Individual Neurons - Computational Explorations in Cognitive Neuroscience - page 33

2.4.4 The Neural Environs and Ions

Inhibitory
Synaptic
Input

We can now put the basic principles of electricity and
diffusion to work in understanding how the neuron inte-
grates information. To do this, we must understand the
nature of the internal and external environment of the
neuron, specifically in terms of the types and concen-
trations of ions present. Neurons live in a liquid envi-
ronment in the brain called the extracellular space that
is very similar to seawater (which is interesting because
life is thought to have originated in seawater). Thus,
as in seawater, there is a certain amount of dissolved
salt (NaCl), which results in a reasonable concentration
of the ions Na + and Cl ￿ . The other major ions of rel-
evance to the neuron are potassium (K + ) and calcium
(Ca ++ ).
If these ions were simply allowed to flow at will
across open membrane channels, the ion concentration
on the inside of a neuron would end up being very sim-
ilar to that of the extracellular space. However, there
are two critical factors that create and preserve an im-
balance in ion concentrations across the cell membrane.
The first such factor is a mechanism called the sodium-
potassium pump that actively works to create an im-
balance in the concentrations of the sodium and potas-
sium ions. It does this by pumping Na + ions out of the
neuron and a smaller amount of K + ions into it. The
second factor is the selective permeability of channels,
where a given channel will only allow one or a few types
of ions to pass. Furthermore, many of these channels
are usually closed unless specifically activated (e.g., by
neurotransmitter released by a sending neuron). Thus,
the sodium-potassium pump creates an imbalance of ion
concentrations, and the selective channels serve to both
maintain this imbalance and dynamically alter it (and
thus the membrane potential) by opening and closing as
a function of inputs coming into the neuron.
The sodium-potassium pump uses energy, and can be
thought of as charging up the battery that runs the neu-
ron. As we will see in a moment, just by creating an im-
balance in two types of ions, other imbalances are also
created. Thus, in some sense, everything follows from
this initial imbalance. Perhaps the most direct conse-
quence of the relatively low internal concentration of
Na + that is produced by the pump is the negative rest-

Cl−

−70

Leak

Excitatory
Synaptic
Input

Cl−

Vm

K+

−70

K+

Vm

Na+

+55

Vm

Na+

Na/K
Pump

−70mV

Vm

0mV

Figure 2.8: Summary of the three major activation ions and
their channels.

ing potential of the neuron. The resting potential is the
membrane potential that holds when no inputs are com-
ing into the neuron, and because there are more positive
Na + ions outside the cell than inside, the inside of the
neuron will have a net negative charge. This negative
charge is typically around -70 millivolts or ￿70mV ,
where 1mV is one thousandth of a volt — not much!
In the following listing, we will enumerate the inter-
nal and external concentrations of each of the four main
ions, assess the electrical and diffusion forces acting on
it, and discuss the channels that allow it to flow into or
out of the neuron (also see figure 2.8).

Na + Because of the sodium-potassium pump, sodium
exists in greater concentration outside the neuron
than inside. Thus, the diffusion force pushes it into
the neuron. To counteract this diffusion force with an
electrical force, the neuron would have to have a pos-
itive charge inside relative to outside (thus repelling
any other positive charges that might otherwise want
to come in). Thus, the equilibrium potential of Na +
is positive, with a typical value of around +55mV .
There are two primary types of channels that pass
Na + . The most important for our purposes is the ex-
citatory synaptic input channel that is usually closed
(preserving the imbalance), but is opened by the
binding of the neurotransmitter glutamate ,asdis-
cussed previously. There is also a voltage-gated Na +
channel (i.e., its opening and closing is dependent