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142 Cards in this Set

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Tinbergen’s Questions
About Causes of Behavior
involve 4 levels of analysis. What are they?
IMMEDIATE CAUSATION
DEVELOPMENT
EVOLUTION/NATURAL SELECTION
ADAPTIVE FUNCTION
Advantages of Animal Models (7)
1.Continuity of behavior from one species to another through evolution
2. Simplest system with qualities of interest
3.More economical to develop methods on animals that may be useful for people.
4.Ethical considerations; some manipulations are not permissible in humans
5.Function of behavior often becomes clearer when viewed in a comparative manner
6.Places human system in perspective
7.Generalizations can be gained from animal models then tested in humans
Disadvantages of Animal Models (4)
1. Enormous species differences
(behaviors may represent adaptations to very specific environments)
2. Behavioral gap between animals and humans is great (Language & Cultural evolution)
3.To get the “cleanest” answers about simplest analyses, the less cognition, the better.
4.Ethical considerations
What steps do you take in order to make a strong inference?
1) Devise alternative/multiple hypotheses
2) Design and execute experiment that will permit rejection of as many of the hypotheses as possible
3) Retain surviving hypotheses
4) Recycle
Science advances only by disproofs. Common misconception is that goal of scientific enterprise is to develop theories that cannot be disproved. Wrong. Marxist, Freudian, Darwinian theories cannot be disproved
What is chronobiology
Scientific study of biological clocks and their associated rhythms
Also called Biochronometry
Began in the early 1960s
Had to counteract the dogma of homeostasis in biology and medicine
Core body temperature varies across the 24 h day
-Borrowed terms and concepts extensively from engineering disciplines to describe biological clocks and their associated rhythms
RHYTHM
A recurrent event that is characterized by its period, frequency, amplitude, and phase.
period
The length of time required to complete one cycle of the rhythm in question.
For instance, the amount of time required to go from peak to peak or trough to trough.
FREQUENCY
Computed as the number of completed cycles per unit of time.
For example, 2 cycles per day.
amplitude
The amount of change above and below the average value.
That is, the distance of the peak or nadir from the average.
phase
Represents a point on the rhythm relative to some objective time point during the cycle
Exogenous versus Endogenous Control of Biological Clocks
Some behavioral rhythms have been recognized since ancient times, but they have generally been attributed to exogenous (outside the organism) factors.
Recent evidence indicates that endogenous (inside the organism) timing mechanisms mediate many of the observed rhythms in physiology and behavior.
How is it determined whether a rhythm is the result of exogenous factors or an endogenous clock?
Isolation experiments
Exogenous versus Endogenous Control of Biological Clocks
Jean Jacques d’Ortous de Mairan found that the tension–relaxation pattern of a heliotropic plant persisted when isolated from exogenous factors.
Recent Evidence that Biological Clocks are Endogenous
Animals maintained in constant conditions aboard a spacecraft orbiting far above the earth, and presumably away from subtle geophysical cues, display biological rhythms with periods similar to those observed on earth.
Recent Evidence that Biological Clocks are Endogenous
Animals maintained in adjacent, but individual, cages in the absence of environmental cues display biological rhythms with slightly different periods, suggesting that they are not being driven by the same subtle geophysical cue;
Recent Evidence that Biological Clocks are Endogenous (3)
Animals maintained in constant conditions aboard a spacecraft orbiting far above the earth, and presumably away from subtle geophysical cues, display biological rhythms with periods similar to those observed on earth.

Animals maintained in adjacent, but individual, cages in the absence of environmental cues display biological rhythms with slightly different periods, suggesting that they are not being driven by the same subtle geophysical cue;

The period (and phase) of the biological rhythms of one individual can be transferred to another individual by means of tissue transplants.
Circadian
Revolution of planet
24 h (22-26 h)
Circatidal
Tides
12.4 h (11-14 h)
Circalunar
Phases of the moon
29.5 days (26-32 days)
Circannual
Seasons of the year
365.25 days (300-400 days)
COMPARISON OF BIOLOGICAL RHYTHMS
Circadian, Revolution of planet
24 h (22-26 h)
Circatidal, Tides
12.4 h (11-14 h)
Circalunar, Phases of the moon
29.5 days (26-32 days)
Circannual, Seasons of the year
365.25 days (300-400 days)
Figure 10.6; page 505
The rhythms persist in the absence of the geophysical cue to which they are normally synchronized (entrained), with a period that approximates that of the geophysical cycle.
Ultradian (shorter than circadian) rhythms
Infradian (longer than circadian) rhythms (persist in constant conditions, but do not correspond to any known geophysical cue)
Usefulness of biological clocks (2)
Synchronizing the activities of animals with their environments (including social)
To prepare for predictable events (e.g., winter, night, etc.)
Synchronizing the internal physiological and biochemical processes of animals
To promote efficient functioning
General Characteristics of biological clocks and rhythms (6)
inherited, tempature independent, relatively resistant to the influence of chemicals, entrainment is limited to specific ranges, independent of behavioral feedback, bio clocks are found at every level of organization in an organism,
One of theGeneral Characteristics of Biological Clocks and Rhythms is that they are
inherited. Describe this.
When mutant animals with free-running circadian rhythms > 25 hrs are mated with each other, their offspring tend to have longer free-running periods than the offspring of mutants with free-running rhythms < 23 hrs (and vice versa)
General Characteristics of Biological Clocks and Rhythms
Temperature independence
Describe
activities or events that change body temperature don't significantly alter circadian clocks
otherwise there would be speed-ups and slow-downs and eventually all resemblance to a 24 hr period would be lost
General Characteristics of Biological Clocks and Rhythms
Relatively resistant to the influence of chemicals -describe
If not, the food consumed would constantly be altering biological clocks.
A few pharmacological manipulations have been shown, however, to affect clocks
Protein synthesis inhibitors
Alcohol (EtOH)
Lithium
Heavy water (deuterium)
General Characteristics of Biological Clocks and Rhythms
Entrainment is limited to specific ranges-describe
A circadian rhythm can be entrained to a 23 h day by providing 11.5 h of light and 11.5 h of dark (same for 12.5 h light/12.5 h dark).
However, 10 h light and 10 h dark does not result in entrainment to a 20 hr day
Instead, results in free-running with sporadic entrainment attained at irregular intervals.
In hamsters, wheel running can be entrained to the following range: 18-26 hrs.
General Characteristics of Biological Clocks and Rhythms
Independence from behavioral feedback. Describe.
Suppose a hamster housed in DD is expressing a 24.25 h cycle of wheel running onset.
Then, the hamster's wheel is locked for 10 days.
Then it is unlocked.
What time will the hamster begin to run?
2 possible predictions:
1) 15 min after last time-- suggesting that the clock suspended time-keeping while the rhythm of wheel running activity was not being expressed
2) 150 min after last time suggesting that the clock continued to run even in the absence of behavioral feedback. Prediction #2 is the correct answer.
General Characteristics of Biological Clocks and Rhythms
Biological clocks are found at every level of organization within an organism. Describe.
Single-celled organisms possess circadian rhythms so the machinery necessary to generate a rhythm must exist at the level of individual cells.
So, in multi-cellular organisms, does every cell possess its own biological clock?
Perhaps, but in multi-cellular organisms, it appears as if these individual biological clocks have been organized into some sort of hierarchical fashion with feedback imposed from above.
E.g., cells taken from hamster adrenals and maintained in culture will free-run at different rates. In the intact hamster, they free-run at the same rate.
Circadian "master" clocks have been isolated after demonstrating that lesions eliminate circadian rhythms. What are examples of some of these ciradian clocks?
Eyes of amphibians
Pineal glands of fish, reptiles, and birds
Suprachiasmatic nuclei (SCN) of the anterior hypothalamus in mammals
Suprachiasmatic nuclei (SCN) of the anterior hypothalamus in mammals is considered a circadian "master" clock. Discuss some of the features of the SCN.
Slices from the SCN maintain circadian rhythms of electrical activity
SCN transplants cause recipient's rhythm to match that of donor
Environmental light entrains oscillations of SCN
SCN are the master clocks
of which animals?
mammals
What pathway for output from the master clock has been extensively studied in mammals?
SCN-->PVN-->MFB-->SCG-->Pineal pathway


(where neural information is transduced into a hormonal message)
Are there different clocks for rhythms of different lengths or are longer rhythms simply the result of the multiplication of shorter rhythms?
E.g., is the 4-day (96 h) estrous cycle of a hamster the result of a 4-day clock cycling once or a 1-day clock cycling 4 times?
Evaluated in studies of free-running rhythms in female hamsters.
Phase shifts of entrainment were accompanied by proportionate shifts in estrous cycle (light- dark cycle changed to 25 h, then estrous cycle changed to 100 h-- if reduced to 20 h, then estrous cycle switched to 80 h)
Is the 4-day (96 h) estrous cycle of a hamster the result of a 4-day clock cycling once or a 1-day clock cycling 4 times?
Evaluated in studies of free-running rhythms in female hamsters.
Phase shifts of entrainment were accompanied by proportionate shifts in estrous cycle (light- dark cycle changed to 25 h, then estrous cycle changed to 100 h-- if reduced to 20 h, then estrous cycle switched to 80 h)
Day length affects
melatonin
Biological rhythms affect
physiology
A zeitgeber is any environmental factor that provides a temporal cue. In the absence of a zeitgeber, you'll see what type of activity?
No zeitgeber means free running activity
Without zeitgebers, behaviors that show rhythm are driven by..
the biological clock
entraining
process by which brief exposure to light resets the clock
It is easier to phase ___ than to phase ___
easier to phase DELAY than to phase ADVANCE
What is happening when you phase Delay?
retraining to light is occuring later

phase delay occurs when you travel east to west
Phase delay occurs when you travel from __ to __.
east to west.
When this occurs, the subjective day is lengthened.
Traveling from west to East is an example of phase ___.
Is this easy or difficult?
west to east = advance.
This is very difficult.
Subjective day is shorter
What is a skeleton period?
A skeleton period is a light dark cycle where dawn and dusk are represented by 2 separate light periods

For example,
A regular period would be 15L:9D
but a skeleton period would be
15L:1D:7L:1D
Define subjective day.
Subjective day is when you are active; what you usually do during the light per
When you sever the SCN and activity changes, what is this called?
splitting
What does tau allow you to determine?
Tau allows you to determine circadian time.
Discuss potential values of tau.
Tau NEVER equals 24.

Taus are either <24 or >24.
What information explains how we entrain to 24 hours?
late/early subjective night differences explain how we entrain to 24 hours.
Why does the dead zone appear on a prc and how is it represented graphically?
The dead zone occurs because any light observed during the subjective day doesn't effect activity.

The dead zone is represented as a flat, horizontal line on the PRC.
Under what circumstances does a phase delay occur?
Phase delays occur if we delay the onset of the light flash such that the light flash occurs early in the subjective day (might want to ask about this)
To cause a phase advance, what should you do?
Put light at the end of the subjective night
What will happen if you have add light during subjective day?
Nothing. Addition of light during the subjective day has no effect.
How are phase delays and phase advances represented on a phase response curve?
delays = valleys
advances = peaks
If you have light at the beginning of subjective night..
you'll sleep longer the next day (I think that this would represent phase delay).
If you have light at the end of subjective night..
you'll go to bed earlier
This is phase advancing.
phase delay
occurs if we delay the onset of the light flash
Why doesn't tau equal 24?
It's not easy to entrain to changing day lengths.
Tau is either less than 24 or greater than 24
What are three inferences that we can make from the phase response curve?
1. there must be a max T cycle
2. Minimum T is 20-22 hours
3. when you use brief light pulses, a process called masking occurs
What is a t cycle?
the length of day
What are the min and max t cycles (lengths of day)?
min 20-22
max 27 hrs
Describe Aschoff's rule.
1. Nocturnal animals have constant dark taus (per of circad rhythm) and constant light taus. Diurnal animals have DD taus> LL taus.
2. Period of tau shortens as light intensity increases
If you have a long tau (or if you extend the length of tau)..
you will become active later and later
When is melatonin secreted?
at night
If you cut the neural connection between the optic lobes and the rest of the nervous system in a cockroach, what happens?
the ability to generate circadian rhythms is lost.

Thus, cockroach rhythm generation is neural, not endocrine.
What kind of pacemaker is the SCN?
The SCN is a master pacemaker.
What innervates the SCN?
the retinohypothalamic tract
Mammals are the only creatures that don't....
have photoreceptors outside the eyes.
When light is introduced, what neurotransmitter is released to the retinohypothalamic tract?
glutamate
Where does the IGL get its light information?
the hypothalamus
What is the fuction of the IGL?
The IGL gets light info from the hypothalamus and entrains.
What is the neurotransmitter associated with the IGL?
neuropeptide Y
What are the two inputs to the SCN?
1. RHT
2. IGL
What type of synapse is the RHT?
RHT is monosynaptic.
The IGL and the RHT both provide inputs to the SCN. Do they use the same pathway to do this?
No. IGL uses a separate path.
What are the functions of the IGL?
entrainment, synchronization, nonphotic cues (like motor activity).
What is one of the regulatory functions of the SCN?
The SCN is able to regulate the secretion of melatonin
What are the sources of output from the SCN?
1. ant hypothal
2. lat and dorsal medial hypothal
3. IGL
4. lat septal nucleus = motivates agression
If you take the SCN out of its host, what will happen to the mechanism by which it fires?
Nothing. If you take out the SCN it will still fire in a rhythmic manner.
What is true of cells from the same SCN that are separated from one another?
Cels from the same SCN that are separated from one another are NOT phase locked.
When you transpant the SCN of one hamster into another hamster...
You implant the phase relationship of the donor into the recipient.
Do circadian rhythms require neural efferents?
No. Circadian rhythms do not require neural efferents but some of the endocrine ones do.
What are the functional characteristics of all biological clocks (ultradian, circadian, circannual clocks, etc.)?
inherited, temp independent, relatively resistant to the influence of chemicals, Entrainment is limited to specific ranges, Independence from behavioral feedback
Biological clocks are found at every level of organization within an organism.
( in other words, Single-celled organisms possess circadian rhythms so the machinery necessary to generate a rhythm must exist at the level of individual cells)
How have past investigators experimentally confirmed that the daily clock is indeed an endogenous timer?
isolation experiments, I think
1.Jean Jacques d’Ortous de Mairan found that the tension–relaxation pattern of a heliotropic plant persisted when isolated from exogenous factors
2. Jurgen Aschoff, a medical doctor and physiologist, expanded on Kleitman's human isolation experiments.He concluded that humans — like the plants investigated by de Mairan — have endogenous circadian cycles.
3. Animals maintained in constant conditions aboard a spacecraft orbiting far above the earth, and presumably away from subtle geophysical cues, display biological rhythms with periods similar to those observed on earth.
Animals maintained in adjacent, but individual, cages in the absence of environmental cues display biological rhythms with slightly different periods, suggesting that they are not being driven by the same subtle geophysical cue;
The period (and phase) of the biological rhythms of one individual can be transferred to another individual by means of tissue transplants.
What is the experimental evidence that the SCN is an important biological clock? What is the evidence that other extra-SCN clocks might also exist in mammals? --in other vertebrates?
Circadian clocks have been isolated after demonstrating that lesions eliminate circadian rhythms
Other extra-SCN clocks that might also exist in mamals:
Eyes of amphibians
Pineal glands of fish, reptiles, and birds
Suprachiasmatic nuclei (SCN) of the anterior hypothalamus in mammals
Flying squirrels become active at dusk, but dusk comes earlier and earlier in the fall and later and later in the spring. Yet, a flying squirrel follows these seasonal changes in timing of dusk. What is it about clock function that permits this flexibility? Explain.
????????
What is the pathway by which there is output from the clock?
SCN-->PVN-->MFB-->SCG-->Pineal pathway
Usefulness of biological clocks
Synchronizing the activities of animals with their environments (including social)
To prepare for predictable events (e.g., winter, night, etc.)
Synchronizing the internal physiological and biochemical processes of animals
To promote efficient functioning
Need to look up definitions of ultradian & infradian, be/c I have this approaching 9 hrs. Definitions I do have are on the back of this card
Circadian, Revolution of planet
24 h (22-26 h)
Circatidal, Tides
12.4 h (11-14 h)
Circalunar, Phases of the moon
29.5 days (26-32 days)
Circannual, Seasons of the year
365.25 days (300-400 days)
Ultradian (shorter than circadian) rhythms
Infradian (longer than circadian) rhythms (persist in constant conditions, but do not correspond to any known geophysical cue)
Circadian, Revolution of planet
24 h (22-26 h)
Circatidal, Tides
12.4 h (11-14 h)
Circalunar, Phases of the moon
29.5 days (26-32 days)
Circannual, Seasons of the year
365.25 days (300-400 days)
What is something that is important to note about all of these?
The rhythms persist in the absence of the geophysical cue to which they are normally synchronized (entrained), with a period that approximates that of the geophysical cycle
So, in multi-cellular organisms, does every cell possess its own biological clock?
Perhaps, but in multi-cellular organisms, it appears as if these individual biological clocks have been organized into some sort of hierarchical fashion with feedback imposed from above.
E.g., cells taken from hamster adrenals and maintained in culture will free-run at different rates. In the intact hamster, they free-run at the same rate.
Discuss the idea of the autonomous clock.(Peifer)
He studied the daily movements of plants; he found that when these plants grew from the seed in continuous darkness they displayed no movement
However, when exposed to brief periods of illumination where light acts as a zeitgeber, the plant was easily able to induce such movement, and when returned to darkness, the plant possessed a persisting daily rhythm of leaf movement.
It was then postulated that plants and animals possess independent cyclical biochemical systems with inherited natural periods of oscillations and when other factors are constant, the period length is a characteristic of the genetic strain.
This period (rarely found to be exactly 24 hours) was called the "free running period” or “t”.
If daily light signals are not important, then one might be tempted to conclude that some less obvious difference between night and day is cueing the plant and driving the rhythm of leaf movements (e.g., changes in temperature or other cues). Why is this not the case?
the daily periodicity of plants housed in continuous illumination was slightly different from the exact 24-hour periodicity seen in plants kept under natural daily illumination.
What conclusions could they make if the bees continued to feed at the same time in unchanging environmental conditions?
How did they know what time it was?
some reliable clock to compensate for its constantly shifting ‘compass’.
, but I am not sure...does zeitgeist have anything to do with this?
What does it mean when we say that the biological clock is temperature compensated?
varies very little at different constant temperatures.
Discuss the temporal organization of bio clocks.
They control an organisms’ behavior and activities so that they occur at the "right time".
For example, clocks coordinate when males and females of the same species become fertile so that the period of maximum fertility coincides, thus establishing a breeding season.
In cyanobacteria (blue-green algae), there is both photosynthesis and nitrogen fixation but the latter process cannot occur in the presence of oxygen.
These processes therefore need to be separated.
Under the control of a biological clock these 2 processes are temporally segregated: photosynthesis occurs during the day, fixation during the night

Temporal Organization between Species.
Biological clocks function such that certain species are active, for example, during the day, while others are active during the night.
If a prey species is active during the night, then the clock of its predator will ensure it “wakes up" so it does not miss out on a potential meal.
In the same way, prey species may become active at times when the predators are usually not active (to avoid becoming a meal!)






Evolution of Biological Clocks


Anticipation of an Organisms Environment.
By registering local time and events (such as high and low tide), biological clocks allow organisms to prepare for and anticipate their environment.
Predictive Homeostasis is a good example of this –
an organism can activate certain systems preempting the changes that will occur in the environment e.g. in humans cortisol levels are raised before we wake up, increasing the metabolic rate so we have the energy necessary for the day ahead
Describe Aschoff's rule
Aschoff's rule


Nocturnal animals have DD taus less than LL taus, diurnal animals have DD taus greater than LL taus.


Period of tau shortens as light intensity increases in diurnal (day-active) animals and lengthens as light intensity increases in nocturnal (night-active) animals.
Output from mamalian clock
SCN-->PVN-->MFB-->SCG-->Pineal pathway
Environmental factors such as light-dark cycles, clocks etc. that give clues as to external cycles are called...
zeitgebers
In the absence of any zeitgeber, behaviors that show rhythmicity are driven by an ...
internal timing device - a biological clock.
What happens if an animal is removed from exposure to zeitgebers?
Many studies in humans and other mammals have shown that, in the absence of zeitgebers, i.e. when the biological clock is allowed to free-run, activity has a period of 22.5 to 26 hours.
This period is incredibly robust, it does not deviate by more than a few minutes in a year and is unaffected by food/water intake, exposure to LSD, removal of endocrine glands or electroconvulsive shock (Richter, 1967).
However the clock can be reset by brief exposure to light - a phenomenon called entraining.
entraining is...
resetting the clock by brief exposure to light
In the absence of zeitgebers, the biological clock will..
free run
There is a number of ways in which the environment or changes in behavior can make the internal clock 'out of synch' with the external world.
Sudden mismatches between 'subjective' time and the external LD cycle are called....
phase shifts
what are some of the situations in which phase shifts occur?
jet lag and ‘rapid rotation’ shift work.
When traveling from East to West the subjective day is _____, re-entraining to light occurs _____ and adaptation is ________.
When traveling from East to West the subjective day is lengthened, re-entraining to light occurs later (delay shift) and adaptation is easier (although it can still take several days
Discuss skeleton photoperiods
A light-dark cycle in which dawn and dusk are represented by two separate light periods, e.g, LDLD1: 7: 1: 15 (the skeleton of LD9 : 15 or of LD17 : 7)
If the two light periods are of equal length, then the skeleton is considered to be symmetric
Subjective Day
The time during which the organism exhibits the behavior usually associated with light in an LD cycle
Splitting
Hamsters housed in LL
show ‘splitting’ of their

locomotor activity patterns.

Often the two circadian
oscillations appear to free-run with different t’s.









Splitting


Two oscillators
Pittendrigh and Daan introduced the hypothesis of a pacemaker consisting of two coupled oscillators.
Their proposal of an evening oscillator (E) and a morning oscillator (M) was based on the splitting of activity rhythms in hamsters under constant conditions.
Recently, Illnerova and Vanecek explained the different sensitivity of onset and offset of N-acetyltransferase activity towards light pulses with this model of two coupled oscillators
circadian time
To understand how light entrains free-running circadian rhythms
CT-12
CT-12is defined as the onset of locomotor activity (or beginning of subjective night)
To convert RT to CT (RT=CTxt/24)
To convert CT to RT (CT=RTx24/t)
To convert RT to CT (RT=CTxt/24)
To convert CT to RT (CT=RTx24/t)
Imagine that one hamster t =27.00 h and another has t = 14.00 h (impossible)
Both are free-running in DD
A light pulse is given 4 real hours after wheel-running starts
Determine the CT for each animal…
Animal #1 CT=RTx24/t = 4 x 24/27= 4 x .89 = 3.56 hCT = CT 15.56 (early subj night)


Animal #2 CT=RTx24/t = 4 x 24/14= 4 x 1.71 = 6.86 hCT = CT 18.86 (late subj night)
Because of its short t, Animal #2 has a much shorter subjective day than Animal #1. An individuals t is necessary to convert from RT to CT.
Because of its short t, Animal #2 has a much shorter subjective day than Animal #1. An individuals t is necessary to convert from RT to CT.
Phase Response Curve (PRC)
Graphical representation of how specific light pulses phase shift free-running rhythms as a function of the time that they are given.
Because of its short t, Animal #2 has a much shorter subjective day than Animal #1. An individuals t is necessary to convert from RT to CT.
Because of its short t, Animal #2 has a much shorter subjective day than Animal #1. An individuals t is necessary to convert from RT to CT.
Cockroach Optic Lobes


If one cuts the neural connection between the optic lobes and the rest of the nervous system,
the ability to generate circadian rhythms in behavior is lost.
If one cuts the neural connection between the optic lobes and the rest of the nervous system, the ability to generate circadian rhythms in behavior is lost.
By contrast, if one simply cuts the optic nerves that bring light information from the eyes to the optic lobes,
By contrast, if one simply cuts the optic nerves that bring light information from the eyes to the optic lobes, the locomotor activity rhythm remains intact, but it free-runs under all lighting conditions. The hypothesis that emerges from these results is that the oscillator is somewhere within the optic lobes
The idea that the connection of the pacemaker to the rest of the nervous system is neural is reinforced by
the lack of rhythmicity if the brain is excised and simply implanted in the abdomen
Terry Page found that if one studied roaches with transected optic lobes left in situ for about 40 days, all of the arrhythmic roaches became rhythmic again with no further intervention.
The period of the restored rhythm was similar to that of the original rhythm.
Page first showed that the restoration of rhythmicity was coincident with and dependent on the regrowth of fibers from the optic lobes to the rest of the brain.
The restoration of behavioral rhythmicity is coincident with the restoration of light-evoked responses in the rest of the brain, which depend on an intact neural route from the retina to the optic lobe to the protocerebrum. What does all of this indicate?
These results indicate that the oscillator in the optic lobe continues to function after being separated from the brain and that the output mechanism from the pacemaker to the driven systems in the brain can regenerate spontaneously.
eyes
are the only photoreceptors in mammals
Characteristics of SCN
. bilateral, paired nuclei
2. a cluster of about 10,000 neurons
3. different cell types
4. presence of neuropeptides
Glutamate is the main neurotransmitter in the...
RHT---GLU is released at synapses in SCN in response to light stimulation
Intergeniculate leaflet (IGL) as an additional source of photic information for entrainment of SCN---projections from IGL to SCN release......
neuropeptide Y
INPUT into SCN -2 ways
Monosynaptic pathway: Retinohypothalamic Tract (RHT

The second passage is the intergeniculate leaflet (IGL)
Monosynaptic pathway: Retinohypothalamic Tract (RHT
This derives from specialized photoreceptors that sense light.
The RHT connects to the core of the SCN.
The detection of light is transmitted via the RHT.
The second passage is the intergeniculate leaflet (IGL)
Receives data directly from the retina, but the route separates from the RHT.
The IGL terminates in the geniculo-hypothalamic tract (GHT)
The IGL path appears to play an important role in the entrainment, synchronization, by non-photic zeitgebers such as motor activity
the SCN is able to regulate the production of...
melatonin
Where is melatonin secreted?
pineal gland
Lesions of the SCN abolish...
circadian rhythms
Types of evidence that SCN is circadian pacemaker:
a. SCNx (destruction of SCN by lesion)----> arrhythmicity
b. electrical or chemical stimulation of SCN----> phase shifts
c. rhythm of neural firing activity and 2DG uptake in SCN in vivo
1. rhythm of neural firing persists after surgical isolation of SCN from neural connections with brain
2. rhythm of neural firing persists in vitro
3. rhythm of 2DG (2-deoxyglucose) uptake indicates rhythm in metabolic rate
SCN transplant studies
1. tau mutant hamster has taudd = 20 h (very different from wild-type hamster taudd, which is very close to 24 h
2. lesion SCN of wild-type hamster and implant SCN taken from tau mutant hamster---transplant restores rhythm and tau = 20h, similar to tau of donor hamster)
3. lesion SCN of tau mutant and implant SCN taken from wild- type hamster---rhythmicity is restored and tau is close to 24h (again like that of donor)
4. transplanted SCN restores locomotor activity rhythm, but rhythm can no longer be entrained to light---probably because connections from RHT to SCN are not restored
5. transplanted SCN does not restore pineal melatonin rhythm---neural connections (output) from SCN to pineal are not restored
Seasonal Gonadal Response is or is not Restored by SCN Transplants
is not
Circadian Behavioral Rhythms do or do not Require Neural Efferents
DO NOT
SCN Transplants Do or Do Not Restore
the Glucocorticoid Rhythm
DO NOT
What can be said about the structure of the SCN?
It is structurally heterogenous.
Two main hormones that carry temporal information are...
melatonin and cortisol
When does the Peak daily cortisol concentrations occur?
Peak usually occurs just prior to or immediately after awakening,
Coincides with the onset of locomotor activities in the morning.
Programmed elevation of blood levels of cortisol increases blood pressure and cardiac output prior to the active phase of the day.
We know that the increased cortisol concentrations are not driven by the increased activity levels, because the same circadian rhythm is observed in bedridden patients under constant conditions (Aschoff, 1965).
Female hamsters display an interesting pattern of activity onset that has been termed “scalloping.” Describe.
Every 4th night, coincident with estrus, females show a spontaneous phase advance in their activity onset






Estrogen Phase Advances Tau







Estrogens shorten tau


Hamsters are solitary creatures and it has been speculated that the earlier onset of locomotor activity during estrus increases the female’s odds of locating a male.
The scalloping pattern is eliminated by ovariectomy.
Furthermore, estradiol treatment of free-running, ovariectomized hamsters or rats reduces the period of locomotor activity onset, suggesting a direct effect on the clock itself
Progesterone does what to the period of circadian rhythms?
Progesterone lengthens the period of circadian rhythms, possibly by counteracting the effects of estradiol