Medulla oblongata

Medulla oblongata. Anatomical structure also known as medulla oblongata or mylencephalon ; It constitutes the lowest of the three segments of the brain stem , located between the brain stem bridge or annular protuberance (above) and the spinal cord (below). On its anterior face is the anterior median fissure, continuation of that described in the spinal cord. This is interrupted by the decusation of the pyramids, a detail produced when the fibers of the corticospinal tract of the pyramidal tract cross the midline [1] .

Laterally are the pyramids and lateral to these are the anterolateral groove where the apparent origin of the hypoglossal nerve (XII cranial nerve) is observed . The olive is also described and behind it the posterolateral sulcus with the emergence of the glossopharyngeal (IX cranial nerve), vagus (X cranial nerve) and accessory (XI cranial nerve) nerves. In the bulbopontine sulcus the emergence of the abductor nerve (VI cranial nerve) is observed , and in the lateral fossa of the sulcus itself laterally to the VII cranial nerve or facial nerve and to the VIII cranial nerve or vestiblochlear nerve .

It has the shape of a truncated cone with a lower vertex, approximately three centimeters long. It limits with the spinal cord in the decusation of the pyramids or Misticheli. While the division with the annular protuberance is clear and is given by the bulb- protuberance groove , but only on the anterior and lateral faces, on the posterior face the limit becomes less clear, since on said face only the lower half is visible and the upper half constitutes part of the floor of the fourth ventricle.

 

Summary

[ hide ]

  • 1 Situation and functions
    • 1 Situation
    • 2 Functions
  • 2 Anatomy
    • 1 External organization
    • 2 Internal organization
  • 3 Pyramids
  • 4 Pyramidal Decussion
    • 1 Level of decusation of the pyramids
    • 2 Level of sensitive decusation
      • 2.1 Fourth ventricle
    • 5 cores
    • 6 References
    • 7 Sources

Situation and functions

Situation

External view of the oblong medulla.

It is located between the spinal cord and the brain. For its macroscopic description, it can be divided into an anterior face in whose midline it presents a longitudinal groove, continuation of the anterior medial groove of the spinal cord that ends, upwards, in the bulb-protuberance groove in a depression known as foramen caecum or blind hole. On both sides of this groove there are two reliefs, the pyramids, which represent the pyramidal path, which exchange nerve fibers crossing the midline and form the decusation of the pyramids.

Continuing outwards is the preolivar or anterolateral sulcus, continuation of the homonym of the spinal cord, where the major hypoglossal nerve or XII cranial nerve has its apparent origin. From here the lateral face begins, which presents an ovoid eminence of 15 mm in length by 4 mm in width, the bulbar olive, behind it is the retroolivar or posterolateral groove in which they have their apparent origin, from top down, the glossopharyngeal nerves or IX cranial nerve, vagus or X cranial nerve, and spinal accessory or XI cranial nerve.

And finally, a posterior aspect that presents a posterior median groove in the midline, continuation of the posterior medullary sulcus bordered on each side by the posterior cord, subdivided in turn by a small paramedian groove into the Goll cord (medial) or fascicle. graceful and Burdach cord or cuneiform fasciculus (lateral) [2] .

Features

Transmission of impulses from the spinal cord to the brain. In case of injury, it causes immediate death due to cardiac or respiratory arrest. Regulation of heartbeat. Through the apneustic center, and the pneumotaxic center. Regulation of respiratory movement. Through the apneustic and pneumotaxic centers. Regulation of the secretion of digestive juices.

Anatomy

Three-dimensional image of the medulla oblongata.

For its study, it is usually divided into three thirds, a lower one, characterized by pyramidal decusation, a middle one, where sensory decusation occurs, and an upper one, where bulbar olives are located.

External organization

The medulla oblongata constitutes the lower portion of the brain stem. The transition from the spinal cord to the medulla oblongata is gradual in its external appearance, and there is no precise macroscopic limit. In any case, it is considered that the medulla oblongata continues inferiorly with the spinal cord at a point immediately above the exit of the anterior and posterior roots of the first spinal nerve, in the vicinity of the level of the foramen magnum . The differences in the external appearance of the spinal cord and medulla oblongata are mainly due to the development of the fourth ventricle., which determines that the posterior structures begin to be located posterolaterally, and the appearance of the pyramids and other prominences. Superiorly, the medulla oblongata communicates with the bridge.

On the anterior face of the bulb there is a deep cleft called the anterior median fissure [3] , which is the continuation of the structure of the same name present in the spinal cord. On either side of it, the pyramids form bulky columns of white matter containing bundles of descending motor fibers that will form the corticospinal tracts in the spinal cord. About 2.5 cm. below the bridge, in the depths of the anterior median fissure, it is possible to observe the decusation of the pyramids. It is in this place where the majority of the corticospinal fibers (90%) cross to the opposite side to constitute the lateral corticospinal tract in the lateral cord.of the spinal cord. A smaller proportion of pyramidal fibers (10%) descends ipsilaterally to form the anterior corticospinal tract in the anterior cord of the spinal cord. Posterolaterally to each pyramid is an oval zone called an olive , which indicates the position of the lower olive grove .

In a region posterior to the olives are the inferior cerebellar peduncles (restiform bodies) forming the floor of the lateral recess of the fourth ventricle. In the longitudinal groove that remains between the pyramid and the olive (anterior lateral groove or pre-olive grove) emerge the roots of the hypoglossal nerve (XII) cranial nerve. Since this cranial nerve is formed by the motor roots of the nerves of the embryonic occipital segments, its roots continue in series with the anterior roots of the spinal nerves of the cervical segments. The glossopharyngeal (IX), vagus (X), and accessory cranial root (X) nerves emerge from the anterolateral surface of the bulb between the olive and the inferior cerebellar peduncle (posterior lateral groove or retro-olive grove).

On the posterior surface of the lower half of the medulla oblongata, the posterior median sulcus continues with the homonymous structure of the posterior surface of the spinal cord. On each side of this groove there are two prominences (gracilis tubers) that indicate the position of the gracilis nucleus . Lateral to each tubercle gracilis is the posterior lateral medial groove and lateral to it is the tuber cuneatus, a less obvious prominence that determines the position of the underlying cuneatus nucleus. At the level of these tubers, the medulla oblongata opens in its dorsal region and constitutes the lateral limits of the fourth ventricle. The upper half of the posterior surface of the medulla oblongata constitutes the floor of the fourth ventricle. In a sagittal section of the brain stem, the central canal of the spinal cord is observed to extend into the lower half of the medulla oblongata until it communicates with the fourth ventricular cavity. In a posterior view it is possible to observe an acute angle located posterior to the site where the central canal joins the rhomboid cavity of the fourth ventricle, this is the obex , a common reference site for neurosurgeons.

Internal organization

The medulla oblongata does not have an internal structure as uniform as the spinal cord, which is why the cross sections of the bulb at different levels show important modifications in the spatial arrangement of the gray and white matter. The appearance and expansion of the fourth ventricle during the embryological development of the rhombusbrain alters the position of the base and wing plates. Unlike the spinal cord, at the bulbar level the wing plates are located laterally and the basal plates medially with respect to the limiting sulcus.

Pyramids

Magnetic resonance image .

Pyramids separated from each other by the anterior median fissure are located in the anterior portion of the medulla oblongata. Each of them carries corticospinal fibers that eventually reach the spinal cord, and some corticonuclear fibers that are distributed between the motor nuclei of the cranial pairs of the medulla oblongata. The inferior cerebellar peduncles are located in the posterolateral region, lateral to the fourth ventricle. The posterior spinocerebellar tract is near this peduncle, close to its junction with it. The anterior spinocerebellar tract is located superficially, between the lower olive complex and the trigeminal spinal nucleus. The medial lemniscusforms an elongated structure on either side of the midline, posterior to the pyramids. The region immediately posterior to each medial lemniscus is occupied by a conglomerate of ascending and descending fibers that make up the medial longitudinal bundle , a very important structure in the coordination of conjugated eye movements and in the regulation of changes in head position. It constitutes the most important connection path between the vestibular nuclei and the nuclei that control the extraocular muscles: oculomotor, trochlear, abductive and spinal nuclei of the accessory.

Pyramidal Decusation

Level of decusation of the pyramids

The organization and anatomical relationships of the different structures present in a cross section of the lower portion of the medulla oblongata passing through the decusation of the pyramids will be analyzed. A large part of the fibers of the pyramidal tract (about 90%) cross the midline in the posterolateral direction until finally locating in the lateral cord of the spinal cord to constitute the lateral corticospinal tract .

The decusation of these motor fibers (decusation of the pyramids) causes the gray matter of the anterior horn to disconnect with the one that surrounds the central canal. In the posterior region, as in the spinal cord, is the fascicle gracilis (medially) and the fascicle cuneatus (laterally). The gracilis and cuneatus nuclei, immersed in the middle of their respective fascicle, appear to be an extension of the central gray matter.

In the posterolateral portion of the bulb, near the surface, is the spinal nucleus of the trigeminal nerve. The fibers that make up the spinal tract are located between this nucleus and the bulbar surface. Inferiorly, the trigeminal spinal nucleus is continued with the gelatinous substance of the posterior horns of the spinal cord. The anterolateral region of the bulb has an arrangement very similar to that of the anterior and lateral cords of the spinal cord.

Level of sensitive decusation

The cross section of a region superior to the decusation of the pyramids allows us to observe the nucleus gracilis and the elevation it produces on the bulbar surface. Laterally to it is the cuneatus nucleus superficially surrounded by terminal fibers of the cuneatus fascicle. From both nuclei a set of fibers originate that pass anterolaterally surrounding the central gray matter (internal arciform fibers); These fibers are then directed medially until they succeed in decussing in the midline (sensory decusation) and then ascending the brainstem, constituting a well-defined tract, the medial lemniscus. In a region more lateral to the cuneatus nucleus is the accessory nucleus cuneatus, which receives proprioceptive impulses from the neck region and sends its output to thecerebellum . In the lateral region to the internal arcuate fibers is the spinal nucleus of the trigeminal nerve superficially covered by the fibers that make up the spinal tract of the same nerve.

The anterior region contains the corticospinal tracts. In the anterolateral portion of the bulb, it is possible to appreciate the spinocerebellar, rubrospinal, and vestibulospinal tracts. Lateral to the sensory decusation, the close relationship between the spinothalamic and spinotectal tracts is observed, which together are called lemniscus. Level of the lower olive groves. The cross section of the medulla oblongata at the level of the olives passes through the lower portion of the fourth ventricle. The amount of gray matter increases compared to the previous cut due to the appearance of various nuclear clusters. A distinctive fact of this brain segment is the presence of the lower olive complex. This complex is formed by the inferior olive grove and the accessory dorsal and medial olive groves.

The lower olive nucleus is a very folded U-shaped sheet of gray matter, the hilum of which is oriented medially. From this last structure emerges a bundle of fibers that decuse in the midline and then continue through the vicinity of the opposite lower olive complex until it penetrates the contralateral cerebellar hemisphere through the lower cerebellar peduncle. These fibers carry excitatory messages related to voluntary muscle movements to the neocerebellar cortex. The accessory olive nuclei are located inside the space formed by the gray matter lamina of the main olive nucleus and send their axons mainly to the cerebellar vermis . The lower olive complex receives input from the spinal cord through the spinoolivar tract and from thered nucleus , mesencephalic periaqueductal gray matter, deep cerebellar nuclei, and cerebral cortex of all lobes.

Fourth ventricle

In the region of the floor of the fourth ventricle a set of nuclei is observed arranged in a regular pattern: a nucleus medial to the limiting groove; the nucleus of the hypoglossal (somatomotor), a little more lateral the dorsal nucleus of the vagus (visceromotor); more laterally we have three nuclear groups: the nucleus of the solitary tract (viscerosensitive), the medial and inferior vestibular nuclei (special proprioceptive) and the trigeminal spinal nucleus (somatosensory). The ambiguous core(Branchiomotor) is located deep in the lateral region, posterior to the lower olive complex. In the anterolateral aspect of the inferior cerebellar peduncle it is possible to find the ventral (auditory) cochlear nucleus. The dorsal (auditory) cochlear nucleus is located in the posterolateral aspect of this peduncle. At this level is also the lower salivary nucleus (visceromotor), which by means of axons that join the glossopharyngeal nerve gives secretomotor (parasympathetic) innervation to the parotid gland .

Cores

Deep nuclei of the oblong medulla, real origin of cranial nerves.

The arcuate nuclei [4] are located on the anterior surface of the pyramids. It is possible that this structure is an inferiorly displaced pontine nucleus. This nucleus receives input from the cerebral cortex and sends impulses to the cerebellum via anterior external arcuate fibers that travel through the middle cerebellar peduncles. After the lower olive complex, a diffuse network of fibers and cells is observed, which constitute the reticular formationmedulla oblongata. In it we can find reticular nuclei, ascending and descending reticular pathways and local connections of the cranial nerves. It is possible to distinguish three groups: in the superior portion of the medulla oblongata the medial reticular nuclear group occupies the two medial thirds of the reticular formation, constituting at this level the gigantocellular reticular nucleus . It receives input from: the cerebral cortex, the upper brain stem, and the parvicellular reticular nucleus. Its outputs are directed to: high brain stem, intralaminar nuclei of the thalamus , hypothalamus and spinal cord through the reticulospinal tract. The lateral reticular nuclear group is composed of the lateral reticular nucleus and the parvicellular nucleus..

The parvicellular nucleus receives input from: the cerebral cortex, spinoreticular and collateral fibers that carry impulses from the auditory, vestibular, trigeminal, and visceral pathways. The paramedian reticular nuclear group is formed by a set of nuclei located laterally to the medial lemnisks that send fibers to the cerebellum via inferior cerebellar peduncles. Serotonin-secreting (serotonergic) secretory fibers project from the nuclei of the raphe magnum of the paramedian group to the interneurons of the gray matter of the spinal cord. These neurons are part of the so-called descending pain inhibitory pathway , an essential element in analgesic processes .

 

by Abdullah Sam
I’m a teacher, researcher and writer. I write about study subjects to improve the learning of college and university students. I write top Quality study notes Mostly, Tech, Games, Education, And Solutions/Tips and Tricks. I am a person who helps students to acquire knowledge, competence or virtue.

Leave a Comment