Threshold Potential and Its Role in Depolarization and Repolarization

Key Concepts

Threshold Potential: The critical level to which a membrane potential must be depolarized to initiate an action potential.
Depolarization: The process of the membrane potential becoming less negative (more positive).
Repolarization: The process of the membrane potential returning to a more negative value after depolarization.
Resting Membrane Potential: Typically around -70 mV for neurons, it's the stable, negative charge of an inactive neuron.

Strong Stimulus: Example - someone tapping on your wrist.
- Strong enough to depolarize the axon membrane to +30 mV, generating an action potential.
- Action potential sends signals to the brain; you feel the tap.
Weak Stimulus: Example - a tiny ant walking on your wrist.
- Not strong enough to depolarize the axon membrane to the threshold potential of -55 mV.
- No action potential generated; no signal sent to the brain, so you don't feel the ant.

Resting State: Axon has a resting membrane potential of about -70 mV.
Sodium Ion Channels: Key to depolarization. More channels opening leads to more sodium ions rushing into the cell, making the membrane potential less negative.
- Weak Stimulus: Opens few sodium ion channels, insufficient to reach the threshold potential.
- Strong Stimulus: Opens many sodium ion channels, sufficient to reach the threshold potential of -55 mV and generate an action potential.
Threshold Potential (-55 mV): The point at which enough sodium ion channels open to allow sufficient sodium ions into the axon to further depolarize the membrane to about +30 mV, triggering an action potential.
Action Potential: When the membrane depolarizes to +30 mV, the action potential is generated and signals are sent to the brain.

Energy Conservation: Prevents unnecessary action potentials, conserving ATP.
Selective Response: Ensures that only important stimuli generate action potentials, avoiding sensory overload.

The threshold potential is crucial for determining which stimuli are strong enough to warrant a response from the nervous system. It balances the need to respond to important stimuli while conserving energy by ignoring insignificant ones.