Homeostasis: A Framework for Human Physiology
11
certain nerve cells in various parts of the body. They gener-
ate electrical signals in the nerve cells at a rate determined by
the temperature. These electrical signals are conducted by
the nerve fi bers—the afferent pathway—to the brain, where
the integrating center for temperature regulation is located.
The integrating center, in turn, sends signals out along those
nerve cells that cause skeletal muscles and the muscles in skin
blood vessels to contract. The nerve fi bers to the muscles are
the efferent pathway, and the muscles are the effectors. The
dashed arrow and the
E
indicate the negative feedback nature
of the refl ex.
Almost all body cells can act as effectors in homeostatic
refl exes. There are, however, two specialized classes of tis-
sues—muscle and gland—that are the major effectors of bio-
logical control systems. In the case of glands, for example, the
effector may be a hormone secreted into the blood. A
hor-
mone
is a type of chemical messenger secreted into the blood
by cells of the endocrine system (see Table 1–1). Hormones
may act on many different cells simultaneously because they
circulate throughout the body.
Traditionally, the term
refl
ex
was restricted to situations
in which the receptors, afferent pathway, integrating center,
and efferent pathway were all parts of the nervous system, as
in the thermoregulatory refl ex. However, the principles are
essentially the same when a blood-borne chemical messen-
ger, rather than a nerve fi ber, serves as the efferent pathway, or
when a hormone-secreting gland (called an
endocrine gland
)
serves as the integrating center. Thus, in the thermoregulation
example, the integrating center in the brain not only sends
signals by way of nerve fi bers, as shown in Figure 1–7, but also
causes the release of a hormone that travels via the blood to
many cells, where it increases the amount of heat these cells
produce. This hormone therefore also serves as an efferent
pathway in thermoregulatory refl exes.
In our use of the term
refl
ex,
therefore, we include hor-
mones as refl ex components. Moreover, depending on the
specifi c nature of the refl ex, the integrating center may reside
either in the nervous system or in an endocrine gland. In addi-
tion, an endocrine gland may act as both receptor and inte-
grating center in a refl ex. For example, the endocrine gland
cells that secrete the hormone insulin, which lowers plasma
glucose concentration, themselves detect increases in the
plasma glucose concentration.
In conclusion, many refl exes function in a homeostatic
manner to keep a physical or chemical variable of the body
within its normal range. Any such system can be analyzed by
answering the questions listed in
Table 1–3
.
Local Homeostatic Responses
In addition to refl exes, another group of biological responses,
called
local homeostatic responses,
is of great importance
for homeostasis. They are initiated by a change in the external
or internal environment (that is, a stimulus), and they induce
an alteration of cell activity with the net effect of counteract-
ing the stimulus. Like a refl ex, therefore, a local response is
the result of a sequence of events proceeding from a stimulus.
Unlike a refl ex, however, the entire sequence occurs only in
the area of the stimulus. For example, when cells of a tissue
become very metabolically active, they secrete substances into
the interstitial fl uid that dilate local blood vessels. The result-
ing increased blood fl ow increases the rate at which nutrients
and oxygen are delivered to that area. The signifi cance of local
responses is that they provide individual areas of the body with
mechanisms for local self-regulation.
Intercellular Chemical Messengers
Essential to refl exes and local homeostatic responses, and
therefore to homeostasis, is the ability of cells to communicate
with one another. In this way, cells in the brain, for exam-
ple, can be made aware of the status of activities of structures
outside the brain, such as the heart, and help regulate those
activities to meet new challenges. In the majority of cases,
this communication between cells—intercellular communica-
tion—is performed by chemical messengers. There are three
categories of such messengers: hormones, neurotransmitters,
and paracrine/autocrine agents (
Figure 1–8
).
As noted earlier, a hormone functions as a chemical
messenger that enables the hormone-secreting cell to com-
municate with cells acted upon by the hormone—its
target
cells
—with the blood acting as the delivery system. Most
nerve cells communicate with each other or with effector
cells, such as muscles, by means of chemical messengers called
neurotransmitters.
Thus, one nerve cell alters the activity of
another by releasing from its ending a neurotransmitter that
diffuses through the extracellular fl uid separating the two
nerve cells and acts upon the second. Similarly, neurotrans-
mitters released from nerve cells into the extracellular fl uid in
the immediate vicinity of effector cells constitute the control-
ling input to the effector cells. Neurotransmitters and their
roles in nerve cell signaling will be covered in Chapter 6.
Chemical messengers participate not only in refl exes,
but also in local responses. Chemical messengers involved in
local communication between cells are known as
paracrine
agents.
Paracrine agents are synthesized by cells and released,
Table 1–3
Questions to Be Asked About Any
Homeostatic Response
1. What is the variable (for example, plasma potassium
concentration, body temperature, blood pressure) that is
maintained within a normal range in the face of changing
conditions?
2. Where are the receptors that detect changes in the state of
this variable?
3. Where is the integrating center to which these receptors send
information and from which information is sent out to the
effectors, and what is the nature of these afferent and efferent
pathways?
4. What are the effectors, and how do they alter their activities
so as to maintain the regulated variable near the set point of
the system?
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