Graded v. Action 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.
No refractory period is associated with graded potentials.
Absolute and relative refractory periods are important aspects of action potentials.
Duration of graded potentials may be a few milliseconds to seconds.
Action potential duration is relatively short; 3-5 ms.
Graded potentials
Action potentials
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.
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.
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.
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).
Amplitude is generally small (a few mV to tens of mV).
Large amplitude of ~100 mV.
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.
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).
The ions involved are usually Na+, K+, or Cl−.
The ions involved are Na+ and K+ (for neuronal action potentials).
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.
