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Information processing by the central nervous is based
on electrical signals. The developmental regulation of
each neuron's resting potential and voltage-gated ion
channels is essential for the emergence of adult function.
The resting membrane potential becomes more negative
during development (Kullberg et al., 1977; Burgard and
Hablitz, 1993; Tepper and Trent, 1993; Sanes, 1993; Ramoa
and McCormick, 1994; Warren and Jones, 1997). This is
due to regulation of extracellular K + by glial cells which
are proliferating and differentiating throughout the brain
(Connors et al., 1982; Skoff et al., 1976; Sykov√° et al., 1992).
For example, extracellular K + drops from about 35 mM in
the cortex of newborn rabbits to 3 mM in adults (Mutani
et al., 1974). This difference translates into a shift of almost
35 mV in membrane potential.
A few simple properties determine the size and speed
of electrical events (Figure 8.A). The first, membrane input
resistance, determines how much the membrane potential
will change for a given current pulse. The second, mem-
brane time constant, determines how rapidly the mem-
brane will reach a new potential when current is injected.
Both of these properties tend to decrease with age, proba-
bly reflecting an increase in cell size (i.e., total membrane).
Thus, input resistance decreases because the number of
resistors (i.e., channels) increase as membrane is added to
a cell. For example, the potassium channel-blocker,
cesium, has the greatest effect on input resistance and time
constant just when these values are decreasing in devel-
oping motor neurons (Cameron et al., 2000). Thus, the
mature neuron is able to process information rapidly and
accurately because the synaptic currents elicit brief
changes in membrane potential.
In many developing systems, the action potential is
first carried by calcium ions. Since the calcium channels
tend to remain open for a longer time, the action poten-
tials can be very slow. Thus, Xenopus neurons begin life
with 60-90 ms action potentials, although they quickly
decrease to about 1 msec in duration (Figure 8.B). Two
basic changes explain this decrease. First, sodium chan-
nels become the primary conduit for inward current
(Spitzer and Lamborghini, 1976; Baccaglini and Spitzer,
1977). Second, there is a 3.5-fold increase in a potassium
channel current, called the delayed rectifier , that is acti-
vated during membrane depolarization (Barish, 1986).
The maturation of this large outward current brings the
100 msec
The Action Potential: Sodium and
Potasssium Channels
When a neuron becomes slightly depolarized, perhaps
owing to a synaptic potential, the opening of voltage-
gated sodium channels permits a large depolarizing
current due to the relatively high extracellular sodium
concentration. As the neuron depolarizes, a second set of
voltage-gated channels are activated that permit potas-
sium to leave the cell, thus returning the membrane
potential to rest. In many cases, the initial depolarization
recruits a third type of voltage-gated channel that permits
calcium to enter the neuron. When do these channels
appear during development?
Postnatal age (days)
FIGURE 8.A Development of passive membrane properties.
(Left) The intracellularly recorded voltage response to positive
and negative current pulses in a P0 and a P30 neuron from ferret
lateral geniculate nucleus (LGN) brain slices. The P0 neuron dis-
played a longer time constant and larger voltage deflection.
(Right) Plots of membrane potential, input resistance, and time
constant from LGN neurons during postnatal development.
Membrane potential becomes about 10 mV more negative, input
resistance decreases by about 200 MW, and the time constant
decreases by about 10 ms. (From Ramoa and McCormick, 1994)
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