Ganong's Review of Medical Physiology, 23rd Edition

86 SECTION IIPhysiology of Nerve & Muscle Cells

Once threshold intensity is reached, a full-fledged action
potential is produced. Further increases in the intensity of a
stimulus produce no increment or other change in the action
potential as long as the other experimental conditions remain
constant. The action potential fails to occur if the stimulus is
subthreshold in magnitude, and it occurs with constant
amplitude and form regardless of the strength of the stimulus
if the stimulus is at or above threshold intensity. The action
potential is therefore “all or none” in character and is said to
obey the all-or-none law.

ELECTROTONIC POTENTIALS, LOCAL

RESPONSE, & FIRING LEVEL

Although subthreshold stimuli do not produce an action po-
tential, they do have an effect on the membrane potential.
This can be demonstrated by placing recording electrodes
within a few millimeters of a stimulating electrode and ap-
plying subthreshold stimuli of fixed duration. Application
of such currents leads to a localized depolarizing potential

change that rises sharply and decays exponentially with

time. The magnitude of this response drops off rapidly as

the distance between the stimulating and recording elec-

trodes is increased. Conversely, an anodal current produces

a hyperpolarizing potential change of similar duration.

These potential changes are called electrotonic potentials.

As the strength of the current is increased, the response is

greater due to the increasing addition of a local response of

the membrane (Figure 4–8). Finally, at 7–15 mV of depolar-

ization (potential of –55 mV), the firing level is reached and

an action potential occurs.

CHANGES IN EXCITABILITY DURING

ELECTROTONIC POTENTIALS & THE

ACTION POTENTIAL

During the action potential, as well as during electrotonic po-

tentials and the local response, the threshold of the neuron to

stimulation changes. Hyperpolarizing responses elevate the

threshold, and depolarizing potentials lower it as they move

FIGURE 4–7 Feedback control in voltage-gated ion channels in the membrane. (a) Na+ channels exert positive feedback. (b) K+ chan-
nels exert negative feedback. (From Widmaier EP, Raff H, Strang KT: Vander’s Human Physiology. McGraw-Hill, 2008.)

Depolarization

of membrane

potential

Increased flow

of Na+ into

the cell

Opening of

voltage-gated

Na+ channels

Depolarizing

stimulus

Inactivation

of Na+ channels

Increased PNa

Start

Stop

Positive

feedback

Repolarization

of membrane

potential

Increased flow

of K+ out of

the cell

Opening of

voltage-gated

K+ channels

Depolarization

of membrane

by Na+ influx

Increased PK

Start

Negative

feedback

(a)

(b)

+