FoundationalConceptsNeuroscience

channels. If the channel is open, then specific ions can flow through
from one side of the cell membrane to the other.
Some ion channels are continuously open, and many ion channels
open or close depending upon various conditions, such as changes
in membrane voltage, the binding of a neurotransmitter molecule,
or interaction with various other molecular regulators (phosphory-
lation, cyclic nucleotides, and so on). Consider this: if an ion-channel
protein selective for potassium ions opens, then potassium ions will
diffuse through the channel and across the cell membrane from inside
the cell (where K is more concentrated) to outside the cell (where K
is less concentrated). Because potassium ions are positively charged,
this decreases the amount of positive charge inside the cell relative to
the outside; that is, the membrane potential (voltage) becomes less
positive (more negative) inside relative to outside. If we measure the
membrane voltage before, during, and after the transient opening of
potassium channels, we would see a change toward a more negative
value (Fig. 5.4).
Similarly, if an ion-channel protein selective for chloride ions
opens, chloride ions will flow through the channel and across the cell
membrane from outside the cell (where Cl" is more concentrated) to
inside the cell (where CI is less concentrated). Because chloride ions
are negatively charged, the amount of negative charge inside the cell
increases relative to the outside. Thus, the membrane potential will
become more negative, inside relative to outside.
In both the K* and CI cases, the membrane potential becomes
more negative and the actual magnitude of the membrane voltage be-
comes larger. Larger magnitude means a greater separation of charge
between inside and outside. The term polarity is used to describe a
separation of charge. The creation of a greater separation of charge
across a cell membrane is referred to as a hyperpolarization, and open-