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E. Carvell, PhD, PT
University of Pittsburgh
SIGNALS: ACTION POTENTIALS AND LOCAL GRADED POTENTIALS
BC&P Fig. 2.15, p. 37
BC&P Fig. 2.23, p. 47
Action Potentials (APs) represent non-graded, all-or-none,
faithful transmission of signals from the soma to the axon termination at some
distance from the cell body. The AP is generated at the axon hillock (initial segment) and
regenerated at each Node of Ranvier in a myelinated axon such as the one in fig
2.23. This ”guaranteed delivery” of messages ensures that if the summed local
depolarizing events at the soma meet threshold, the information will be passed on to
other neurons without fail.
Local Potentials are non-propogated, graded potentials found at a number of locations. Axon terminals utilize local potentials at synapses that lead to interactions between cells (synaptic transmission). Dendrites and the soma are other locations of local potentials that may be either depolarizing or hyperpolarizing. In addition, local potentials are located in the periphery where sensory transduction leads to a generator potential in a sensory receptor (e.g., skin, muscle, ear, eye, etc.)
Action Potentials are self-regenerative depolarizations of the axon. In a myelinated axon this regenerative process occurs at each successive Node of Ranvier (saltatory conduction). The current generated by an AP is typically far in excess of threshold under ‘normal’ physiological conditions (~5x threshold safety factor for transmission). Myelin is produced by Oligodendroglia in the CNS and by Schwann Cells in the PNS.