14
Chapter 1
Subjects were put in experimental chambers that com-
pletely isolated them from their usual external environment,
including knowledge of the time of day. For the fi rst few days,
they were exposed to a 24 h rest-activity cycle in which the
room lights were turned on and off at the same time each
day. Under these conditions, their sleep-wake cycles were 24
h long. Then, all environmental time cues were eliminated,
and the subjects were allowed to control the lights themselves.
Immediately, their sleep-wake patterns began to change. On
average, bedtime began about 30 min later each day, and so did
wake-up time. Thus a sleep-wake cycle persisted in the com-
plete absence of environmental cues. Such a rhythm is called a
free-running rhythm.
In this case it was approximately 25 h
rather than 24. This indicates that cues are required to entrain
or set a circadian rhythm to 24 h.
The light-dark cycle is the most important environmen-
tal time cue in our lives, but not the only one. Others include
external environmental temperature, meal timing, and many
social cues. Thus, if several people were undergoing the exper-
iment just described in isolation from each other, their free-
running rhythms would be somewhat different, but if they
were all in the same room, social cues would entrain all of
them to the same rhythm.
Environmental time cues also function to
phase-shift
rhythms—in other words, to reset the internal clock. Thus if
you jet west or east to a different time zone, your sleep-wake
cycle and other circadian rhythms slowly shift to the new
light-dark cycle. These shifts take time, however, and the dis-
parity between external time and internal time is one of the
causes of the symptoms of jet lag—a disruption of homeosta-
sis that leads to gastrointestinal disturbances, decreased vigi-
lance and attention span, sleep problems, and a general feeling
of malaise.
Similar symptoms occur in workers on permanent or
rotating night shifts. These people generally do not adapt
to their schedules even after several years because they are
exposed to the usual outdoor light-dark cycle (normal indoor
lighting is too dim to function as a good entrainer). In recent
experiments, night-shift workers were exposed to extremely
bright indoor lighting while they worked and 8 h of total
darkness during the day when they slept. This schedule pro-
duced total adaptation to night-shift work within 5 days.
What is the neural basis of body rhythms? In the part
of the brain called the hypothalamus, a specifi c collection of
nerve cells (the suprachiasmatic nucleus) functions as the prin-
cipal
pacemaker,
or time clock, for circadian rhythms. How it
keeps time independent of any external environmental cues is
not fully understood, but it appears to involve the rhythmical
turning on and off of critical genes in the pacemaker cells.
The pacemaker receives input from the eyes and many
other parts of the nervous system, and these inputs mediate
the entrainment effects exerted by the external environment.
In turn, the pacemaker sends out neural signals to other parts
of the brain, which then infl uence the various body systems,
activating some and inhibiting others. One output of the pace-
maker goes to the
pineal gland,
a gland within the brain that
secretes the hormone
melatonin.
These neural signals from
the pacemaker cause the pineal to secrete melatonin during
darkness but not during daylight. It has been hypothesized,
therefore, that melatonin may act as an important mediator to
infl uence other organs either directly or by altering the activ-
ity of the parts of the brain that control these organs.
Balance in the Homeostasis of Chemical
Substances in the Body
Many homeostatic systems regulate the balance between
addition and removal of a chemical substance from the body.
Figure 1–10
is a generalized schema of the possible pathways
involved in maintaining such balance. The
pool
occupies a
position of central importance in the balance sheet. It is the
body’s readily available quantity of the substance and is often
identical to the amount present in the extracellular fl
uid. The
pool receives substances from and redistributes them to all the
pathways.
The pathways on the left of Figure 1–10 are sources of
net gain to the body. A substance may enter the body through
Metabolism
POOL
Storage depots
Reversible
incorporation
into other
molecules
GI tract
Lungs
NET GAIN TO BODY
DISTRIBUTION WITHIN
BODY
NET LOSS FROM
BODY
Excretion from body
via lungs, GI tract,
kidneys, skin,
menstrual flow
Air
Food
Synthesis in body
Figure 1–10
Balance diagram for a chemical substance.
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