Consciousness Flashcards
Define consciousness
The brain state the enables us to experience the world around us and within one-self
Distinct from autonomic behaviours that occur in an unconsciousness manner (e.g. breathing)
Elements of consciousness?
Distinction between LEVELS (alertness) vs. CONTENTS (subjective experience) of consciousness
What does LEVELS incorporate?
Alertness invovles the RETICULAR FORMATION
o this regulates vital functions
o projects into the thalamus & cortex
o allowing it to ‘control’ if sensory signals reach cortical sites of conscious awareness e.g. posterior parietal cortex
RAS?
Reticular Activating System
What is the RAS?
A polysynaptic network in the core of the midbrain, pons and upper medulla
What does the RF contain?
o NA neurones - locus coeruleus (pons)
o Dopaminergic neurons - ventral tegmental nucleus/area (midbrain)
o Cholinergic neurons - project into the cortex to boost the level of activity via. the thamalus (maintains awareness)
o Serotonergic projections - raphe nucleus (midline)
Functions of the RF?
Besides control of awareness, also:
o centres for body systems e.g. CVS, respiratory, bladder
Degree of activity in the RS is associated with alertness/levels of consciousness
The RF receives sensory information from ALL sensory pathways - what are they?
Touch & Pain
o from ASCENDING TRACTS
Vestibular
o from MEDIAL VESTIBULAR tracts
Auditory
o from INFERIOR COLLICULUS
Visual
o from SUPERIOR COLLICULUS
Olfactory
o from MEDIAL FOREBRAIN BUNDLE
Which part of the RF has the most important role in regulating levels of arousal?
Cholinergic neurones!
project to thalamus
How do the cholinergic neurones regulate level of arousal?
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
How can you monitor levels of arousal?
via. ECG rhythms OR GCS
What are the different waves seen in ECG rhythms of levels of arousal?
DELTA waves = seen during SLEEP (>4Hz)
THETA waves = associated w. drowsiness (4-8Hz)
ALPHA waves = subject is RELAXED with eyes closed (8-13Hz)
BETA waves = subject is AWAKE with normal consciousness (13-30Hz)
HIGHER Fqs waves (gamma range)
o associated with CREATION of CONSCIOUS CONTENTS in the focus of the mind’s eye
o via. the recurrent thalamo-cortical feedback loops
o ~40Hz
GCS?
Glasgow Coma Scale!
What can alter the state of consciousness?
o Contusions (a bruise) - localised bleeding within the brain
o Concussion (diffuse, widespread homogenous impairment of brain tissue due to brain trauma)
o 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
o Stupor
- lack of critical cognitive function & consciousness
- only responsive to pain
What can lead to a coma?
Damage to RF/thalamus
What is a coma?
State of unconsciousness in which the subject cannot be roused even by strong sensory stimuli
How is a coma different to sleep?
Metabolic activity of the brain is DEPRESSED in a coma
Causes of coma?
- Metabolic alteration
o drug OD
o hypoglycaemia
o hypoxia - Bilateral lesions in cerebral hemispheres
- Lesions in thalamus OR brain stem (raised ICP)
What are the 2 forms of coma?
- Persistent vegetative state
- irreversible coma due to disconnection of cortex from brainstem
OR
- widespread cortical damage - Brain death
- irreversible coma due to brainstem death
- body kept alive artificially
Explain the form of coma where person is in ‘persistent vegetative state’?
Brainstem still functions SO
- reflexes, postural movements & sleep-wake cycle may be present
- can be aroused from but are still unconscious
Explain the form of coma where person is ‘brain dead’?
Decision to cease treatment depends on demonstration of absence of brainstem reflexes and response to HYPERCAPNIA
Spinal reflexes & some postural movement may still be present
How does Achromatopsia arise?
Lesions to extra-striate cortex eliminates awareness of colour
Left visual neglect in terms of disorders of consciousness?
Breakdown of consciousness awareness after RIGHT PARIETAL damage
o patient remain unconscious of info. from L visual field (despite 1o visual cortex being intact in occipital lobe)
- info being received by occipital lobe BUT not processed
o damage to parietal cortex means info. does NOT reach awareness as the RF is disrupted
How can you investigate left visual neglect?
o Star cancellation test
- patient cancels fewer left stars
o Visual explorations
- patient will NOT cross midline to look left
What can be used to study consciousness?
fMRIs!
What is meant by blind-sight in terms of disorders of consciousness?
Are perceptually blind of their visual field due to occipital damage
BUT
can respond to visual stimuli
Feed-forward processing vs. top-down recurrent processing?
There is NO SINGLE AREA for consciousness
o feed-forward processing (subliminal or non-conscious)
o top-down recurrent processing (conscious access)
ONENOTE!!
Visual neglect vs. Hemianopia?
Hemianopia
o patient can NOT see half of their field of vision
o patient WILL KNOW
Visual neglect
o patient would ‘THINK’ they can see whole field of vision
o BUT can NOT see half their visual field
