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| Like any other discipline in
biology and medicine, chronobiology has developed its own nomenclature.
In relation to rhythms numerous terms describing their properties and
parameters were borrowed from physics. Biologic rhythms, however, do not
show the same precision as their counterparts in physics and thus cannot
be characterized by point estimates. Every parameter of a biologic
rhythm is a statistical entity which always has to be viewed with its
variance estimate. This qualification has to be kept in mind if terms
used in physics are applied to biologic rhythms and is expressed in the
term "circa" which is used to describe frequencies which are
known to change their cycle length under certain conditions. Although
the terms used in physics are mostly well-defined and described in
mathematical terms, their adaptation to chronobiology has led to some
differences in their use by different investigators.
Other terms were adopted or coined (often derived from
Latin or Greek) to describe aspects of biologic rhythms for which no
suitable term was available and/or in order to avoid a lengthy
descriptive phrase, the frequent use of which in a chronobiologic text
could be quite cumbersome. Many of these terms were necessary and have
been introduced similar to every other subspecialty of medicine and have
been widely accepted by the specialists in the field. However, since
chronobiology is a new and rapidly developing specialty, many of the
terms introduced recently are still unknown to many investigators and
physicians who might benefit from the application of chronobiologic
principles and findings to their work. We have tried to help the reader
of this book by presenting many of the more widely used terms with a
definition which is kept as simple and generally understandable as
possible. We are aware that some chronobiologists may be using more
complicated definitions which may have some merit as such but often make
the understanding of chronobiologic texts rather difficult for the
nonspecialized reader. We have not included terms which have not yet
widely accepted and which are not generally used in the field. It is
certain that, like in any living branch of science, some of these terms
will be accepted while others will disappear as unnecessary or
cumbersome. The following glossary is thus a snapshot of terms widely
used today in chronobiology in a presentation aimed at being short and
understandable (although sometimes perhaps simplified) and does not make
any claim to completeness. |
Acrophase.
Measure of timing of a rhythm in relation to a defined reference time point
selected by the investigator (e. g., local midnight for
circadian rhythms); used for data which can be described by the fitting
of a mathematical model, e.g., a cosine curve, and represents the crest
time of the cosine curve best fitting to the data; may be expressed in
(negative) degrees as the lag from the acrophase reference (360'C =1
period) or in calendar time units (e.g., hours and minutes for circadian
rhythms, days or months for infradian rhythms).
Amplitude
(A). The measure of one half of the extent of the rhythmic change
estimated by the mathematical model (e.g., cosine curve) best fitting to
the data (e.g., the difference between the maximum and the
rhythm-adjusted mean (MESOR) of the best fitting curve).
Autorhythmometry. Self-measurement of biologic rhythms by the subject examined.
Bathyphase.
The time of the lowest point of a mathematical model (e. g., cosine
curve) fitted to a time series and describing a rhythm. If a sine or a
cosine curve is fitted, the bathyphase will differ 180' from the
acrophase, measured in relation to a defined reference time point
selected by the investigator (e.g., local midnight for circadian
rhythms); may be expressed in degrees as the lag from the phase
reference (360' = I period) or in calendar time units (e. g., hours and
minutes for circadian rhythms, days or months for infradian rhythm).
Biologic
time structure. The sum of nonrandom time-dependent biologic
changes, including growth, development, and aging, and a spectrum of
rhythms with different frequencies.
Biological clocks.Self-sustained oscillators which generate biologic rhythms
in absence of external periodic input (e. g., at the gene level in
individual cells)
Biologic rhythm. A regularly recurring (periodic) component in a series of
measurements of a biologic variable obtained as a function of time.
Chronergy. Represents the rhythmic change of the response of the organism to a drug
(its total effect) according to its chronokinetics and its chronesthesy
(see below).
Chronesthesy. Rhythmic (thus predictable-in-time) changes in the susceptibility or
sensitivity of a target biosystem (cell or organism) to an agent. May be
caused by temporal changes in receptors of target cells or organs,
membrane permeability, etc.
Chronobiology.The science of investigating and objectively quantifying phenomena and
mechanisms of the biologic time structure, including the rhythmic
manifestations of life. Term derived from: Chronos (time), bios (life),
and logos (science).
Chronobiotic. An agent capable of influencing biologic rhythm parameters (e.g., the
phase setting).
Chronodesm. Time-qualified reference intervals. Reference intervals constructed
along the time scale by Gaussian or non-Gaussian methods. Include time
qualified prediction and tolerance intervals.
Chronogram. Display of data as a function of time.
Chronopathology. Changes in an individual's biologic time structure preceding, coincident
or following functional disorders or organic disease and/or
time-dependent manifestation of disease.
Chronopharmacodynamics. Temporal variations in the mode of action of a drug.
Chronopharmacokinetics.
The study of the temporal changes in absorption, distribution,
metabolism, and elimination of a drug. Describes the influence of the
time of administration of a drug on the mathematical parameters which
describe these processes in terms of absorption rate, peak drug
concentration (Cmax), time-to-peak drug concentration (Emax), area under
the concentration time curve (AUC), half-life (t1/2), etc.
Chronotherapy. Use of treatment timed according to the stages in the
sensitivity-resistance cycles of target (or nontarget) tissues and
organs (or of the organism as a whole) to enhance the desired
pharmacologic effect and/or reduce undesirable side effects of drugs or
other therapeutic agents.
Chronotolerance. Time-dependent tolerance of an organism to environmental stimuli and
xenobiotics. Chronotoxicology.
Time-dependent variation in toxicity.
Circadian. About 24 h. The term describes rhythms with an about 24-h
( > 20 to < 28 h) cycle length whether they are synchronized with
a 24-h periodic surrounding or not.
Circadiseptan. A rhythm with a period of about 14 ( ± 3)
days.
Circannual. A rhythm with a period of about 1 year ( ± 2
months), synchronized with or desynchronized from the calendar year.
Circaseptan. A rhythm with a period of about 7 ( ± 3) days,
which may or may not be synchronized with the calendar week.
Circatrigintan. A rhythm with a period of about 30 (±5)
days. Includes, in mature women during the time of ovarian activity, the
menstrual cycle. The term is preferred to the term "menstrual"
because rhythms of this frequency are found in premenarchal girls,
postmenopausal women and in men.
Circavigintan. A rhythm with a period of about 20 ( ± 3
days).
Clinospectrometry. Resolving of a spectrum of rhythms and
trends (cline) by (computer implemented) time series collection and
analysis. With rhythms quantified as algorithmically formulated
phenomena validated in inferential statistical terms.
Cosinor procedure. A mathematical-statistical method of
describing a rhythm by determining by least squares technique the cosine
curve best fitting to the data and exploring the presence of a rhythm by
examining the null hypothesis for amplitude in an F-test. If a rhythm
can be described by this procedure the cosinor yields a rhythm-adjusted
mean (MESOR), an amplitude as measure of the extent of the rhythm, and
an acrophase as indication of its timing with variance estimates for
each.
Cosinor. Single cosinor - a cosinor procedure
applicable to single biologic time series.
Population mean cosinor
- the cosinor procedure applicable to parameter estimates from three or
more biologic time series for assessing the rhythm characteristics of a
population. The parameter estimates are based on the means of estimates
obtained from individuals in the samples.
Daily. Occurring every day.
Dampened oscillation. Oscillation decreasing (dampened) in
amplitude due to inevitable loss of energy.
Desynchronization.
State of two or more previously synchronized rhythmic variables that
have ceased to exhibit the same frequency and/or the same acrophase
relationships and show different than usual and/or changing time
relations.
Diurnal. Day related (in contrast to nocturnal), e. g.,
diurnal (vs nocturnal) activity pattern.
Endogenous rhythm. Presumably
genetically fixed biologic rhythm, persisting in an environment without
outside time cues.
Entrainment. Coupling of two rhythms of the same frequency to
one of them (the entraining agent or synchronizer) determining the phase
of the other. e. g., Coupling of endogenous rhythms to environmental
oscillator of the same frequency and/or determination of the phase of
biologic rhythms by an internal pacemaker.
Episodic variation. Apparently irregular (nonrhythmic)
variation of a biologic variable, e. g., episodic secretion of certain
hormones (used by some as synonymous with "pulsatile").
External desynchronization. Desynchronization of a biologic
rhythm from an environmental cycle.
Feedsideward coordination.
Interaction of several rhythms (multifrequency coordination). Involves
rhythmic and to that extent predictable sequences of effects depending
upon the phase of each of the rhythms involved. "Feedsideward"
may manifest itself as rhythmic alteration of stimulation, no effect or
inhibition by an action of a rhythmic entity upon two other interacting
entities.
Free running. Continuance of an endogenous bioperiodicity at
least slightly but consistently different from any known environmental
schedule, i. e., from its usual synchronizer or usual pacemaker rhythm.
Frequency (f). The number of cycles occurring per time unit; f is the
reciprocal of the period (t).
Frequency ranges. Groups of frequencies (or periods)
frequently encountered in biologic rhythms. (Circadian frequency range:
rhythm with periods of about one day, i. e., by definition > 20 to
< 28 h).
Infradian rhythm. Rhythm with a period
longer (by definition > 28 h) than the circadian range; the term
includes circaseptan, circatrigintan, circannual, and other rhythms of
lower frequency.
Internal desynchronization. State in which two or more
previously synchronized variables within the same organism have ceased
to exhibit the same frequency and/or the same acrophase relationships
and show different than usual and/or changing time relations.
Jet lag. Desynchronization and its clinical effect
after rapid movement over several time zones (after trans- meridian
flights).
Lighting regimen. The light-dark cycle (LD),
or constant light (LL), or constant dark (DD) conditions used for
chronobiologic studies.
Longitudinal sampling. Study of the same subject or of a
group of subjects over numerous cycles.
Longitudinal study. Study of the same individual over a
prolonged time span (e. g., aging).
Marker rhythm. Rhythm of use in monitoring an
organism's biologic timing and/or the timing of a related rhythm showing
a fixed time relation to the rhythm used as "marker". Can be
used where appropriate for decision-making in applied or basic
physiologic or pharmacologic work, e.g., for time of sampling, timing of
therapy, or for assessing therapeutic response (without any implication
of causal relations between the rhythmic process and its marker). See
also "reference rhythm".
Masking of a rhythm. Alteration of the usual shape and/or parameters
of a rhythm due to random or nonrandom environmental stimuli, persisting
for the duration of the stimulus only (without persistent alteration of
endogenous rhythm components). e.g., change in body temperature after a
hot bath.
MESOR. Midline Estimating Statistic of Rhythm. The value midway
between the highest and the lowest values of the (cosine) function best
fitting to the data. The "M" is equal to the arithmetic mean
only for equidistant data covering an integral number of cycles.
Pacemaker. A functional entity capable of
self-sustaining oscillations which synchronize other rhythms (e.g., the
suprachiasmatic nucleus in man).
Peak. The highest point in a series of measurements obtained as a
function of time.
Period (t). Duration of one complete
cycle in a rhythmic variation.
Phase. The value of a rhythmic biological variable at a certain
time. Each instantaneous state of an oscillation represents a phase.
Phase advance. Involves the earlier occurrence of a rhythm's phase,
usually the acrophase (denoted by a plus sign).
Phase delay. Involves the later occurrence of a rhythm's (acro)phase
(denoted by a minus sign).
Phase drift. During free running of an endogenous rhythm with a
period slightly but consistently different from its usual environmental
synchronizer, the rhythm's acrophase will occur during every
synchronizer cycle at a different time (e. g., clock hour in the case of
circadian rhythms) in relation to the phase reference.
Phase reference. Time point chosen by the investigator as reference
for the estimation of the timing of a rhythm (e. g., local midnight for
circadian rhythms).
Phase response curve. Graphical plot indicating how the amount and
the duration of a phase shift, induced by a single stimulus, depend upon
the rhythm's stage at which the stimulus is applied.
Phase shift. Single relatively abrupt or gradual change in the
timing of a rhythm (completed within a finite time span) and described
by the difference between the initial and final (acro)phase.
Photoperiod. In a light-dark regimen the duration of the light span
(e.g., in light-dark = LD 12:12 h, the photoperiod L = 12 h).
Plexogram. Display of original data covering spans longer than the
period of a rhythm investigated along an abscissa of a single period
(irrespective of time order of data collection).
Pulsatile variation. Variation of a biologic function with an
irregular period higher than circadian of which a regular recurrence
(rhythm) cannot be documented. May be the result of circadian-ultradian
interactions or of other rhythmic and/or nonrhythmic mechanisms (used by
some as synonymous with "episodic").
Reference rhythm. A rhythm in one variable
used as a time reference for other rhythms, events, or actions. (See
also "marker rhythms").
Rhythm. A regularly recurring and thus, to a certain degree,
predictable (periodic) component of a (biologic) time series,
demonstrated by inferential statistical means.
Scotoperiod. In a light-dark regimen the
duration of the dark span (e. g., light-dark = LD 12:12 h the
scotoperiod D = 12 h).
Seasonal variation. Change in a biologic system brought about by
seasonal changes of temperature, light-span, etc, and not observed in
the absence of such changes.
Self-sustained oscillation. System that can make use of a constant
source of energy (to counteract energy losses) and is able to continue
to oscillate without outside energy input.
Shift work. Transient or permanent change in work schedule in
relation to the social surroundings (e.g., 3 x 8-h work shifts).
Suprachiasmatic nucleus. Group of hypothalamic neurons situated
above the optic chiasm exhibiting an endogenous circadian oscillation
acting as circadian pacemaker, receiving external phase information via
the retina.
Synchronization. State of a system when two or more
variables exhibit periodicity with the same frequency and specifiable
acrophase and phase relation.
Synchronizer. Environmental periodicity determining the temporal
placement of a biologic rhythm along an appropriate time scale.
Synonyms: entraining agent, time giver, Zeitgeber.
Synchronizing agent. See Synchronizer.
Time giver See Synchronizer.
Time series. A series of measurements obtained as a function of
time.
Transmeridian flight. Movement over time zones (see Jet lag).
Transverse sampling. Sampling of a group of subjects over one cycle
of a rhythm.
Transverse study. Comparison of two groups differing by a parameter
(e. g., age, sex, etc.) studied at one time (e. g., over one cycle).
Trough. The lowest point in a series of measurements obtained as a
function of time.
Ultradian rhythm. Biologic rhythm with a
period shorter than circadian (less than 20 h).
Zeitgeber See Synchronizer. It has to be
understood that the "Zeitgeber" does not "give time"
(does not induce a rhythm) but determines its arrangement in time
(synchronizes). |
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