Medulla oblongata

The medulla oblongata or medulla oblongata (medulla oblongata, in Latin) is the lowest of the three segments of the brainstem, It is located between the brain stem bridge or annular protuberance, above, and the spinal cord, below. It has the shape of a truncated cone with a lower vertex. Contains sensory nerve fibers ascending from the spinal cord to the cerebral cortex and its related nuclei. They also contain the descending motor pathways from the cortex to the spinal cord and part of the nerve fibers that connect the brainstem to the cerebellum. In addition, the medulla oblongata contains a large number of nuclei that regulate cardiac, respiratory, and gastrointestinal functions. and the autonomic nervous system.

Anatomy

Limits

It lies between the spinal cord and the pons. The limit with the spinal cord is not precise and is approximately at the decussation of the pyramids and hypoglossal nerve roots, just below the foramen magnum. Rostrally it widens to the pons, whose division with it is clear from the bulbopontine groove on the anterior face. On its posterior face, the medulla oblongata is visible in a lower part and hidden by the cerebellum in its upper part with which it is joined by the inferior cerebellar peduncles. In humans it is approximately 3 centimeters long and 2 centimeters wide. width. With the cerebellum removed, the cavity that forms the fourth ventricle between the medulla oblongata and the pons is observed.

External topography

The medulla oblongata is shaped like an inverted cone between the spinal cord and the pons. For its macroscopic description it can be divided into three faces: anterior, lateral and posterior.

Front face

On the anterior face, there is a longitudinal groove in the midline called the anterior median fissure. It is a continuation of the median groove of the spinal cord up to the bulbopontine groove that delimits the division between the medulla oblongata and the pons. The groove is interrupted at the bottom by the decussation of the pyramids, which are the two reliefs on both sides of the sulcus representing the descending corticospinal pathways of the pyramidal pathway, that is, the axons that carry nerve impulses from the primary motor cortex. The pyramids are bounded laterally by the ventrolateral or pre-olivary sulcus. The roots of the hypoglossal nerve (cranial nerve XII) and the abducens nerve (cranial nerve VI) originate from this groove at the limit with the pons.

Side Face

Following the ventrolateral sulcus is an ovoid bulge on the surface of the medulla oblongata bounded by the posterolateral or retrolivar sulcus containing the inferior olivary nucleus. Along this groove are the origins of the roots of the glossopharyngeal (IX), vagus (X), and accessory (XI) nerves.

Back side

In the midline of the posterior aspect, the posterior median sulcus is located below the equivalent of the spinal cord. In this part of the medulla oblongata, on either side of the dorsal median sulcus is the pons that contains the ascending axons. of the gracilis and cuneate tracts, which receive conscious sensory input from the trunk and extremities and are bounded laterally by the paramedian sulcus.

Internal configuration

The medulla oblongata presents different structures inside that change their configuration from caudal to rostral. Inside, several bundles of nerve bundles and the sensory, motor and autonomic nuclei that control different vital functions of the organism. The sharp separation between gray matter and white matter seen in the spinal cord is lost in the medulla oblongata. The main tracts of the spinal cord continue in the brainstem. To facilitate their study, the internal structure can be divided into several sections: the first at the level of the decussation of the pyramids, the second at the level of the decussation of the lemnisci and the third at the level of the inferior olivary nuclei.

Decussation level of the pyramids

Roughly in the center of the medulla oblongata is the central canal. In the anterior region is the decussation of the pyramidal pathways which continue caudally into the anterior corticospinal tracts of the spinal cord. The continuity of the gray matter of the ventral horn is lost and while the persistent gray matter of the central region is continued with the hypoglossal nucleus, the gray matter of the ventral region is continuous with the nucleus of the accessory nerve and more rostrally with the nucleus ambiguus which supplies motor nerve fibers to the vagus nerve. In the dorsal region the gray matter is continuous with the gracile and cuneate nuclei, where the respective fascicles synapse. Ventrolateral to these nuclei is the spinal nucleus of the trigeminal nerve (cranial nerve V).

Lemniscus decussation level

The central canal becomes progressively more dorsal. In the ventral region, the pyramids contain ipsilateral axons from the primary motor cortex, as they have not yet crossed the medial sulcus. The inferior olivary nucleus and various tracts such as the posterior and anterior spinocerebellar tract, and the lateral spinothalamic tract begin to form.. Dorsally the gracile and cuneate nuclei are more defined. The medial lemniscus is also formed, consisting of fibers emerging from these nuclei towards the primary sensory cortex by following a ventrolateral trajectory and then crossing the midline towards the contralateral medulla oblongata. This tract progresses rostrally between the spinal tract of the trigeminal nerve and the gray matter around the central canal.

Level of the caudal apex of the fourth ventricle

The central canal opens in the lower part of the fourth ventricle. In the ventral region is the olivary complex that participates in the functions of coordinating the movement carried out by the cerebellum. The nucleus also remains at this level ambiguous which is continuous rostrally with the facial nucleus. In the dorsal region, the gray matter associated with the central canal, which is now part of the floor of the fourth ventricle, contains the hypoglossal motor nucleus and other sensory nuclei such as the vestibular nucleus (cranial nerve VIII), which receives auditory and balance information, and the solitary nucleus, which receives impulses from taste receptors. There is also the dorsal nucleus of the vagus nerve that contains preganglionic parasympathetic neurons. In addition to the fascicles of these nuclei such as the nucleus solitaire tract, at this level the reticular formation that continues in the pons stands out and is a mesh of fibers that connect the nuclei with each other and with other interneurons to perform functions related to cardiovascular control, states of wakefulness and sleep, and respiratory rate.

Vascularization

Arterial supply

On the anterior aspect of the medulla oblongata, the anterior spinal arteries, which arise from the vertebral arteries, join at the anterior medial sulcus and descend through the medulla. On the other hand, the vertebral arteries, which arise from the subclavian arteries, converge on the basilar artery of the median sulcus at the pons. The posterior aspect is supplied by the posterior cerebellar artery which ascends behind the vagus and glossopharyngeal nerve roots to the pons. Subsequently, it supplies the choroid plexus of the fourth ventricle and the cerebellum laterally.

Venous return

Veins in the brainstem anastomose to form a plexus to drain blood into the veins of the spinal cord or the dural sinuses of the occipital region.

Embryonic development

Before closing the neural tube, rostrally to the spinal cord, the cephalic region will expand and subsequently three vesicles will be formed: prosencephalon, mesencephalon, and rhombencephalon, which will continue with the spinal cord. Around the fourth week of development the head begins to fold in the region of the midbrain, while another flexion appears between the rhombencephalon and the spinal cord. These flexions widen the rhomboncephalon that will constitute the fourth ventricle. Laterally, grooves will be formed that will be the rhombomeres that will participate in the constitution of the cranial nerves and a lateralization of the dorsal columns that will give rise to sensory nuclei.

In subsequent stages of development, the rhombencephalon will form the myelencephalon, metencephalon, and cerebellum. The myelencephalon will be the origin of the medulla oblongata. Around the fourth month of development, the descending corticospinal fibers will reach the ventral region of the myelencephalon to form the pyramids. At the same time, the ascending fibers from the medulla constitute the inferior cerebellar peduncle.

Clinical significance

The medulla oblongata performs several vital functions for the body:

• Transmission of impulses from the brain to the spinal cord for the motor response.
• It is intermediary in the transmission of sensitive sensitive information from the spinal cord to the sensitive cortex.
• It participates in the regulation of digestion and other functions of the autonomous nervous system.
• It controls the reflexes of cough, vomiting, sneezing, swallowing, and in line with the same muscles that are needed for swallowing and voice production.
• Controls the heart and respiratory rate. In case of injury, immediate death is caused by cardiac and/or respiratory arrest.
• Participates in the coordination control of movement and motor learning.
• It is responsible for receiving the sense of hearing and taste.

There are a number of conditions of the medulla oblongata that have neurological symptoms related to the nuclei and fasciculi. The appearance of vertigo, nausea, and the vomiting reflex together is usually related to a pathology that affects the rostral part of the medulla oblongata or caudal part of the pons, such as an obstruction in one of the arteries that supply the medulla oblongata. Thus, the Wallenberg syndrome is triggered by an obstruction of the inferior cerebellar artery that supplies the medulla oblongata and cerebellum, affecting the nuclei that control the movement of the muscles of the pharynx, larynx, and various sensory nuclei. Symptoms are dysphasia, dysarthria, vertigo, nausea, loss of sensation of the ipsilateral face and the contralateral extremities. As the cerebellum is affected, there is also a loss of muscle control in the movement of the limbs. A blockage of the vertebral artery affects the corticospinal tract of the contralateral pyramidal pathway and sense of touch. However, there is ipsilateral paralysis of the muscles that control the tongue, causing a deviation of the tip of the tongue to the opposite side.

Arnold Chiari malformation during embryonic development of the medulla oblongata and cerebellum blocks the flow of cerebrospinal fluid in the fourth ventricle. As a consequence, internal hydrocephalus is triggered by fluid accumulation in the four ventricles. This increase in internal pressure can prove fatal to the newborn.