592
Chapter 16
hypothalamus and the other brain areas to the effectors is via:
(1) sympathetic nerves to the sweat glands, skin arterioles, and
the adrenal medulla; and (2) motor neurons to the skeletal
muscles.
Control of Heat Production
Changes in muscle activity constitute the major control of
heat production for temperature regulation. The fi rst muscle
changes in response to a decrease in core body temperature
are a gradual and general increase in skeletal muscle contrac-
tion. This may lead to shivering, which consists of oscillating,
rhythmical muscle contractions and relaxations occurring at
a rapid rate. During shivering, the efferent motor nerves to
the skeletal muscles are infl uenced by descending pathways
under the primary control of the hypothalamus. Because
almost no external work is performed by shivering, virtually
all the energy liberated by the metabolic machinery appears as
internal heat, a process known as
shivering thermogenesis.
People also use their muscles for voluntary heat-producing
activities such as foot stamping and hand rubbing. The oppo-
site muscle reactions occur in response to heat. Basal muscle
contraction is refl exly decreased, and voluntary movement is
also diminished. These attempts to reduce heat production
are limited, however, both because basal muscle contraction
is quite low to start with and because any increased core tem-
perature produced by the heat acts
directly
on cells to increase
metabolic rate. In other words, an increase in cellular tempera-
ture directly accelerates the rate at which all of its chemical
reactions occur. This is due to the increased thermal motion
of dissolved molecules, making it more likely that they will
encounter each other. The result is that ATP is expended at a
higher rate because ATP is needed for many of a cell’s chemical
reactions. This, in turn, results in a compensatory increase in
ATP production from cellular fuel stores, which also generates
heat as a by-product of fuel metabolism (Chapter 3). Thus,
increasing cellular temperature can itself result in the produc-
tion of additional heat through increased metabolism.
Muscle contraction is not the only process controlled in
temperature-regulating refl exes. In most experimental animals,
chronic cold exposure induces an increase in metabolic rate
(heat production) that is not due to increased muscle activity
and is termed
nonshivering thermogenesis.
Its causes are an
increased adrenal secretion of epinephrine and increased sym-
pathetic activity to adipose tissue, with some contribution by
thyroid hormone as well. However, nonshivering thermogene-
sis is quite minimal, if present at all, in adult human beings, and
there is no increased secretion of thyroid hormone in response
to cold. Nonshivering thermogenesis does occur in infants,
however, whose shivering mechanism is not yet fully devel-
oped. Newborn infants possess in addition to normal (white)
adipocytes a type of adipose tissue called brown fat, or
brown
adipose tissue.
This type of adipose tissue is responsive to thy-
roid hormone, epinephrine, and the sympathetic nervous sys-
tem, and it contains large amounts of a class of proteins called
Epinephrine
Adrenal medulla
Sweat glands
Hypothalamus
Peripheral
thermoreceptors
Central
thermoreceptors
Cerebral cortex
Skin arterioles
Skeletal muscles
Core temperature
Skin temperature
Voluntary motor responses
Via
sympathetic
nerves
Involuntary motor responses
Via
motor
nerves
Begin
Begin
Figure 16–18
Summary of temperature-regulating mechanisms beginning with peripheral thermoreceptors and central thermoreceptors. The dashed arrow
from the adrenal medulla indicates that this hormonal pathway is of minor importance in adult human beings. The solid arrows denote neural
pathways. The hypothalamus infl uences sympathetic nerves via descending pathways.
previous page 620 Vander's Human Physiology The Mechanisms of Body Function read online next page 622 Vander's Human Physiology The Mechanisms of Body Function read online Home Toggle text on/off