Lecture 16: Brain Arousal Systems Flashcards
What’s the hierachy (4 components) of Consciousness?
Coma —> Arousal/Wakefulness —> Awarness —> Alertness
Disruptions in consciousness result from what type of lesions; what is the common misconception?
- Smaller, subcortical lesions: brainstem, midbrain, and hypothalamus
- Common misconception that comatose state results from damage to cerebral cortices, but cortex has NO intrinsic mech. for arousal. Requires activation!
What are the characteristics of a coma vs. persistent vegetable state?
Coma: neither awake or aware
Persistent Veg: physiologically identifiable sleep/wake cycles, no evidence of awarness
What does “modal specifity is lost” refer to in regards to the RAS; how does the RAS respond to multiple sensory modalities?
- There is sufficient synaptic convergence of input to the neurons of the RAS
- So much sensory input going in, the specificity of this information is lost.
- RAS responds equally well to multiple sensory modalities
The outputs from the RAS are via 2 pathways, what are they?
- Dorsal - via the non-specifc nuclei of the thalamus, including intralaminar nucleus of thalamus, then diffuses to cortex
- Ventral - straight to cortex or through forebrain and hypothalamus then cortex
Other than the RAS what is the other subgroup of EAA neurons that are crucial for arousal/cortical activation?
Parabrachial nuclei
What is one big difference between the EAAs of RAS and Parabrachial Nuclei?
The outputs of the parabrachial nuclei are exclusively via the Ventral pathway
What is the major EAA NT utilized by both the parabrachial and the RAS neurons?
- Glutamate
- Is true for both dorsal and ventral pathways
What is the role of the EAA system in arousal?
- Provides a baseline excitation that is crucial to cortical activity
What is the Cholinergic contribution to the arousal system?
PPT/LDT nuclei of the Pons
The outputs of the cholinergic pathway: PPT/LDT are via which pathway and what is the major NT?
- Via the dorsal and ventral pathways
- Major NT is Acetylcholine!
Damage specifically to the PPN/DLT has what affect on brain arousal systems?
- Doesn’t necessarily cause coma
- Does produce severe cognitive deficits, associated w/ generalized slowing of cortical processes
What are the 2 components of brain arousal system necessary to get us from coma state to arousal/wakefulness; the NT used by each?
- EAA: RAS and Parabrachial nuclei - Glutamate
- Cholinergic: PPT/LDT - Acetylcholine
The source of the nonadrenergic NT’s for the arousal system is what?
Locus Coeruleus
The ascending output from the Locus Coereuleus travel via?
Dorsal and ventral with the RAS
The ascending fibers from the locus coereuleus (dorsal and ventral) become what bundle?
Dorsal noradrenergic bundle
What is the function of the Locus Coeruleus, NA (norepinephrine) system in arousal?
- Startle and alerting: responses on the EEG
- Sleep-wake
- Behavioral vigilance
The startle and alerting responses on the EEG are an indicator of what system functioning?
Noradrenergic: Locus Coeruleus
The serotonergic: Raphe Nuclei, use what path(s) for output in the arousal system?
- Both dorsal and ventral
- Ventral is the predominate path (bypasses thalamus)
What is the function of Serotonin from the Raphe nuclie in the arousal system?
- QUIET AWARNESS
- Other (non-RAS activites) include:
- Mood and Affect
- Modulation of pain
What NT are necessary to get us from the arousal/wakefulness state to the awarness state?
- NE from noradrenergic: locus coeruleus
- Serotonin from serotonergic: raphe nuclei
Also found embedded in the RAS is the VTA, which provides what important NT and what are its functions?
- Dopamine
- Cognitive functions
- Motor activity
- Emotion (pleausre)
What part of the arousal system is necessary to get us from awarness to alertness?
Dopaminergic: VTA
What is the dorsal pathway of the arousal system, where are the synapses?
- Arousal systems send axons to synapse on non-specific nuclei of thalamus (i.e., intralaminar)
- Axons from thalamus go to cortex
What is the ventral pathway of the arousal system, where are the synapses?
- Arousal systems send axons straight to cortex
- Synapse directly on cortical neurons
From the intralaminar nucleus of the thalamus there is a diffuse projection to the entire cortex, what is the NT of choice?
Utilizing EAA as the NT
The neurons from the thalamus interact with a series of intracortical neurons that release what NT and why is this significant?
- Neurons that release GABA (inhibitory)
- This cycle of EAA (excitation) and GABA (inhibition) produces the oscillations seen in the EEG
In a persistent vegetative state the rostral regions of the pons/midbrain/thalamus show neuronal loss exceeding that of the cortex, what is the threshold of excitability like?
Cortical neurons are HYPERPOLARIZED
What are the thalamocortical neurons like during sleep and why?
- Are hyperpolarized and show occasional bursts (spindle-like)
- Hyperpolarization cuts the cortex off from the excitatory influence during the deepest levels of sleep
