Homeostasis, circadian rhythms, and sleep

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What is homeostasis

process by which the body maintains a stable internal environment, despite changes in external conditions, e.g.:

• cardiovascular function (blood pressure, heart rate)

• body temperature

• food and energy regulation

• fluid regulation

Many homeostatic functions show

daily rhythms

Example of homeostasis

during sleep body temperature decreases, heart rate decreases, respiration rate decreases

• energy conservation

What is the circadian rhythm

Your body’s internal 24-hour biological clock that regulates a wide range of physiological processes.

• Including sleep and wake cycles, hormone release, body temperature, and metabolism

• Based on the cycle of light and darkness in your environment

• e.g. body temperature, heart rate, respiration, sleep

What is circannual rhythms

biological cycles that occur roughly once per year

• These rhythms help organisms adapt to seasonal changes in the environment, such as daylight, temperature, and food availability.

The circadian rhythm is maintained in

constant light

• periodicity changed

What would happen without the suprachiasmatic nucleus

• circadian rhythm abolished

• no periodicity

Where is the suprachiasmatic nucleus (SCN) located

in the hypothalamus, above the optic chiasm

The cells in the suprachiasmatic nucleus (SCN) show

oscillations of activity

• related to circadian rhythm

• believed to form the ‘biological clock’

How does light information reach the suprachiasmatic nucleus in non-mammalian species

they have photoreceptors outside the eye

How does light information reach the suprachiasmatic nucleus in mammals

• carries light information to suprachiasmatic nucleus (SCN)

• rods and cones do influence the SCN function

• light sensitive information still reached SCN in the absence of rods and cones, therefore other light receptors also present in eye

• photoreceptive ganglion cells in the retina

Describe the surgery made by Bremer

• Surgically separated midbrain from forebrain in cats

  • Animals remained permanently asleep

• Proposed that in the absence of sensory input the cortex became quiescent (i.e. sleep)

Describe the study made by Moruzzi & Magoun

• Electrical stimuli of the midbrain woke sleeping animals

• Lesions to this area caused persistent sleep

Lack of tonic activating influence of midbrain causes

cortical neurones to cease firing, and sleep to ensue

What did electroencephalographic (EEG) recordings show when asleep

showed abundant neuronal activity in cortex

Pattern of the EEG was very different in sleep than in

walking

• waves of activity, indicating synchronous firing of cortical neurones

• synchronising stimulus coming from sub-cortical areas

• midbrain reticular formation still seen as important

Characteristics of slow-wave sleep

• Progressive decrease in spinal reflexes

• Progressive reduction in heart rate and breathing rate

• Reduced brain temperature and cerebral blood flow

• Increased hormone secretion (e.g. growth hormone)

• Synchronised cortical activity

Characteristics of REM sleep

• Spinal reflexes absent

• Rapid eye movements behind closed eyelids

• Increased body temperature and cerebral blood flow

• Desynchronised cortical activity

• Dreams

Brain activity during sleep

• awake = low amplitude high frequency EEG

• light sleep = increasing amplitude decreasing frequency EEG

• deep sleep = high amplitude low frequency EEG

• rapid eye movement (REM) sleep = low amplitude high frequency EEG

How does our brain promote sleep

• cortex is ‘kept awake’ by ascending activation from midbrain

• 5HT inputs inhibit midbrain ‘activating system’ areas

Stimulation of … induces slow wave sleep

area surrounding SCN

Neurotransmitters used in sleep

• 5HT = promotes slow wave sleep, inhibition of ‘activating system’

• noradrenaline = inhibition of muscle tone during REM sleep

• dopamine = general arousal

• acetylcholine = induces REM sleep

Name a few ‘sleep-promoting substances’

• Factor S

• DSIP (delta-sleep inducing peptide)

• melatonin

they may modulate circadian rhythmicity rather than sleep

Name a few disorders of sleep

• insomnia = reduction or absence of sleep

• hypersonic (narcolepsy) = excessive drowsiness and falling asleep

• sleep-wake schedule disturbance = transient or persistent

  • (e.g. jet lag, shift work)

• partial arousal = e.g. sleep walking, nightmares

Disorders of sleep are often associated with:

• anxiety

• psychological disturbance

• drug taking

Name hypnotic (somnogenic) drugs

• morphine = widely used as a sedative

• barbiturates = widely used as sedatives and anaesthetics

• benzodiazepines = widely used as hypnotics (anxiolytic)

none of these induce natural sleep patterns:

• decreased REM sleep

• increased drowsiness during waking

___

• melatonin = weakly hypnotic

• serotonin precursor, tryptophan = weakly hypnotic

both induce natural sleep patterns

What cycle is important for SCN

light/dark cycle