Reticular Formation

Dr. Sam David

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

Reticular Formation

5 groups of Nuclei in the MEDULLA:

Lateral reticular nucleus

Paramedian reticular nucleus

Ventral reticular nucleus

Magnocellular reticular nucleus

Parvicellular reticular nucleus

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.

Lateral Reticular NucleusVentral Reticular Nucleus

Paramedian Reticular Nucleus

Midbrain

Pons

Medulla

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

Lateral Reticular NucleusVentral Reticular Nucleus

Magnocellular Reticular Nucleus

Parvicellular Reticular Nucleus

Paramedian Reticular Nucleus

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

Lateral Reticular Nucleus

Paramedian Reticular Nucleus

Parvicellular Reticular NucleusMagnocellular Reticular

Nucleus

Ventral Reticular Nucleus

Caudal Pontine Reticular Nucleus

Rostral Pontine Reticular Nucleus

Midbrain

Mesencephalic RN: Consists of scattered cells in area bounded by tectum, Red nucleus and ascending lemniscus.

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

Functional OrganizationConnections with the Cerebellum

Spinoreticular tract

Collaterals from Spinal lemniscus

Red Nucleus

Lateral Reticular Nucleus

Functional OrganizationConnections with the Cerebellum

Spinoreticular tract

Collaterals from Spinal lemniscus

Red Nucleus

Lateral Reticular Nucleus

CerebellumICP

Paramedian reticular nucleus

Cerebellum

Functional OrganizationConnections with the Cerebellum

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

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.

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

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.

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)

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

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

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.)

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

(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

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