Sleep and Circadian Rhythms

Question 1

what are the characteristics of sleep?

Answer 1

• change in behaviour
• change in brain activity
• structural changes in neurons and glia
• gene expression

Question 2

what does sleep consist of?

Answer 2

• cycles of non-REM and REM sleep

Question 3

what is the behavioural definition of sleep?

Answer 3

• reduced motor activity
• reduced response to stimulation
• stereotypic (eyes closed in humans)
• easy reversibility

Question 4

how can physiological activity be measured non-invasively?

Answer 4

1. Electromyography (EMG): muscle movements
2. electro-oculography (EOG): eye movements
3. electroencephalography (EEG): brain activity

Question 5

what is EEG?

Answer 5

• measures synchronous electrical activity from populations of neurons in brain
• electric field created by ionic movements
• electrodes are placed on scalp to detect electric fields
• linked to electrical amplifier and to a monitor
• different electrodes record different patterns of activity

Question 6

what are the advantages of EEG?

Answer 6

• non-invasive
• easy to administer
• data easily generated
• large amounts of data from different brain areas
• high temporal resolution (milliseconds) so can detect quick changes in brain activity

Question 7

what are the disadvantages of EEG?

Answer 7

low spatial resolution:

• electric fields follow an inverse square law so only cortical activity is detectable
• as distance increases from the electrical field, charge intensity decreases (detection decreases)
• distance from neurons to electrode is ~1cm through the scalp

Question 8

what are the 4 main types of brain waves, from high to low frequency?

Answer 8

1. beta = 13-30Hz
2. alpha = 8-13Hz
3. theta = 4-8Hz
4. delta = 0.5-4Hz (large amplitude, slow oscillations)

Question 9

what is non-REM sleep?

Answer 9

• low neuronal activity, little AP firing
• metabolic rate and brain temperature at their lowest
• heart rate and blood pressure decline
• decreased sympathetic outflow
• increased parasympathetic outflow
• muscle tone and reflexes in tact
• some muscular activity in all non-REM stages

Question 10

what are the 4 main stages in non-REM?

Answer 10

1. drowsiness - theta waves
2. light-sleep
3. deep-sleep - delta waves
4. very-deep sleep - delta waves

Question 11

what happens in drowsiness stage (1) during non-REM?

Answer 11

• awakened easily
• eyes move slowly and muscle activity slows
• transition from wakefulness to onset of sleep (lasts several mins)
• sinusoidal alpha activity 20-40microvolts at 10Hz
• EEG = low voltage activity and theta waves
• some muscular activity
• people have sensation of falling

Question 12

what happens in light-sleep stage (2) during non-REM?

Answer 12

• eye movement stops
• brain waves slower with burst of rapid brain waves
• body temp drops and HR slows
• bursts of sinusoidal waves called sleep spindles (12-14Hz) and biphasic waves called K complexes

Question 13

what are K complexes?

Answer 13

• occur episodically against background of continuing low-voltage EEG activity

Question 14

what happens in deep-sleep stage (3) during non-REM?

Answer 14

• slow delta waves interspersed with small faster waves
• sleep-walking, night-terrors, talking during sleep and parasomnia
• occurs during transitions between non-REM and REM sleep
• slow delta waves (0.5-2Hz)

Question 15

what happens in very deep-sleep stage (4) during non-REM?

Answer 15

• brain produces delta waves exclusively
• disorientation or several minutes following arousal
• slow wave activity dominates EEG record

Question 16

what are stages 3 and 4 in non-REM sleep known as?

Answer 16

slow wave sleep

Question 17

how are the waves in each stages compared?

Answer 17

stage 1 = theta waves

stage 2 = low frequency waves with sleep spindles and K complexes

stage 3 and 4 = slow wave brain activity

Question 18

what is stage 5 of sleep?

Answer 18

REM sleep

Question 19

what is REM?

Answer 19

• EEG mimics wakefulness
• closed eyes move rapidly from side to side
• similar to stage 1: low voltage and mixed frequency
• paradoxical sleep: REM-neuronal firing similar to wakefulness, but the muscles are still relaxed so there is no movement

Question 20

what happens during REM?

Answer 20

• brain temp and metabolic rate rise due to increased neuronal activity
• some areas more neuronal activity than there would be when awake
• all skeletal muscles are atonic (paralysed)
• muscles controlling eye, ear ossicles and diaphragm remain active to enable breathing and hearing of danger

Question 21

what is the normal sleeping pattern in humans?

Answer 21

REM and non-REM sleep alternate cyclically:

• at 70-80 mins, sleeper returns to stage 2 or 3 before entering first REM phase which lasts 8-10 mins
• time from first stage to end of REM = 90-100 mins
• repeated 4 or 5 times per night
• during each repetition, stages 3 and 4 decrease in duration and REM increases

Question 22

how much time is spent per stage of sleep in young adults?

Answer 22

• 5% in stage 1
• 50-60% in stage 2
• 15-20% in stage 3 and 4
• 20-25% in REM

Question 23

how is sleep regulated?

Answer 23

• noradrenaline and serotonin neurons in brainstem
• diffuse modulatory system controls rhythmic behaviour in hypothalamus
• inhibition of motor neurons

Question 24

what causes wakefulness?

Answer 24

• activation of neurons in brainstem precedes awakening
• stimulation of brainstem causes awakening, as depolarisation causes neurons to fire many APs

lesion of brainstem causes coma

Question 25

what causes non-REM?

Answer 25

• decreased AP firing in brainstem

- sleep spindles correlate with increased activity in thalamus, creating delta rhythms

Question 26

what causes REM?

Answer 26

• similar to AP firing in an awake stage, - no activity in frontal lobe, raphe nuclei or locus coeruleus
• inhibition of motor neurons

Question 27

what are the theories of dreaming?

Answer 27

• strange imagery due to the brain not fully functioning
• sleep exercises synapses when there is no external activity
• memory consolidation
• circuit testing to check that everything is working correclty

Question 28

what are circadian rhythms?

Answer 28

endogenous periodicity of 24 hours between sleep and wakefulness

• persist without environmental cues
• modulated by external timing cues called zeitgeers which adapt the rhythm to the environment

Question 29

what do circadian rhythms require?

Answer 29

sensory information
- light going off is an external cue that maintains the circadian clock

if the external cue is removed, the circadian rhythm is shifted

Question 30

what is the major internal clock of the circadian rhythm?

Answer 30

Suprachiasmatic nucleus of the anterior hypothalamus

• lesioning of the suprachiasmatic tract interferes with circadian rhythm
• the suprachiasmatic nucleus regulates timing of sleep, but is not responsible for sleep itself

Question 31

what inputs into the suprachiasmatic nucleus?

Answer 31

intrinsically photosensitive ganglion cells send info from retina to the nucleus

• these cells respond to light
• even if photoreceptors are removed, they can still detect changes in light intensity