biological clocks and rhythms

chronobiology

scientific study of biological clocks and their associated rhythms

rhythm

a recurrent event characterized by:

1. period

2. frequency

3. amplitude 

4. phase

period

length of time to complete one cycle of rhythm; varied range

frequency

how often the rhythm recurs or the number of completed cycles per unit of time

amplitude

the amount of change from the average

phase

the point of the rhythm relative to some objective timepoint

exogenous

factors outside the organism

endogenous

factors inside the organism

rhythm persists

if environment cues are removed to create constant conditions/biological rhythm

• rhythm is endogenous or generated within organism

rhythm disappears

if environmental cue is removed to create constant conditions

• rhythm is exogenous or driven by environmental cues 

coupled to

although endogenous and not driven by the environment, rhythms can be BLANK BLANK environmental cues

entrainment

process of synchronizing an endogenous biological rhythm to an environmental cue

free-runningh

rhythm that represents the endogenous rhythm; rhythm that persists when environmental cues are removed

phase shift 

shift in activity in response to synchronizing stimulus of light

phase advance

shift clock 1 hour ahead/travel east - produces symptoms such as malaise, impaired attention and motivation, sleep disruption

phase delay

set clock 1 hour back/travel west - easier adjustment; can go to bed later and sleep in later

mood disorders

individuals with this have significant disruption in circadian rhythms

cardiovasicular disease, obesity and cancer

disrupted circadian rhythm are associated with these chronic illnesses

ultradian

infradian

rhythms that do not correspond to any known geophysical cues

ultradian rhythm 

shorter than circadian, repeat more than once per day 

infradian rhythm

longer than a day but shorter than a month 

predictable events

By synchronizing the activities of animals with their environments, biological clocks prepare for BLANK BLANK and coordinate essential activities when appropriate time of day

efficient functioning

By synchronizing the internal physiological and biochemical processes of animals, biological clocks promote BLANK BLANK

6 general characteristics of circadian clocks

1. Inherited 

2. Temperature independent 

3. Relatively resistant to the influence of chemicals 

4. Entrained to only limited cycle lengths 

5. Independent from behavioral feedback 

6. Common across phyla and found at every level of organization within an organism 

SCN

mammalian master biological clock; contains heterogeneous population of neurons that express variety of neuropeptides

Lesions in SCN

abolishs circadian rhythms; damage disrupts the daily sleep-wake cycle in humans 

retinohypothalamic tract

consist of retinal ganglion cells that project light information from the eye to the SCN

melanopsin

chemical found in retinal ganglion cells that allows them to encode and transmit light information directly to the SCN

vertebrates 

have photoreceptors that detect light outside of the eye in the pineal gland 

SCN

PVN

SCG

Pineal gland

neural circuitary to and from SCN in mammals:

melatonin

released by the pineal gland; tells the body that it is dark and thus plays a role in sleep onset 

short wavelength light

ex. blue light; exposure to this type of light at night can disrupt melatonin and has adverse effects on health

photoperiodism

use of day length to time annual cycle and seasons

melatonin and photoperiodism

melatonin secretion duration varies with length of nights; many species use this information to assess time of year and time reproductive physiology/behavior to seasons that favor reproductive success

fall and winter

shorter days - increased melatonin

• Causes hypothalamus to become more sensitive to negative feedback of gonadal steroids 

• Decreased GnRH and gonadotropin secretion = gonadal regression

spring and summer

long days - decreased melatonin

• Causes hypothalamus to become less sensitive to negative feedback of gonadal steroids

• Increased GnRH and gonadotropin secretion 

• Gonads regrow