Download Ch 13: Homeostasis: Active regulation of internal states

Ch 13: Homeostasis:
Active regulation of internal states
Millions of years of evolution have endowed our bodies
with a complex physiological system devoted to
producing a stable internal environment.
}  Food energy, body temperature, fluid balance, fat storage,
nutrients are carefully regulated, and although we may be
unaware of some of these processes, the nervous system is
intimately involved in every stage.
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Homeostasis maintains internal states within a
critical range
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Redundancy: The property of having a particular process
monitored and regulated by more than one mechanism.
Several different means of monitoring our stores, of conserving
remaining supplies, etc. Loss of one part of the system usually
can be compensated for by the remaining parts.
Homeostasis: Referring to the active process of maintaining a
particular physiological parameter relatively constant. The
nervous system coordinates these actions.
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Negative feedback for homeostasis
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Negative feedback: The property by which some of the output of
a system feeds back to reduce the effect of input signals.
Set point: The point of reference in a feedback system [set zone].
Fig. 13.1
Homeostasis
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Body temperature regulation.
Fluid regulation.
Food and energy regulation.
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Body temperature is a critical condition for all
biological processes
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The rate of chemical reactions is temperature-dependent.
The enzyme systems of mammals and birds are most efficient within
a narrow range around 37 ℃. [protein fold/fuse process in high
temperature; lipid bilayers of cellular membranes become disrupted
by the formation of ice molecules at very low temperature
(antifreeze protein)].
Brain cells are especially sensitive to high temperature.
Fig. 13.2
Thermoregulation in humans
Fig. 13.3
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Brain monitors and regulates body temperature
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Preoptic area (POA) of the hypothalamus [lesion & stimulation studies].
Lesions in the lateral hypothalamus abolished behavioral regulation; Lesions of
POA impaired the physiological regulation of temperature (shivering,
vasoconstriction).
The set zones of thermoregulatory systems are narrower at higher levels of the
nervous system than at lower levels.
Fig. 13.4
Fluid regulation
Our cells evolved to function in seawater.
}  Land animals had to prevent dehydration.
}  Water in the human body moves back and forth between
two major compartments.
}  Aquaporins(水通道蛋白): a single aquaporin-1 channel
can selectively conduct about 3 billion molecules of water
per second.
}  Osmosis(滲透): a passive movement of molecules from
one place to another.
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Osmosis (滲透)
Fig. 13.9
Two internal cues trigger thirst
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Physiological saline is 0.9% NaCl. A solution with more salt is hypertonic; a solution
lower in salt is hypotonic.
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Hypovolemic thirst: thirsty caused by decreasing volume; serious blood loss
(hemorrhage), the blood pressure drops à thirty. Baroreceptors in major blood vessels
detect any pressure drop from fluid loss.
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Osmotic thirst: thirsty caused by increasing solute concentration; consume salty food
à thirty. Osmosensory neurons in the brain detect any increased osmolality of
extracellular fluid.
Fig. 13.10
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Circumventricular organs
•  The circumventricular organs mediate between the brain and the cerebrospinal fluid (CSF).
•  The BBB is weak in the subfornical organ, organum vasculosum of the lamina terminalis
(OVLT), and area postrema, so neurons there can monitor the osmolality of blood.
Fig. 13.12
An Overview of Fluid Regulation
Fig. 13.15
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Food and energy regulation
Nutrients, a chemical that is needed for growth,
maintenance, and repair of the body but is not used as a
source of energy.
}  Of 20 amino acids found in our bodies, 9 are difficult or
impossible for us to manufacture, so we must find these
essential amino acids in our diet.
}  Most of our food is used to provide us with energy.
}  ~33% of the energy in food is lost in the digestive process,
~55% of food energy is consumed by basal metabolic
process, and ~12% for active behavioral process.
} 
Why losing weight is so difficult
Fig. 13.17
At the start of a diet (less nutrition), the basal metabolic rate
will fall to prevent losing weight.
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Figure 13.18 The Benefits of Caloric Restriction in
Monkeys
Figure 13.19 The Role of Insulin in Energy Utilization
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The Hypothalamus Coordinates
Multiple Systems That Control Hunger
No single brain region has control of appetite, but the
hypothalamus is important to regulation of:
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Metabolic rate
Food intake
Body weight
A dual-center hypothesis proposed two appetite centers in
the hypothalamus:
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One for signaling hunger
(internal state of an animal seeking food)
One for signaling satiety
(feeling of fulfillment or satisfaction)
Information regarding feeding status was thought to be
integrated here.
Anorexia or
Aphagia
(厭食症)
(Hunger center)
(Satiety center)
Obesity
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The Hypothalamus Coordinates Multiple
Systems That Control Hunger
Ventromedial hypothalamus (VMH) lesions cause animals
to eat to excess (hyperphagia) and become obese,
suggesting the VMH is a satiety center.
Lateral hypothalamus (LH) lesions cause aphagia—
refusal to eat—suggesting LH is a hunger center.
Fig. 13.20
The dual-center hypothesis proved to be
too simple.
VMH-lesioned animals exhibit a dynamic phase of obesity
with hyperphagia (overeating) until they become
obese, on a rich diet.
Their increased weight stabilizes in a static phase of
obesity; this weight is maintained even after food
manipulations.
LH-lesioned animals stop eating and drinking but will
resume and stabilize their weight at a new, lower level.
As with VMH-lesioned animals, they can be forced to gain
or lose weight but will regulate their body weight
when offered standard food.
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Hypothalamic nuclei in the appetite control
The arcuate nucleus of the hypothalamus contains an appetite controller
governed by hormones, like insulin.
The arcuate nucleus of the hypothalamus
The arcuate nucleus of the hypothalamus
contains an appetite controller governed by
hormones, like insulin.
Other peripheral peptide hormones
leptin: produced by fat cells and secrete it into the
bloodstream.
}  Ghrelin: released into the blood by endocrine cells
in the stomach. an appetite stimulant.
}  PYY3–36: Peptide YY3-36 is secreted by cells in the
intestines. an appetite suppressor
} 
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The brain can monitor circulating leptin levels !
body’s energy reserves in fat.
•  Mice with two copies of the obese gene
(ob/ob),which have defective leptin genes,
become obese.
•  Defects in leptin production or sensitivity
give a false reporting of body fat, causing
animals to overeat
Fig. 13.22
Figure 13.23 Integration of Appetite
Signals in the Hypothalamus (Part 1)
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The arcuate appetite system The arcuate appetite system relies on two sets of
neurons with opposing effects:
1.  NPY/AgRP neurons produce neuropeptide Y
(NPY) and agouti-related peptide (AgRP).
•  These neurons stimulate appetite and lower
metabolism, promoting weight gain.
2.  POMC/CART neurons produce proopiomelanocortin (POMC) and cocaine- and
amphetamine-related transcript (CART).
•  These neurons inhibit appetite and raise
metabolism, promoting weight loss.
Figure 13.23 Integration of Appetite
Signals in the Hypothalamus (Part 2)
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The NST in the brainstem receives and integrates
appetite signals from many sources, some via the
vagus nerve.
Orexin is a peptide produced in the LH that also
causes increased feeding.
Cholecystokinin (CCK) is a peptide hormone
released by the gut after high intake and acts on
receptors on the vagus nerve to inhibit appetite.
The endocannabinoid—an endogenous homolog
of marijuana—system is a major regulator of
appetite and feeding, primarily stimulating hunger.
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