Biomedical Engineering Reference
In-Depth Information
livestock particularly in ruminants are incon-
sistent; the levels have been observed to
fluctuate episodically, or peaks and troughs
have been found at varying times of the day
depending on the physiological status and
conditions. Increased glucocorticoid secretion
at the circadian peak depends on increased
hypothalamic-pituitary activity (HPA). Leptin
is also a major regulator of neuroendocrine
function and has an overall inhibitory effect
on HPA activity and suppress the appetite-
stimulating effects of glucocorticoid. Leptin
secretion is pulsatile but leptin pulses are
irregular in cattle. Chronobiologically, to
maximise nutrient efficiency and optimise
health, nutrient supply to reticulorumen,
splanchnic and peripheral tissues needs to be
synchronised with endogenous rhythms in
hormone production and nutrient metabolism.
The circadian system or oscillator coordinates
the metabolic and hormonal changes needed
to initiate and sustain milk synthesis or lacta-
tion. The animal or cow's capacity to produce
milk and cope with metabolic stresses in early
lactation is related to animal's ability to set
circadian rhythms in order particularly dur-
ing the transition period or early lactation.
Circadian variations are also observed in many
other biological functions like reactive oxygen
species (ROS), defence systems, thermoregu-
lation, the cardiovascular system and other
functions in humans and domestic animals.
period similar to, but not identical, 24 h. The length
of cycle is under the control of a circadian
oscillator (Ikonomov et al.
1998
) . Most living
organisms organise their activities coordinating
with 24-h light and dark cycle associated with
sunrise and sunset. Circadian rhythms like sleep-
ing and waking in animals, brain wave activity,
production of hormones and flower closing
and opening in angiosperms, tissue growth and
differentiation in fungi and other biological
activities in plants and animals are related to
the daily sunrise and sunset cycle. The circadian
rhythms are generated by an internal clock(s) that
is synchronised to light-dark cycles and other
cues in an organism's environment. This internal
clock accounts for waking up at the same time
every day spontaneously even without an alarm.
The clock also causes nocturnal animals to func-
tion at night when diurnal creatures are resting.
Circadian rhythms to an extent can be disrupted
by changes in daily schedule. In mammals, the
circadian clock or oscillator is located within the
brain's hypothalamus, and pineal gland helps
coordination through release of melatonin in
response to the information it receives from
retinal photoreceptors in the eye. In the retinal
photoreceptors, the light energy perceived is
transformed into nerve impulses and transmitted
to the hypothalamic nuclei of the central nervous
system for further processing. The suprachias-
matic nuclei (SCN) in the hypothalamus are the
anatomical loci of the circadian pacemaker
(Moore
1997
; Fig.
1
) and are the generators of
circadian rhythms (Klein et al.
1991
) . The daily
alternation in light and dark observed is the regu-
latory factor of the pineal hormone melatonin.
The typical daily pattern of melatonin secretion
also depends on the function of the suprachias-
matic circadian clock (Moore and Klein
1974
) .
The alterations or changes in long-term lighting
conditions that occur during the year result in
metabolic and behavioural changes of mammali-
ans. For instance, the reproductive cycles with
concurrent hormonal changes are entrained in
many mammalian species and livestock like
sheep by annual lighting conditions (Karsch et al.
1984
). The SCN-pineal complex is believed to
be a probable component in the mechanism
1
Introduction
A biological rhythm is a cyclic change in the bio-
logical or a chemical function. Biological rhythms
affect a variety of activities, such as the sleep-
wake cycle, migration behaviour in birds and
seasonal fattening, hibernation and reproductive
cycles in wild animals (Piccione and Caola
2002
) .
Biological rhythms are endogenously controlled
by self-contained circadian clocks or oscillators.
Circadian rhythm implies that under constant
external conditions (without time cues), the
rhythms follow a pattern with an endogenous
