Download Reticular Activating System

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.