88 SECTION IIPhysiology of Nerve & Muscle Cells
larization in myelinated axons jumps from one node of
Ranvier to the next, with the current sink at the active node
serving to electrotonically depolarize the node ahead of the ac-
tion potential to the firing level (Figure 4–10). This jumping of
depolarization from node to node is called saltatory conduc-
tion. It is a rapid process that allows myelinated axons to con-
duct up to 50 times faster than the fastest unmyelinated fibers.
ORTHODROMIC & ANTIDROMIC
CONDUCTION
An axon can conduct in either direction. When an action po-
tential is initiated in the middle of it, two impulses traveling in
opposite directions are set up by electrotonic depolarization
on either side of the initial current sink. In the natural situa-
tion, impulses pass in one direction only, ie, from synaptic
junctions or receptors along axons to their termination. Such
conduction is called orthodromic. Conduction in the oppo-
site direction is called antidromic. Because synapses, unlike
axons, permit conduction in one direction only, an antidrom-
ic impulse will fail to pass the first synapse they encounter and
die out at that point.
BIPHASIC ACTION POTENTIALS
The descriptions of the resting membrane potential and ac-
tion potential outlined above are based on recording with
two electrodes, one in the extracellular space and the other
inside it. If both recording electrodes are placed on the sur-
face of the axon, there is no potential difference between
them at rest. When the nerve is stimulated and an impulse is
conducted past the two electrodes, a characteristic sequence
of potential changes results. As the wave of depolarization
reaches the electrode nearest the stimulator, this electrode be-
comes negative relative to the other electrode (Figure 4–11).
When the impulse passes to the portion of the nerve between
the two electrodes, the potential returns to zero, and then, as
it passes the second electrode, the first electrode becomes
positive relative to the second. It is conventional to connect
the leads in such a way that when the first electrode becomes
negative relative to the second, an upward deflection is re-
corded. Therefore, the record shows an upward deflection
followed by an isoelectric interval and then a downward de-
flection. This sequence is called a biphasic action potential
(Figure 4–11).
PROPERTIES OF MIXED NERVES
Peripheral nerves in mammals are made up of many axons
bound together in a fibrous envelope called the epineurium.
Potential changes recorded extracellularly from such nerves
therefore represent an algebraic summation of the all-or-none
action potentials of many axons. The thresholds of the indi-
vidual axons in the nerve and their distance from the stimulat-
ing electrodes vary. With subthreshold stimuli, none of the
axons are stimulated and no response occurs. When the stim-
uli are of threshold intensity, axons with low thresholds fire
and a small potential change is observed. As the intensity of
the stimulating current is increased, the axons with higher
thresholds are also discharged. The electrical response in-
creases proportionately until the stimulus is strong enough to
excite all of the axons in the nerve. The stimulus that produces
excitation of all the axons is the maximal stimulus, and appli-
cation of greater, supramaximal stimuli produces no further
increase in the size of the observed potential.NERVE FIBER TYPES & FUNCTION
After a stimulus is applied to a nerve, there is a latent period
before the start of the action potential. This interval corre-
sponds to the time it takes the impulse to travel along the axon
from the site of stimulation to the recording electrodes. Its du-
ration is proportionate to the distance between the stimulating
and recording electrodes and inversely proportionate to the
speed of conduction. If the duration of the latent period and
the distance between the stimulating and recording electrodes
are known, axonal conduction velocity can be calculated.
Erlanger and Gasser divided mammalian nerve fibers into
A, B, and C groups, further subdividing the A group into α, β,
γ, and δ fibers. In Table 4–1, the various fiber types are listedFIGURE 4–11 Biphasic action potential. Both recording elec-
trodes are on the outside of the nerve membrane. It is conventional to
connect the leads in such a way that when the first electrode becomes
negative relative to the second, an upward deflection is recorded.
Therefore, the record shows an upward deflection followed by an iso-
electric interval and then a downward deflection.+
_+
_−
++
_+
_+
_+
_+
_+
_+
_+
_+
_+
_+
_+
_−
++
_+
_+
_+
_+
_+
_+
_+
_+
_+
_+
_+
_+
_+
_+
_+
_−
++
_+
_+
_+
_+
−+
_+
_+
_−
++
_+
_NerveTimemV