Action Potentials

Lecture 6

Action Potentials

Course Information

  • Course: PSC 101 - Biological Psychology

  • Date: January 27, 2026

Learning Objectives

  • 6.1: Define what is an action potential (AP) and how it can convey information across a neuron.

  • 6.2: Predict if a cell will fire an AP based on how the membrane potential changes from resting membrane potential (Em). Explain how an Excitatory Post-Synaptic Potential (EPSP) or Inhibitory Post-Synaptic Potential (IPSP) influences the likelihood of firing an AP.

  • 6.3: Compare and contrast the ion channels needed to elicit an AP and identify their locations.

  • 6.4: Draw an AP waveform and identify key landmarks: resting membrane potential (Em), AP threshold, rising phase, falling phase, and refractory period. At each point of the waveform, identify which ion has the greatest movement across the membrane and which ion channels are open, closed, or inactivated.

  • 6.5: Describe how a refractory period can influence the AP firing rate of a neuron.

  • 6.6: Describe how an AP travels along an axon and how myelin influences AP propagation speed.

Action Potential (AP) Overview

  • Definition: An AP is a large change in membrane potential that travels down an axon.

  • Function: Conveys information from the cell body to the synaptic terminal.

  • When the large depolarization reaches the synaptic terminal, it triggers neurotransmitter release, facilitating communication between neurons.

AP Behavior

AP Signaling Characteristics

  • Binary Behavior:

    • APs function as "all-or-nothing" signals. A neuron fires an AP only if the membrane potential reaches or exceeds a certain threshold (AP threshold).

Amplitude

  • Consistency: The amplitude of an AP for a given neuron is consistent and does not vary.

    • Amplitude Measurement: Defined as the amount of voltage change.

  • Contrasts with Graded Potentials:

    • EPSPs and IPSPs can vary in amplitude, allowing different types of signals to be conveyed.

Firing Rate

  • Communication Mechanism: Neurons communicate by varying the number of APs fired, known as the firing rate, measured in APs per second.

  • Baseline Spontaneous Firing:

    • Most cells have a baseline of random spontaneous firing that does not convey information, but variations in firing can signal different information.

AP Threshold

  • Threshold Voltage: Firing of an AP occurs when the membrane potential reaches approximately -55 mV (15 mV depolarization from Em).

  • Firing Condition: Neurons will NOT fire an AP if the potential is more negative than -55 mV.

EPSP and IPSP Explained

  • EPSP (Excitatory Post-Synaptic Potential):

    • Increases the likelihood of firing an AP by bringing the membrane potential closer to AP threshold, typically caused by depolarization.

  • IPSP (Inhibitory Post-Synaptic Potential):

    • Decreases the likelihood of firing an AP by preventing the membrane potential from reaching the threshold, typically caused by hyperpolarization.

Membrane Potential Behavior

  • Illustrative graph displays membrane potential in mV over time in relation to EPSPs and the influence on AP firing.

Ion Channels in Action Potentials

Key Ion Channels

  • Voltage-Gated Na+ Channels

  • Voltage-Gated K+ Channels

Similarities and Differences

  • Similarities:

    • Both channels utilize chemical gradients for ion movement across the membrane.

    • Both channels are voltage-gated, opening during neuron depolarization.

    • Both channels are ion-specific and essential for AP generation.

  • Differences:

    • Na+ Channels: Open at -55 mV, fast to open, have an inactivation mechanism.

    • K+ Channels: Open at later stages, slower to open and close, do not have an inactivation mechanism.

Inactivation of Na+ Channels

  • Na+ channels can become inactivated after a brief period of activation.

  • Ball and Chain Model:

    • Unique to Na+ channels, a special segment on the intracellular side enters the channel pore and blocks it, preventing ion flow regardless of voltage.

  • The channel can return to the opened state once the blocking segment is removed following the refractory period.

Importance of Leak Channels and Transport Proteins

  • Additional channels and proteins operate in the background to maintain Em and ion concentration gradients.

Action Potential Morphology

Drawing Required Elements

  • Students should be able to identify and label a drawing of the AP waveform including:

    • Resting membrane potential (Em)

    • AP Threshold

    • Rising Phase

    • Falling Phase

    • Refractory Period

Self-Assessment Activity

  • Practice drawing AP waveforms based on provided graphs and feedback from peers.

Refractory Period Overview

  • Definition: Time frame post-AP firing during which another AP is unlikely or impossible.

  • Phases:

    • Absolute Refractory Period: Na+ channels are inactive, making it impossible to fire another AP.

    • Relative Refractory Period: Neurons can fire another AP, but it requires a stronger stimulus due to membrane hyperpolarization.

Mechanism of Refractory Phases

  • The absolute refractory phase occurs when Na+ channels are still inactivated.

  • In the relative refractory phase, a strong enough stimulus can lead to firing an AP, but it's less likely due to the hyperpolarized state of the cell.

Propagation of Action Potentials

Axonal Propagation

  • APs do not travel passively; they actively propagate down the axon.

  • Initiated at the axon hillock, the depolarization spreads to adjacent areas of the membrane, stimulating APs along the axonal length.

Myelination and AP Speed

  • Myelin Segments: Insulate axons to prevent ion leaks.

    • Short axons can rely on simple AP transmission.

    • Long axons utilize myelin for faster transmission.

  • Nodes of Ranvier: Unmyelinated areas between myelin segments, which are rich in voltage-gated sodium and potassium channels, where APs are regenerated.

Saltatory Conduction

  • Mechanism: APs 'jump' from node to node via saltatory conduction, significantly increasing transmission speed compared to unmyelinated axons.

Synaptic Transmission

  • Upon reaching the synaptic terminal, the AP facilitates the release of neurotransmitters into the synaptic cleft, preparing for further communication with other neurons.

Summary and Resources

  • Additional Study: Students are encouraged to review provided videos for a deeper understanding of AP dynamics and can use supplementary resources recommended during the lecture for self-assessment and clarification of complex concepts.

Self-Assessment Encouragement: Regular self-assessment will help reinforce material and clarify misconceptions during learning. Questions and slide references promote engagement and deeper understanding of the lecture content.