Consciousness Flashcards
What is consciousness?
the brain state that enables us to experience the world around us and within one-self
What is the difference between levels and contents of consciousness?
levels (alertness) involves the reticular formation; this regulates vital functions. It projects into the thalamus and the cortex (allowing it to ‘control’ whether or not sensory signals reach cortical sites of conscious awareness – such as the posterior parietal cortex)
What is the reticular activating system?
a polysynaptic network in the core of the midbrain, pons and upper medulla
reticular formation contains:
- NA neurones – Locus coeruleus
- Dopaminergic neurons – Ventral tegmental area
- Cholinergic neurons – these project into the cortex to boost the level of activity via the thalamus (this maintains awareness)
functions:
- control of alertness
- centres for body systems like the CVS, respiratory, bladder and motor patterns
Where does the RF receive information from?
ALL sensory pathways:
- Touch and pain – from ascending tracts
- Vestibular – from medial vestibular tracts
- Auditory – from inferior colliculus
- Visual – from superior colliculus
- Olfactory – from medial forebrain bundle
How does the RF modulate cerebral activity?
via projections:
Nucleus coeruleus – consists of NA neurones which project directly into the cerebral cortex
Ventral tegmental nucleus – consists of DA neurones which project directly into the cerebral cortex
Cholinergic neurones – project to the thalamus.
- These have the most important role in regulating level of arousal, which involves 3 mechanisms:
• Excitation of individual thalamic relay nuclei -> activation of the cortex
• Projections to intralaminar nuclei -> project to all areas of the cortex
• Projections to reticular nucleus -> regulates flow of information through thalamic nuclei to cortex
Raphe nucleus – in the midline and is the main source of serotonergic projections to the brain and spinal cord
What are the waves in EEGs?
- Delta waves - often seen during sleep - >4Hz
- Theta waves – associated with drowsiness – 4-8Hz
- Alpha waves – subject relaxed with eyes closed – 8-13Hz
- Beta waves – subject is awake with normal consciousness (mental activity) – 13-30Hz
- Higher frequency waves (gamma range) – associated with creation of conscious contents in the focus of the mind’s eye, via the recurrent thalamo-cortical feedback loops. - ~40Hz
What are altered states of consciousness in the GCS?
Contusion – a bruise -> localised bleeding within the brain
Concussion – diffuse, widespread homogenous impairment of brain tissue due to brain trauma
Delirium – or acute confusion – sustained disturbance of consciousness, where mental processes are slowed. Subject may be inattentive, disorientated or have difficulty carrying out simple commands
Stupor – lack of critical cognitive function and consciousness – only responsive to pain
What is a coma? What are causes and types of coma?
damage to the RF/thalamus can lead to this: A state of unconsciousness in which the subject cannot be roused even by strong sensory stimuli. Different from sleep as the metabolic activity of the brain is depressed.
Causes:
- Metabolic alteration – e.g. hypoglycaemia, hypoxia, drug overdose
- Bilateral lesions in cerebral hemispheres
- Lesions in thalamus or brainstem (raised ICP)
- diffuse intracranial - e.g head injury, meningitis, encephalitis
- hemisphere lesion - e.g cerebral infarct, cerebral haemorrhage, tumour
- brain stem - BS infarct, tumour, abscess
- GCS – Glasgow Coma Scale – lowest score is 3
- 3 sections – eyes, verbal response, motor response
Forms of coma:
- Persistent vegetative state – irreversible coma due to disconnection of cortex from brainstem or widespread cortical damage
> Brainstem still functions therefore there are reflexes, postural movements and sleep-wake cycle may be present
> Thought to differ from patients in coma as the former can be aroused eventually yet both groups are thought to be unconscious
- Brain death – irreversible coma due to brainstem death – body kept alive artificially
> Decision to cease treatment depends on demonstration of absence of brainstem reflexes and response to hypercapnia
> Spinal reflexes and some postural movements may be present
What are neural correlates of consciousness?
minimum neuronal mechanisms jointly sufficient for any one specific conscious experience
What’s the difference between visual neglect and heminopia?
Hemianopia would cause the patient to not see half their field of vision and they’d know it too
Visual neglect would cause the patient to ‘think’ they see their whole field of vision whereas they actually cannot see half their field of vision
How can you test different disorders of consciousness?
Left visual neglect – breakdown of consciousness awareness after right parietal damage,
- Patients remain unconscious of information from left visual field (despite primary visual cortex being intact in the occipital lobe)
- The damage to the parietal cortex means information does not reach awareness as the RF is disrupted
Investigation:
- Star cancellation test – patient cancels fewer left stars
- Visual exploration – patient will not cross midline to look left.
Blind-sight – patients that are perceptually blind of their visual field due to occipital damage but can respond to visual stimuli
- They can demonstrate responses to visual stimuli so can manually interact with “unseen” objects.
Vegetative state awareness detection
- MRIs have been used to detect whether there is a level of consciousness in a vegetative state
