Reticular Activating System

Reticular Formation

forms most of the core of the brainstem

reticular formation is made up of heterogeneous neurons that have different functions, but do not form distinct nuclei

exceptions:

raphe nuclei – located along midline of brainstem and release serotonin;

substantia nigra (DOPA) and locus coeruleus (NEpi) – also considered part of the reticular formation

Anatomy of the Reticular Formation

Neurons: reticular means ‘net’; this refers to highly branched nature of the axons and dendrites of the reticular formation

dendrites: mostly oriented in the transverse plane; act like antennae to pick up signals travelling up and down

axons: also highly branched; neurons send signals up and down

Chemoanatomy: many new neurotransmitters are still being discovered, but some generalizations can be made

cells in the medial group are cholinergic and/or cholinoceptive
many raphe neurons contains serotonin; also substance P, and TRH; thought to regulate selective attention

locus coeruleus neurons are all noradrenergic; has a part in the sleep/wake cycle

Afferent Connections: reticular formation neruons get signals from many sensory modalities at once

spinoreticular fibers: sensory input of all the modalities from the spinal cord

cranial nerve sensory nuclei project second order neurons to the reticular formation

cerebellar fibers: mostly from the fastigal nucleus; project to 2/3 of the reticular formation

corticoreticular fibers: descending fibers parallel to the corticospinal; mostly from the sensorimotor cortex

Efferent Connections:

descending reticulospinal fibers: run from reticular formation to spinal cord where they activate motor pathways

ascending fibers go to: thalamus, hypothalamus, preoptic area, medial septal nucleus, striatum, and other structures, some branches go as far forward as the forebrain

collateral branches of the reticular formation are found in all cranial nerve nuclei
many efferent axons branch to make both ascending and descending branches
ascending fibers generally are more caudal, descending are generally more rostral; this allows for the integration of ascending and descending signals

note on sensory systems: sensory info travels in two parallel paths (experimental evidence in physiology section below)

specific sensory sytems: the classical paths (i.e. DCML); go directly to the thalamus

thalamic efferents project to specific targets in the cortex
this system is concerned with recognition and processing of stimuli (i.e. recognizing sounds)

reticular sensory systems: parallel to the classical path, but the second order neurons go first to the reticular formation, where they synapse and efferent fibers are sent from the reticular formation to parts of the thalamus that are distinct from the specific system (mainly the intralaminar nuclei)

thalamic efferents project diffusely to the cortex

this system is evolutionarily older and is concerned with quick response to important and appropriate stimuli (i.e. bringing the cortex up to full speed when you hear a tiger bearing down on your ass)

control of the sensitivity of this system is important Þ oversensitivity can lead to panic disorder in which normally mild stimulus leads to an inappropriate response (i.e. intense fear evoked by a benign sound)

Reticular Activating System: a subset of the reticular formation concerned with consciousness

during sleep and drowsiness: EEG shows high amplitude, low frequency waves (synchronized waves)
during waking and alertness: EEG shows low amplitude, high frequency waves (desynchronized waves)

note: desynchronized is really a misnomer; synchrony is present, just not as obvious

the transition between the two is termed activation; controlled by the reticular activating system

theory: thalamus and cortex communicate in a loop and this accounts for the slow waves during sleep; during conciousness, other inputs modulate this communication leading to the desynchronized waves

Physiology of the Reticular Activating System

Moruzzi and Magoun (1949): stimulated reticular formation of anesthetized cats Þ EEG changes identical to arousal

reticular formation stimulation mimicked effects of sensory stimulation (which also lead to desynchronized EEG)

sensory stimulation no longer resulted in arousal if the reticular formation was destroyed

arousal could still be obtained from reticular formation stimulation even if the ascending sensory structures or the corresponding sensory cortex were destroyed

conclusion: arousal to sensory stimuli is mediated by an intervening system between the ascending paths and cortex Þ Reticular Activating System (see note on sensory pathways above)

The Midbrain Reticular Formation (MRF)

the primary source for EEG changes evoked by the reticular activating system

evidence: MRF fibers begin discharging at a high rate just before EEG becomes dissynchronous on arousal

midbrain RF neurons have ipsilateral ascending projections to the midline and intralaminar cell groups of the thalamus
afferent and efferent connections are somatotopically organized

most MRF neurons are mulitmodal and map to a point on the body
topography is maintained in the reticulothalamic projections

it is thought that the MRF acts as a gate, allowing only certain sensory stimuli to reach the cortex; selective attention arises from inhibition or facilitation of thalamocortical responses to a given stimulus