Lecture 14(P2) + 15(P1)- Medulla Oblongata

Brain Stem - Medulla Oblongata

Overview of Brain Stem

  • Definition: The brain stem connects the cerebrum to the spinal cord.

  • Components: Includes three main parts:

    • Midbrain

    • Pons

    • Medulla Oblongata

  • Location: Situated in the posterior cranial fossa.

  • Cerebellar Connections: Each part of the brainstem is connected to the cerebellum via cerebellar peduncles (superior, middle, and inferior).

  • Histology: Comprises deep grey matter surrounded by white matter fiber tracts.

Functions of the Brain Stem

  1. Pathways: Contains ascending and descending tracts linking the spinal cord to higher centers in the forebrain.

  2. Cranial Nerves: Contains sensory and motor nuclei of cranial nerves.

  3. Nuclei: Houses olivary nuclei (in medulla), pontine nuclei (in pons), and red nucleus/substantia nigra (in midbrain).

  4. Reticular Formation: Groups of nuclei associated with:

    • Control of consciousness

    • Pain perception

    • Regulation of cardiovascular and respiratory systems.

Medulla Oblongata Specifics

  • Location: Most inferior portion of the brainstem, about 3 cm long, transitioning into the spinal cord at the foramen magnum.

  • Shape: Conical, broader above (joins with pons) and narrows below (continuous with spinal cord).

  • Parts: Divided into:

    • Lower Closed Part: Contains a central canal.

    • Upper Open Part: The central canal widens to form the lower half of the fourth ventricle floor.

External Features of the Medulla

Anterolateral Surface

  • Anterior Median Fissure: Prominent groove down the front.

  • Three Elevations:

    • Pyramid: Formed by corticospinal tract.

    • Olive: Formed by the inferior olivary nucleus.

    • Inferior Cerebellar Peduncle (ICP): Fibers connecting the medulla with the cerebellum.

Posterior Surface

  • Lower Half (Closed Medulla): Features a posterior median sulcus with two columns:

    • Gracile Fasciculus: Ends at gracile tubercle associated with grading nuclei for fine touch and proprioception.

    • Cuneate Fasciculus: Ends at cuneate tubercle, also involved in fine touch and proprioception.

  • Upper Half (Open Medulla): Contributes to the floor of the fourth ventricle with an inverted V-shaped sulcus detailing several nuclei.

Internal Structures of the Medulla

Levels

  1. Level of Pyramidal Decussation:

    • 80% of corticospinal fibers cross to opposites sides.

    • Cuneate and gracile nuclei present.

    • Contains trigeminal sensory nucleus.

  2. Level of Lemniscal Decussation:

    • Involved in sensation pathways.