Homeostasis, biorhythms and sleep Flashcards
What is homeostasis?
Maintenance of equilibrium by active regulation of internal states - multiple mechanisms control the balance, emphasising its importance to survival
Set points not fixed - many homeostatic functions show daily rhythms which maintain levels appropriate for the level of activity - efficient in energy use e.g. heart rate dropping during sleep to conserve energy
What is meant by a circadian rhythm?
Bodily functions linked to day length - light/dark cycle important but not critical e.g. during winter dark hours are longer but we still sleep roughly the same amount so the light/dark time is not exclusively determining our body functions
24hr cycle
What are humans and most other primates like?
Diurnal i.e. active during light hours
What is a common way of studying circadian rhythms?
Place a rodent on a wheel and register each revolution –> an activity rhythm
Each revolution indicated as a dark mark - see activity begins shortly before dark period each day, remaining active during the dark
When PHASE SHIFT occurs i.e. light timing altered so lights go off later each day, there was also a phase shift in activity
When constant dim light, activity gets later each subsequent day - this is a FREE-RUNNING ACTIVITY RHYTHM and indicates that the endogenous clock has a periodicity slightly more than 24hrs
What is meant by periodicity?
Interval of time between two similar points of successive cycles e.g. sunset to sunset
How does the internal clock get set normally?
Using light cues - synchronise free running rhythm to, for example, the beginning of the dark period
What is the phase shift?
This shift of activity produced by a synchronising stimulus, and the process of shifting the biological rhythm to an environmental stimulus (entrainment)
We experience phase shifts when fly between time zones e.g. sunlight might wake us several hours earlier than we were expecting
What is a zeitgeber?
Any cue that an animal uses to synchronise its activity with the environment e.g. the light/dark cycle
Because light stimuli can entrain circadian rhythms, the endogenous clock must have inputs from the visual system
In normal light/dark cycles we say the rhythm is PHASE LOCKED
What is the main value of having circadian rhythms?
Allow us to synchronise our behaviour and body states to changes in the environment - day and night are significant for survival, and having an endogenous clock allows anticipation of an event such as darkness so we can begin physiological preparations in advance
So we say these rhythms provide the TEMPORAL ORGANISATION of animal behaviour
What is thought to be important in establishing circadian rhythms?
The suprachiasmatic nucleus
If we see a lesion of the optic tract, and light info no longer reaching visual cortex, rhythm is maintained i.e. light fluctuations aren’t essential. Periodicity does change, however - free-running rhythm rather than 24hr cycle
Lesions of the SCN fully abolish the circadian rhythm, no periodicity at all, no endogenous rhythm (some daily rhythms still able to be established using external cues, but without these behaviour becomes random) and arrhythmic
The SCN is the MASTER CLOCK
How does light actually reach the SCN?
Most vertebrates have photoreceptors outside the eyes e.g. the pineal gland in amphibians is light sensitive ad helps entrain rhythms; it is a secretory gland and main source of melatonin release, hormone responsible for regulating sleep-wake cycles
In mammals, it is photoreceptors in the eye that alert SCN to changing light levels - certain retinal ganglion cells send axons along the RETINOHYPOTHALMIC pathway and at the optic chiasm these split off to synapse directly with SCN
What are the retinal ganglion cells of the retinohypothalmic pathway like?
Don’t rely on traditional rods and cones but rather contain photopigment themselves - melanopsin
So even in the absence of rods/cones (essentially blindness), behaviour could still be entrained to light
(Melanopsin is most sensitive to light freq in blue range, which is why blue light has the largest effect on human circadian systems)
What is the sleep rhythm like?
Free-running rhythm is actually around 25 hrs but entrainment to light/dark cycle maintains 24hr periodicity (mediated by SCN)
What is jet lag the result of?
Rapid shifts in light/dark cycle - takes a few days for the endogenous rhythm to re-entrain
What was initially thought about the onset of sleep?
That it was a passive process - when midbrain surgically separated from forebrain –> state of permanent sleep
It was suggested that, in absence of sensory input, the cortex becomes quiescent i.e. sleep
What did Moruzzi and Magoun do?
Electrically stimulated the midbrain and woke sleeping animals
Lesions to this area –> persistent sleep
Suggested some kind of activating system in midbrain which acts on the cortex - where there is a lack of tonic activating influence of midbrain, cortical neurones cease their firing and sleep ensues
How has this passive view of sleep been challenged?
By electroencephalographic recordings which show abundant neuronal activity in a sleeping cortex
What are the characteristics of SLOW WAVE SLEEP?
Can be further divided into several stages, all marked by slow waves of electrical potential representing widespread synchronisation of cortical activity (contrasting with the different functions different regions all fulfil separately during waking)
What are the 4 stages of slow wave sleep?
1) Drowsy/relaxed - alpha waves, distinctive rhythm regularly oscillating around 8-12Hz
2) Stage N1 (Theta waves) - time spent at alpha rhythm starts to decrease, events of smaller amplitude and irregular frequency occur along with sharp waves, heart rate slows and muscles relax
3) Stage N2 (sleep spindles) - waves of around 12-14Hz occur in periodic bursts
4) Stage N3 (delta waves) - Larger amplitude and lower freq delta waves (transition from light to deep sleep)
What physiological changes occur during slow wave sleep?
Progressive decrease in spinal reflexes, heart rate, breathing rate, brain temp and cerebral blood flow. Hormone secretion increases
