Chapter 6 The Nervous System 203
known as a nerve impulse, or action potential, and
it executes in an all-or-none fashion. This means
that the electrical charge of the action potential is
always the same size, and once initiated, it always
travels the full length of the axon.
Following the discharge of the action
potential, the membrane becomes permeable to
(or accepting of) potassium ions, which rapidly
diffuse out of the cell. This begins the process of
restoring the membrane to its original, polarized
resting state, a process called repolarization.
Until the cell membrane is repolarized, it cannot
respond to another stimulus. The time between
the completion of the action potential and
repolarization is called the refractory (ree-FRAK-
toh-ree) period. During the refractory period the
neuron is temporarily “fatigued.”
Impulse Transmission
Two factors—the presence or absence of a
myelin sheath and the diameter of the axon—
have a major impact on the speed at which a
nerve impulse travels. Because the fatty myelin
sheath is an electrical insulator, action potentials
in a myelinated axon “jump over” the myelinated
regions of the axon. Depolarization occurs only at
the nodes of Ranvier, where the axon is exposed
(Figure 6.3). This process, known as saltatory
(SAWL-ta-TOH-ree) conduction, results in
signifi cantly faster impulse transmission than is
possible in nonmyelinated axons.
Impulse conduction is much faster in
nonmyelinated axons with larger diameters than
in those with smaller diameters. The larger the
axon, the greater the number of ions there will
be to conduct current. This is somewhat like a
large-diameter pipe versus a small-diameter
pipe when transferring water from one place
to another. The water moves through the large-
diameter pipe faster than it does through the
pipe with a small diameter.
A third factor infl uencing conduction speed
is body temperature. Warmer temperatures
increase ion diffusion rates, whereas local
cooling, which occurs when holding an ice cube,
for example, decreases ion diffusion rates.
So how fast do nerve impulses travel? As is
clear from our recent discussion, the type and
size of the nerve axon have much to do with this.
Impulses that signal limb position to the brain
travel extremely fast—up to 119 m/s (meters per
second). Information or impulses from the
objects that we touch travel more slowly, at
around 76 m/s (Figure 6.5). By contrast, the
sensation of pain moves more slowly, at less than
1 m/s. Thought signals, which are happening
right now as you are reading, transmit at 20–
30 m/s. For a nerve to transmit impulses at
speeds greater than 1 m/s, it must have a
myelinated axon.
Transmission at Synapses
Communication between some cells occurs
through direct transfer of electrical charges at
electrical synapses within specialized sites called gap
junctions. The intercalated discs between cardiac
muscle fi bers, for example, serve as gap junctions.
1. What is meant when a cell membrane is
said to be polarized?
2. Do action potentials occur when
neuron cell membranes are polarized or
depolarized?
3. What two factors infl uence the speed at
which a nerve impulse travels?
Check Your Understanding
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Figure 6.5 Although it may seem as though we feel
pain instantly, nerve impulses communicating pain
travel more slowly than other nerve impulses.