Table 1. Features of graded potentials and action potentials
|Depending on the stimulus, graded potentials can be depolarizing or hyperpolarizing.
||Action potentials always lead to depolarization of membrane and reversal of the membrane potential.
|Amplitude is proportional to the strength of the stimulus.
||Amplitude is all-or-none; strength of the stimulus is coded in the frequency of all-or-none action potentials generated.
|Amplitude is generally small (a few mV to tens of mV).
||Large amplitude of ~100 mV.
|Duration of graded potentials may be a few milliseconds to seconds.
||Action potential duration is relatively short; 3-5 ms.
|Ion channels responsible for graded potentials may be ligand-gated (extracellular ligands such as neurotransmitters), mechanosensitive, or temperature sensitive channels, or may be channels that are gated by cytoplasmic signaling molecules.
||Voltage-gated Na+ and voltage-gated K+ channels are responsible for the neuronal action potential.
|The ions involved are usually Na+, K+, or Cl−.
||The ions involved are Na+ and K+ (for neuronal action potentials).
|No refractory period is associated with graded potentials.
||Absolute and relative refractory periods are important aspects of action potentials.
|Graded potentials can be summed over time (temporal summation) and across space (spatial summation).
||Summation is not possible with action potentials (due to the all-or-none nature, and the presence of refractory periods).
|Graded potentials travel by passive spread (electrotonic spread) to neighboring membrane regions.
||Action potential propagation to neighboring membrane regions is characterized by regeneration of a new action potential at every point along the way.
|Amplitude diminishes as graded potentials travel away from the initial site (decremental).
||Amplitude does not diminish as action potentials propagate along neuronal projections (non-decremental).
|Graded potentials are brought about by external stimuli (in sensory neurons) or by neurotransmitters released in synapses, where they cause graded potentials in the post-synaptic cell.
||Action potentials are triggered by membrane depolarization to threshold. Graded potentials are responsible for the initial membrane depolarization to threshold.
|In principle, graded potentials can occur in any region of the cell plasma membrane, however, in neurons, graded potentials occur in specialized regions of synaptic contact with other cells (post-synaptic plasma membrane in dendrites or soma), or membrane regions involved in receiving sensory stimuli.
||Occur in plasma membrane regions where voltage-gated Na+ and K+ channels are highly concentrated.