S8) The Brainstem, Arousal, Sleep and Consciousness

Question 1

What is arousal?

Answer 1

Arousal is the emotional state associated with some kind of goal or avoidance of something noxious

Question 2

What is consciousness?

Answer 2

Consciousness has something to do with ‘awareness’ of both external world and internal states (difficult to define, but measurable)

Question 3

Two basic ingredients are required for consciousness.

What are they?

Answer 3

Consciousness = cerebral cortex + reticular formation

Question 4

What is the reticular formation?

Answer 4

The reticular formation is a population of specialised interneurones in the brainstem

Question 5

Numerous inputs regulate the level of arousal.

Where do they come from?

Answer 5

• Sensory system
• Cortex

Question 6

The reticular formation has widespread outputs.

Where do these go to?

Answer 6

• Thalamus (sensory gating)
• Hypothalamus
• Basal forebrain nuclei
• Spinal cord (muscle tone)

Question 7

Which assessment might one use to assess consciousness?

Answer 7

Glasgow Coma Scale

Question 8

Which investigation might one use to assess consciousness?

Answer 8

The electroencephalogram (EEG) measures the combined activity of thousands of neurones in a given part of the cortex to a very high temporal resolution

Question 9

How many different stages are there in sleep? Explain

Answer 9

5 stages – during the night you typically pass through around 6 cycles of sleep, progressing from an awake state down through to stage 4 and then periodically going from stage 4 rapidly up into REM sleep

Question 10

Provide a brief interpretation of the sleep on an EEG

Answer 10

Going down through the 4 stages, the EEG shows decreasing frequency and increasing amplitude as neuronal populations in the cortex become synchronous

Question 11

Describe the following tracings of the different stages of sleep in this EEG:

Answer 11

• Awake – low voltage, random, fast (beta waves)
• Drowsy – alpha waves
• Stage I – theta waves
• Stage II/III – sleep spindles and K complexes
• Stage IV – delta waves
• REM sleep (beta waves)

Question 12

Describe the neural mechanism of sleep

Answer 12

Deactivating the reticular activating system and inhibiting the thalamus:

• The positive feedback loop between RAS and cortex is inhibited, leading to decreased cortical activity
• Inhibition of the positive feedback loop is assisted by removal of sensory inputs

Question 13

What initiates REM sleep?

Answer 13

REM sleep is initiated by groups of neurones in the pons

Question 14

In four steps, describe the paradox of REM sleep

Answer 14

⇒ EEG similar to that seen during arousal (beta waves), but hard to rouse due to strong inhibition of the thalamus

⇒ Majority of muscle tone is lost due to descending inhibition of LMNs by glycinergic fibres arising from the reticular formation

⇒ Preservation of eye movements and some other cranial nerve functions

⇒ Autonomic effects are seen e.g. penile erection, loss of thermoregulation

Question 15

What are the functions of sleep?

Answer 15

Enigmatic:

• Energy conservation and bodily repair
• Memory consolidation
• Clearance of extracellular debris

Question 16

Identify three sleep disorders

Answer 16

• Insomnia
• Narcolepsy
• Sleep apnoea

Question 17

What is insomnia?

Answer 17

Insomnia is a sleep disorder where people have trouble sleeping and is often due to psychiatric/psychological causes

Question 18

What is narcolepsy?

Answer 18

• Narcolepsy is a long-term neurological disorder that involves a decreased ability to regulate sleep-wake cycles
• It is characterised by excessive sleepiness, sleep paralysis, hallucinations

Question 19

What is sleep apnoea?

Answer 19

Sleep apnoea is a serious sleep disorder that occurs when a person’s breathing is interrupted during sleep

Question 20

Identify four disorders of consciousness

Answer 20

• Brain death
• Coma
• Persistent vegetative state
• Locked in syndrome

Question 21

What is brain death?

Answer 21

Brain death is widespread cortical and brainstem damage (flat EEG)

Question 22

What is a coma?

Answer 22

• A coma is a disorder of consciousness due to widespread brainstem and cortical damage, with various (disordered) EEG patterns detectable
• The patient is unarousable and unresponsive to psychologically meaningful stimuli
• No sleep-wake cycle detectable

Question 23

What is locked in syndrome?

Answer 23

• Locked-in syndrome is a condition that can be caused by basilar/pontine artery occlusion
• Eye movements can be preserved, but all other somatic motor functions lost from the pons down

Question 24

What is PVS?

Answer 24

• PVS is a disorder of conscious due to widespread cortical damage, with various (disordered) EEG patterns detectable
• Similar to coma but with some spontaneous eye opening, can also localise to stimuli via brainstem reflexes
• Sleep-wake cycle detectable