The Reticular Formation: Control Of Consciousness

Question 1

What is consciousness an awareness of?

Answer 1

External environment and internal states

Question 2

What is arousal?

Answer 2

The emotional state Associated with goal-seeking behaviour and avoidance of noxious stimuli

Question 3

What two neural components are required for consciousness and what is the role of each?

Answer 3

Cerebral cortex - where conscious thoughts arise, receives many inputs including from reticular formation

Reticular formation - particularly reticular activating system in Brainstem, circuitry keep cortex awake, receives many inputs including from cortex and sensory systems

Both connected via reciprocal excitatory projections forming a positive feedback loop

Question 4

What outcomes use positive feedback loops?

Answer 4

Binary outcomes e.g. sleep OR awake, ovulating Or not

Question 5

How does the reticular formation send outputs to the cortex?

Answer 5

Recticukar fromation sends cholinergic (excitatory) projections to three relay nuclei:

• basal forebrain nuclei -> excitatory cholinergic fibres -> cortex
• hypothalamus -> histaminergic excitatory fibres -> cortex
• thalamus -> excitatory glutamatergic fibres -> cortex

Also sends projections down the cord (maintain muscle tone)

Question 6

What is the reticular formation?

Answer 6

A diffuse network of of nerve pathways in the Brainstem connecting the spinal cord, cerebrum and cerebellum and mediating the overall level of consciousness

Question 7

How do we clinically asses consciousness?

Answer 7

Glasgow coma scale commonly used
3 components looking for the best response:

• eye opening (spontaneous 4/ speech 3/ pain 2/ 1 none)
• motor response (obeys commands 6/ localised to stimuli 5/ withdraws to pain 4/ flexor response to pain 3 (lesion above level red nuclei)/ extensor response to pain 2 (lesion below RN)/ none 1)

Highest score- 15, lowest - 3

• verbal response (orientated time & place 5/ confused convo 4/ inappropriate words 3/ incomprehensible sounds 2/ none 1)

1 = severe damage to Brainstem &/or cortex

Question 8

What does an electroencephalogram measure? What’s it good for detecting?

Answer 8

EEG

Measures combined activity of thousands of neurones in particular region of cortex

High temporal resolution (real time)
Low spatial resolution (can’t localise)

Good for detecting neuronal synchrony (occurs both physiological e.g. sleep and pathological e.g. epilepsy processes)

Assess consciousness

Question 9

If deprived of sensory input how will the neurones in the brain fire?

Answer 9

Synchronously

Question 10

Functions of sleep

Answer 10

Generally unknown

Energy conservation
Repair
Memory consolidation 
Clearance of extracellular debris 
Resetting CNS

Question 11

What are the four stages of sleep and what are the typical EEG patterns of these? What are the two stages of being awake that come first and what is the final added stage of sleep?

Answer 11

Typically pass through around 6 cycles per night:

1. Awake with eyes open - beta waves, irregular, 50Hz
2. Awake eyes closed - alpha waves, regular, 10Hz
3. Stage 1 sleep - background of alpha + interspersed theta waves of 5Hz, regular
4. Stages 2/3 sleep - background theta + interspersed sleep spindles and k-complexes (sleep spindles high frequency bursts arising from thalamus, k-complexes represent emergence of intrinsic rate of cortex)
5. Stage 4 sleep - delta waves, regular, 1Hz (related to k-complexes from stages 2/3)

Slide 9

REM sleep - similar to beta waves, dreaming occurs so similar to EEG in a conscious patient (cortex provides sensory inputs)

Question 12

The neural mechanism of non-REM sleep

Answer 12

Complex, deactivation of reticular activating system and hence cortex + inhibition of thalamus

Deactivation is facilitated by removal of sensory inputs (fewer positive influences on positive feedback loop)

Question 13

Neural mechanism of REM sleep

Answer 13

Initiated by neurones in pons (appears to be active process)

Similar EEG to when awake with eyes open (beta waves) but difficult to rouse due strong thalamic inhibition (stops us acting out dreams, inhibits LMNs)

Decreased muscle tone - glycinergic inhibition of lower motor neurones

Eye movements and some other cranial nerve functions can be preserved e.g. nocturnal bruxism

Autonomic effects are seen (loss thermoregulatory, penile erection)

Essential for life - long term deprivation= death

Question 14

What’s insomnia normally caused by?

Answer 14

Commonly caused by underlying psychiatric disorder as opposed to primary insomnia

Question 15

What’s narcolepsy caused by?

Answer 15

Rare

Some caused mutations in orexin gene - peptide transmitter involved in sleep

Question 16

Causes of sleep apnoea

Answer 16

Common

Often excess neck fat -> compression airways during sleep and frequent waking

Causes excessive daytime sleepiness

Question 17

Which part of the brain is thought to cause sleep spindles?

Answer 17

Thalamus

Question 18

Why is REM sleep called paradoxacle?

Answer 18

Can move, semi conscious as dreaming but difficult to rouse

Question 19

Disorders of consciousness list

Answer 19

Brian death - widespread cortical and Brainstem damage flat EEG

Coma - widespread Brainstem and cortical damage, various EEG patterns, unarousal and unresponsive to psychological stimuli, no sleep wake cycle

PVS (persistent vegetative state) - widespread cortical damage with various EEG patterns, some spontaneous eye opening, sleep wake cycle detectable

Locked in syndrome - caused basilar/ pontine artery occlusion can cause, eye movements can preserved but other somatic motor functions lost from pons down