by Talib F. Abbas
 In fact, the word glia is Greek for glue. However, today
theses cells are recognized for their role in
communication within the CNS in partnership with
neurons. There are two major types of glial cells in the
vertebrate nervous system: microglia and macrogli.
 Microglia are scavenger cells that resemble tissue
macrophages.
 Macroglia: oligodendrocytes, Schwann cells, and
astrocytes.
 Fibrous astrocytes, which contain many
intermediate filaments, are found primarily in white
matter. Protoplasmic astrocytes are found in gray
matter.
 Electrical potentials exist across the membranes of
virtually all cells of the body. In addition, some cells,
such as nerve and muscle cells, are capable of
generating rapidly changing electrochemical impulses
at their membranes, and these impulses are used to
transmit signals along the nerve or muscle
membranes. In other types of cells, such as glandular
cells, macrophages, and ciliated cells, local changes in
membrane potentials also activate many of the cells’
functions.
 “Diffusion Potential” Caused by an Ion Concentration
Difference on the Two Sides of the Membrane. In , the
potassium concentration is great inside a nerve fiber
membrane but very low outside the membrane. Let us
assume that the membrane in this instance is permeable to
the potassium ions but not to any other ions. Because of
the large potassium concentration gradient from inside
toward outside, there is a strong tendency for extra
numbers of potassium ions to diffuse outward through the
membrane. As they do so, they carry positive electrical
charges to the outside, thus creating electropositivity
outside the membrane and electronegativity inside because
of negative anions that remain behind and do not diffuse
outward with the potassium.
 [K+x] > [K+y], that is, more osmotically active particles





are on side X than on side Y.
Donnan and Gibbs showed that in the presence of a
nondiffusible ion.
[K+x]/ [K+y] =[Cl–y]/ [Cl–x]
Cross-multiplying,
[K+x] + [Cl–x] = [K+y] + [Cl–y]
This is the Gibbs–Donnan equation. It holds for any
pair of cations and anions of the same valence.
 The interior of the cell is negative relative to the exterior,









and chloride ions are pushed out of the cell along this
electrical gradient. An equilibrium is reached between Cl–
influx and Cl– .
ECl = RT/ FZCl ln [Clo–][Cli–]
where
ECl = equilibrium potential for Cl–
R = gas constant
T = absolute temperature
F = the faraday (number of coulombs per mole of charge).
ZCl = valence of Cl– (–1)
[Clo–] = Cl– concentration outside the cell
[Cli–] = Cl– concentration inside the cell
 the concentration gradient is outward and the electrical
gradient inward. In mammalian spinal motor neurons, EK
is –90 mV. Because the resting membrane potential is –70
mV, there is somewhat more K+ in the neurons than can be
accounted for by the electrical and chemical gradients. The
situation for Na+ is quite different from that for K+ and Cl–
. The direction of the chemical gradient for Na+ is inward,
to the area where it is in lesser concentration, and the
electrical gradient is in the same direction. ENa is +60 mV
Because neither EK nor ENa is equal to the membrane
potential, one would expect the cell to gradually gain Na+
and lose K+ if only passive electrical and chemical forces
were acting across the membrane.
 “uphill” against an electrical or pressure gradient), the
process is called active transport.
 some energy source must cause excess movement of
potassium ions to the inside of cells and excess
movement of sodium ions to the outside of cells.
 The basic mechanism for transport of a substance
through a cellular sheet shows that the epithelial cells
are connected together tightly at the luminal pole by
means of junctions called “kisses.” The brush border
on the luminal surfaces of the cells is permeable to
both sodium ions and water.
 The resting membrane potential of large nerve fibers when
not transmitting nerve signals is about –90 millivolts. That
is, the potential inside the fiber is 90 millivolts more
negative than the potential in the extracellular fluid on the
outside of the fiber. all cell membranes of the body have a
powerful Na+-K+ that continually pumps sodium ions to
the outside of the cell and potassium ions to the inside, this
is an electrogenic pump because more positive charges are
pumped to the outside than to the inside (three Na+ ions to
the outside for each two K+ ions to the inside), leaving a
net deficit of positive ions on the inside; this causes a
negative potential inside the cell membrane.
 Nerve signals are transmitted by action potentials,
which are rapid changes in the membrane potential
that spread rapidly along the nerve fiber membrane.
 The cell membranes of nerves, like those of other cells,
contain many different types of ion channels. Some of
these are voltage- gated and others are ligand-gated.
 In response to a depolarizing stimulus, some of the
voltage-gated Na+ channels become active, and when
the threshold potential is reached, the voltagegated Na+channels overwhelm the K+ and other
channels and an action potential results (a positive
feedbackloop) .
 The membrane potential moves toward the equilibrium
potential for Na+(+60 mV) but does not reach it during the
action potential, primarily because the increase in Na+
conductance is short-lived. The Na+ channels rapidly enter
aclosed state called the inactivated state.
 In addition, the direction of the electrical gradient for Na+
is reversed during the overshoot because the membrane
potential is reversed, and this limits Na+ influx. A third
factor producing repolarization is the opening of voltagegated K+ channels.
 The slow return of the K+ channels to the closed state also
explains the after-hyperpolarization, followed by a
return to the resting membrane potential. Thus, voltagegated K + channels bring the action potential to an end and
causeclosure of their gates through anegative feedback
process.
 Two types of physicochemical disturbances are
produced: local, nonpropagated potentials called,
depending on their location, synaptic, generator, Or
electrotonic potentials; and propagated potentials,
the action potentials (or nerve impulses ).
 The impulse is normally transmitted (conducted)
along the axon to its termination. Nerves are not
“telephone wires” that transmit impulses passively;
conduction of nerve impulses, although rapid, is much
slower than that of electricity.
 The transmission of each action potential along a nerve
fiber reduces very slightly the concentration differences of
sodium and potassium inside and outside the membrane,
because sodium ions diffuse to the inside during
depolarization and potassium ions diffuse to the outside
during repolarization. For a single action potential, this
effect is so minute that it cannot be measured. Indeed,
100,000 to 50 million impulses can be transmitted by large
nerve fibers before the concentration differences reach the
point that action potential conduction ceases. Even so,
with time, it becomes necessary to re-establish the sodium
and potassium membrane concentration differences. This
is achieved by action of the Na+-K+ pump in the same way
as described previously in the chapter for the original
establishment of the resting potential.
 Epineurium
 all-or-none action potentials.
 maximal stimulus
 Neurotrophins