The Medulla Oblongata: Structure and Function

Rubbia Khalid July 19, 2024

Brainstem

The brainstem is the stalk-like part of the brain that connects the spinal cord with the forebrain, consisting of three parts: medulla oblongata, pons, and midbrain. Located in the posterior cranial fossa, the brainstem has several key functions and structures:

Functions:

  • Pathway for Tracts: Passes ascending and descending tracts between the spinal cord and forebrain.
  • Autonomic Reflex Centers: Controls vital functions like respiration, heart rate, and blood pressure.
  • Reticular Activating System: Regulates consciousness.
  • Cranial Nerve Nuclei: Contains nuclei for cranial nerves III to XII.

Structure:

  • The brainstem's ventral surface lies on the clivus, and posteriorly, the pons and medulla are separated from the cerebellum by the fourth ventricle.
  • Composed of nerve fibers and nerve cells, with fibers arranged in tracts and cells forming well-defined nuclei and diffuse reticular formation.

Note: Bilateral destructive lesions in the brainstem can be fatal due to damage to vital centers controlling respiration, heart rate, and blood pressure. Impairment of the reticular activating system can lead to loss of consciousness, stupor, and coma.



Medulla Oblongata

The medulla oblongata is the lowest part of the brainstem, continuing from the spinal cord to the lower border of the pons. It has the following key features:

Structure:

  • Shape: Truncated cone (bulb-like).
  • Dimensions: About 3 cm in length, 2 cm in breadth, and 1.25 cm in thickness.
  • Location: Lies in the anterior part of the posterior cranial fossa between the clivus in front and the vallecula of the cerebellum behind.
  • Division: Lower closed part (contains central canal) and upper open part (forms lower part of the fourth ventricle).

Functions:

  • Vital Centers: Contains cardiac, vasomotor, and respiratory centers essential for life.
  • Cranial Nerves: Provides attachment to cranial nerves IX to XII.
  • Nuclei and Tracts: Contains nuclei that relay sensory and motor information and tracts that connect the spinal cord with other brain parts.

External Features of the Medulla Oblongata

The medulla oblongata is divided into right and left symmetrical halves by the anterior median fissure and posterior median sulcus.

Dividing Features:

Anterior Median Fissure:

  • Continuous with the spinal cord’s fissure below.
  • Ends in a small triangular depression (foramen caecum) at the lower border of the pons.
  • Interrupted by crossing fibers (decussation of pyramids).

Posterior Median Sulcus:

  • Continuous with the spinal cord’s sulcus below.
  • Present only in the lower half of the medulla.
  • Lips diverge above to form part of the fourth ventricle floor.

Lateral Sulci:

  • Anterolateral Sulcus:
  • Extends along the lateral border of the pyramid.
  • Emerges rootlets of the hypoglossal (XII cranial) nerve.

Posterolateral Sulcus:

  • Located between the olive and the inferior cerebellar peduncle.
  • Emerges rootlets of the glossopharyngeal (IX), vagus (X), and accessory (XI) cranial nerves.

These sulci divide each half of the medulla into anterior, posterior, and lateral regions.


Ventral Features:

Pyramids:

  • Two elongated elevations on either side of the anterior median fissure.
  • Contain corticospinal (pyramidal) fibers, with about 75% crossing to the opposite side in the lower medulla (pyramidal decussation).
  • Fibers descend as lateral corticospinal tract (75%), anterior corticospinal tract (20%), and along with uncrossed fibers in the lateral white column of the spinal cord (5%).

Olives:

  • Oval elevations posterolateral to the pyramids.
  • Produced by the underlying inferior olivary nucleus.

Rootlets of the Hypoglossal Nerve:

  • Emerge from the anterolateral sulcus between the pyramid and the olive.

Inferior Cerebellar Peduncles:

  • Thick bundles of fibers posterolateral to the olive.
  • Attach the medulla to the cerebellum.

Rootlets of Cranial Nerves IX, X, XI:

  • Emerge through the posterolateral sulcus, separating the olive from the inferior cerebellar peduncle.


Features on the Posterior (Dorsal) Aspect of Medulla

The dorsal aspect of the medulla is divided into lower closed and upper open parts:

Closed Part:

  • Longitudinal Elevations:
  • Fasciculus Gracilis: Medial elevation.
  • Fasciculus Cuneatus: Lateral to the fasciculus gracilis.
  • Inferior Cerebellar Peduncle: Most lateral.
  • Gracile and Cuneate Tubercles: Formed by the expanded upper ends of the fasciculus gracilis and fasciculus cuneatus due to underlying nuclei.
  • Tuber Cinereum: Lateral to the cuneate tubercle, produced by the spinal nucleus of the trigeminal nerve.

Open Part:

Forms the lower part of the floor of the fourth ventricle.

Features include:

  • Median Sulcus
  • Hypoglossal and Vagal Triangles
  • Vestibular Areas
  • Area Postrema
  • Stria Medullaris





Internal Structure of the Medulla

The internal structure of the medulla is examined at four levels through transverse sections:

At the Level of Decussation of Pyramids:

  • Nucleus Gracilis and Nucleus Cuneatus: Appear as narrow strip-like projections from the posterior aspect of the central grey matter.
  • Nucleus of Spinal Tract of Trigeminal Nerve: Swollen apex of the posterior horn, continuation of the substantia gelatinosa.
  • Spinal Tract of Trigeminal Nerve: Bundle of fibers capping the nucleus of the spinal tract of trigeminal nerve.
  • Decussation of Pyramidal Tracts: About 75% of fibers cross to the opposite side, forming the lateral corticospinal tract. This process detaches the anterior horns from the central grey matter.
  • Each detached anterior horn forms the spinal nucleus of the accessory nerve and the supraspinal nucleus of the first cervical nerve.
  • Accessory Nerve Nucleus: Extends down to the fifth cervical spinal segment.
  • Supraspinal Nucleus: Provides efferent fibers for the first cervical nerve and is continuous with the hypoglossal nerve nucleus.
  • Reticular Formation: Diffuse zone with a network of fibers and scattered nerve cells, located in the lateral white column adjacent to the nucleus of the spinal tract of the trigeminal nerve.



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Rubbia Khalid

Posted by Rubbia Khalid

BSc in Biomedical Science & Physiology from UNISA.

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