Understanding Threshold Potential in Neurons

Aug 11, 2024

Threshold Potential and Its Role in Action Potential Generation

Introduction to Threshold Potential

  • Threshold Potential: A critical concept tied to depolarization and repolarization in neurons.
  • Depolarization: Process where the neuron's membrane potential becomes more positive.
  • Repolarization: Process of returning the membrane potential to its resting state.

Situational Examples

  • Strong Stimulus: Tapping on the wrist generates a noticeable signal.
  • Weak Stimulus: An ant walking on the wrist is not felt.

Key Questions

  1. Can you feel a strong stimulus (tap) with eyes closed? Yes.
  2. Can you feel a weak stimulus (ant) with eyes closed? Probably not.

Explanation of Stimulus Strength

  • Strong Stimulus: Depolarizes the axon to +30 mV, generating an action potential sent to the brain.
  • Weak Stimulus: Fails to depolarize sufficiently, thus no action potential is generated, and no signal is sent to the brain.

Discussion Points

  • Good or Bad? Opinions vary; conserving ATP by not responding to every stimulus is seen as beneficial.
  • Example: Not waking up from an insignificant stimulus like an insect crawling on you while sleeping.

Mechanism of Threshold Potential

  • Resting Membrane Potential: -70 mV, maintained by low internal and high external sodium ion concentrations.
  • Voltage-Gated Sodium Ion Channels: Critical for changing membrane potential.

Weak Stimulus Mechanism

  • Few Sodium Channels Open: Not enough sodium ions enter the axon.
  • Result: Membrane potential increases slightly but does not reach the threshold potential (~ -55 mV).
  • Outcome: No action potential generated; no signal sent to the brain.

Strong Stimulus Mechanism

  • More Sodium Channels Open: More sodium ions rush into the axon.
  • Result: Membrane potential reaches threshold potential and depolarizes to +30 mV.
  • Outcome: Action potential is generated; signal sent to the brain.

Importance of Threshold Potential

  • Function: Allows the nervous system to conserve ATP by not generating action potentials for insignificant stimuli.
  • Example: Ensures that only important stimuli are processed, avoiding unnecessary energy expenditure.