Reticular Activating System - University of Kansas Medical
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Reticular Activating System
RAS
Reticular Formation Functions
Modulates sensation of pain.
Modulates certain postural reflexes and
muscle tone.
Helps control breathing and heartbeat.
Regulates level of brain arousal and
consciousness.
Raphe Nuclei
Lie along midline of medulla and midbrain.
Important in maintaining wakefulness.
Damage may result in permanent coma.
Reticular Formation
All sensory input that enters brain via the
medulla is also sent to neurons of the
reticular formation.
These neurons may monitor sensory input
for importance.
May alert higher brain centers when
critical input is detected.
Reticular Formation
Reticular formation receives input from
cerebral cortex.
May use this input as a basis for its
decisions.
Characteristics of RAS
Made up of diffuse aggregations of cells.
Magnocellular zone:
Large cells restricted to medial two-thirds of
reticular formation.
Parvocellular zone:
Small cells found in lateral regions.
Magnocellular Zone
Characterized by a specific type of neuron
with long radiating dendrites:
Spread out in a plane perpendicular to long
axis of brain stem.
Seem to be suited to pick up information
from a variety of sources.
Magnocellular Zone Afferents
Spinoreticular fibers.
Sensory cranial nerves.
Cerebellum.
Hypothalamus.
Basal nuclei.
Cerebral cortex:
Esp. premotor cortex.
Magnocellular Zone
Long ascending/descending pathways:
Terminate in:
Hypothalamus.
Midline and intralaminar thalamic nuclei.
Segments of spinal cord.
Long ascending fibers from:
Lower pons and medulla.
Monoaminergic Pathways
Serotonin.
Norepinephrine (noradrenalin).
Dopamine
Serotonergic Pathways
Originate in raphe nuclei.
Extensive continuous collection of cell
groups throughout the brainstem.
One part terminates in substantia
gelatinosa:
Implicated in pain mechanisms.
Serotonergic Pathways
Another part projects to limbic structures:
May be associated with changes in mood
and behavior.
Serotonin pathways may be part of a
sleep-inducing mechanism:
Inhibition of serotonin synthesis or
destruction of raphe nuclei leads to
insomnia:
Can be cured by administration of serotonin.
Noradrenergic Pathways
One system arises from lateral reticular
formation and innervates the
hypothalamus and other limbic structures.
Best-known group of cells is in the locus
ceruleus (heavily pigmented cells):
Projects to every major region of the brain
and spinal cord.
Noradrenergic Pathways
Most adrenergic fibers terminate on small
blood vessels and capillaries in the brain:
May help regulate blood flow in brain.
Dopaminergic Pathways
Majority of dopaminergic cells are located
in the substantia-nigra.