Biomedical Engineering Reference
In-Depth Information
Fig. 1
The biological clock
located within the suprachias-
matic nucleus in the brain
(Moore
1997
; BSCS
2003
,
Copyright © 2003 BSCS.
All rights reserved. Used
with permission)
Suprachismatic nucleus
Hypothalamus
Thalamus
Lateral geniculate
nucleus
Cerebral
cortex
Pineal gland
Retina
Pons
Light
falling
on
retina
Optic nerve
Spinal cord
signals transmitted to
body
entraining annual rhythms (Zucker et al.
1991
;
Scott et al.
1995
). Pineal gland and its secretion
facilitates regulation of seasonal breeding in
mammalians by synchronising the hypothalamo-
hypophyseal-gonadal axis reproductive functions.
The melatonin serves the reproductive system as a
messenger of night duration or length (Karsch
et al.
1991
; Bartness et al.
1993
) . In addition to
the melatonin rhythm and pattern, the daily varia-
tion in glucocorticoid levels in blood is also a
classical example of circadian rhythms in humans
(Bliss et al.
1953
; Orth and Island
1969
) , rats
(
Rattus norvegicus
, Guillemin et al.
1959
; Moore
and Eichler
1972
) rhesus monkey (
Macaca
mulatta
, Perlow et al.
1981
) , Syrian hamsters
(
Mesocricetus auratus
, de Souza and Meier
1987
)
and red-backed voles (
Clethrionomys gapperi
,
Kramer and Sothern
2001
) . Though the phenom-
enon of circadian rhythm of glucocorticoids is
classical in many mammalian species, but in
ruminants, like cattle (MacAdam and Eberhart
1972
; Wagner and Oxenreider
1972
; Abilay and
Johnson
1973
; Lefcourt et al.
1993
) and sheep
(Simonetta et al.
1991
), they do not exhibit any
particular pattern. The inconsistent, absence of
rhythm or discrepant results in ruminants could
be attributed to differences in metabolism of
nutrients and to the role of glucocorticoids in the
regulation of metabolism in ruminants. The special
digestive system of ruminants stores and processes
a large amount of undigested, partially digested
Table 1
Periods of biological rhythms
Type of rhythm
Period (
t
)
Ultradian
t
< 20 h
Circadian
20 h £
t
£ 28 h
Infradian
t
> 28 h
Circaseptan
t
= 7 ± 3 days
Circannual
t
= 1 year ± 3 months
Source: Piccione and Caola (
2002
)
and digested food; nutrients continue to absorb
into the blood evenly during the 24-h period.
Therefore, variations in metabolites and metabolic
hormones are little in ruminants, and characteris-
tic patterns are either absent or inconsistent in
ruminants. The lighting conditions affect regula-
tion of lipid metabolism in mammals (Clarke
2001
; Bartness et al.
2002
; Morgan et al.
2003
) ,
and the melatonin is a principal mediator and
regulator of photoperiodic information (Lincoln
et al.
2003
) . The variation in the periods of bio-
logical rhythm has been presented in Table
1
.
Biological rhythms in mammals are of two
general types:
•
Exogenous rhythms
: The exogenous rhythms
as the name indicates are directly produced by
an external influence such as sunlight, food,
noise or social interaction and are not generated
internally by the organism itself, for example,
environmental cue. Since these rhythms are
not generated internally, in the absence of
environmental cues, the exogenous rhythm is
