Medulla oblongata

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Medulla oblongata

  • Like the pons, the rostral portion of the medulla is open, separated from the cerebellum by the 4th ventricle
  • In contrast, the arrangement of gray and while matter in the caudal portion of the medulla is closed (i.e., it resembles the spinal cord)
  • In reference to the fourth ventricle, the open portion of the fourth ventricle is dorsal to the rostral medulla
  • the closed portion is within the caudal medulla.

Surface landmarks and major structures

Ventral

  • The bulging vertical striations form the pyramids
    • These represent the descending fibres of the corticospinal tract
    • The bilateral pyramids are separated by the median sulcus
    • Most caudally, 90% of the fibres in the corticospinal tract cross the midline at the "decussation of the pyramid"
    • Continue to descend on the contralateral side of the spinal cord as the lateral corticospinal tract
    • The remaining 10% do not cross and descend ipsilaterally as the anterior corticospinal tract

Dorsal

  • The rostral portion of the medulla resembles the dorsal surface of the pons
  • The median sulcus and the bilateral sulcus limitans extend into this region
  • The medially placed nuclei include the motor nuclei of CN IX, X, and XI and the hypoglossal nucleus (XII)
  • The laterally placed nuclei include the descending portion of trigeminal nucleus and the sensory nucleus of CN IX, X, and XI
  • The caudal portion of the medulla resembles the spinal cord
  • the dorsal structures, similar to those in the spinal cord and midbrain, serve sensory function
  • The dorsal surface features four linearly arranged elevations:
    1. Tubercle gracile: the medial pair, separated by the median sulcus
    2. Tubercle cuneate: the lateral pair
  • The elevations represent groups of nuclei within the tubercles
  • Ascending sensory fibres from the spinal cord synapse with cell bodies in these nuclei which in turn projects to tertiary sensory neurons in the ventral-posterior nucleus in the thalamus
  • In the medulla oblongata, the remaining fibres of the corticobulbar tract decussate and terminate in the cranial nerve nuclei of the glossopharyngeal (CN IX), vagus (CN X), accessory (CN XI), and hypoglossal (CN XII) nerves