MEDULLA OBLONGATA

The medulla oblongata is the portion of the brain stem located caudal to the pons and continuous with the spinal cord through the foramen magnum. It can be divided in a dorsal part or tegmentum (located dorsally, forms the floor of the fourth ventricle), a ventral part (formed by the olive and the pyramids) and a raphe (Middle wall with nuclei and crossed fibers).

 

            The trapezoid body[1] is formed by transverse fibers that, from the ventral cochlear nucleus, reach the ipsi- and contralateral dorsal nucleus of the trapezoid body  (this nucleus was formerly known as superior olivary nucleus). Axons from the dorsal cochlear nucleus form the acoustic stria that reaches also the ipsi- and contralateral dorsal nucleus of the trapezoid body. The neurons from this nucleus ascend toward the caudal colliculus forming the lateral lemniscus. From that colliculus, acoustic information reaches the medial geniculate body of the metathalamus through the brachium of the caudal colliculus. Some axons from the dorsal nucleus of the trapezoid body incorporate back to the vestibulocochlear nerve to inhibit receptorscells of the cochlear duct. These axons form the olivocochlear fascicle. Scattered among the transverse fibers of the trapezoid body are the ventral nucleui of the trapezoid body that receive axons from the ventral cochlear nucleus.

 

            The tegmentum is the portion of the medulla oblongata which is found ventrally to the fourth ventricle. The following nuclei and tracts are located in it:

 

  • Nucleus gracilis[2]

  • Medial cuneate nucleus[3]

  • Lateral cuneate nucleus[4]

  • Motor nucleus of the abducens nerve[5]

  • Motor nucleus of the facial nerve[6]

  • Nucleus ambiguus[7] (formed by the somatic motor nuclei of the glossopharyngeal nerve[8], the vagus nerve[9] and the neurons that form the cranial roots[10] of the accessory nerve)

  • Motor nucleus of the hypoglossal nerve[11]

  • Parasympathetic nucleus of the facial nerve[12]

  • Parasympathetic nucleus of the glossopharyngeal nerve

  • Parasympathetic nucleus of the vagus nerve

  • Vestibular nuclei[13] (the caudal vestibular nucleus is medial to the caudal cerebellar peduncle; the medial vestibular nucleus is located medial to the caudal vestibular nucleus; the lateral vestibular nucleus is dorsal to the caudal vestibular nucleus and the rostral vestibular nucleus is found in front of all the aforementioned vestibular nuclei)

  • Dorsal nucleus of the trapezoid body (Formerly known superior olivary nucleus)

  • Ventral nuclei of the trapezoid body (scattered among the transverse trapezoid fibers)

  • Solitary tract[14] descends to the cranial cervical spinal segments. Transmits signals from the walls of heart, vessels and intestine

  • Nucleus of the solitary tract [15] (consisting of the neuronal bodies of the solitary tract neurons)

  • Raphe nuclei[16]

  • Nucleus and spinal tract of the trigeminal nerve[17]

  • Solitary tract[18]

  • Rubrospinal tract[19]

  • Tectospinal tract[20]

  • Spinothalamic tract[21]

  • Dorsal spinocerebellar tract[22]

  • Ventral spinocerebellar tract[23]

  • Medial lemniscus[24]

  • Lateral lemniscus[25]

  • Dorsal longitudinal fasciculus[26]

  • Medial longitudinal fasciculus[27]

  • Ventral reticular nucleus[28]

  • Lateral reticular nucleus[29]

  • Superficial arcuate fibers[30]

  • Area postrema[31]

 

           

The following are localized in the ventral part:

 

[1] Derived from trapezoid. From the Greek trapezion, the diminutive of trapeza, “table”.

[2] From the Latin gracilis, “thin, fine”. It is, along with the medial cuneate nucleus, the origin of the fibers that form the medial lemniscus.

[3] From the Latin cuneus, “wedge-shaped”.

[4] The lateral cuneate nucleus, or accessory nucleus, is part of one of the spinocerebellar tracts. It receives propioceptive fibers from the spinal roots of the brachial plexus and its axons are projected toward the ipsilateral portion of the cerebellum, via the caudal cerebellar peduncle.

[5] From the Latin, abducens, “drawing away”.

[6] Its neurons innervate the facial muscles.

[7] It forms a column of cells that are localized ventromedially to the nucleus of the spinal tract of the trigeminal nerve. It innervates the striated muscles of the pharynx, larynx and esophagus.

[8] From the Greek glossa, “tongue”; pharynx, “pharynx”.

[9] From the Latin vagari, “vagabond”. The tenth cranial nerve receives this name because of its long path and ample distribution. It was described by Marino around 100 A.D. The name was coined by the Italian anatomist, Domenico de Marchetti (1622-1688).

[10] Fibers from the cranial roots of the accessory nerve join the vagus nerve and form part of the recurrent laryngeal nerve.

[11] From the Greek hypo, “below”; glossa, “tongue”.

[12] Located caudal to the genu of the facial nerve, in front of the parasympathetic nuclei of the glossopharyngeal and vagus nerves. It innervates the lacrimal gland, the nasal glands and the mandibular and sublingual salivary glands.

[13] The vestibular nuclei are found on the ventrolateral surface of the fourth ventricle.

[14] Descends to the cranial cervical spinal segments. Transmits signals from the walls of heart, vessels and intestine.

[15] Dorsolateral to the parasympathetic nuclei of the facial, glossopharyngeal and vagus nerves. It is connected to the general visceral efferent column and interneurons in the reticular formation. It participates in visceral reflex activity and in regulating visceral functions.

[16] From the Greek raphe, “suture”. The axons from the raphe nuclei release serotonin and they decrease the excitation threshold of the neurons that transmit painful stimuli.  They also influence the excitability of the alpha motor neurons.

[17] The nucleus of the spinal tract of the trigeminal nerve is located medially to the spinal tract of the trigeminal nerve. Both extend caudally from the sensory pontine nucleus of the trigeminal nerve towards the first two cervical segments of spinal cord, where they mix with the dorsolateral fasciculus, the marginal nucleus and the gelatinous substance of the spinal cord. Transmits touch and pressure impulses.

[18] The solitary tract is formed by the sensory (afferent) visceral axons of the facial, glossopharyngeal and vagus nerves.

[19] The rubrospinal tract was described by Von Monakow. It is composed of fibers that originate in the red nucleus, cross in the ventral portion of the mesencephalic tegmental decussation, and terminate in the ventral horn of the spinal cord.

[20] The tectospinal tract is a compilation of fibers that decussate in the dorsal portion of the mesencephalic tegmental decussation and descend from the rostral colliculus, ventrally to the medial longitudinal fasciculus, to reach the spinal cord. They establish connections with motor neurons of the cranial and spinal nerves.

[21] The spinothalamic tract was described by Edinger. In the spinal cord, it is located in the lateral and ventral funiculi. It transmits tactile, thermal and painful impulses to the thalamus.

[22] The dorsal spinocerebellar tract transmits proprioceptive information to the cerebellum. In the spinal cord, it is located in the lateral funiculus.

[23] The ventral spinocerebellar tract transmits proprioceptive information to the cerebellum. In the spinal cord, it is located in the lateral funiculus.

[24] It is a bundle of myelinated axons that decussate in the brainstem giving rise to the deep arched fibers, and ascend to the thalamus. They transmit proprioceptive impulses from the gracilis and medial cuneate nuclei.

[25] It is a bunch of fibers that carries sound from the dorsal nucleus of the trapezoid boy to the caudal colliculus

[26] Schütz’s fasciculus. Between hypothalamus and visceral general efferent nuclei.

[27] Connects the vestibular nuclei with eye motor nuclei and extends through the spinal cord in the ventral funiculus

[28] To mediate in respiration along with the lateral reticular nucleus.

[29] The lateral reticular nucleus, also named dorsolateral olivary nucleus. It is located at the most caudal part of the medulla oblongata. It receives fibers from the red nucleus and spinal cord. Its axons join the superficial arcuate fibers to enter the cerebellum via the corpus restiforme as mossy fibers. It receives spinoreticular neurons controlling posture, reaching, grasping, locomotion, scratching and respiration.

[30] They emerge from the lateral reticular nucleus and join the caudal cerebellar peduncle.

[31] Corresponds to a circumventricular organ with chemoreceptors involved in vomiting.

[32] The olivary nucleus was described by Willis and named corpora olivaria by Vieussens. The surface of this formation is referred to as olive. It receives central motor fibers. Its axons cross the midline and reach the contralateral cerebellar hemisphere via the caudal cerebellar peduncle.

[33] They are two prominent longitudinal columns separated by the ventral medial fissure. They are composed of descending motor fibers forming the pyramidal or corticospinal tract.. It is made up of corticonuclear and corticospinal fibers that, from the primary motor cortex, reach the motor nuclei of the cranial nerves and spinal nerves, respectively.

 

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