Absolute refractory period: No stimulus can elicit another action potential, as Na+ channels are in an inactive state.
      • Relative refractory period: A stronger than normal stimulus is needed to evoke another action potential, as Na+ channels are resetting and K+ channels are still open.

    Synaptic Transmission

    • Synapses are junctions between a neuron and another cell, where communication occurs.
    • The presynaptic neuron transmits the nerve impulse, while the postsynaptic neuron receives it.
    • Communication can be electrical or chemical.
      • Electrical synapses transmit impulses directly via gap junctions.
      • Chemical synapses release neurotransmitters from the presynaptic neuron into the synaptic cleft.
    • Neurotransmitters bind to postsynaptic membrane receptors, which often causes postsynaptic ligand-gated ion channels to open, resulting in ion movement into or out of the postsynaptic cell.
    • Neurotransmitter levels can be reduced via diffusion, reuptake into the presynaptic neuron, or enzymatic destruction.

    Neurotransmitters

    • Neurotransmitters are signaling molecules responsible for chemical synapse communication.
    • They can elicit a variety of responses by binding to postsynaptic receptors and influencing the membrane potential.
    • Excitatory neurotransmitters cause depolarization of the postsynaptic membrane, making an action potential more likely.
    • Inhibitory neurotransmitters cause hyperpolarization of the postsynaptic membrane, making an action potential less likely.

    Summation of Postsynaptic Potentials

    • The combined effect of multiple synaptic inputs on a postsynaptic neuron is called summation.
    • Spatial summation: Multiple presynaptic neurons release neurotransmitters simultaneously, creating a larger postsynaptic potential.
    • Temporal summation: A single presynaptic neuron releases neurotransmitters rapidly, causing the postsynaptic potential to build up over time.
    • Summation determines whether or not an action potential is generated in the postsynaptic neuron.

    Common Neurotransmitters

    • Glutamate (Glu): Primary excitatory neurotransmitter in the central nervous system, involved in learning and memory.
    • Gamma-aminobutyric acid (GABA): Primary inhibitory neurotransmitter in the brain.
    • Glycine (Gly): Primary inhibitory neurotransmitter in the spinal cord.
    • Dopamine (DA): Involved in cognition, attention, movement, and reward.
    • Serotonin (5-HT): Involved in sleep, appetite, and mood.
    • Epinephrine and Norepinephrine (NE): Involved in sympathetic signaling in the autonomic nervous system.
    • Acetylcholine (ACh): Involved in parasympathetic signaling in the autonomic nervous system and released by motor neurons at neuromuscular junctions to excite skeletal muscle.
    • Endorphins: Opiates produced by the body that modulate pain and contribute to elevated mood following exercise.

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    Test your understanding of action potentials in neurons. This quiz covers the phases of action potentials, including the rising and falling phases, and the role of ion channels. Explore how these electric impulses are initiated and propagated.

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