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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.

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Reticular Activating System

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  1. Reticular Activating System RAS

  2. 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.

  3. Raphe Nuclei • Lie along midline of medulla and midbrain. • Important in maintaining wakefulness. • Damage may result in permanent coma.

  4. 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.

  5. Reticular Formation • Reticular formation receives input from cerebral cortex. • May use this input as a basis for its decisions.

  6. 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.

  7. 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.

  8. Magnocellular Zone Afferents • Spinoreticular fibers. • Sensory cranial nerves. • Cerebellum. • Hypothalamus. • Basal nuclei. • Cerebral cortex: Esp. premotor cortex.

  9. 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.

  10. Monoaminergic Pathways • Serotonin. • Norepinephrine (noradrenalin). • Dopamine

  11. 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.

  12. 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.

  13. 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.

  14. Noradrenergic Pathways • Most adrenergic fibers terminate on small blood vessels and capillaries in the brain: May help regulate blood flow in brain.

  15. Dopaminergic Pathways • Majority of dopaminergic cells are located in the substantia-nigra.

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