Neurology 17 - Consciousness

Question 1

List the structures involved in regulating the level of arousal

Answer 1

• Reticular formation

- Polysynaptic network in the core of the midbrain, pons and upper medulla

Question 2

List the functions of the reticular formation

Answer 2

• Control of alertness

- Centres which regulate cardiovascular, respiratory, bladder and motor patterns

Question 3

Which sensory pathways does the reticular formation receive information from?

Answer 3

• Touch and pain from ascending tracts
• Vestibular from medial vestibular nucleus
• Auditory from inferior colliculus
• Visual from superior colliculs
• Olfactory via medial forebrain bundle

Question 4

List the ways the reticular formation modulates cerebral activity

Answer 4

• Locus coeruleus (noradrenergic neurons - pons)
• Ventral tegmental nucleus (dopaminergic neurons - midbrain)
• Cholinergic neurons (project to thalamus)
• Raphe nuclei (in the midline, main source of serotonergic projections to the brain and spinal cord

Question 5

Which neurons are most important for regulating the level of arousal?

Answer 5

• Cholinergic neurons

- Increase the level of activity in the cerebral cortex via the thalamus

Question 6

List the three mechanisms of the cholinergic projections of the RAS regulating sleep

Answer 6

• Cholinergic projections excite individual thalamic relay nuclei leading to activation of cortex
• Cholinergic projections to intralaminar nuclei, which in turn project to all areas of cortex
• Cholinergic projections to reticular nucleus, which regulates flow of information through other thalamic nuclei to cortex

Question 7

What is the function of the tuberomammilary nucleus?

Answer 7

• Histaminergic
• In the hypothalamus
• Projects widely to the cortex, involved in maintaining an awake state

Question 8

How to the levels of activity in the reticular activating system compare with awakeness?

Answer 8

• Always some activity
• Level correlates with alertness
• Reduced activity results in sleep

Question 9

How is the level of arousal measured?

Answer 9

• Seen as a change of waveform in EEG (records activity from the cerebral cortex)
• Different basic rhythms are defined by frequency

Question 10

List the 4 basic rhythms of arousal and their frequency

Answer 10

• Delta (0.5-4Hz) present during sleep
• Theta (4-8Hz) associated with drowsiness
• Alpha (8-13Hz) subject relaxed with eyes closed
• Beta (13-30Hz) indicates mental activity and attention

Question 11

What do altered states of consciousness refer to?

Answer 11

• The level of consciousness

- Not the contents

Question 12

List the altered states of consciousness

Answer 12

• Concussion or contusion (temporary loss, only lasts a few minutes)
• Confusion (sustained disturbance of consciousness)
• Stupor
• Coma (cannot be roused by even strong sensory stimuli)

Question 13

Compare confusion and stupor

Answer 13

• In confusion mental processes are slowerd
• Confusion results in people being inattentive, disoriented, or having difficulty carrying out simple commands or speaking
• Stupor can only be roused by strong sensory stimuli

Question 14

What is the Glasgow Coma Scale?

Answer 14

• International standard measure of level of consciousness
• Eyes (4 - none, pain, speech, spontaneous)
• Verbal responses (5 - no verbal response, incomprehensible, inappropriate words, disoriented speech, oriented speech)
• Motor response (6 - extensor/flexor pain response, withdrawal from pain, localisation of pain, obeys commands)

Question 15

What can coma be produced by?

Answer 15

Metabolic causes

• Drug overdose
• Hypoglycaemia
• Diabetes
• Hypercalcaemia

Diffuse intracranial

• Head injury
• Meningitis
• SAH
• Encephalitis
• Epilepsy
• Hypoxic brain injury

Hemisphere lesion (cerebral infarct, haemorrhage or tumour)

Brainstem (infarct, tumour, haemorrhage)

Question 16

What is brain death?

Answer 16

• Irreversible coma due to brainstem death
• Body kept alive artificially
• Decision to cease treatment depends on brainstem reflexes and response to hypercapnia
• EEG is not diagnostic
• Spinal reflexes and postural movements may still be present

Question 17

What is a persistent vegetative state?

Answer 17

• Irreversible coma due to disconnection of the cortex from the brainstem, or widespread disease in the cerebral hemispheres
• Brainstem still functioning, so reflexes, postural movements and sleep-wake cycle may still be present

Question 18

How are vegetative state patients differed from those in a coma?

Answer 18

• Vegetative state patients can be aroused

- Both groups are believed to be unconscious

Question 19

What are the neural correlates of consciousness?

Answer 19

• Minimal brain activity sufficient to produce conscious awareness of information
• Primarily localised to a posterior cotical hot zone that includes sensory areas

Question 20

Give examples of how brain lesions can alter the contents of consciousness

Answer 20

• Lesions to the extra-striae cortex can eliminate awareness of colour (achromatopsia)
• Parietal lesions may lead to lack of awareness for stimuli in the contralateral visual field relative to the side of the brain lesion (neglect)

Question 21

Are perceptual brain regions necessary for conscious experience?

Answer 21

• Debated
• Patients with damage to the primary visual cortex report being unaware but can still display non-conscious visual processing)

Question 22

Define consciousness

Answer 22

• The state of being aware of and responsive to ones surroundings
• Consists of level, content and self

Question 23

Explain the mechanisms of consciousness

Answer 23

• High integration (neurones working together, multiple brain regions are involved)
• High differentiation (different brain regions doing a range of things)

Question 24

Describe minimally conscious state

Answer 24

• Severely altered consciousness
• Minimal but clearly discernible behavioural evidence of self- or environmental awareness is demonstrated
• Eg. dementia