Session 8 L2: Consciousness Flashcards
What is arousal?
The emotional state associated with some kind of goal or avoidance of something noxious. Regulated by inputs from sensory system and cortex
What is consciousness?
-Related to awareness of both external environment and internal states.
What is the requirement of consciousness?
- Cerebral cortex
- Reticular formation.
How does the cerebral cortex affect consciousness?
- The site where conscious thoughts arise
- Receives many inputs, including from the reticular formation
How does the reticular formation affect consciousness?
-Circuitry to keep cortex awake, particularly the reticular activating system in the brain stem
What is the reticular formation?
- A population of specialised interneurons in the brainstem. It is a diffuse network of grey matter and is poorly defined. Lives in all 3 divisions of the brainstem
- Receives many inputs, including from the cortex and sensory systems
How are the cortex and reticular formation associated?
- Cortex and reticular formation are connected by reciprocal excitatory projections, forming a positive feedback loop
- Binary outcome of sleep/awake
What are the outputs to the cortex from the reticular formation?
-Occurs via three major relay nuclei
What are the types of projections from the reticular formation?
Cholinergic Projections (excitatory)
Which three major relay nuclei do reticular formation send fibres by?
- Thalamus sends excitatory glutamatergic fibres to the cortex (sensory gating)
- Hypothalamus sends excitatory histaminergic fibres to the cortex (think sedative side-effects of seating antihistamines)
- Basal forebrain nuclei sends excitatory cholinergic fibres to cortex (think sedative side effect of sedating anticholinergics)
What other fibres are sent by the reticular formation?
-Projections sent down the cord, responsible for mainting muscle tone
What is used to assess cortical function?
- EEG
- Glasgow Coma Scale
What is an EEG?
- Measures combined activity of thousands of neurones in a given part of cortex to very high temporal resolution but low spatial resolution
- Neurones in the brain tend to fire synchronously when deprived of sensory input
- Good for detecting neuronal synchrony
What are the responses tested in the Glasgow Coma Scale?
- Eye opening response
- Verbal response
- Motor response
What are the function of sleep?
- Energy conservation and bodily repair
- Memory consolidation
- Clearance of extracellular debris
- Resetting of the CNS
How many stages do you pass through in sleep?
- Typically pass through around 6 cycles of sleep per night
- 4 major stages + rapid eyes movement
- As you progress from Stage 1 to Stage 4, amplitude increases and frequency decreases
What are the stages of Sleep?
Awake with eyes open - Beta waves (50Hz). Irregular
Awake with eyes closed - Alpha waves(10Hz). Regular
Stage 1 - Background of alpha waves + interspersed theta waves (5Hz)
Stage 2/3 - Background of theta waves with interspersed sleep spindles and k-complexes
Stage 4 - Delta waves (1Hz). Regular
REM sleep - EEG similar to beta waves. Dreaming occurs here
What are Sleep spindles and K complexes seen in stage 2/3?
- K complexes represent the emergence of the intrinsic rate of the cortex
- Sleep spindles are high frequency burst arising from the thalamus
What are the mechanisms of the sleep?
- Complex
- Deactivation of the reticular activating system and hence the cortex and inhibiting the thalamus.
- Positive feedback loop between RAS and cortex is inhibited, leading to decreased cortical activity
- Inhibition of the positive feedback loop by removal of sensory inputs (fewer positive influences on positive feedback loop).
How is REM sleep initiated?
-Initiated by neurones in the pons
Why is the person difficult to rouse in REM sleep?
-Although EEG activity is similar to that seen during arousal, there is strong inhibition of the thalamus
Why is Muscle Tone affect in the body in REM sleep?
-Muscle tone in most of the body is lost due to descending inhibition of LMNs by glycinergic fibres arising from the reticular formation and running down the reticulospinal tracts
What are autonomic effect in REM sleep?
- Penile erection seen
- Loss of thermoregulation
What is important about REM sleep?
-REM sleep is essential for life. Long deprivation leads to death
Describe cranial nerve function during REM sleep?
-Eye movement and some other cranial nerve functions are preserved
What are some sleep disorders?
- Insomnia
- Narcolepsy
- Sleep apnoea
What is insomnia?
- Inability to sleep
- Due to psychological or psychiatric disorder (depression and anxiousness) as opposed to ‘primary insomnia’
What is narcolepsy?
- Rare disorder
- Some cases are caused by mutations in orexin gene affecting neurotransmission. Positive loop feedback can’t work. Orexin is a peptide transmitter involved in sleep
- Caused by neurotransmission. Can’t keep the positive loop feedback working. (look at orexin)
- Inability to keep wakefulness
What cause sleep apnoea?
- Common condition. Affects men and often caused by excess neck fat leading to compression of airways during sleep and frequent waking
- Obstruction of structures in the neck causing a loss of sleep due to hypoxia.
- Causes excessive daytime sleepiness
What are some contains of disordered consciousness?
- Brain death
- Coma
- PVS
- Locked in syndrome
What is a coma?
Widespread brainstem and cortical damage, with various (disordered) EEG patterns detectable. Unarousable and unresponsive to psychologically meaningful stimuli. No sleep-wake cycle detectable
What is PVS?
Widespread cortical damage, with various (disordered) EEG patterns detectable. Like coma but with some spontaneous eye opening. Can even localise to stimuli via brainstem reflexes. Sleep-wake cycle detectable
What is Locked In Syndrome?
Can be caused by basilar/pontine artery occlusion. Eye movements can be preserved, but all other somatic motor functions lost from the pons down.
