Brain arousal systems Flashcards
Consciousness has 2 parts. What are they?
arousal and awareness
are there any sleep/wake cycles in the comatose state?
no
what is characteristic of the persistent vegetative state?
sleep/wake cycles but no evidence of awareness
what is characteristic of the minimally conscious/ awareness state?
there are sleep/wake cycles, the patient has reproducible evidence of awareness (ability to respond to simple commands) but limited or absent communication
Most often, disruptions of consciousness result from what?
small lesions in the brainstem, midbrain, or hypothalamus
patients in a persistent vegetative state have cortical neurons that are what?
30 mv below threshold than under normal conditions (they are incredibly hyperpolarized)
different levels of consciousness/ awareness are the result of what?
different levels of cortical excitation
What is critical for arousing the cortex?
brainstem regions
What does the cortex require for activation since it does not have an intrinsic mechanism for activation?
it requires input from the midbrain/hindbrain regions
What system must be present for a patient to move from a comatose state to at least a persistent vegetative state?
the EAA system
What contributes to the EAA system (reticular activating system (RAS)) but if the EAA system it cannot move the patient from the comatose state to the PVS on its own?
the cholinergic system
Where is the RAS located?
mid ventral portion of the medulla and midbrain
where is the cholinergic system located?
in the pons
Where is the noradrenergic system located?
the locus ceruleus
where is the serotonergic system located?
raphe nuclei
where is the dopaminergic system located?
the ventral tegmental area
What inputs does the RAS receive?
all the ascending sensory tracts and also the cranial nerves: trigeminal, auditory, and visual
What is the result of the sufficient synaptic convergence of input to the neurons of the RAS?
modal specificity is lost “ I just know something happened not what happened”
There are two pathways from the EAAs system to the cortex. What are they?
dorsal pathway and ventral pathway
What happens in the dorsal pathway?
a neuron in the RAS sends an axon to the thalamus and then from there it heads to the cortex
what happens in the ventral pathway?
an axon goes straight from the RAS to the cortex
there is a second group of EAA neurons arising. From where?
from the pons in the parabrachial nuclei
What are the output pathways from the parabrachial nuclei?
it is exclusively ventral–> so it bypasses the thalamus and goes straight to the cortex
What is the major neurotransmitter utilized by both the parabrachial and the RAS neurons?
EAA/Glutatmate
within the RAS there is a substantial number of interneurons (neurons intrinsic to the RAS) that release what?
GABA
There are 2 nuclei in the pons that are associated with the cholinergic excitation of the cortex. What are they?
the pedunculopontine tegmental and laterodorsal nuclei (PPT/ LDT)
What are the output options for the cholinergic system?
dorsal (goes to the thalamus) or ventral (bypasses the thalamus)
What is the major neurotransmitter used by the cholinergic system?
acetylcholine (ACh)
What is the main purpose of the cholinergic system?
provides a baseline excitation that is crucial to cortical activity
What would damage to the PPT or LDT nuclei result in?
it doesn’t necessarily cause coma, but it does produce severe cognitive deficits that are associated with a generalized slowing of cortical processes
Where is the locus ceruleus located?
in the pons
Once i have moved from the comatose state to the aroused/wakeful state, what is the first NT needed to get closer to being fully aware?
noradrenaline
What is different about the input into the locus ceruleus compared to the RAS?
the input to the LC has undergone more neuronal processing
What are the pathways used for the outputs from the LC related to consciousness?
they ascend to the cortex using both the dorsal and ventral pathways
the locus ceruleus system has a very specific function. What is it?
it is responsible for producing the startle and alerting responses on the EEG (waking you up)
Where are the serotenergic nuclei located?
raphe nuclei (in the pons)
What is the input into the raphe nuclei?
sensory input from the spinal cord (fine proprioception) and the trigeminal nerve input
What are the output pathways of the raphe nuclei in regards to arousal?
both the dorsal and ventral pathways are used
What is the function of serotonin in the arousal pathway?
quiet awareness “i’m aware of everything around me”/ general awareness
How do we get to the final step of being really alert?
we have to add yet another level of excitation in–> dopaminergic systems
the ventral tegmental area provides a dopaminergic input that is important to many functions including what?
cognitive functions, motor activity, and emotion
Axons from the thalamus to the cortex are known as what?
thalamocortical neurons
in the dorsal pathway, it does not matter what NT is released in the thalamus, the neuron that leaves the thalamus and synapses in the cortex will release what?
EAAs
in the ventral pathway, what is the NT that is released once it synapses directly onto the cortical neurons?
the one that is specific to that system
The thalamocortical neurons synapse on the cortical neurons and what else?
the intracortical neurons
what do the intracortical neurons release on other cortical neurons?
GABA
What is believed to lead to the waves recorded on the EEG?
the alternating waves of excitation (due to EAA being released) and inhibition (due to the GABA being released)
In Alzheimer’s disease, which systems are particularly hit hard?
the cholinergic systems
the noradrenergic and serotenergic systems move us from what to what?
from being awake to being more generally aware of incoming information
the alerting response on the EEG is an early indicator of what?
that the cortex is looking for sensory input
