Anatomy of Medulla Oblongata, Pathways, Divisions, and Structures Overview.

MEDULLA OBLONGATA:

The pons and spinal cord are connected superiorly and inferiorly by the medulla oblongata. The origin of the anterior and posterior roots of the first cervical spinal nerve, which roughly corresponds to the level of the foramen magnum, is where the medulla and spinal cord join. The broad extremity of the medulla oblongata is pointed superiorly, giving it a conical form. The spinal cord's central canal extends upward into the medulla's lower half and widens into the fourth ventricle's chamber in the upper half.

Medullary Pyramids and Corticospinal Pathway:

The anterior median fissure, which is located on the medulla's anterior surface, is continuous with the spinal cord's anterior median fissure inferiorly. The pyramids are swellings on either side of the median fissure. The corticospinal fibers, which start in giant nerve cells in the precentral gyrus of the cerebral cortex, are the nerve fiber bundles that make up the pyramids. The decussation of the pyramids is formed by the pyramids tapering inferiorly, where most of the descending fibers cross over to the opposite side.

Structures and Nerve Pathways:

The anterior external arcuate fibers cross the surface of the medulla oblongata laterally before emerging from the anterior median fissure above the decussation and entering the cerebellum. The olives are oval elevations created by the underlying inferior olivary nuclei, and they are located posteriorly to the pyramids. The hypoglossal nerve's rootlets emerge in the groove between the pyramid and the olive. The inferior cerebellar peduncles, which link the medulla and cerebellum, are located posterior to the olives. The cranial roots of the accessory nerve as well as the glossopharyngeal and vagus nerves emerge in the groove formed between the inferior cerebellar peduncle and the olive.

Divisions of the Medulla Oblongata:

The bottom portion of the fourth ventricle's floor is formed by the posterior surface of the superior half of the medulla oblongata. There is a posterior median sulcus on the posterior surface of the inferior half of the medulla, which is continuous with the posterior side of the spinal cord. The underlying gracile nucleus produces an extended swelling called the gracile tubercle on either side of the median sulcus. Similar in size to the gracile tubercle, the cuneate tubercle is generated by the underlying cuneate nucleus.

Internal structure:

The medulla oblongata is made up of white and grey matter, just like a spinal cord, however a transverse section analysis of this area reveals that the arrangement of these brain regions has changed significantly. The neural tube's growth to generate the hindbrain vesicle, which eventually develops into the fourth ventricle, provides an embryological explanation for this rearrangement. The derivatives of the embryo's alar and basal plates are repositioned as a result of the fourth ventricle's wide lateral spread.

Levels of Decussation and Primary Structures:

To help with this understanding, keep in mind that the derivatives of the alar and basal plates are located in the spinal cord posteriorly and anteriorly to the sulcus limitans, respectively, and that they are located laterally and medially to the sulcus limitans, respectively, in the medulla oblongata.

There are four levels to the internal anatomy of the medulla oblongata:

1. Pyramidal level of decussation;
2.  Lemnisci level of decussation
3. Olive level
4. Level just inferior to the pons.

It contrasts the primary structures seen at each of the medulla oblongata levels.

Level of Pyramid Decussation:

The great motor decussation, or decussation of the pyramids, is traversed by a transverse portion that runs across the inferior part of the medulla oblongata. The corticospinal fibers occupy and form the pyramid in the superior section of the medulla, but approximately three-fourths of the fibers cross the median plane inferiorly and continue down the spinal cord as the lateral corticospinal tract, which is located in the lateral white column. The connection between the gray matter surrounding the central canal and the spinal cord's anterior column is severed as these fibers cross the midline.

Nuclei Connectivity:

Beyond the core gray matter, the fasciculus gracilis and the fasciculus cuneatus keep ascending superiorly posteriorly. The posterior extensions of the central gray matter are represented by the nuclei gracilis and cuneatus. The inferior end of the trigeminal nerve's spinal tract nucleus becomes contiguous with the substantia gelatinosa in the posterior gray column of the spinal cord. The nucleus and the surface of the medulla oblongata are separated by the fibers of the nucleus' tract. In these slices, the lateral and anterior white columns of the spinal cord are clearly visible, and their fiber arrangement remains unaltered.

Level of Lemnisci Decussation

The fasciculus gracilis and the fasciculus cuneatus continue to climb superiorly and posteriorly beyond the core gray matter. The nuclei gracilis and cuneatus represent the posterior expansions of the central gray matter. In the posterior gray column of the spinal cord, the substantia gelatinosa and the inferior end of the trigeminal nerve's spinal tract nucleus unite. The fibers of the nucleus' tract divide the nucleus from the surface of the medulla oblongata.

White Columns and Pathway Localization:

These slices clearly show the anterior and lateral white columns of the spinal cord, which remain intact in terms of their fiber organization. The internal arcuate fibers are lateral to the trigeminal nerve's spinal tract nucleus. The trigeminal nerve's spinal tract is located lateral to the nucleus. A region lateral to the lemnisci's decussation is home to the spinotectal and anterior spinothalamic pathways. Together, they are referred to as the spinal lemniscus because of their close proximity to one another. The vestibulospinal, spinocerebellar, and rubrospinal tracts are located in the medulla oblongata's anterolateral area.

Level of the Olives:

The inferior region of the fourth ventricle is crossed by a transverse portion that goes through the olives. The olivary nuclear complex, which includes the arcuate nuclei and the vestibulocochlear, glossopharyngeal, vagus, accessory, and hypoglossal nerve nuclei, is responsible for the expansion in gray matter at this level.

Olivary Nuclear Complex:

The inferior olivary nucleus is the biggest nucleus in this complex. The gray matter, which has the appearance of a rumpled bag with its mouth pointing medially, is what gives the medulla's surface its olive-shaped elevation. There are additional smaller auxiliary olivary nuclei in the dorsal and medial regions. Via the inferior cerebellar peduncle, the cells of the inferior olivary nucleus transmit fibers medially across the midline and into the cerebellum. Afferent fibers from the cerebellum, cerebral cortex, and spinal cord (the spino-olivary tracts) go to the interior olivary nuclei. Voluntary muscular action and the function of the olivary nuclei are related.

Vestibulocochlear Nuclei:

The lower vestibular nucleus, the lateral vestibular nucleus, the superior vestibular nucleus, and the medial vestibular nucleus comprise the vestibular nuclear complex.

Later on, we will talk about these nuclei's specifics and how they are related.

This level's part displays the inferior and medial vestibular nuclei.

The two cochlear nuclei are the posterior cochlear nucleus, which is located on the posterior aspect of the peduncie lateral to the floor of the fourth ventricle, and the anterior cochlear nucleus, which is located on the anterolateral aspect of the inferior cerebellar peduncle. Later on, the links between these nuclei are explained.

Nucleus Ambigus:

Large motor neurons make up the nucleus ambigus, which is located deep within the reticular formation. The developing nerve fibers are distributed throughout voluntary skeletal muscle and connect to the glossopharyngeal, vagus, and cranial portions of the accessory nerve.

Central Gray Matter:

At this level, the fourth ventricle's floor is beneath the central gray matter. Identifying the following significant structures can help you move from medial to lateral.

1. The tractus solitarius nucleus.
2. The dorsal vagus nucleus.
3. The hypoglossal nucleus
4. The inferior and medial vestibular nuclei.

Within the reticular formation, the nucleus ambiguus has taken up a deep position. The relationships and practical importance of these nuclei.

Neural Pathways:

The anterior surface of the pyramids contains arcuate nuclei, which are thought to be inferiorly displaced pontine nuclei. They receive nerve fibers from the cerebral cortex and then transmit efferent fibers to the cerebellum through the anterior external arcuate fibers.

Neural Tracts in the Medulla:

The corticospinal fibers descend to the spinal cord, while the corticonuclear fibers are distributed to the motor nuclei of the CNs located within the medulla. The pyramids containing the corticospinal and some corticonuclear fibers are located in the anterior part of the medulla, divided by the anterior median fissure. Posterior to the pyramid, the medial lemniscus creates a flattened tract on both sides of the midline. These fibers connect the thalamus with sensory data, emerging from the lemnisci's decussation.

Neural Pathways in the Brainstem:

Located on both sides of the midline, anterior to the hypoglossal nucleus and posterior to the medial lemniscus, the medial longitudinal fasciculus is a tiny tract of nerve fibers. The connections of the inferior cerebellar peduncle, which is made up of ascending and descending fibers, are located in the section's posterolateral corner on the lateral side of the fourth ventricle. The anteromedial aspect of the inferior cerebellar peduncle houses the trigeminal nerve's spinal tract and nucleus.

Spinocerebellar Tracts and Reticular Formation:

The anterior spinocerebellar tract is located close to the surface, between the nucleus of the spinal tract of the trigeminal nerve and the inferior olivary nucleus. The posterior spinothalamic, lateral spinothalamic, and spinotectal tracts make up the deeply situated spinal lemniscus. The reticular formation is positioned deeply posterior to the olivary nucleus and is made up of tiny clusters of nerve cells and a diffuse combination of nerve fibers. At this stage, the reticular formation only makes up a tiny portion of this system, which is also found in the midbrain and pons.

Cranial Nerve Connectivity:

Through the reticular formation, the glossopharyngeal, vagus, and cranial portions of the accessory nerves can be observed flowing forward and laterally. The inferior cerebellar peduncles and the olives are where the nerve fibers arise. Additionally, the hypoglossal nerves emerge between the pyramids and the olives after passing anteriorly and laterally via the reticular formation.

Level just inferior to pons:

There are minimal alterations in the gray and white matter distribution as compared to the preceding level. The cochlear nuclei are now visible on the anterior and posterior sides of the inferior cerebellar peduncle, as the lateral vestibular nucleus has superseded the inferior vestibular nucleus.