368
UNIT THREE
neuron. If threshold is reached, an
action potential
results,
which is the basis for the nerve impulse.
In many cases, a single depolarizing stimulus is not sufF
-
cient to bring the postsynaptic cell to threshold. ±or example,
if presynaptic neurons release enough neurotransmitter to
open some chemically-gated sodium channels for a moment,
the depolarization that results might be insufF
cient to reach
threshold
(f g. 10.15
a
)
. Such a subthreshold depolarization
will not result in an action potential.
If the presynaptic neurons release more neurotransmit-
ter, or if other neurons that synapse with the same cell join
in the effort to depolarize, threshold may be reached, and an
action potential results. The mechanism uses another type
of ion channel, a voltage-gated sodium channel that opens
when threshold is reached (F
g. 10.15
b
).
Action Potentials
In a multipolar neuron, the F
rst part of the axon, the
initial
segment,
is often referred to as the
trigger zone
because it
contains many voltage-gated sodium channels. At the rest-
ing membrane potential, these sodium channels remain
closed, but when threshold is reached, they open for an
Local Potential Changes
Neurons are excitable; that is, they can respond to changes
in their surroundings. Some neurons, for example, detect
changes in temperature, light, or pressure outside the body,
whereas others respond to signals from inside the body,
often from other neurons. In either case, such changes or
stimuli usually affect the membrane potential in the region
of the membrane exposed to the stimulus, causing a local
potential change.
Typically, the environmental change affects the mem-
brane potential by opening a gated ion channel. If, as a
result, the membrane potential becomes more negative than
the resting potential, the membrane is
hyperpolarized.
If the
membrane becomes less negative (more positive) than the
resting potential, the membrane is
depolarized.
Local potential changes are graded. This means that
the degree of change in the resting potential is directly pro-
portional to the intensity of the stimulation. ±or example, if
the membrane is being depolarized, the greater the stimu-
lus, the greater the depolarization. If neurons are sufF
ciently
depolarized, the membrane potential reaches a level called
the
threshold
potential,
approximately –55 millivolts in a
–62 mV
Presynaptic
neuron
Na
+
Na
+
Chemically-gated
Na
+
channel
Neurotransmitter
(a)
–55 mV
Na
+
Na
+
Na
+
Na
+
Na
+
Voltage-gated
Na
+
channel
Trigger zone
(b)
FIGURE 10.15
Action potentials. (
a
) A
subthreshold depolarization will not result in
an action potential. (
b
) Multiple stimulation by
presynaptic neurons may reach threshold, opening
voltage-gated channels at the trigger zone.
previous page 398 David Shier Hole's Human Anatomy and Physiology 2010 read online next page 400 David Shier Hole's Human Anatomy and Physiology 2010 read online Home Toggle text on/off