Reticular Formation
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Reticular Formation
Dr. Sam David
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Reticular Formation
Consists of neurons within the central core of the brainstem
Multi-neuronal, polysynaptic pathway
Receives input from almost all sensory system (except the dorsal column pathway)
Has efferent connections, either direct or indirect, with all levels of the CNS
Hence has multiple functions and affects Motor, Sensory, Autonomic functions and responsiveness of the cortex
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Reticular Formation
5 groups of Nuclei in the MEDULLA:
Lateral reticular nucleus
Paramedian reticular nucleus
Ventral reticular nucleus
Magnocellular reticular nucleus
Parvicellular reticular nucleus
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Caudal Medulla Oblongata
Ventral RN: caudal 1/2 of medulla. Has small and large neurons.
Lat. RN: discrete, well defined. Located near lateral surface from caudal medulla to mid olivary nucleus level.
Paramedian RN: adjacent to midline. Entire length of medulla.
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Lateral Reticular NucleusVentral Reticular Nucleus
Paramedian Reticular Nucleus
Midbrain
Pons
Medulla
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Rostral Medulla Oblongata
Parvicellular RN: contains small neurons. Located immediately lateral to magnocellular nucleus. From mid-olivary level to rostral limit of medulla.
Magnocellular RN: contains very large neurons
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Lateral Reticular NucleusVentral Reticular Nucleus
Magnocellular Reticular Nucleus
Parvicellular Reticular Nucleus
Paramedian Reticular Nucleus
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Pons
Parvicellular RN: Located in lateral tegmentum. Extension of Parvicellular nucleus in medulla
Caudal Pontine RN: extension of magnocellular nucleus
Rostral Pontine RN: lacks large neurons
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Lateral Reticular Nucleus
Paramedian Reticular Nucleus
Parvicellular Reticular NucleusMagnocellular Reticular
Nucleus
Ventral Reticular Nucleus
Caudal Pontine Reticular Nucleus
Rostral Pontine Reticular Nucleus
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Midbrain
Mesencephalic RN: Consists of scattered cells in area bounded by tectum, Red nucleus and ascending lemniscus.
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Lateral Reticular Nucleus
Paramedian Reticular Nucleus
Ventral Reticular Nucleus
Rostral Pontine Reticular Nucleus
Mesencephalic Reticular Nucleus
Magnocellular Reticular Nucleus
Caudal Pontine Reticular Nucleus
Parvicellular Reticular Nucleus
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Functional OrganizationConnections with the Cerebellum
Spinoreticular tract
Collaterals from Spinal lemniscus
Red Nucleus
Lateral Reticular Nucleus
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Functional OrganizationConnections with the Cerebellum
Spinoreticular tract
Collaterals from Spinal lemniscus
Red Nucleus
Lateral Reticular Nucleus
CerebellumICP
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Paramedian reticular nucleus
Cerebellum
Functional OrganizationConnections with the Cerebellum
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Connections with the Spinal Cord
Motor areas of Cerebral Cortex
Red Nucleus
Substantia nigra
Medial areas of RF
Medulla & Pons
Cerebellum
SPINAL CORDTerminates directly or indirectly on and motor neurons
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Visceral functional connections -1
Regulates visceral functions through connections with nuclei of Autonomic outflow
Regulates respiration through connections with motor neurons in Phrenic nucleus and thoracic spinal cord.
Reticular formation neurons involved in Respiratory & Cardiovascular control are intermingled.
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Maximal Inspiratory response evoked by Magnocellular Ret. Nu. in Medulla
Maximal Expiratory response evoked by Parvicellular Ret. Nu. in Medulla
Normal respiratory rhythm controlled by Pontine RF (Pneumotactic centre).
Visceral functional connections -2
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Visceral functional connections -3
Ventral & Magnocellular reticular nuclei have depressor effects on Heart rate and Blood pressure
Lateral reticular nucleus in Medulla has opposite effects on heart rate and blood pressure.
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Ascending Reticular Activating System
Vestibular & Cochlear Nuclei
Nucleus of Spinal Tract of V
Solitary Nucleus
Spinal Lemniscus
Superior Colliculus (retinal input)
Spinoreticular Tract (magnocellular RF, Rostral pontine RF)
Olfactory input (mesencephalic RF)
Cortical input
Intralaminar nuclei send fibers to other thalamic nuclei that then project to widespread areas of the cerebral cortex including non-specific association areas and areas involved in emotions.
Medial reticular areas send collaterals that synapse with other reticular neurons. Via this repetition of relays (polysynaptic), axons reach the diencephalon ending in Hypothalamus and Thalamus (Intralaminar nuclei, and Nucleus of the midline)
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Functions of the Ascending Reticular Activating System
Relatively non-specific
Sensory modalities are merged in a polysynaptic pathway
Only provides a vague awareness of any particular sensory modality
Results in cortical stimulation with profound effects on: levels of Consciousness and Alerting reactions to sensory stimuli
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When the cortex is stimulated by the ARAS during sleep: EEG activity of the cortex changes from sleep pattern to waking state
When the cortex is stimulated by the ARAS while awake: Sharpens attentiveness and creates optimal conditions for the perception of other sensory data conveyed via more direct pathways.
Damage to RF results in COMA
Functions of the Ascending Reticular Activating System
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Simultaneously recorded surface EEG, EEG 1.2 mm into the cortex in a lightly anesthetized cat. Stimulation of the midbrain reticular formation (MRF) at 300 Hz at the bar produced an alerting response. (Reproduced, with permission, from Steriade M, Amzica F, Contreras D: Synchronization of fast (30–40 Hz) spontaneous cortical rhythms during brain activation. J Neurosci 1996;16:392.)
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In situ hybridization images show matched coronal brain sections of the lateral hypothalamus hybridized with a 33P-labeled antisense riboprobe for orexin (a neurpeptide).
Chemelli RM, et al., (1999) Narcolepsy in orexin knockout mice: molecular genetics of sleep regulation. Cell 98:437-451
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(E) EEG/EMG recording during a typical narcoleptic episode. The EEG shows that the start of this episode corresponds to two high-amplitude spindling epochs, marked with arrows, associated with phasic EMG activity as muscle tone declines at the onset of attack. The star marks the onset of observable immobility.
Chemelli RM, et al., (1999) Narcolepsy in orexin knockout mice: molecular genetics of sleep regulation. Cell 98:437-451
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General anesthetics are thought to suppress transmission through the RF
Cutaneous stimuli and olfactory stimuli are especially important in maintaining Consciousness
While visual and auditory stimuli are important in levels of alertness and attention
Functions of the Ascending Reticular Activating System