PhysioEx Neurophysiology Lab: Action Potential Threshold Notes

Neurophysiology of Nerve Impulses: Activity 3 - The Action Potential Threshold

• Student Name: Madison Hida
• Date: 13 May 2026
• Session ID: session-491aa1df-fdf0-aa26-76cb-949aafadc6f4
• Objective: To investigate the threshold voltage required to trigger an action potential and the "all-or-nothing" nature of neuronal firing.

Fundamental Definitions and Key Concepts

• Threshold:
  • Defined as the "trigger point" value that a neuronal membrane must reach to initiate an action potential.
  • It is the specific voltage level where the neuron reaches the "all or none" point of firing.
  • In an axon, the threshold voltage is typically less negative than the resting membrane potential.
• Depolarization:
  • A change in membrane potential where the interior of the cell becomes more positive (less negative).
  • This is the specific type of membrane potential change that triggers an action potential.
• The "All-or-Nothing" Principle:
  • This principle states that once the threshold voltage is reached, an action potential will fire completely and in the same manner, regardless of how much the stimulus voltage exceeds the threshold.
  • Metaphor/Analogy: An action potential is like firing a gun; regardless of how softly or forcefully you pull the trigger, as long as the trigger's engagement point is met, the gun fires with the same intensity every time.
• Anatomical Focus:
  • The specific part of the neuron investigated in this activity was the trigger zone, located between the axon hillock and the initial segment of the axon.

Experimental Data and Results

• Stimulus vs. Response Metrics:
  • The experiment measured the peak values at two different recording electrodes, designated as $R1$ and $R2$, across a range of stimulus voltages.

• Observations on Voltage and Amplitude:
  • Threshold Identification: The threshold voltage for this specific axon was identified as $20\,mV$. Stimuli at or above this level produced an action potential, while a stimulus of $10\,mV$ failed to do so.
  • Effect of Suprathreshold Stimuli: As the stimulus voltage was increased from $20\,mV$ to $50\,mV$, the peak value of the action potential remained constant at $100\,\mu V$ at both $R1$ and $R2$.
  • Timing: For stimulus voltages between $20\,mV$ and $50\,mV$, the action potential occurred between the $3$-$4$ second and $5$-$6$ second markers.

Propagation and Electrode Dynamics

• Recording Latency:
  • The action potential recorded at the second electrode ($R2$) is delayed relative to the recording at the first electrode ($R1$).
  • Reasoning: This delay accounts for the time required for the action potential to propagate (travel) along the length of the axon from the location of $R1$ to the location of $R2$.

Factors Affecting Threshold and Excitation

• Extracellular Potassium ($K^+$) Levels:
  • An increase in extracellular $K^+$ serves to depolarize a neuron.
  • This depolarization typically occurs if neurons are damaged.
  • Effect on Nearby Axons: Increased extracellular $K^+$ causes nearby axonal membranes to be depolarized to values near or above their threshold voltages, potentially triggering unintended action potentials.
• Changes in Resting Membrane Potential:
  • If a graded receptor potential causes the resting membrane potential to become more negative (hyperpolarization), it becomes more difficult for the axon to reach the threshold voltage.
  • Example: A shift from a resting potential of $-70\,mV$ to a more negative $-75\,mV$ increases the distance to the threshold, requiring a stronger stimulus to fire.
• Failure to Reach Threshold:
  • In a sensory neuron, failure to reach the threshold voltage may be caused by various factors, effectively preventing the transmission of a signal to the central nervous system.

Post-lab Quiz Findings

• Threshold Characteristics: The threshold is confirmed to be less negative than the resting potential.
• Difficulty of Activation: Increasing the negativity of the resting potential directly correlates with increased difficulty in reaching the threshold.
• All-or-None Confirmation: Experimental results confirmed the prediction that increasing stimulus voltage beyond the threshold does not change the amplitude of the action potential recorded at $R1$ or $R2$.