Brain Arousal System Lecture (Dr. Karius) Flashcards
Consciousness is recognized for have two parts
1) Arousal (being awake)
2) Awareness (Conscious processing of Inputs)
**In a person with an intact Nervous System, we don’t often this about this separation
After an insult to the Brain, these two parts of Consciousness can dissociate
1) COMA: Neither awake or aware
2) PERSISTENT VEGETATIVE STATE:
- Physiologically identifiable sleep/wake cycles appear
- No evidence of Awareness
3) MINIMALLY CONSCIOUS STATE:
- Sleep/wake cycles
- Reproducible evidence of awareness, ability to respond to simple commands
- Limit or absent communication
Consciousness steps
1) Coma —>
2) Arousal/ Wakefulness —>
3) Awareness —>
4) Alertness
Consciousness
- It is a common misperception the the COMATOSE STATE results from DAMAGE to the Cerebral Cortices
a) Although this is true, the damage must be MASSIVE - Disruptions of Consciousness result from much smaller lesions in the BRAINSTEM, MIDBRAIN, or HYPOTHALAMUS
a) This finding led to the Hypothesis that Brainstem regions were CRITICAL for Arousing the Cortex
Consciousness explained
- Both Arousal and Awareness REQUIRE ACTIVATION of the Cortex but the Cortex has NO INTRINSIC Mechanism for Activation
- There are multiple SUB-CORTICAL structures that function to provide the Activation required for the Cortex to Function
Arousal Systems
1) EAA:
- Reticular Activating System (RAS)
- Parabrachial Nuclei
2) CHOLINERGIC: Pedunculopontine Tegmental and Lateraldorsal Nuclei (PPT/ LDT)
3) NORADRENERGIC: Locus Ceruleus
4) SEROTONERGIC: Raphe Nuclei
5) DOPAMINERGIC: Ventral Tegmental Area
EAA: Reticular Activating System (RAS)
- Occupies the “MID-VENTRAL PORTION” of Medulla and Midbrain
- It is a LOOSE COLLECTION of Neuron and Fiber Tracts
EAA: Inputs to the Reticular Activating System
1) *****ALL ASCENDING SENSORY Tracts send information to the RAS!!!!!
As do:
- Trigeminal
- Auditory
- Visual
2) ****There is sufficient Synaptic Convergence of INPUT to the Neurons of the RAS that MODEL SPECIFICITY is LOST!!!!!!!!!!!!!!!!!!!!!!!!!
(This means that all this information converges on the Same Neurons, so the Brain only knows something happened, not what happened)
3) So the Neurons of the RAS response equally well to MULTIPLE SENSORY MODALITIES
EAA: Outputs from the Reticular Activating System
TWO PATHWAYS:
1) Dorsal Pathway
- Via the Non-specific Nuclei of the Thalamus including the INTRALAMINAR NUCLEUS OF THE THALAMUS
- From there, diffuse pathway to all Higher Levels
2) Ventral Pathway
- Via BASAL FOREBRAN and HYPOTHALAMUS
- From there, diffuse pathway to all Higher Levels
EAA: Parabrachial Nuclei
- Located in the PONS, the Parabrachial Nuclei (Medial, Intermediate, and Lateral) are CRUCIAL for Arousal/ Activation
- The sensory inputs are similar to those seen in the RAS, ALL/MOST of the Sensory inputs to the Body!!!!!!!
EAA: Outputs from the Parabrachial Nuclei
ONE BIG DIFFERENCE:
- The outputs are likely EXCLUSIVELY via the VENTRAL PATHWAY!!!!!!
- With extensive, very diffuse Innervation of the entire Cortex
EAA: RAS and Parabrachial Nuclei
- The major Neurotransmitter utilized by BOTH the Parochial and the RAS Neurons is the EAA/ Glutamate***. That is true for both DORSAL and VENTRAL Pathways
A couple of things to be aware of:
1) Within the RAS, there is a substantial number of Interneurons (Neurons intrinsic to the RAS) that release GABA
2) The RAS also has a Neuronal population that releases ACETYLCHOLINE
EAA: Its role in Arousal
- Regardless of path (Dorsal or Ventral), the EAA System appears to provide a BASELINE EXCITATION that is Crucial to Cortical activity
Cholinergic: Pedunculopontine Tegmental and Laterodorsal Nuclei
- Like the RAS, the PPT/LDT Nuclei receive so much input that all modality-specific information is lost
- Outputs are via the Dorsal and Ventral Pathways used by the RAS….
- But the Major Neurotransmitter is ACETYLCHOLINE!!!!!!!
Cholinergic: Its role in Arousal
- Regardless of Path (Dorsal or Ventral), the Cholinergic System ALSO appears to provide a Baseline EXCITATION that is CRUCIAL to CORTICAL ACTIVITY
Cholinergic: Role in Arousal/ Awareness
- Activity in the Cholinergic inputs from the Pons is also associated with Arousal and Awareness
- Damage specifically 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
Consciousness Steps with EAA and ACh
1) COMA —— (EAA/ ACh)—->
2) Arousal/ Wakefulness
3) Awareness
4) Alertness
Noradrenergic: Lucus Coereuleus
OUTPUTS from the Locus Coereules:
- Bost ASCENDING and DESCENDING
- Ascending: Dorsal and Ventral with the RAS
***The Ascending Fibers from this group of Cells become the DORSAL NORADRENERGIC BUNDLE!!!!!!!!!!!!!!!!
Noradrenergic: Functions
Functions of the L.C. NA System:
1) STARTLE and ALERTING responses on the EGG!!!!!!!
2) Sleep-Wake
3) BEHAVIORAL VILIGANCE
Serotonergic: Raphe Nuclei
INPUTS: Multiple, Difficult to decide which ones relate to Arousal Systems
OUTPUTS: For Arousal, BOTHE THE DORSAL AND VENTRAL PATHS ARE USED (There other outputs that relate to other functions of the Raphe)
Serotonergic: Functions
- QUIET AWARENESS
- Other (Non-RAS activities) include:
a) Mood and Affect
b) Modulation of Pain
Consciousness Steps with EAA, ACh, NE, and 5HT
1) COMA —-(EAA/ACh*)—->
2) Arousal/ Wakefulness — (NE/5HT)—–>
3) Awareness
4) Alertness
Dopaminergic: Ventral Tegmental Area
- The VTA (Along with the SUBSTANTIA NIGRA) provides a DOPAMINERGIC Input that is Important to many functions, including:
1) Cognitive Functions
2) Motor Activity
3) Emotion
Consciousness Steps with EAA, ACh, NE, 5HT, and DA
1) COMA —-(EAA/ACh)—->
2) Arousal/ Wakefulness — (NE/5HT)—–>
3) Awareness —– (DA)——->
4) Alertness
Reminder: Dorsal Pathway
- Arousal Systems send axons to THALAMUS
- Synapse
- Axons from Thalamus to Cortex
Reminder: Ventral Pathway
- Arousal Systems send axons STRAIGHT to CORTEX
- Synapse DIRECTLY onto Cortical Neurons
Thalamic Arousal Systems = Dorsal Pathway
- DORSAL PATHWAY synapses in the Non-specific Nuclei of the Thalamus (Ex Intralaminar)
- Non specific is a Misnomer, it si in Contrast to the SENSORY-SPECIFIC Nuclei of the Thalamus and reflects and old belied that these Thalamic Nuclei did not have Specific Projections/ Functions
Thalamic Arousal Systems
- From the Thalamus, there is a DIFFUSE PROJECTION to the ENTIRE CORTEX, utilizing EAA as the Neurotransmitter
- These Neurons (With their EAA input) interact with a series of INTRACORTICAL Neurons that release GABA, to create Osciltations that are seen in the EEG!!!
Functions of all Arousal Systems
1) The RAS/ Parabrachial EAA System is CRUCIAL for Increasing general Excitability of Cortical Neurons
- **In a persistent Vegetative state, the Rostral regions of the Pons/Midbrain/ Thalamus show Neuronal LOSS that exceed that of the the CORTEX.
- *** Cortical Neurons are 10 to 30 Millivolts——> HYPERPOLARIZED relative to their Threshold!!!!!!
2) The Cholinergic System adds to that General Excitation
***In ALZHEIMER’S Disease, the Cholinergic Systems are partially HARD HIT! Mental processes slow Dramatically and Memory formation is Profoundly Impaired
3) The Noradrenergic and Serotonergic Systems move us from being awake to being more Generally Aware of Incoming Information
***The ALERTING Response in the EEG is an Early indicator the the Cortex is “Looking” for or “Expecting” Sensory Input
4) The Dopaminergic System adds to that Awareness, particularly focused awareness associated with NOVEL STIMULI, but its role is not as well defined
***In a limited number of people in a Persistent Vegetative State, treatment with LEVODOPA has produced DRAMATIC INCREASES in Cognitive Function!!!!!!!!!
Thalamic Arousal Systems during Sleep
- During SLEEP, the THALAMOCORTICAL Neurons are HYPERPOLARIZED and show OCCASION BURSTS (Spindle-like Discharges)
- This HYPERPOLARIZATION essentially CUTS the Cortex OFF from the Excitatory Influence during the DEEPEST Levels of SLEEP
Match the Specific Component of the Arousal System to the level of Consciousness/Awareness produced by its activity
1) COMA —-(EAA/ACh)—->
2) Arousal/ Wakefulness — (NE/5HT)—–>
3) Awareness —– (DA)——->
4) Alertness
A) EAA/ ACh: HYPERPOLARIZES/ Memory
B) NE/5HT: Startle/ Awareness
C) Dopamine: Agonists IMPROVE Cognitive Function
