Neurosurgery notes/Anatomy/Brainstem/Reticular formations

Reticular formations

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Brainstem

General

  • Is a Diffuse Aggregation of Cells in the Central Brainstem that Possesses an Unusually Wide Range of Neural Connections and Functions
  • Has widespread network of axonal projections from the brainstem are distributed rostrally as far as higher regions of the brain, and caudally as far as the spinal cord
  • Reticular neurons may be viewed as the rostral extension of spinal interneurons.
  • Made of different nuclei (therefore it is not reticular i.e. diffuse in nature) except the following
    • Red nucleus
    • Substantia nigra
    • Cranial nerve nuclei (they are closely link to reticular formation)

Location

  • Within tegmentum

Classification in 3 different ways

  • Morphology
  • Neurochemically
  • Functional centres

Has two types of zones

  • Magnocellular zone:
    • Contains large cells
    • Give rise to long ascending and descending pathways,
    • Located in the medial two thirds of the reticular formation.
  • Parvocellular zone:
    • Contains predominantly small cells
    • That send axons to the medial central nuclei.
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Function

Motor control

  • Influence muscle tone (faciliatory and inhibitory)
    • Reticular neurons in medial portion of the medulla and pons → reticulospinal tracts → alpha and gamma spinal motor neurons of extensor muscle.
    • Many afferents to reticular neurons
      • Hence, Many factors influence muscle tone
      • Ascending spinal impulses
      • Descending impulses from
        • Cerebellum
        • Cerebral cortex
    • Pontine reticular spinal tract stimulates facilitatory to extensive tone
    • Medullary reticular spinal tract inhibit this facilitatory action.
      • Medullary reticular spinal tract is under facilitatory control of the cerebral cortex.
      • Disease
        • An injury to the brainstem caudal to the red nucleus but rostral to the vestibular and reticular spinal nuclei results in extensor posturing.
          • This occurs because the red nuclei projections (reticular spinal tract, flexor facilitator) are severed, and the cortical input to the medullary reticular spinal tract is also severed.
          • This results in unopposed extensor tone initiated by the
            • Lateral vestibulospinal
            • Pontine reticulospinal tracts.

Autonomic control: control of respiratory and cardiovascular systems

  • Respiratory
    • Respiratory-related reticular neurons are spread throughout the brainstem.
      • Dorsal respiratory center
        • Located in the dorsal medulla
        • Controls inspiration.
        • Main respiratory center.
      • Ventral respiratory center
        • Located in ventrolateral medulla
        • Controls the rate and pattern of breathing but only during significant respiratory effects
      • Pneumotaxic center
        • Located in the dorsal rostral pons.
        • Controls the rate and pattern of breathing
    • Afferent impulses from many sources and are directly influenced as well by the carbon dioxide content of the blood (more accurately the H + content of the blood).
    • Efferent impulses: reticulospinal pathways → spinal motor neurons → respiratory muscles.
  • Cardiovascular
    • Cardiovascular reticular neurons are involved in complex polysynaptic pathways.
    • Afferent impulses from many sources, including
      • Peripheral sensory receptors such as the
        • Carotid sinus
      • Higher regions of the brain such as the hypothalamus.
    • Efferent impulses: reticulospinal tracts → spinal neurons that innervate both the heart and the peripheral circulation.

Sensory control

  • Reticulospinal pathways modulate the sense of pain at the level of the dorsal horn of the spinal cord.

Consciousness

  • Ascending reticular formation activating system (ARAS) modulates wakefulness and arousal.
    • Afferent: Receives collaterals from the long ascending sensory pathways
      • Medial lemniscal
      • Spinothalamic tracts.
    • Efferent: towards thalamic nuclei → widespread areas of the cerebral cortex.
    • Stimulation of the reticular neurons of the ARAS induces wakefulness.
      • Can be seen on EEG showing electrical activity in the cerebral cortex.
        • The low voltage–high frequency activity of arousal replaces the high voltage–slow wave activity of somnolence.
    • Brainstem lesions affecting the ARAS typically result in the impairment of consciousness in the most extreme cases, causing coma.
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