Neurobiology of Consciousness Flashcards
What is the “content of consciousness”?
Arousal (level of consciousness)
Awareness of self/environment (content of consciousness)
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Motivation to respond
What is arousal?
Global state of wakefulness
(deep sleep to high vigilance)
What is awareness?
Ability to perceive one or more specific stimuli:
Visual, Tactile, Auditory, Gustatory, Olfactory, Vestibular, Proprioceptive, Interoceptive
Define motivation
Drive to act on internal or external stimuli that have entered conscious awareness
What is the “modern day” ARAS (Ascending reticular activating system)?
Ascending arousal system
What is the ascending arousal system?
Complex and diffuse network of neurons projecting from multiple brainstem source nuclei (within and adjacent of RF) to the cortex via thalamic and extrathalamic pathways
What functions does the reticular formations have?
Wakefulness
Eye movements
Swallowing/vomiting
Posture/locomotion
Respiration
Blood pressure
Sensory awareness
What are the two pathways of the ascending arousal system
Thalamic and extrathalamic
Two pathways provides redundancy!
What is the general pathway of the thalamic branch?
Starting in the reticular formation
Projects to the thalamus
Then spreads to various cortical regions via what are known as thalamocortical projections
What is the general pathway of the extrathalamic branch?
Starting in the brain stems reticular formation
A little bit of input to the thalamus, but by and large, the projections bypass the thalamus and go to the hypothalamus and the basal forebrain
Then continue to project on to other areas of the cerebral cortex.
What are the nuclei for the thalamic pathway?
Cholinergic (pedunculopontine and laterodorsal tegmental nuclei)
(PPT/LDT)
What are the nuclei for the extrathalamic pathway?
Serotonergic, some dopaminergic (dorsal and median raphe subnuclei)
Noradrenergic (locus coeruleus) (LC)
Dopaminergic (ventral tegmentum, ventral periaqueductal gray) (vPAG)
Glutamatergic (parabrachial complex in the rostral pons)
Histaminergic (tuberomamillary nucleus in hypothalamus) (TMN)
Orexin and melanin-concentrating hormone (lateral hypothalamus (LH)
Cholinergic and GABA-ergic (basal forebrain) (BF)
How does one become aware of stimuli?
Conscious awareness and arousal states interact
Awareness requires interactions between the cortex and specific and nonspecific thalamic nuclei (e.g., reticular, intralaminar)
What is the function of the thalamus?
Relay station and filters and modulates information
Coordinates activity in widespread areas
1. Cortico-striatopallidal-thalamocortical loops
2. Cortico-thalamocortical loops
How do the thalamus and striatum interact?
Largest thalamic efference is to the striatum (caudate, putamen, nucleus accumbens)
Provide connections to large cerebral networks: potential mechanism for translating sensory/motor activity to awareness
What role does the thalamus play with the cortical and subcortical regions?
Frontal/parietal system: Movement planning/execution
Brainstem attentional capture mechanisms: Redirect attention to peripheral sensory inputs
What is a connectomes?
spontaneous resting brain activity, functionally connected brain regions
What are some examples of resting state networks?
Default mode
Frontoparietal
Salience
Auditory
Sensorimotor
Visual
What is the pathophysiology of a coma?
Global dysfunction of corticothalamic loops from diffuse cellular dysfunction, disconnection, or loss of upper brainstem arousal tone.
If the entire brain or brainstem is permanently nonfunctional, then diagnosis is brain death rather than coma
What is the pathophysiology of vegetative state?
Same as coma, except that it implies some functioning of the upper brainstem
What is the pathophysiology of minimally conscious state?
Diverse but typically diffuse injury to white matter and/or thalamus.
Varying degrees of cortical injury
What is the pathophysiology of akinetic mutism (disorder of motivation)?
Dysfunction of prefrontal cortex or its subcortical connections or of white matter connecting these areas
What is the pathophysiology of locked-in state?
Classically the loss of corticospinal tract in ventral pons, but can also be from diffuse white matter injury in the setting of trauma
What pathophysiology presents as decorticate positioning?
Bilateral damage to the diencephalon-upper midbrain
What pathophysiology presents as decerebrate positioning?
Bilateral damage to the upper midbrain
What pathophysiology presents as localization?
Swelling in one hemisphere compression the diencephalon
What is a potential mechanism to restore consciousness?
Amantadine hydrochloride - the gold standard medication for stimulation of arousal and subsequent awareness
How does amantadine hydrochloride aid in restoring consciousness?
Increases the excitation of the striatum to the frontal cortex
This increase the inhibitory input to the globus pallidus, which then limits the amount of inhibitory influence it previously had on the thalamus
Restores the cyclical behavior of the thalamus - providing excitatory input to the cortices as well as the striatum
