Medulla Oblongata is the connection between the brainstem and the spinal cord, carrying multiple important functional centers. It is comprised of the cardiovascular-respiratory regulation system, descending motor tracts, ascending sensory tracts, and origin of cranial nerves IX, X, XI, and XII. Motor neurons cross from the left motor cortex to the right side of the spinal cord in the medulla. The medulla is the most caudal aspect of the brainstem, approximately at the level of the foramen magnum. Anterior to the medulla oblongata is the median fissure, which connects with the median fissure of the spinal cord. The posterior surface of the medulla can divide into two parts, the inferior part, which has median sulcus continuous with the spinal cord, and the superior part, which forms the lower floor of the fourth ventricle. The medulla, including the pons and the midbrain, is divided into three laminae, from dorsal to ventral, called the tectum, tegmentum, and basis, respectively. The tectum of the medulla involves the inferior medullary velum, which is the most inferior posterior part of the fourth ventricle. The tegmentum consists of the inferior olivary nucleus and the cranial nerve nuclei of IX, X, XI, XII. The basis, most ventral layer, has the pyramid decussation at the medulla.

Surface of Medulla Oblongata

Anterior surface

  • The anterior and posterior surfaces of the medulla oblongata have unique protuberances that are formed from the presence of different nuclei and their associated tracts. Along the midline of the anterior surface of the medulla is the anterior median fissure.
  • This is a continuation of the anterior median fissure of the spinal cord. On either side of the anterior median fissure is a vertical protuberance known as the pyramids. These areas contain efferent corticospinal fibers from the precentral gyrus (Brodmann area 4) that are responsible for muscle movement.
  • Owing to the fact that a substantial amount of these fibers cross the midline with its counterpart on the opposite side – forming the pyramidal decussations – fibers from the precentral gyrus on one side innervate muscles of the contralateral (opposite) side. Just lateral and posterior to each pyramid is another excrescent structure known as the olive. This bulge can be attributed to the presence of the inferior olivary nucleus.
  • The anterior surface of the medulla is also littered by the protrusion of cranial nerve fibers emerging from the brainstem. The rootlets of the CN XII (hypoglossal) emerge posterolateral to the pyramid and anteromedial to the olive, after which they unite to form the single nerve. Similarly, posterolateral to the olive, but anteromedial to the inferior cerebellar peduncle are the rootlets of CN IX (glossopharyngeal) and CN X (vagus). The respective rootlets fuse with each other to form their resultant nerve.

Posterior surface

  • The raised areas of the posterior medulla are not only arranged differently from those of the anterior surface, but they also arise as a result of different underlying structures. As was observed on the anterior surface, the dorsal median sulcus of the spinal cord continues cranially and passes through the obex (inferior apex of the fourth ventricle).
  • Bilateral to the dorsal median sulcus are vertical raised areas known as the fasciculus gracilis. This is a continuation of the fasciculus gracilis of the dorsal column of the spinal cord. The cranial part of the fasciculus gracilis becomes enlarged and is referred to as the gracile tubercle, which is formed by the gracile nucleus.
  • Fasciculus cuneatus runs laterally to the fasciculus gracilis on either side of the medulla and also transitions into the larger cranial portion, the cuneate tubercle, formed also by its own nuclei.
  • Posterolateral to the gracile tubercle is the trigeminal tubercle (formed by the spinal nucleus of the trigeminal nerve). The lateral funiculus (lateral white matter bundle of fibers) also continues cranially in the same order it courses in the spinal cord.
  • The posterior superior part of the medulla overlaps with the inferior compartment of the rhomboid fossa (fourth ventricle). The dorsal median sulcus continues through the midline of the fossa to divide it into halves. The facial colliculus is the largest midline protrusion of the floor of the fourth ventricle (caudal end of the medial eminence). It is located superiorly to the horizontally coursing striae medullaris fibers, the hypoglossal trigone, and the vagal trigone (in that order).
  • The superior and inferior fovea forms the respective limits of the sulcus limitans that is laterally related to the facial colliculus. The sulcus limitans is the embryological border between the alar and basal plates of the fetal neural tube.
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Structure and Function of Medulla Oblongata

The medulla includes multiple nuclei and tracts that have information from the spinal cord as well as the higher cortex. Each nucleus in the medulla or tract passing through will have a separate explanation below.

The cardiovascular-respiratory function of the medulla

  • Multiple studies show that the cardiovascular system and the respiratory system unite as one regulating system within a specific part of the medulla. The rostral ventral lateral medulla (RVLM) has been found to have the excitatory neurons that carry information to the pre-sympathetic neurons in the spinal cord, which maintain baseline arterial pressure.
  • Within the RVLM, there is also the ventral respiratory column, which is known to be the center of control of respiratory rhythm and pattern. The ventral respiratory column divides into multiple sub-nuclei that establish connections with the presynaptic fibers in the RVLM and regulate the oscillating respiratory pattern that allows us to perfuse our tissues with oxygen.
  • The caudal ventrolateral medulla contains synaptic input from tonic inhibitory baroreflex control. Together both the caudal and the rostral part of the ventral medulla have been known to be an essential location for input and convergence for controlling respiration with cardiovascular regulation.

The Nucleus of the Solitary Tract (NTS)

  • The NTS is in the dorsolateral medulla, from the caudal part of the facial nucleus to the caudal aspect of the pyramidal decussation. The organization of this nucleus is by the type of information transmitted, and the pathways needed to be activated.
  • It is essential for analyzing and coordinating visceral afferent information. The afferent cardiorespiratory signals from peripheral chemoreceptors, pulmonary stretch receptors, and baroreceptors, synapse in the caudal and intermediate sections of the NTS, before reaching the RVLM in the medulla.
  • They work in the regulation of respiration. Neurons carrying taste sensation also synapse in the NTS first before traveling to the thalamus and later on to the cortex.

Area Postrema

  • This structure lies on the dorsal surface of the medulla at the floor of the 4th ventricle, rostral to the obex, adjacent to the solitary nucleus. It is also known as the vomiting center. It is positioned to detect emetic toxins in the blood and the CSF.
  • The cells in the area postrema are unique because they don’t have a blood-brain barrier to keep out large polar molecules. These cells receive innervation via vagal afferents leading to the solitary nucleus, which is one way a person can feel nauseous and vomit.
  • Studies have shown a lesion in the AP does not entirely inhibit vomiting from all of the emetic drugs. Vomiting from motion sickness is also reported to be not associated with the area postrema.

Spinal trigeminal Nucleus

  • This brainstem nucleus is in the lateral medulla. It further subdivides into the trigeminal nucleus pars oralis, pars interpolates, and pars caudalis, named rostral to caudal, respectively.
  • The caudal portion predominantly receives nociceptive afferents from the face. The other nuclei incorporate sensory information from the sensory branches of the trigeminal cranial nerve (V) of the ipsilateral side of the face.
  • The sensation of temperature, pain, and a deep or crude touch of the ipsilateral face are sent to this nucleus as well. It is the first central synapse and relay in the orofacial pain nerve fibers, which later reach the ventral posteromedial nucleus of the thalamus.

Inferior Olivary nuclei

  • The inferior olivary nuclei are in the superior medulla. They are two structures lateral to the pyramidal columns in a C shape, composed of grey matter. It is associated with the inferior cerebellar peduncles (ICP), and many neurons travel through the ICP to the cerebellar cortex.
  • The inferior olivary nuclei receive information from multiple sources carrying proprioception, muscle tension, and motor intention. The neurons from this nuclei are important because they synapse directly on the Purkinje cell bodies in the cerebellum.
  • Due to the critical connection with the cerebellum, atrophy of the inferior olivary nuclei will lead to a cerebellar loss as well.

Reticular formation

  • This is a net-like system found in the tegmentum of the midbrain, pons, and medulla, as well as the subthalamus and thalamus. This system is a complex organization divided into four regions.
  • The 4th zone is the only one in the medulla between the medial and lateral columns. This zone plays a role in autonomic regulation of respiration, heart rate, and blood pressure.
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Pyramidal decussation of the motor pathway

  • This is the most central and most caudal part of the medulla, which is where the majority of the motor fibers from the motor cortex in the cerebrum decussate in the medulla and form the lateral corticospinal tract in the spinal cord. Some of the fibers that don’t cross the midline become the anterior corticospinal tract.

The Cuneate Nucleus and Gracilias Nucleus

  • These nuclei are most dorsal and caudal in medulla on the same level as the pyramidal decussation. The dorsal column- medial lemniscus pathway synapse into second-order neurons from their perspective dorsal root ganglia neurons at these nuclei.
  • This pathway conveys conscious proprioception, fine tactile discrimination, and vibration sensations from the body. The cuneate nucleus, lateral to the gracile nucleus, receives sensory information from the upper extremities. The gracile nucleus receives sensory input from the lower extremities. Both nuclei travel cephalad and from the medial lemniscus once they decussate in the medulla.

Medial Leminscus

  • The neurons that form the medial lemniscus are the internal arcuate fibers from the cuneate and gracile nucleus in the caudal medulla. Once they decussate, they form the medial lemniscus pathway and terminate in the ventral posterior nucleus of the thalamus, carrying proprioception, vibration, and fine touch modalities.
  • The medial lemniscus pathway is located in between the two inferior olivary nuclei and is dorsal to the pyramidal motor pathway. It travels through the rest of the medulla up to the level of the pons and midbrain.

Spinothalamic tract

  • The spinothalamic tract includes the anterior and lateral tracts. Pain and temperature sensation travel through the lateral tract, and crude touch is through the anterior tract. The second-order neurons reaching the medulla from the spinal cord have already crossed over through the anterior white commissure in the spinal cord segment.
  • In the medulla, the spinothalamic tract is between the inferior olivary nucleus and the spinal trigeminal nucleus. This tract goes through the whole brain stem terminating in the ventral posterior lateral nucleus in the thalamus.

Blood Supply of Medulla Oblongata

  • The blood supply to the medulla can divide into two groups, which are the paramedian bulbar and lateral bulbar arteries. The paramedian bulbar arteries arise from the vertebral arteries and supply the medial aspect of the medulla. At the most caudal part of the medulla, the paramedian bulbar arteries can also arise from the anterior spinal artery. The lateral bulbar branches arise from the vertebral artery or the posterior inferior cerebellar artery and supply the lateral part of the medulla.

Blood to the medulla is supplied by a number of arteries.

  • Anterior spinal artery – This supplies the whole medial part of the medulla oblongata.
  • Posterior inferior cerebellar artery – This is a major branch of the vertebral artery, and supplies the posterolateral part of the medulla, where the main sensory tracts run and synapse. It also supplies part of the cerebellum.
  • Direct branches of the vertebral artery – The vertebral artery supplies an area between the other two main arteries, including the solitary nucleus and other sensory nuclei and fibers.

Venous drainage

There are several main venous drainage routes for the medulla oblongata.

  • The petrosal vein is one such vessel; it will end up draining to the superior petrosal sinus.
  • Both the anterior and anterolateral medullary veins drain into the corresponding spinal veins inferiorly.
  • There are also anterior and posterior median medullary veins that travel along their respective sulci to drain the medulla.

Development

The medulla oblongata forms in fetal development from the myelencephalon. The final differentiation of the medulla is seen at week 20 gestation.

Neuroblasts from the alar plate of the neural tube at this level will produce the sensory nuclei of the medulla. The basal plate neuroblasts will give rise to the motor nuclei.

Alar plate neuroblasts give rise to

  • The solitary nucleus, which contains the general visceral afferent fibers for taste, as well as the special visceral afferent column.
  • The spinal trigeminal nerve nuclei which contains the general somatic afferent column.
  • The cochlear and vestibular nuclei, which contain the special somatic afferent column.
  • The inferior olivary nucleus, which relays to the cerebellum.
  • The dorsal column nuclei, which contain the gracile and cuneate nuclei.

Basal plate neuroblasts give rise to

  • The hypoglossal nucleus, which contains general somatic efferent fibers.
  • The nucleus ambiguus, which form the special visceral efferent.
  • The dorsal nucleus of vagus nerve and the inferior salivatory nucleus, both of which form the general visceral efferent fibers.
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Nerves of Medulla Oblongata

Glossopharyngeal Nerve (IX)

  • This nerve has three functions from three different nuclei, the main motor nucleus, parasympathetic nucleus, and sensory nucleus. The motor nucleus is located in the nucleus ambiguous, found within the reticular formation of the medulla, just below the pontomedullary junction.
  • The nerve runs through the subarachnoid space, exiting the skull through the jugular foramen. The motor component of the glossopharyngeal nerve only supplies the stylopharyngeus muscle, which contributes to the gag reflex and helps elevate the pharynx when talking or swallowing.
  • The parasympathetic preganglionic fibers originate from the inferior salivatory nucleus, located in the pons. These fibers synapse in the otic ganglion and proceed to innervate the parotid glands to stimulate salivation. The sensory fibers of the glossopharyngeal nerve innervate the chemoreceptors and baroreceptors of the carotid body and send their information to the caudal nucleus solitaries. Cranial nerve IX sensory fibers also innervate the posterior one-third of the tongue, carrying taste, touch, and pain information to the rostral nucleus solitaires.

Vagus Nerve (X)

  • The tenth cranial nerve contains four nuclei in the medulla. The dorsal motor nucleus of the vagus nerve, near the floor of the fourth ventricle, is the largest nuclei of the vagus nerve, and it runs from the rostral to the caudal medulla. The dorsal nucleus receives afferent fibers from the hypothalamus and has efferent fibers sending parasympathetic signals to the heart, stomach, bronchi, esophagus, pancreas, and proximal intestines.
  • The motor nucleus is within the nucleus ambiguous, which is within the reticular formation of the medulla. The vagal fibers from the nucleus ambiguous innervate the muscles of the palate, pharynx, upper esophagus, and larynx and stylopharyngeus. The motor innervation controls speech and swallowing with the help of the glossopharyngeal.
    The general sensory fibers of the vagus that receives information from the pharynx, larynx meninges of the posterior fossa, and the region near the external auditory canal reach the caudal nucleus solitaries. The general visceral sensation from the chemoreceptors and baroreceptors of the aortic arch and digestive tract is carried back to the nucleus solitaries as well.
  • The gag reflex, cough reflex, and carotid sinus reflex occur via the combination of sensory and motor fibers of the vagus nerve from the nucleus solitaries. The taste sensation of the epiglottitis and posterior pharynx gets carried to the rostral nucleus solitaries.

Accessory Nerve Nucleus (XI)

  • The accessory nerve is the combination of a cranial nerve arising from the medulla and cervical roots up to C5. The cranial nerve nucleus starts from the nucleus ambiguous.
  • Nerve rootlets until C5 leave the lateral aspect of the spinal cord between the dorsal and ventral nerve roots and ascend through the foramen magnum to exit back through the jugular foramen and supply the sternocleidomastoid and trapezius muscles.

Hypoglossal Nerve (XII)

  • The hypoglossal nucleus is situated midline near the vagus nerve nuclei and caudal to the bottom of the fourth ventricle. It produces a bulge in the floor of the fourth ventricle, named the hypoglossal eminence/trigone. The hypoglossal nerve supplies the intrinsic muscles of the tongue and the styloglossus.
  • A lesion in the hypoglossal nerve nuclei causes ipsilateral tongue weakness, which manifests as the tongue deviating towards the weak side.

Function of Medulla Oblongata

The medulla oblongata connects the higher levels of the brain to the spinal cord and is responsible for several functions of the autonomous nervous system which include:

  • The control of ventilation via signals from the carotid and aortic bodies. Respiration is regulated by groups of chemoreceptors. These sensors detect changes in the acidity of the blood; if, for example, the blood becomes too acidic, the medulla oblongata sends electrical signals to intercostal and phrenic muscle tissue to increase their contraction rate and increase oxygenation of the blood. The ventral respiratory group and the dorsal respiratory group are neurons involved in this regulation. The pre-Bötzinger complex is a cluster of interneurons involved in the respiratory function of the medulla.
  • Cardiovascular center – sympathetic, parasympathetic nervous system
  • Vasomotor center – baroreceptors
  • Reflex centers of vomiting, coughing, sneezing, and swallowing. These reflexes which include the pharyngeal reflex, the swallowing reflex (also known as the palatal reflex), and the masseter reflex can be termed, bulbar reflexes.

References

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