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Neuronal Action Potential -
Graded Potentials versus Action Potentials
There are important differences between graded potentials and action potentials of neurons (see Introduction to this lecture). Table 1 lists the main differences between graded potentials and action potentials. As discussed in this lecture and upcoming lectures, most of these differences are due to the fact that graded potentials result from the passive electrical property of the neuronal membrane, whereas action potentials result from an orchestrated response to depolarizing stimuli, and involve a coordinated activity of voltage-gated ion channels. Graded potentials must occur to depolarize the neuron to threshold before action potentials can occur. Depending on the cell and type and the nature of stimulus, graded potentials that lead to action potentials are called synaptic potentials (i.e., post-synaptic potential changes in neurons), generator potentials or receptor potentials (graded potentials in sensory cells causes by adequate stimuli), or end-plate potentials (i.e., synaptic potentials in skeletal muscle cells). These graded potentials will be discussed in later lectures. In the next lecture, we will consider the propagation of neuronal action potentials and we will see that additional neuronal adaptations allow action potentials to travel over long distances without losing any strength (i.e., amplitude). In yet another later lecture, we will see how summation of graded potentials is responsible for much of information processing at specialized contact regions between neurons (synapses).
Table 1. Features of graded potentials and action potentials
Graded potentials 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.
Note: The details of action potentials noted here refer to those of neuronal action potentials. As we will see throughout our study of physiology, other action potentials (for example, in skeletal, cardiac, and smooth myocytes, and in some endocrine cells) exhibit different features than those mentioned here.

Posted: Thursday, July 5, 2012
Last updated: Friday, January 17, 2014