Developmental psychology.pdf

Physiology and Behavior 93
—f
Resting
potential
i
i
Action
potential
Absolute
refractory
period
QTT.
Na
+ + I-
J
- + + + +
Figure 4.3
Nerve Impulse. The left figure
depicts the electrical impulse as it is
experienced at a single point on the
nerve fiber. The right figure shows
what happens as it moves along the
fiber from left to right. Sodium (Na)
ions enter the fiber, making it
positive, whereas in the normal
resting state potassium (K) ions keep
it negative (Katz, 1952).
current. This part of the nerve fiber is momentarily depolarized or activated. This change
in electrical potential then produces a similar increase in permeability, or depolarization,
in the immediately adjacent part of the nerve, causing an influx there, and so on,
until the electrochemical disturbance has traveled the length of the fiber. Its maximum
speed in human nervous tissue is about four miles per minute, and it is fastest in the
relatively thick fibers.
This brief shift in electrical energy is called the action potential, or nerve impulse,
and all impulses traveling along a particular nerve fiber have the same energy,
regardless of the nature or intensity of the activating stimulus. The reason is that a
stimulus does no more than release the electrical energy already in the fiber; it does
not contribute energy. This property of the nerve fiber, responding at full strength or
not at all, is known as the all-or-none law, which is illustrated whenever a doorbell is
pressed. If you press it hard enough, the bell sounds; if you press harder, it does not
sound any louder. But you must press hard enough, just as a stimulus must be strong
enough to activate the nerve. The stimulus intensity must be above a critical point,
called the threshold, in order to activate the fiber. If it is sufficient to do so, the resulting
action potential travels the full length of the fiber at maximum strength. If not, there
is no impulse at all.
Types of Refractory Periods The nerve fiber is like a doorbell in another sense.
Most doorbells cannot be reused until the button pops out again, and a waiting or
restoration period, known as the refractory period, is also necessary with the nerve
fiber. This refractory period has two phases. Immediately after activation, during the
absolute refractory phase, no stimulus of any strength can start a nerve impulse. This
phase differs from fiber to fiber, but it is very brief, usually only about one-thousandth
of a second.
After this phase, there is a progressive increase in excitability so that stronger
than normal stimulation can produce another response. This interval between the absolute
refractory phase and restoration of the normal resting state is known as the
relative refractory phase, and it lists a few thousandths of a second. During this phase,
the nerve responds only to intense stimulation (Figure 4.3).
Actually, nerve fibers typically appear in bundles, and thousands of fibers can
be activated simultaneously, depending upon the intensity of the stimulus. The optic
nerve, for example, has an estimated 400,000 fibers, and increasing the stimulus intensity
can have two effects. It can increase the frequency of discharge in each responding
fiber during the relative refractory phase, and it can activate more and more
receptors and fibers.