Tone of Voice and Mind : The Connections between Intonation ...

172 Chapter 6
to move away from high concentrations of similar ions, the momentary transparency
of the cellular membrane induces an uncharacteristically-rapid flow of
charge. At the outset, the bulk of the ionic flow is that of positive sodium ions
flowing into the cell. After 1–2 milliseconds of such ionic diffusion, the ion
channels close and this process repeats itself at the next step along the axon.
Potassium channels then briefly open for an outflow of positive charge, and
eventually all channels are closed and the neuron returns to the slower process
of importing and exporting ions to re-establish the normal gradient of ionic
charge, the impulse proceeds step by step down the axon (Figure 6-6).
The free diffusion of ions across the neuronal membrane during the action
potential is a process that occurs only when the ion-channels are opened (Figure
6-7). Although there are other mechanisms for importing and exporting of
ions against the potential gradient, ion diffusion with the potential gradient is a
massive movement of charged particles occurring when the difference between
intra- and extracellular potential gradients has exceeded a threshold value.
M
e
m
b
r
n
e
P
o
t
e
n
t
i
a
l
+30 mV
0 mV
–60 mV
–70 mV
–80 mV
Na+
Influx
Na+
Channels
Open
a
+ K
Channels
Open
b
c
K+
efflux
+ Na Channels
Close
Action
Potential
d
+ K Channels
Close
0 1 2 3
Time in milliseconds
Figure 6-6. Details of the action potential (after Grilly 2002:59 ). Sodium channels are
opened due to a small decrease in the resting potential from –80 to –70 milliVolts. The
influx of positive charges triggers the opening of potassium channels and the closing of
the sodium channels. Eventually, the resting potential is re-established.
e