Question

Describe, in order, the events that create a local potential that (a) doesn’t result in an action potential, and (b) does result in an action potential

Answer 1

GRADED POTENTIAL OR LOCAL POTENTIAL

Graded or Local potentials are primarily generated by sensory input, causing a change in the conductance of the membrane of the sensory receptor cell. Local potentials also are those generated at a localized place on the cell membrane where an excitatory or inhibitory synapse has taken place. Local potentials which are excitatory in nature, are generated in the same way an AP is generated at the axon hillock, that is, by the influx of positively charged sodium ions into the intracellular fluid, decreasing the negativity of the charge inside relative to the outside of the cell. If this change in difference across the membrane reaches a certain threshold, a local potential is generated at a segment of the cell membrane of the neuron or dendrite at which the stimulation occurred; that is, the change in the positive to negative ratio causes a local flow of current.

Unlike an AP, which is an all-or-nothing phenomenon, a local potential is not likely to be fully propagated along the membrane; rather, the voltage will tend to decrease as the current spreads due to leakage of sodium ions through the resistance and capacitance of the membrane. The current may remain more localized, and potential may decrease in strength the further along the membrane it spreads.

a) No action potential is produced when:

If the graded or local potential initially is not strong enough to reach the threshold required for generation of an AP, it likely will decrease by the time it travels along the membrane to the axon hillock and an AP will not be generated at that time.

b) Action potential is generated when:

Local potentials summate either in time (a number of graded potentials occurring on the cell membrane at once) or in space (a number of graded potentials occurring at the same point on the membrane) to generate a signal strong enough to generate an AP.