Brain Arousal Systems Flashcards
Consciousness
Arousal + Awareness
Coma
- neither awake nor aware
- no sleep/wake cycles
- no awareness/verbal responses
Persistent Vegetative State
- physiologically identifiable sleep/wake cycles
- no evidence of awareness
- some evidence for inability to send motor response vs lack of awareness
- eye/head motions
- no awareness/verbal responses
- rostral regions of pons, midbrain, and thalamus show neuronal loss exceeding that of cortex; these areas are above RAS so cannot get further than vegetative state
Minimally Conscious State
- sleep/wake cycles
- evidence of awareness ie can respond to simple commands but not consistent
- limited/absent communication
- verbal responses (inconsistent)
- eye/head motions (inconsistent)
- inconsistencies due to spinal/cortical reflexes
Hierarchy of Consciousness
Coma > Arousal/Wakefulness > Awareness > Alertness
Diff levels of consciousness result from different levels of cortical excitation
What type of injuries disrupt consciousness?
Comatose: damage to cerebral cortices that is MASSIVE and BILATERAL
Disruptions in consciousness: small lesions in brainstem, midbrain, hypothalamus
What is the potential for cortical neurons in unconscious patients?
cortical neurons in persistent vegetative state are very hyperpolarized and can be up to 30 mV below threshold?
Cortex Activation
needed for arousal and awareness
requires brainstem regions/multiple subcortical structures bc cortex has no intrinsic mechanism for activation
EAA Arousal System
- BASIC
- must be functional to move from comatose to vegetative
- reticular activating system
- parabrachial nuclei in pons
Cholinergic Arousal System
- must also have EAA activation
- Location = pedunculopontine tegmental and laterodorsal nuclei (treat as one nuclei)
- NT = Ach
- output = via dorsal and ventral pathways used by RAS
Arousal Systems
EAA Cholinergic Noradrenergic Serotonergic Dopaminergic movement into more and more consciousness with each system
Noradrenergic Arousal System
- Location = Locus Coereuleus
- input = LC gets input from Paragigantocellularis n in rostal medulla, Periaqueductal grey, and higher centers -info = sensory info that has been more processed than inputs from EAA/Ach systems; much more specific info adds new level of awareness
- NT = NE
- output: from LC ascend to cortex using ventral and dorsal pathways; also separate outputs to spinal cord
Serotonergic Arousal System
- Raphe nuclei = source
- input = very specific sensory info from spinal cord; proprioception, trigeminal N
- output = dorsal and ventral path; other outputs related to other functions of raphe
- NT = serotonin
Dopaminergic arousal system
- location = ventral tegmental area provides dopamine input
- function = cognitive functions, motor activity, emotion, mood/pleasure
Reticular Activating System
- EAA
- diffuse system; loose collection of neurons/tracts
- mid-ventral portion of medulla and midbrain
Inputs to RAS
- receives all ascending sensory tracts
- trigeminal, auditory, visual, tactile
- modal specificity is lost due to synaptic convergence of many inputs
- brain knows something has happened, but not what happened
- thus neurons of RAS respond equally well to multiple sensory modalities
Outputs from RAS: Dorsal Path
-neuron in RAS sends axon to thalamus synapse on second axon, sends info to cortex
NT = EAA/glutamate
via nonspecific nuclei of thalamus + intralaminar nucleus of thalamus
then diffuse to all higher levels
Outputs from RAS: Ventral Path
- bypasses thalamus via basal forebrain and hypothalamus
- straight to cortex
- NT = EAA/glutamate
- diffuse to higher levels
Parabrachial Nuclei
- area of the pons crucial for arousal/activation
- most sensory inputs to the body like RAS
- inputs EXCLUSIVELY via ventral pathway aka bypasses thalamus to cortex but may synapse on hypothalamus
- NT = EAA/glutamate
RAS Neurotransmitters
- major is EAA/glutamate
- some interneurons of RAS release GABA
- some release acetylcholine
Ach Role in Arousal
- cholinergic system provides a baseline excitation important for cortical activity
- damage doesn’t cause coma, but causes slowing of activities
Locus Coereuleus Noradrengergic System Functions
startle and alert responses on EEG
sleep/wake
behavioral vigilance
Serotonin Functions
quiet awareness
non RAS activities = mood and affect, pain modulation
Dorsal Pathway
arousal systems send axons to thalamus
synapse on non-specific nuclei of thalamus
cortical neurons from thalamus to cortex send diffuse signals; can be from EAA, NE, 5HT, DA, Ach
Ventral Pathway
arousal systems send axons straight to cortex
synapse directly on cortical neurons
EAA, parabrachial nuclei outputs, Ach, NE, 5HT, DA
Consciousness Overview and Clinical Indications
- RAS/Parabrachial EAA increases general excitability in cortex; BASIC and has to be present
- Cholinergic System adds to general excitation (Alzheimer’s patients have reduced mental capacity due to cholinergic deficits but are not in vegetative states bc of RAS)
- Noradrenergic and Serotonergic Systems move us from being awake to more generally aware of incoming info (alerting response in EEG indicates cortex is looking for input)
- Dopaminergic System adds focused level of awareness to novel stimuli (levodopa can help cognitive function in some pt’s in vegetative state)
Sleep and Thalamocortical Neurons
hyperpolarization of neurons with occ short bursts of APs
hyperpolarization cuts cortex off from excitatory influence during sleep
EEG and Thalamic Arousal systems
Thalamocortical neurons release EAA
synapse onto intracortical neurons that release GABA
alternating waves of excitation and inhibition cause EEG waves
