Arousal, coma and unconsciousness: Implications for management of head trauma Flashcards
What is consciousness?
At its least, normal human consciousness consists of a serially time-ordered, organized, restricted and reflective awareness of self and the environment. Moreover, it is an experience of graded complexity and quantity.
James, W. The physical basis of emotion. Psychol. Rev. 1, 516–529 (1894).
What is missing from the definition of consciousness?
Arousal level and the awareness of the contents of consciousness, which includes a range of specific functional types of attention, intention, memory, and mood–emotion.
What do disorders of consciousness affect?
They affect both arousal level and awareness of the contents of consciousness.
Where do we start in consciousness?
We start in the Brainstem Nuclei, specifically the Pedunculopontine Nucleus (part of the Ascending Reticular Activating System). From here, tracts go up to the hypothalamus and the basal forebrain, where arousal is driven from.
What are the key neurotransmitters in those pathways?
Orexin, Adrenergic, and Cholinergic are vital in sleep and consciousness.
If you have an antibody to orexin, you can get narcolepsy - rapid onset - cannot resist.
What are the neural substrates of consciousness?
Basal nuclei (RAS), Thalami-cortical circuits (Global workspace, Internal and external awareness), Striatum (stimulated by thalamus and frontal cortex, inhibits globus pallidus interna), and Globus pallidus interna (inhibits thalamus and brain stem nuclei).
Where is the second part of the circuit found?
In the thalamus, frontal and parietal cortex, and the striatum.
What is the thalamo-cortical circuit responsible for?
It is responsible for internal and external awareness.
What is the striatum stimulated by and what does it do?
The striatum is stimulated by the thalamus and frontal cortex and inhibits the Globus Pallidus Interna.
What does the globus pallidus interna do?
It inhibits the thalamus and brain stem nuclei.
Briefly describe how the mesocircuit model enables consciousness.
- RAS actively switches on the cortex and there is to and fro traffic between the two, activating consciousness. 2. Positive feedback in the frontal lobe stimulates the striatum to stay awake. 3. Striatum inhibits globus pallidus interna, maintaining a level of inhibition when we are conscious.
What happens in this mechanism when we start to become bored/tired?
- Stimulation from the cortex to the striatum reduces. 2. Inhibition to globus pallidus interna starts to be removed. 3. GPI becomes more active, inhibiting RAS. 4. RAS switches off, leading to sleepiness and fading consciousness.
What is this circuit called?
This circuit is known as the mesocircuit model, which includes more detail about the conscious network.
What step in this mechanism keeps you awake?
The inhibition of the striatum on the Globus Pallidus Interna keeps you awake.
What would happen if you have an injury that damages this connection?
Loss of inhibition in GPi will cause someone to slip into sleep/coma. Interestingly, the use of an inhibitory neurotransmitter can wake someone from a coma by inhibiting the Globus Pallidus Interna, allowing the brainstem nuclei (RAS) to switch back on and stimulate the cortex.
How does the mesocircuit model translate into clinical responsibilities?
The brainstem nuclei are the engine house of the conscious network of the meso circuit model, responsible for arousal. The cortex is responsible for awareness, including localized movement, language, thinking, and judgement.
Anatomically, how does internal awareness vary from external awareness?
Different parts of the brain are responsible for internal/self-awareness and other parts for external/sensory awareness.
What is brain injury?
It is primarily a disruption of networks, resulting in them not working properly. Some networks have other pathways, so injuries to the same degree in different locations can have different outcomes (neuroplasticity).
Define brain stem death.
It is when a person no longer has any brain stem functions, permanently losing the potential for consciousness and the capacity to breathe.
What are the diagnostic criteria for brain stem death?
The criteria include the absence of brain stem reflexes and the inability to breathe independently.
Define coma.
Coma is a complete failure of the arousal system, where the patient cannot have spontaneous eye opening or be awakened by vigorous sensory stimulation, equating to a Glasgow Coma Score of ≤8.
What is Unresponsive Wakefulness (Vegetative State) characterized by?
It is characterized by the complete absence of behavioral evidence for self or environmental awareness, with preserved capacity for spontaneous or stimulus-induced arousal, evidenced by sleep-wake cycles.
What is minimal conscious state (MCS)?
MCS is when cognitively mediated behavior occurs inconsistently but is reproducible or sustained long enough to be differentiated from reflexive behavior.
What does MCS look like in a patient?
Patients may follow simple commands, provide gestural or verbal yes/no responses, and exhibit purposeful behavior, including appropriate emotional responses and reaching for objects.
