Reticular Formation Flashcards
What is arousal?
The emotional state associated with some kind of goal or avoidance of something noxious
What is consciousness?
Difficult to determine, something to do with ‘awareness’ of both external world and internal states
What parts of the brain are required for consciousness?
- Cerebral cortex
- Reticular formation
What is the reticular formation?
A population of specialised interneurones in the brainstem
What inputs to the reticular system regulate the level of arousal?
- Inputs from sensory system
- Inputs from cortex
What are the widespread outputs from the reticular formation?
- Thalamus
- Hypothalamus
- Basal forebrain
- Spinal cord
What is the part of the reticular formation that is devoted to arousal?
The reticular activating system
Where is the reticular formation found?
In the brainstem
What does the reticular formation consist of?
Diffuse population of interneurones
What are the major inputs to the reticular system?
- Sensory system
- Cerebral cortex
Describe the relationship between the reticular activating system and the cerebral cortex?
The cerebral cortex sends excitatory projections to the reticular formation, and the reticular formation activates the cortex - mutual excitation produces a positive feedback loop
What is the importance of the positive feedback loop produced between the cerebral cortex and the reticular activating formation?
It produces an all or nothing phenomenon which is required to maintain the awake state
Give two methods of assessing consciousness
- The glasgow coma scale
- EEG
What does the EEG measure?
Combined activity of thousands of neurones in a given part of the cortex
Describe the resolution of the EEG
- Very high temporal resolution
- Very poor spatial resolution
What do neurones in the brain tend to do when deprived of sensory input?
Fire synchonously
What happens to the EEG during sleep?
You typically pass through 6 stages of sleep, progressing from an awake state through to stage 4, and then periodically going from stage 4 rapidly up into REM sleep
What does the EEG show going down to the first 4 stages of sleep?
Decreasing frequency and increasing amplitude, as neuronal populations in the cortex become synchronous
Describe the brainwaves in an awake person
- ß-waves at about 50Hz
- Irregular
Why are the brainwaves in an awake person irregular?
Because you are processing sensory input
Describe the brain waves of a person with their eyes closed
- Alpha waves, at about 10Hz frequency
- More regular
What happens with increasing synchronicity amongst neurones?
Increased amplitude
Describe the brainwaves in stage 1 sleep
Background of alpha waves with occasional low frequency θ waves at about 5Hz
Describe the brainwaves in stage 2/3 sleep
Background of θ, but ocassional bursts of activity called sleep spindles. Occassionally get very big waves called K complexes
What are sleep spindles?
Attempt by the thalamus to wake the cortex - K complex is the cortex trying to go back to the intrinsic rate
Describe the brainwaves in stage 4 of sleep
Just see delta waves at a frequency of 1 per second, as cortex is now acting independantly of reticular formation
What is the neural mechanism of sleep?
Complex, but is basically about ‘deactivating’ the reticular activating system, and hence teh cortex, and inhibiting the thalamus
What happens to the positive feedback loop between the reticular activating system and the thalamus in sleep?
It is inhibited, leading to decreased cortical activity
What is inhibition of the positive feedback loop between the reticular activating system and the cortex assisted by?
Removal of sensory inputs
What is REM sleep initiated by?
Groups of neurones in the pons
Describe the EEG activity in REM sleep
SImilar to that seen during arousal, with beta waves
Why is the person difficult to rouse in REM sleep?
Due to strong inhibition of the thalamus
What happens to muscle tone in most of the body during REM sleep?
It is lost due to descending inhibition of LMNs by glycinergic fibres arising from the reticular formation, and running down the reticulospinal tracts
What functions are preserved during REM sleep?
Eye movements and some other cranial nerve functions
What autonomic effects are seen during REM sleep?
- Penile erection
- Loss of thermoregulation
How is penile erection during REM sleep clinically useful?
Good for determining if physiological or psychological impotence
Give three examples of disorders of sleep
- Insomnia
- Nacrolepsy
- Sleep apnoea
What are the common causes of insomnia?
Anxiety and mental health
Rarely neurological
Give four examples of disorders of consciousness
- Brain death
- Coma
- Persistent vegetative state
- Locked in syndrome
What is brain death?
Widespread cortical and braimstem damage
What is shown on the EEG in brain death?
Flatline
What is a coma?
Widespread brainstem and cortical damage, with various (disordered) EEG patterns detectable. Unarousable and unresponsive to psychologically meaningful stimuli.
Is a sleep-wake cycle detectable in coma?
No
How does persistant vegetative state differ from a coma?
- Some spontaneous eye opening
- Can localise stimuli via brainstem reflexes
- Sleep-wake cycle detectable
What can locked in syndrome be caused by?
Basilar/pontine artery occlsuion
What happens to motor functions in locked in syndrome?
Eye movements can be preserved, but all other somatic motor functions are lost from the pons down
