Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 4Excitable Tissue: Nerve 87

the membrane potential closer to the firing level. During the
local response, the threshold is lowered, but during the rising
and much of the falling phases of the spike potential, the neu-
ron is refractory to stimulation. This refractory period is di-
vided into an absolute refractory period, corresponding to
the period from the time the firing level is reached until repo-
larization is about one-third complete, and a relative refrac-
tory period, lasting from this point to the start of after-
depolarization. During the absolute refractory period, no
stimulus, no matter how strong, will excite the nerve, but dur-
ing the relative refractory period, stronger than normal stim-
uli can cause excitation. During after-depolarization, the
threshold is again decreased, and during after-hyperpolariza-
tion, it is increased. These changes in threshold are correlated
with the phases of the action potential in Figure 4–9.

ELECTROGENESIS OF

THE ACTION POTENTIAL

The nerve cell membrane is polarized at rest, with positive
charges lined up along the outside of the membrane and neg-
ative charges along the inside. During the action potential, this
polarity is abolished and for a brief period is actually reversed
(Figure 4–10). Positive charges from the membrane ahead of
and behind the action potential flow into the area of negativity
represented by the action potential (“current sink”). By draw-
ing off positive charges, this flow decreases the polarity of the
membrane ahead of the action potential. Such electrotonic de-
polarization initiates a local response, and when the firing lev-
el is reached, a propagated response occurs that in turn
electrotonically depolarizes the membrane in front of it.

SALTATORY CONDUCTION

Conduction in myelinated axons depends on a similar pattern

of circular current flow. However, myelin is an effective insu-

lator, and current flow through it is negligible. Instead, depo-

FIGURE 4–8 Electrotonic potentials and local response. The
changes in the membrane potential of a neuron following application
of stimuli of 0.2, 0.4, 0.6, 0.8, and 1.0 times threshold intensity are
shown superimposed on the same time scale. The responses below
the horizontal line are those recorded near the anode, and the re-
sponses above the line are those recorded near the cathode. The stim-
ulus of threshold intensity was repeated twice. Once it caused a
propagated action potential (top line), and once it did not.

− 55

− 70

− 85

0.5 1.0 1.5

ms

Resting membrane

potential

Propagated

action potential

Firing level

Local response

Membrane potential (mV)

FIGURE 4–9 Relative changes in excitability of a nerve cell

membrane during the passage of an impulse. Note that excitability

is the reciprocal of threshold. (Modified from Morgan CT: Physiological

Psychology. McGraw-Hill, 1943.)

FIGURE 4–10 Local current flow (movement of positive

charges) around an impulse in an axon. Top: Unmyelinated axon.

Bottom: Myelinated axon. Positive charges from the membrane

ahead of and behind the action potential flow into the area of negativ-

ity represented by the action potential (“current sink”). In myelinated

axons, depolarization jumps from one node of Ranvier to the next (sa-

lutatory conduction).

Spike

potential

After-depolarization

After-hyperpolarization

Local

response

Period of latent addition

Supernormal period

Refractory period

Time

Subnormal

period

Excitability

Potential change

Myelin

Axon

ECF _ +

_ +

Direction of propagation

Active

node

Inactive

node

+

+

_

_

++++––++ +

–––– ++ –––

++++––++ +

–––– ++ –––

Axon

ECF