Brain Arousal Systems Flashcards
What are the 2 parts of consciousness?
1) Arousal (being awake)
2) Awareness (conscious processing of inputs)
Define a coma in terms of consciousness
Neither awake or aware
Define a persistent vegetative state in terms of consciousness
Physiologically identifiable sleep/wake cycles appear; no evidence of awareness
Define a minimally conscious state
Sleep/wake cycles; reproducible evidence of awareness (ability to respond to simple commands); limited or absent communication
What is the hierarchy of consciousness from least conscious to most conscious?
Coma, arousal/wakefulness, awareness, alertness
What usually causes a coma or a disruption of consciousness?
Usually smaller lesions in brainstem, midbrain, or hypothalamus; it takes massive, bilateral damage in the cerebral cortices to alter consciousness or cause a coma
How much further below threshold are neurons in people in a persistent vegetative state?
30 mV
Both arousal and awareness require what?
Activation of the cortex
For patients in a coma, what are their eye/head motions, sleep/wake cycle, awareness, and verbal responses?
Eye/Head Motions: inconsistent or intermittent
Sleep/Wake Cycles: –
Awareness: –
Verbal Responses: –
For patients in a persistent vegetative state, what are their eye/head motions, sleep/wake cycle, awareness, and verbal responses?
Eye/Head Motions: yes
Sleep/Wake Cycles: yes
Awareness: –
Verbal Responses: –
For patients in a minimally conscious, what are their eye/head motions, sleep/wake cycle, awareness, and verbal responses?
Eye/Head Motions: yes
Sleep/Wake Cycle: yes
Awareness: inconsistent or intermittent
Verbal Responses: inconsistent or intermittent
For patients who are aware/alert, what are their eye/head motions, sleep/wake cycle, awareness, and verbal responses?
Eye/Head Motions: yes
Sleep/Wake Cycle: yes
Awareness: yes
Verbal Responses: yes
What are the different arousal systems?
EAA, cholinergic, noradrenergic, serotonergic, and dopaminergic
What makes up the EAA arousal system?
Reticular activating system (RAS) and parabrachial nuclei
What makes up the cholinergic arousal system?
Pedunculopontine tegmental and laterodorsal nuclei (PPT/LDT)
What makes up the noradrenergic arousal system?
Locus ceruleus
What makes up the serotonergic arousal system?
Raphe nuclei
What makes up the dopaminergic arousal system?
Ventral tegmental area
Where is the RAS found?
Occupies the mid-ventral portion of medulla and midbrain
What sends inputs to the RAS?
Trigeminal, auditory, visual, and all ascending sensory tracts; so much convergence that modal specificity is lost, so the brain knows something happened but not what happened
What are the 2 outputs from the RAS?
Dorsal pathway and ventral pathway
What is the dorsal pathway leaving the RAS?
Information leaves via non-specific nuclei of the thalamus, including the intralaminar nucleus of the thalamus; from there, diffuse pathway to all higher levels
What is the ventral pathway leaving the RAS?
Bypasses thalamus via basal forebrain and hypothalamus; from there, diffuse pathway to all higher levels
Where is the parabrachial nuclei located? What is it crucial for?
Pons; crucial for arousal and activation; has medial, intermediate, and lateral parts and inputs are the same as the RAS
What is the output from the parabrachial nuclei?
Strictly the ventral pathway with extensive, very diffuse innervation of the entire cortex
What is the major neurotransmitter used by the RAS and parabrachial nuclei?
EAA/glutamate; RAS also has interneurons that release GABA and a neuronal population that releases acetylcholine
What are the similarities and differences between the RAS and cholinergic (PPT/LDT) arousal systems?
Same: receive input from lots of modalities, so specific information is lost; outputs are via the dorsal and ventral pathways used by RAS
Different: major neurotransmitter is acetylcholine
What does damage to the PPT/LDT (pedunculopontine tegmental and laterodorsal nuclei) cause?
Causes cognitive deficits that are associated with a generalized slowing of cortical processes, but not necessarily a coma
What 2 pathways provide baseline excitation that is crucial to cortical activity?
RAS and cholinergic arousal systems
What are the inputs to the locus coereuleus?
Paragigantocellularis (in rostral medulla), which carries sensory information; periaqueductal grey (PAG); higher centers including the cortex
What is the key difference in information going to the RAS/cholinergic (EAA/Ach) arousal systems and the noradrenergic arousal system?
The information going to the locus coereuleus has undergone more neural processing than the sensory inputs received by the EAA/Ach systems
What are the outputs from the locus coereuleus?
Those related to consciousness that ascend to the cortex using the ventral/dorsal pathways and unrelated outputs to the spinal cord
What are the functions of the locus coereuleus (noradrenergic) arousal system?
Startle and alerting responses on EEG, sleep-wake cycles, and behavioral vigilance
What is the source of serotonin?
Raphe nuclei
What are the inputs to the raphe nuclei?
Sensory, from the spinal cord (fine proprioception and trigeminal nerve)
What are the outputs from the raphe nuclei?
For arousal, both dorsal and ventral paths are used; there are other outputs related to other functions of the raphe
What are the functions of the raphe nuclei?
Quiet awareness and other (non-RAS activities) such as mood and affect and modulation of pain
What is the ventral tegmental area?
Provides a dopaminergic input that is important to many fucntions, including cognitive functions, motor activity, and emotion
What are thalamo-cortical neurons?
Axons from the thalamus to the cortex
What do thalamo-cortical neurons release as their neurotransmitter?
EAA (excitatory amino acids)
Where do thalamo-cortical neurons synapse? What do those neurons release?
Synapse on intracortical neurons that release GABA on other cortical neurons
What leads to the waves that are recorder on an EEG?
Alternating waves of excitation, due to EAA being released through the dorsal pathway and thalamo-cortical neurons, and inhibition, due to GABA being released in the cortex by intracortical neurons
In a persistent vegetative state, the rostral regions of the pons, midbrain, and thalamus show ______ ______ that _______ that of the cortex.
Neuronal loss; exceeds
What happens in Alzheimer’s disease?
Cholinergic systems are particularly hard hit; mental processes slow dramatically and memory formation is profoundly impaired in the absence of the excitation
What does the heirarchy of consciousness suggest about the physiological basis of consciousness?
1) Different levels of consciousness/awareness are the result of different levels of cortical excitation
2) Moving from arousal/wakefulness to awareness takes additional excitation
3) Moving to full awareness takes another layer of excitation to acheive
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