S8) The Brainstem, Arousal, Sleep and Consciousness Flashcards
What is arousal?
Arousal is the emotional state associated with some kind of goal or avoidance of something noxious
What is consciousness?
Consciousness has something to do with ‘awareness’ of both external world and internal states (difficult to define, but measurable)
Two basic ingredients are required for consciousness.
What are they?
Consciousness = cerebral cortex + reticular formation
What is the reticular formation?
The reticular formation is a population of specialised interneurones in the brainstem
Numerous inputs regulate the level of arousal.
Where do they come from?
- Sensory system
- Cortex
The reticular formation has widespread outputs.
Where do these go to?
- Thalamus (sensory gating) (GLUT)
- Hypothalamus (Histamine)
- Basal forebrain nuclei (Ach)
- Spinal cord (muscle tone)
Which assessment might one use to assess consciousness?
Glasgow Coma Scale
Which investigation might one use to assess consciousness?
The electroencephalogram (EEG) measures the combined activity of thousands of neurones in a given part of the cortex to a very high temporal resolution
How many different stages are there in sleep? Explain
5 stages – during the night you typically pass through around 6 cycles of sleep, progressing from an awake state down through to stage 4 and then periodically going from stage 4 rapidly up into REM sleep
Provide a brief interpretation of the sleep on an EEG
Going down through the 4 stages, the EEG shows decreasing frequency and increasing amplitude as neuronal populations in the cortex become synchronous due to reduction of sensory input
Describe the following tracings of the different stages of sleep in this EEG:
- Awake – low voltage, random, fast (beta waves)
- Drowsy – alpha waves; more regular
- Stage I – theta waves; more regular and synchronised
- Stage II/III – sleep spindles and K complexes
- Stage IV – delta waves
- REM sleep (beta waves)
Describe the neural mechanism of sleep
Deactivating the reticular activating system and inhibiting the thalamus:
- The positive feedback loop between RAS and cortex is inhibited, leading to decreased cortical activity
- Inhibition of the positive feedback loop is assisted by removal of sensory inputs
(stimulating cortex stimulates the RF which the stimulates the cortex + feedback)
What initiates REM sleep?
REM sleep is initiated by groups of neurones in the pons
In four steps, describe the paradox of REM sleep
⇒ EEG similar to that seen during arousal (beta waves), but hard to rouse due to strong inhibition of the thalamus
⇒ Majority of muscle tone is lost due to descending inhibition of LMNs by glycinergic fibres arising from the reticular formation (paralysed)
⇒ Preservation of eye movements and some other cranial nerve functions (teeth grinding)
⇒ Autonomic effects are seen e.g. penile erection, loss of thermoregulation
What are the functions of sleep?
Enigmatic:
- Energy conservation and bodily repair
- Memory consolidation
- Clearance of extracellular debris