The Reticular Formation Flashcards
Define arousal and consciousness
- Arousal - the emotional state associated with some kind of goal or avoidance of something noxious
- Consciousness - ‘awareness’ of both external world and internal states
- Requires cortex and reticular formation
Describe the reticular formation, including its inputs and outputs
- A population of specialised interneurones in the brainstem
- Numerous inputs regulate the level of arousal from sensory system and cortex
- Difficult to sleep with lots of stimulus/sensation
- Widespread outputs - thalamus, hypothalamus, basal forebrain nuclei, spinal cord
Describe the relationship between the cortex and reticular formation
- Has a positive feedback loop with cortex - as cortex signalling increases, reticular formation signalling increases which further increases cortex signalling
- Keeps people awake
- Shuts down in sleep
Describe the reticular activating system
- A large part of the reticular formation is devoted to arousal - reticular activating system
- Output to basal forebrain nuclei and then to cortex using ACh neurotransmitter
- Anticholingergic drugs cause drowsiness - inhibit stimulatory output
- Output to hypothalamus and then to cortex using histamine
- Antihistamines also inhibit excitatory ascending fibres
- Output to thalamus and then to cortex using glutamate
- ACh, histamine and glutamate are all excitatory - maintain consciousness by keeping cortex awake
State how consciousness can be assessed
- Glasgow coma scale
- EEG
Describe what is assessed from the Glasgow coma scale
- Eye opening
- Motor response
- Verbal response
Describe what an EEG is
- EEG - electroencephalogram
- Measures the combined activity of thousands of neurones in a given part of the cortex
- Deprived of sensory input, neurones in the brain tend to fire synchronously (sleep)
Describe the EEG of a person who is awake
- ß waves when awake and eyes open
- Random and fast waves
- ~50Hz, irregular (brain processing many inputs)
Describe the EEG of a person who is awake but has their eyes closed
- α waves
- slower synchronised waves
- ~10Hz, regular (no input to brain so synchronised)
Describe the EEG of a person who is at stage 1 sleep
- Background α waves with θ waves
- θ waves ~5Hz (longer than α waves)
Describe the EEG of a person who is at stage 2/3 sleep
- Background θ waves with sleep spindles and K-complexes
- Sleep spindles - thalamus trying to wake you up (fast, large waves)
- K-complex - input taken away so brain produces intrinsic waves (slow large waves)
Describe the EEG for person in stage 4 sleep
- δ wave ~1Hz
- All synchronised neurones as all stimulus removed, so amplitude increased
Describe what REM sleep is and how its EEG looks like
- REM (rapid eye movement) sleep - similar to ß waves
- Dreaming - eyes produce imaginary input to brain
Describe the neural mechanism of sleep
- Involves deactivation of reticular activating system (and hence cortex) and inhibiting thalamus
- Positive feedback loop between RAS and cortex inhibited, leading to decreased cortical activity
- Inhibition of positive feedback loop assisted by removal of sensory inputs
Describe the paradox of REM sleep
- REM sleep is initiated by groups of neurones in the pons
- EEG activity is similar to that seen during arousal (ß-waves), however person is difficult to rouse due to strong inhibition of thalamus
- Muscle tone in most of body is lost due to descending inhibition of LMN by glycinergic fibres arising from the reticular formation
- Eye movements and some other cranial nerve functions are preserved (eg. Nocturnal bruxism - teeth grinding)
- Autonomic effects are seen including penile erection and loss of thermoregulation
