8. Reticular formation & control of consciousness

Question 1

What is consciousness and which brain structures does it require?

Answer 1

“Awareness” of both external world and internal states.

Requires +ve feedback loop between CEREBRAL CORTEX and RETICULAR FORMATION.

Question 2

What is arousal?

Answer 2

Emotional state associated with some kind of goal or avoidance of something noxious.

Question 3

What is the reticular formation? Describe its inputs and outputs.

Answer 3

Reticular formation = population of specialised interneurones in the brainstem involved in consciousness and arousal.

Inputs from:

• sensory system (e.g. can’t sleep when light/noisy)
• cortex (e.g. can’t sleep when anxious)

Outputs to:

• thalamus (sensory gating)
• hypothalamus
• basal forebrain nuclei
• spinal cord (muscle tone)

Question 4

What is the reticular activating system?

Answer 4

Part of the reticular formation devoted to arousal, including stimulation of the cortex via the:

• thalamus: glutaminergic neurones to cortex
• hypothalamus: histaminergic neurones to cortex
• basal forebrain nuclei: receive cholinergic input from RF and project cholinergic neurones to cortex

Question 5

How is level of consciousness assessed?

Answer 5

Glascow Coma Scale (GCS) - assesses:

1. eye opening (spontaneous, response to speech, nil)
2. motor response (obeys, localises, withdraws, abnormal flexor response, extensor response, nil)
3. verbal response (oriented, confused conversation, incomprehensible sound, nil)

Electroencephalograms

Question 6

Do neurones fire synchronously when asleep or awake?

Answer 6

When asleep

Question 7

Describe the wave pattern seen on EEG when awake with eyes open and with eyes closed.

Answer 7

Awake with open eyes = beta waves

• high frequency (50 Hz)
• irregular due to complex processing

Awake with eyes closed = alpha waves

• lower frequency (10 Hz)
• more regular due to deprivation of input

Question 8

Describe the wave pattern seen on EEG during the stages of sleep.

Answer 8

Stage 1

• background alpha waves (10 Hz)
• occasional theta waves (5 Hz)

Stage 2/3

• background of theta waves (5 Hz)
• occasional sleep spindles (short period of increased thalamus activity stimulating cortex)
• occasional K complexes (intrinsic cortical rate)

Stage 4
- delta waves (slow intrinsic cortical rate, high amplitude due to synchronicity of neuronal firing)

REM sleep
- beta waves - dreaming provides visual input to cortex

Question 9

where are the neurones located for REM sleep located?

Answer 9

pons

Question 10

Describe the state of arousal seen in REM sleep

Answer 10

• EEG activity similar to that seen during arousal (beta waves)
• person difficult to rouse due to strong inhibition of thalamus
• muscle tone in most of body is lost due to descending inhibition of LMNs by glycinergic fibres arising from the reticular formation and running down the reticulospinal tracts
• eye movements and some other cranial nerve functions are perserved (e.g. nocturnal bruxism)
• autonomic effects are seen inc. penile erection and loss of thermoregulation

Question 11

describe 3 disorders of consciousness

Answer 11

1. Persistent vegetative state
   - widespread cortical damage
   - various (disordered) EEG patterns detectable
   - unarousable and unresponsive to pyschologically meaningful stimuli but with some spontaneous eye opening (can even localise to stimuli via brainstem reflexes)
   - sleep-wake cycle detectable
2. Coma
   - widespread brainstem and cortical dysfunction
   - various (disordered) EEG patterns detectable
   - unarousable and unresponsive to pyschologically meaningful stimuli
   - no sleep-wake cycle detectable
3. Brain death
   - widespread cortical and brainstem damage
   - flat EEG