During the year there changes in the photo and scotoperiod caused by
seasonal variations. In order to keep its clock synchronised to these cues,
an organisms must be able to reset its clock
In humans, resetting is applicable when considering human travel,
especially the phenomenon of jet lag. When we cross time zones there appears
to be a shift in environmental Zeitgebers, especially light, and to a lesser
extent temperature. This causes a conflict between external rhythms and the
person’s internal circadian clock. As the internal rhythms reset to local
environmental Zeitgebers, they do so at different rates. This results in the
fatigue, gastrointestinal complaints and short attention span that
constitute the symptoms of jet lag. Resetting also has issues with shift
work, as shift schedules often require the frequent resynchronisation of
individuals to new time cues as a result of working shifts.
The reason an organism needs to reset its clock is that the changes in
Zeitgebers may cause a phase shift in the clock of the organism. Therefore,
to understand resetting we need to understand phase shifts, by looking at
phase response curves (PRC).
Perturbing pulses
Phase response curves are constructed by keeping an organism in constant
conditions to establish a freerunning cycle. This is most often constant
dark. Then a change in conditions is used to induce phase shifts. These are
such things as light pulses (in constant dark), dark pulses (in constant
light) or steps from light to dark.
Circadian time
The unit of time used in PRCs is circadian time. This does not
necessarily correspond to “normal” 24-hour time. Circadian time is used
to keep track of time in a freerunning cycle and is used to denote time of
day in a biological clock.
In a freerunning cycle, the period length of a cycle is set as 24
circadian time units. Time 0 is considered to be the time of onset of
activity in diurnal animals. For nocturnal animals, onset of activity is
given as time 12. These times are then used to set subjective day, with
times 0-12 being subjective day, and times 12-0 being subjective night.
Calculation of circadian time
Phase response curves
If organisms kept in constant conditions are exposed to a single
perturbing signals (a light pulse introduced to organisms kept in constant
dark will be used as an example) at different times over a 24-hour period,
the phase shift caused is not always the same. At some times during the
cycle, the phase shift will regress (delay) the cycle; at other times the
phase shift will advance the cycle. At some points in the subjective day
there are little or no shifts in the phase.
If the phase shifts obtained by a perturbing signal are plotted against
circadian time, the result is a phase response curve.
Although the PRCs for each different species is different, examination of
PRCs show that there are certain common features. Delay phase shifts occur
when the light pulse is given early in the subjective night and around
“dusk”. Advancing shifts are achieved when the light pulse happens late
in the subjective night at around “dawn”. There are two places in a PRC
where there appears to be little phase shift – towards the middle of the
subjective day, and area called the dead zone, and around the middle
of the subjective night called the singularity. The singularity
occurs because it is at this point that there is a switch from delaying
phase shifts to advancing phase shift. The reason for the dead zone is less
clear. It may be that that there is a change in the sensitivity of the
circadian pacemaking process. However it may also represent a decrease in
the sensitivity of detecting the perturbing signal.
A generalised phase response curve
in response to light pulses
PRCs with dark pulses
Whilst most PRCs are constructed using light pulses on animals in
constant dark, some are produced using pulses of darkness in otherwise
constant light to induce phase shifts. If this method is used, the pattern
seen with light pulses is reversed, with advances being seen early in the
subjective night and delays being seen late in the subjective night. Such
graphs are referred to as mirror images (despite the fact that the graphs
are not exactly reversed).
Factors that affect PRCs
Phase response curves are not identical for all animals. Different
species have different PRCs. Many of the differences arise due to lifestyle.
Larger phase shifts can be obtained from smaller signals in nocturnal
animals. For example nocturnal hamsters respond to a 15 minute light pulse,
whilst diurnal house sparrows respond only to light pulses of 2-4 hours.
This may be due to nocturnal animals having a greater sensitivity in
detecting light, as their natural environments have only limited light
(Binkley 1997).
As a rule the PRCs for nocturnal animals tend to have a longer delaying
portion. This is because nocturnal animals tend to freerun with a period
shorter than 24 hours and need to delay to correct to a 24-hour cycle.
Diurnal animals tend to have a longer advancing portion and they freerun
with periods longer than 24 hours and thus need to advance their clocks to
reduce the cycle to 24 hours.
Using PRCs to explain phase shifts
PRCs can be used to explain many of the properties of circadian rhythms
Entrainment to an altering photoperiod over the year i.e. short days in
winter and long days in summer have been explained by looking at PRCs
resulting from the use of light and dark pulses. Entrainment to shortening
photoperiod is attributed to the phase shifting sensitivity to dark as
revealed by dark pulse PRCs; whilst the entrainment to long summer days is
explained by the phase shifting sensitivity to light shown by light pulse
PRCs.
The effects of light intensity may be explained by the
differences in the size of advancing and delaying portions in nocturnal and
diurnal animals. In diurnal animals there is a larger advancing portion. The
effect of constant light should therefore be an advance in the phase,
leading to a shorter period length. On the other hand nocturnal animals have
a larger delaying portion, the effects of constant light therefore should be
a delay in the phase with a resultant longer period.
RESETTING
