What are the differences between a graded potential and an action potential?

Basis of differentiation 

Graded potential 

Action potential 

Definition 

Is a variable-strength signal that can be conveyed over small distances and could lead to an action potential if it becomes strong enough to meet the threshold value of -55mv 

Is a massive depolarization that can be transferred over long distances and is the actual message sent by neurons

Trigger

May be brought about by: 

• External stimuli in sensory neurons
• Neurotransmitters released in synapses

Are triggered by membrane depolarization to threshold

Threshold 

No threshold - can occur with any stimulus

Requires threshold to be reached

Occurrence 

May occur in any region of the cell plasma membrane

Occurs in regions of the cell plasma membrane where there is a high concentration of voltage-gated Na+ and K+ channels

Amplitude 

Varies depending on the strength of the stimulus 

Is all-or-nothing and does not vary with stimulus strength 

Duration 

Varies depending on the duration of the stimulus, generally ranges from a few milliseconds to seconds 

Consistent duration, generally relatively short lasting from 3 to 5 milliseconds 

Stimulus strength  

Decreases with distance

Does not decrease with distance

Ion channels

Ion channels responsible may be:

• Temperature sensitive or mechanosensitive channels 
• Ligand-gated (extracellular ligands such as neurotransmitters)
• Channels gated by cytoplasmic signaling molecules

Ion channels responsible are:

• Voltage-gated Sodium ion (Na+)
• Voltage-gated Potassium ion (K+)channels 

Ions involved

Usually Na+, K+, or Cl−

Usually Na+ and K+ 

Refractory period

None

Has absolute and relative refractory periods 

Depolarization

May be depolarizing or hyperpolarizing depending on the stimulus

Always leads to depolarization of membrane and reversal of the membrane potential