3 - Reticular System Flashcards by John Castillo

Reticular System:

Definition

Target

Diffuse collection of >110 nuclei; extends from medulla to the forebrain

Projectiosn to thalamus and cortex

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What does the Reticular System intregrate?

Integrates information from all senses (EXCEPT OLFACTION) to modulate brain functions

Ex: Pain, Motivation, Respiration, Sleep, REM sleep

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Ascending Reticular Activating System (ARAS)

Consciousness, Arousal, Attenion, Wakefullness

Allows purposeful interaction with your environment

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Reticular Formation Divisions: Rostral vs Caudal

Rostral: Supplies cerebral cortex with excitatory inputrs for consciousness/wakefullness/arousal/attening (ARAS SYSTEM)

Caudal: Modulates motor function via medial and lateral reticulospinal tracts (descending tracts)

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Major Functions of the Ascending Reticular Activating System (ARAS)

Prime

Filter

Shunt

Primes - the corect and othe rbrain regions allowing incoming sensory information to be proessed (focuses attention)

Filters - out information that is unimportant

Shunts - information that is life-threatening or fearful directly to amygdala

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Reticular Formation Divisions: Telencephalon

Nucleus Basalis of Meynert

Major source of cholinergic (ACh) neurons for the cortex and amygdala (w/out thalamic relay)

Clinical: Substantial neurodegeneration is always associated with cognitive dysfunction, decline, dementia, AD

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Reticular Formation Divisions: Diencephalon

Reticular Nucleus of the Thalamus

Wide variety of thalamic and hypothalamic nuclei strongly associated with reticular formation

Retocular Nucleus of the Thalamus: Only thalamic nuclei to have no projections outside of the thalamus

Integrates incoming information from cortex, thalamic relay and other ARAS inputs; may control when information is sent to various cortical areas

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Telencephalon - Nucleus Basalis of Meynert

Major source of Cholinergic (ACh) Neurons for cortex and amygdala (without thalamic relay)

Degeneration can be associated with cognitive dysfunction, dementia, AD

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Diencephalon - Reticular Nucleus of the Thalamus

Only thalamic nuclei to have no projections outside the thalamus

Integrates information incoming from cortex, relay, and other ARAS inputs

May control when information is sent to various areas (coordination role)

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Midbrain Pathways

Two (PAG, Dorsal Raphe Nucleus) main pathways that carry nociceptive signal to somatosensory cortex with collateral going to reticular formation

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Midbrain - Periaqueductal Gray (PAG)

Modulation of descending pain transmission

Determines whether pain will be transmitted; contains high desnity of Opiod receptors

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MIdbrain - Dorsal Raphe Nucleus

Primary site of serotonergic neurons (5HT) in reticular formation

Modulates pain transmission; no thalamic relay

Modulates consciousness, attention, and mood (ARAS)

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Midbrain - Ventral Tegental Area (VTA)

Mesolimbic (DA) and mesocortical pathways

Critical in reward circuitry, memory, attention, and motivation

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Midbrain - Substantia NIgra Pars Compacta (SNc)

Nigrostriatal (DA) pathways; modulation of basal ganglia

Parkinson’s Disease

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Pons - Nucleus Locus Coeruleus (blue nucleus)

Majority of Norepinephrine (NE) input in CNS

Ascending / Descending projections to the limbic structures, dorsal horn, and cortex (without relaying in the thalamus)

Modulates ARAS - arousal, selective attention, stress response, pain modulation, and mood

Clinical: Degenerates in both AD / Parkinsons

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Pons - Pedunculopontine Nucleus and Lateraldorsal Tegmental Nucleus

Largest sites of Acetylcholine (ACh) production in brain

Involved in ARAS / REM sleep; part of indirect arousal system

**Must be inhibited for sleep to occur**

Medulla - Nucleus Raphe Magnus (5HT) and Rostral Ventral Medulla (RVM-Glutamate)

Regulate whether information will be transmitted higher in the brain; modulates transmission of pain to the dorsal horn

Medulla - Nuclei of the Medullary Reticular Formation

Critical in vital functions; cardiovascular and respiratory center regulation

- Death can occur from tonsillar herniation compression of these structions

Opiod abuse results in depression of respiratory centers
SIDS possibly factor here

How does the ARAS Prime?

Focuses attention, increase general alertness

How does the ARAS filter?

Bottom-Up control; removes conscious awareness of information perceivd to be unimportant

Modulates how information is represented to the cortex

How does the ARAS shunt?

Sends information that is life-threatening or fearful directly to the amygdala

Where are major ARAS nuclei clustered?

Ponto-mesencephalic diencephalic regions of reticular formation

ARAS facilitated learning

ARAS ACTIVATES the cortex (entorhinal/hippocampus)

Cortex tells ARAS what is important allowing filtering of extraneous information

Memory formed

Clinical: Amnestic Agent

ARAS Depression produces state of unconsciousness; still allows activity in the thalamic and cortical areas, but there will be no arousal = NO formation of memory

Ex. Midazolam (Versad)`

Clinical: Bilateral lesion of Ponto-mesencephalic-diencephalic region of Reticular Formation?

Critical for **maintaining consciousness** ## Footnote **Bilateral Lesion = Coma**

What are the two pathways of consciousness?

**Direct - Aminergic Contributions** - Direct Cortical Projections (no Thalamic relay) - Activated only during **waking state/regulating overall activity of the brain** **- - -** **Indirect - Cholinergic Contributions** - Indirect projection to cortex **VIA thalamic relay** **-** Activated during waking state **and REM sleep**

What are the two parts of consciousness?

Content and Level Content: Sensory, Motor, Memory, Emotion Level: Attention, Alertness, Awareness - - - One side can not maintain consciousness alone!

Clinical: **Bilateral** lesion of upper brainstem affecting ARAS nuclei and pathways, or **bilateral** thalamic lesion?

Loss of consciousness

Clinical: Bilateral compromis or destruction of the hemispheres? Causes?

Toxic insult, metabolic, anoxia, drug or alcohol overdose, barbiturates, encephalopathy/increased brain pressure

Clinical: Focal or Unilateral lesion of the thalamus (as long as the RAS projectings to at least one hemisphere and thalamus are intact)

Generally--do **not** generate coma directly, but can **indirectly via tonsillar herniation**

Clinical: Locked in Syndrome

Bilateral Ventral **Pontine Lesion** Prevents **Corticospinal (trunk/limb) / Corticobulbar (non-oculomotor face, head, neck)** **motor output** Symptoms: Quadriplegia, Aphonia (can't speak) Preserved: **ARAS is unaffected = fully conscious,** sensory function, vertical eye movement, and blinking

Clinical: Akinetic Mutism, Abulia, Catatonia

Distinct constellationof psychomotor disturbances; **bilateral medial frontal lobe lesion with _intact ARAS_** Symptoms: Apathy, Deficits in response initiation, No spontaneous movement, Purposeful response some aversive stimuli, intact reflexes, normal sleep wake cycles **Staring, Mutism, Refusal to eat, abnormal posture, grimace**

Clinical: Minimally Conscious State

Impaired function of cortex and ARAS due to widespread cerebral damage Cause: Diffuse cerebral hypoxia or trauma Symptoms: Periods of responsiveness or wakefullness with minimal and variable awareness, visual tracking intact, sleep/wake cycle intact, **highly variable EEG**

Clinical: Vegetative State

\> Month = Persistent vegetative state Severe impaired cortical function; some ARAS may be preserved Symptoms: Periods of wakefulness, **no period of awareness; NO visual tracking, may react to pain** - Small number of patients can activate parts of brain by thinking of an activity; return of visual tracking may indicate + sign patient is recovering

Clinical: Coma

Severe impairment of cortical function and ARAS; prolonged loss of consciousness Symptoms: Unresponsive, no reaction to sensory input, no awareness, no sleep wake cycle, som primitive reflex activity is present

Clinical: Brain Death

All cortical, subcortical, brainstem function is lost Only spinal cord function may be preserved

What occurs if the reticular system does not prime an area to receive information?

The sensory information will **not be processed**