Mechanisms Of Arousal Flashcards
What are the two parts of consciousness?
Arousal (being awake)
Awareness (conscious processing of inputs, etc)
What is a coma?
Neither being awake or aware
No sleep/wake cycles
What is a persistent vegetative state?
Physiologically identifiable sleep/wake cycles appear
No evidence of awareness
What is 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?
Coma -> arousal/wakefulness -> awareness -> alertness
Disruption of consciousness result from what?
Smaller lesions in the brainstem, midbrain or hypothalamus
A comatose state can result from damage to what?
The cerebral cortices
But the damage must be massive and bilateral to disrupt consciousness
What is the membrane potential of neurons for those in a persistent vegetative state?
Cortical neurons are up to 30 mV below threshold thats under normal conditions
Different levels of consciousness/awareness are the result of what?
Different levels of cortical excitation
Both arousal and awareness require activation of the what?
Cortex
But the Cortex has not intrinsic mechanism for activation
There are multiple sub-cortical structures that function to provide the activation required for the cortex to function
What findings can be present during a coma?
Eye/head movements
No sleep/wake cycle, awareness or verbal response
What findings are present during a persistent vegetative state?
Eye/head movements and sleep/wake cycle
No awareness of verbal response
What findings are present during minimal consciousness?
Eye/head movements, sleep/wake cycle
Inconsistent or intermittent awareness or verbal responses
What findings are present in someone who is aware/alert?
Eye/head movements, sleep/wake cycle, awareness and verbal responses
Which NT systems are involved with the arousal system?
EAA (has to be present in order to move from coma to PVS), cholinergic, noradrenergic, serotonergic and dopaminergic
Last 3 move you more and more conscious
What is the reticular activating system?
Occupies mid ventral portion of medulla and midbrain
An area of loose collection of neurons and fiber tracts
Associated with the EAA
What inputs does the RAS receive?
All ascending sensory tracts send info to the RAS
Trigeminal, auditory and visual inputs also come into RAS
There is so much input to the neurons that modal specificity is lost
What does it mean when the neurons of RAS modal specificity is lost?
All the info converges on the same neurons so the brain only knows something happened not what happened
As a result the neurons of the RAS respond equally well to multiple sensory modalities
What are the two pathways of output from the RAS?
Dorsal pathway and ventral pathway
Describe the dorsal pathway from the RAS
Arousal systems send axons via the non specific nuclei of the thalamus including the intralaminar nucleus of the thalamus
From the thalamus axons diffuse pathway to all higher levels
Describe the ventral pathway from RAS
Bypasses thalamus via basal forebrain and hypothalamus and sends axons straight to cortex
Synapse direct onto cortical neurons
From there diffuse pathway to all higher levels
What is the parabrachial nuclei (middle, intermediate and lateral)?
Associated with EAA
Located in the pons
Crucial for arousal/activation
Sensory inputs are similar to those seen in RAS (all/most of the sensory inputs to the body)
Outputs are exclusively via the ventral pathway (with extensive very diffuse innervation of the entire cortex)
The major NT utilized by both the parabrachial and RAS neurons is what?
EAA/glutamate regardless of the pathway to the brain
What other types of neurons are located in RAS?
Substantial number of interneurons (neurons intrinsic to RAS) that release GABA
Also has neuronal population that release ACh
Regardless of the dorsal or ventral pathway, the EAA system appears to provide what?
A baseline excitation that is crucial to cortical activity
What are the pedunculopontine tegmental and laterodorsal nuclei (PPT/LDT)?
Associated with the cholinergic NT system
Receive so much input that all modality specific info is lost
Output are via the dorsal and ventral pathways by the RAS but the major NT is ACh
Regardless of path (dorsal or ventral) the cholinergic system also provides what?
A baseline excitation that is crucial to cortical activity
Does require the EAA (cant act on its own)
Activity in the cholinergic inputs from the pons is also associated with what?
Arousal and awareness
Damage to the PPN/DLT doesn’t necessarily cause coma, but does produce severe cognitive deficits that are associated with a generalized slowing of cortical processes
Moving from arousal/wakefulness to awareness takes what?
An additional excitation
Need EAA and ACh
What is the locus ceruleus?
Associated with the noradrenergic system
What are the inputs to the locus ceruleus?
Paragigantocellularis n (in rostral medulla)
PAG - periaqueductal grey
Higher centers including the cortex
All sensory info
This information has undergone more neural processing than the sensory inputs received by the EAA/ACh systems
What are the outputs from the locus ceruleus?
Those related to consciousness ascend to the cortex using both the dorsal and ventral pathways
There are unrelated outputs to the SC
What are the functions of the locus ceruleus/NE system?
Startle and alerting response identifiable on an EEG* (much more specific, something happened and you want to pay attention to it)
Sleep-wake
Behavioral vigilance
What is the raphe nucleus?
Associated with serotonergic system (release serotonin)
Receives sensory inputs from the SC (fine propioception) and trigeminal N
What are the outputs from the raphe nuclei?
For arousal, both the dorsal and ventral pathways are used
There are other outputs that relate to other functions of the raphe
What are the functions of the serotonin?
Quiet awareness (aware of surroundings) Other (non-RAS activities) including mood and affect, modulation of pain
What is required for the movement of arousal/wakefulness to awareness?
NE and 5HT
What is the ventral tegmental area?
Associated with the dopaminergic system
Provides a dopaminergic input that is important to many functions including cognitive functions, motor activity and emotion
Dopamine is needed to move from awareness to what?
Alertness
Fully functional
What are thalamocortical neurons?
Axons from the thalamus to the cortex
Describe the thalamic arousal system
Thalamocortical neurons elease EAA as their NT
Also synapse on intracortical neurons that release GABA on other cortical neurons
The alternating waves of excitation (due to EAA released) and inhibition (due to GABA released) are believed to lead to the waves recorded on an EEG
Describe the function of the arousal systems in creating consciousness
- RAS/parabrachial EAA system is crucial for increasing general excitability of cortical neurons
- Cholinergic system adds to that general excitation
- The NE and serotonergic systems move us from being awake to being more generally aware of incoming info
- The dopaminergic system seems to add focused awareness associated with novel stimuli
What is the state of the thalamocortical neurons during sleep?
During sleep they are hyperpolarized and show occasional short bursts of AP
This hyperpolarization essentially cuts the cortex off fro the excitatory influence during the deepest levels of sleep
In a persistent vegetative state the rostral regions of the pons, edulla and thalamus show what?
Neuronal loss that exceeds that of the cortex
What happens in Alzheimer’s disease?
The cholinergic systems are particularly hard hit
Mental processes slow dramatically and memory formation is profoundly impaired in the absence of excitation
The alerting response in the EEG is an early indicator that the cortex is what?
Looking for sensory input
In a limited number of people in a persistent vegetative state treatment with levodopa has produced what?
Increases in cognitive function
