FoundationalConceptsNeuroscience

However, the phospholipid bilayer membrane of a nerve cell, like all
biological membranes, is studded with protein molecules of various
kinds. Some of these proteins are channels that can open and close,
allowing specific ions to pass through them and cross the membrane
when the channels are open. Figure 5.1 shows a lipid bilayer mem-
brane containing channel proteins that allow Cl ions, but not Na
ions, to cross the membrane. On the left, sodium and chloride ions
are distributed in water, and the channel proteins in the lipid bilayer
membrane separating the two compartments are closed. On the
right, channel proteins allowing the passage of chloride ions open,
and Cl flows across the membrane. Given this opportunity, Cl will
tend to move by diffusion from where it is more concentrated (upper
compartment) to where it is less concentrated (lower compartment);
diffusion is said to be “down the gradient of concentration.” As long as
the channels are open, the movement of Cl continues until the ten-
dency to equalize concentration is offset by the tendency of the pos-
itive charge in the upper compartment to pull the Cl back to its side.
An equilibrium balancing these two tendencies is rapidly established.
Biological membranes also contain pump or transporter proteins
that use energy to move specific ions from one side of the membrane
to the other. A very important ion-transporter protein found in all
neurons is the sodium-potassium or Na/K pump, which moves
sodium ions (Na) out of the neuron and potassium ions (K) into the
neuron. This process requires energy because as Na is pumped out,
the concentration of Na outside the cell becomes larger than that
inside the cell—the Na needs to be “pushed” up its concentration gra-
dient. It’s like carrying water uphill—the water tends to flow down,
and energy is required to carry it up. Similarly, with potassium ions
energy is required to move them into the cell, against their concentra-
tion gradient.