10
Chapter 1
most refl exes, no matter how simple they may appear to be, are
subject to alteration by learning.
The pathway mediating a refl
ex is known as the
refl
ex
arc,
and its components are shown in
Figure 1–6
. A
stimu-
lus
is defi ned as a detectable change in the internal or external
environment, such as a change in temperature, plasma potas-
sium concentration, or blood pressure. A
receptor
detects
the environmental change. A stimulus acts upon a receptor to
produce a signal that is relayed to an
integrating center.
The
pathway the signal travels between the receptor and the inte-
grating center is known as the
afferent pathway
(the general
term
afferent
means “to carry to,” in this case, to the integrat-
ing center).
An integrating center often receives signals from many
receptors, some of which may respond to quite different types
of stimuli. Thus, the output of an integrating center refl ects
the net effect of the total afferent input; that is, it represents
an integration of numerous bits of information.
The output of an integrating center is sent to the last
component of the system, whose change in activity constitutes
the overall response of the system. This component is known
as an
effector.
The information going from an integrating
center to an effector is like a command directing the effector
to alter its activity. The pathway along which this information
travels is known as the
efferent pathway
(the general term
efferent
means “to carry away from,” in this case, away from
the integrating center).
Thus far we have described the refl ex arc as the sequence
of events linking a stimulus to a response. If the response pro-
duced by the effector causes a decrease in the magnitude of the
stimulus that triggered the sequence of events, then the refl ex
leads to negative feedback and we have a typical homeostatic
control system. Not all refl exes are associated with such feed-
back. For example, the smell of food stimulates the stomach to
secrete molecules that are important for digestion, but these
molecules do not eliminate the smell of food (the stimulus).
Figure 1–7
demonstrates the components of a nega-
tive feedback homeostatic refl ex arc in the process of ther-
moregulation. The temperature receptors are the endings of
Response
Afferent
pathway
Negative
feedback
Integrating center
Efferent
pathway
Effector
Receptor
Stimulus
Begin
Figure 1–6
General components of a refl ex arc that functions as a negative
feedback control system. The response of the system has the effect
of counteracting or eliminating the stimulus. This phenomenon of
negative feedback is emphasized by the minus sign in the dashed
feedback loop.
Heat loss
INTEGRATING CENTER
Heat
production
STIMULUS
RECEPTORS
AFFERENT PATHWAY
(Nerve fibers)
EFFERENT PATHWAY
(Nerve fibers)
Constriction
Smooth muscle in
skin blood vessels
Shivering
Contraction
Skeletal muscle
Specific nerve cells in brain
Altered rates of firing
Begin
Temperature-sensitive
nerve endings
Signaling rate
Body
temperature
Figure 1–7
Refl ex for minimizing the decrease in body temperature that occurs on exposure to a reduced external environmental temperature. This fi
gure
provides the internal components for the refl ex shown in Figure 1–4. The dashed arrow and the
E
indicate the negative feedback nature of
the refl ex, denoting that the refl ex responses cause the decreased body temperature to return toward normal. An additional fl ow-diagram
convention is shown in this fi gure: Blue boxes always denote events that are occurring in anatomical structures (labeled in blue italic type in the
upper portion of the box).
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