<P> A neuron's ability to generate and propagate an action potential changes during development . How much the membrane potential of a neuron changes as the result of a current impulse is a function of the membrane input resistance . As a cell grows, more channels are added to the membrane, causing a decrease in input resistance . A mature neuron also undergoes shorter changes in membrane potential in response to synaptic currents . Neurons from a ferret lateral geniculate nucleus have a longer time constant and larger voltage deflection at P0 than they do at P30 . One consequence of the decreasing action potential duration is that the fidelity of the signal can be preserved in response to high frequency stimulation . Immature neurons are more prone to synaptic depression than potentiation after high frequency stimulation . </P> <P> In the early development of many organisms, the action potential is actually initially carried by calcium current rather than sodium current . The opening and closing kinetics of calcium channels during development are slower than those of the voltage - gated sodium channels that will carry the action potential in the mature neurons . The longer opening times for the calcium channels can lead to action potentials that are considerably slower than those of mature neurons . Xenopus neurons initially have action potentials that take 60--90 ms . During development, this time decreases to 1 ms . There are two reasons for this drastic decrease . First, the inward current becomes primarily carried by sodium channels . Second, the delayed rectifier, a potassium channel current, increases to 3.5 times its initial strength . </P> <P> In order for the transition from a calcium - dependent action potential to a sodium - dependent action potential to proceed new channels must be added to the membrane . If Xenopus neurons are grown in an environment with RNA synthesis or protein synthesis inhibitors that transition is prevented . Even the electrical activity of the cell itself may play a role in channel expression . If action potentials in Xenopus myocytes are blocked, the typical increase in sodium and potassium current density is prevented or delayed . </P> <P> This maturation of electrical properties is seen across species . Xenopus sodium and potassium currents increase drastically after a neuron goes through its final phase of mitosis . The sodium current density of rat cortical neurons increases by 600% within the first two postnatal weeks . </P>

The depolarization phase of an action potential requires