S8 L2 The reticular formation: control of consciousness Flashcards
What is consciousness?
- Related to awareness of external environment and internal states
→ Exteroreception- external world
→ Interoreception- internal world
What is aurosal?
Emotional state associated with some kind of goal or avoidance of something noxious
What are the basic component involved in consciousness?
Cerebral cortex and reticular formation
Connected to each other by reciprocal excitatory projections, forming a positive loop
Cerebral cortex- site where many conscious thoughts arise - receives many inputs
Reticular formation associated with keeping the cortex awake
What is the cerebral cortex?
Site where conscious thought arises
Receives many inputs, including from the reticular formation
What is the reticular formation?
The circuitry that keeps the cortex awake
Diffuse set of neurones throughout the brainstem
Receives many inputs→ sensory system and cortex
Widespread outputs
Large part devoted to arousal
What are the main outputs from the reticular formation?
3 main relay nuclei
Cholinergic (excitatory) projection to these relaya
1. Basal forebrain → excitatory cholinergic fibres to the cortex (think sedative side effects of anticholinergics)
2. The hypothalamus → excitatory histminergic fibres to the cortex (think sedative side effect of sedating antihistamines)
3. The thalamus → excitatory glutamatergic fibres to the cortex
Also projections down the spinal cord, responsible for maintaining muscle tone → rapid eye movement sleep- effectively paralysed by inhibitory inputs from the RF
How are the cortex and reticular formations connected?
Positive feedback loops are seen when there is a binary outcome
Sleep/awake, ovulating/not ovulating etc….
(binary outcome - only two possible outcomes)
How can you access consciousness?
Glasgow Coma Scale (GCS) Three components → Eye opening → Motor response → Verbal response Looking for the best response in all
What are the different levels of response in the eye opening pathway?
- Spontaneous = 4
→ Suggest normal cortical and brainstem function - Speech response = 3
→ Slightly diminished cortical function but still functioning brainstem
→ e.g. sleeping - To pain = 2
→ Subcortical reflex activity
→ Suggests impaired cortical function but brainstem preserved - No response = 1
→ Suggests severe damage to brainstem +/- cortex
What are the different levels of response in the motor response pathway?
- Obeys command = 6
→ Normal function with working connections from the auditory to brainstem/spinal cord
→ Whole pathway intact - Localises = 5
→ Able to bring hand up to the part of the body that you stimulate
→ Cortical activity but less
→ Connections from sensory to motor cortex still functioning - Withdrawal = 4
→ Remove from painful stimuli
→ Physiological reflex response still working
→ Spinal reflex, brainstem mediated, doesn’t require cortical input - Flexor response = 3
→ Flexion in response to painful stimuli
→ Physiological response
→ Decortical lesion above the level of the red nuclei - Extensor response = 2
→ Lesion below red nuclei
→ Not a physiological response - No response = 1
→ Nothing happens
→ Severe damage
What are the different levels of response in the verbal response pathway?
- Orientated = 5
→ Appropriate answer to question
→ Normal cortical function - Confused conversation = 4
→ Understand question but don’t get the answer right
→ Suggest diminished higher cortical function - Inappropriate word = 3
→ Still verbal
→ Suggests language centres have been damaged - Incomprehensible sounds = 2
→ Cortical damage with brainstem mediated groans - No response = 1
→ Severe damage to brainstem +/- cortex
→ Brainstem damage really bad, unable to activate muscles for phonation
What is the electroencephalogram?
- Measure combined activity of thousands of neurones in a particular region of the cortex
- High temporal resolution → electrical changes in the brain per ms
- Low spatial resolution → unable to localise lesion as it looks at the summed activity of thousands of neurones
- Detects neuronal synchrony, sign of normal cerebral function
→ occurs in the brain during physiological and pathological process such as sleep and epilepsy
What is the normal EEG look like during sleep?
Awake with eyes open → beta waves - irregular 50 Hz
Awake with eyes closed → alpha waves - regular 10 Hz
4 stages and REM stage
Cycles through about 6 times per night
Stage 1 sleep → Back ground alpha + interspersed theta waves (theta at around 5 Hz, regular)
Stage 2/3 sleep → Background of theta + interspersed sleep spindles and k-complexes
Stage 4 sleep → delta waves - regular 1Hz, related to k-complexes seen in stages 2/3
REM sleep → EEG similar to beta waves, dreaming occurs in this stage, so similar to the EEG in a conscious patient
What are the functions of sleep?
Generally unknown - enigmatic Energy conservation/repair Memory consolidation Clearance of extracellular debris Resetting of the CNS
What is the neural mechanism of sleep?
- Complex
- Basically about deactivating the reticular activating system (and hence the cortex) and inhibiting the thalamus
- Facilitated by the removal of sensory inputs (fewer positive influences on positive feedback loops) → from the reticular formation
