7 propagated signaling: the action potential

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abstract

the action potential is generated by the flow of ions through voltage-gated channels

sodium and potassium currents through voltage-gated channels are recorded with the voltage clamp

voltage-gated sodium and potassium conductances are calculated from their currents

the action potential can be reconstructed from the properties of sodium and potassium channels

variations in the properties of voltage-gated ion channels expand the signaling capabilities of neurons

the nervous system expresses a rich variety of voltage-gated ion channels

gating of voltage-sensitive ion channels can be influenced by various cytoplasmic factors

excitability properties vary between regions of the neuron

excitability properties vary between types of neurons

the mechanisms of voltage-gating and ion permeation have been inferred from electrophysiological measurements

voltage-gated sodium channels open and close in response to redistribution of charges within the channel

voltage-gated sodium channels select for sodium on the basis of size, charge, and energy of hydration of the ion

voltage-gated potassium, sodium, and calcium channels stem from a common ancestor and have similar structures

X-ray crystallographic analysis of voltage-gated channel structures provides insight into voltage-gating

the diversity of voltage-gated channel types is generated by several gentic mechanisms

an overall view

content

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action potentials have four properties important for neuronal signaling. first, they have a threshold for initiation. as we saw in chapter 6, in many nerve cells the membrane behaves as a simple resistor in response to small hyperpolarizing or depolarizing current steps. second, the action potential is an all-or-none event. the size and shape of an action potential initiated by a large depolarizing current is the same as that of an action potential evoked by a current that just surpasses the threshold. third, the cation potential is conducted without decrement. it has a self-regenerative feature that keeps the amplitude constant, even when it is conducted over great distances. fourth, the action potential is followed by a refractory period. for a brief time after an action potential is generated, the neuron’s ability to fire a second action potential is suppressed. the refractory period limits the frequency at which a nerve can fire action potentials, and thus limits the information-carrying capacity of the axon