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What is the location and extent of medulla oblongata.

Medulla oblongata is the lowermost part of the brainstem (other parts are pons and midbrain). It is:

  • continuous below with the spinal cord and above with pons.
  • located in the  posterior cranial fossa  and is  related to clivus anteriorly and cerebellum posteriorly.
  • separated from the cerebellum by the cavity of fourth ventricle.

medulla oblongata - location

 

 Describe the external features of medulla oblongata.

 Medulla oblongata is

  • pyriform shaped, broad  in the upper part and narrow in the lower part.
  • approximately 3 cm in length.
  • divided into two part:
    • Lower closed part:  It has a central canal.
    • Upper open part : In this part the central canal widens and opens dorsally to form the lower half of the floor of fourth ventricle.

Ventral Aspect of Medulla Oblongata

The ventral aspect of medulla oblongata shows the following features from medial to lateral:

  • Anterior median fissure/sulcus in the median plane (divides ventral aspect into two symmetrical halves).
  • Pyramids: Elongated elevation produced by underlying corticospinal tract.
  • Anterolateral sulcus: Hypoglossal nerve rootlets emerge along this sulcus.
  • Olive: Oval elevation produced by underlying Inferior olivary nucleus.
  • Posterolateral sulcus: Rootlets of glossopharyngeal, vagus and cranial part of accessory nerves emerge along this sulcus from above downward.

Dorsal Aspect of Medulla Oblongata

Features of the dorsal surface of lower closed part of medulla oblongata from medial to lateral are:

  • Posterior median sulcus: In the median plane.
  • Fasciculus gracilis (lowerpart) and gracile tubercle (in the upper part): gracile tubercle is produces by the underlying nucleus gracilis, where the axons comprising fasciculus gracilis terminate.
  • Fasciculus cuneatus (lowerpart) and cuneate tubercle (in the upper part): cuneate tubercle is produces by the underlying nucleus cuneatus, where the axons comprising fasciculus cuneatus terminate.
  • Inferior cerebellar peduncle:  Connects medulla to cerebellum.

Dorsal surface of upper open part of medulla oblongata

Dorsal surface of upper part of medulla forms the lower half of the floor of fourth ventricle. The transversely running stria medullaris fibers separate the dorsal surface of medulla from that of pons. These fibers originate from arcuate nucleus ( dislodged pontine nuclei) and reach the cerebellum  via inferior cerebellar peduncle. Features of the dorsal surface of upper open part of medulla oblongata from medial to lateral are:

  • Median sulcus: in the median plane.
  • Medial eminence: shows two triangular areas, hypoglossal triangle above and vagal triangle below, which overlie the nucleus of  hypoglossal nerve and dorsal motor nucleus of vagus respectively. Below the vagal triangle lies area postrema ( chemoreceptor trigger zone (CTZ) which trigger vomiting in response to  presence of emetic substances in the blood). Area postrema is devoid of blood brain barrier.
  • Sulcus limitans: separates medial eminence from vestibular area.
  • Vestibular area: inferior and medial verstibular nuclei lie deep to this area.

Describe the internal structure of medulla oblongata.

Structural components forming grey and white matter of medulla oblongata are 

Grey MatterWhite Matter
Nuclei of last four cranial nerves

Glossopharyngeal
Vagus
Cranial part of accessory
Hypoglossal
Ascending tracts

Fasciculus cuneatus and Fasciculus gracilis
Anterior and posterior spinocerebellar
Lateral and anterior spinothalamic
Spinal tract of trigeminal
Vital centres

Cardiac centre
Respiratory center
Vasomotor centre
Descending tracts

Corticospinal
Rubrospinal
Tectospinal
Spinal nucleus of Trigeminal nerveMedial longitudinal fasciculus
Parts of vestibular and cochlear nuclei
Nucleus gracilis and Nucleus cuneatus
Inferior olivary nucleus, Arcuate nucleus and Accessory cuneate nucleus

Transverse sections of the medulla oblongata at the level of pyramidal decussation.

Transverse sections of the medulla oblongata at the level of sensory decussation

Transverse sections of the medulla oblongata at the level of olive (open part of medulla)

Describe the arterial supply of medulla oblongata.

The medulla oblongata is supplied by  the following arteries:

  • Vertebral arteries.
  • Anterior and posterior spinal arteries.
  • Anterior and posterior inferior cerebellar arteries.
  • Basilar artery.

Applied Aspects

Medial medullary syndrome

It occurs due to injury to the branches of anterior spinal artery supplying the paramedian region of medulla. Following structures are Affected:

  • Hypoglossal nucleus
  • Medial lemniscus
  • Corticospinal tract (pyramid)

structures involved in medial medullary syndrome

The symptoms are as follows:

Structure InvolvedResultant Signs and Symptoms
PyramidContraleral hemiplegia (UMN Paralysis)
Hypoglossal NucleusIpsilateral paralysis of tongue muscles (LMN paralysis)
Medial lemniscusContralateral loss of conscious proprioception-sense of position and vibration

 Lateral medullary syndrome

It occurs due to thrombosis of posterior inferior cerebellar artery,  which supplies a wedge-shaped area on the dorsolateral aspect of the medulla. The following structures are affected:

  • Vestibular nucleus
  • Inferior cerebellar peduncle
  • Spinal nucleus and tract of trigeminal
  • Nucleus ambiguus
  • Reticular formation
  • Lateral spinothalamic tract

structures affected in lateral medullary syndrome

Following are the signs and symptoms:

Structure InvolvedResultant Signs and Symptoms
Spinothalamic TractContralateral loss of pain & temperature sensation from the body below the head.
Spinal tract and nucleusIpsilateral loss of pain & temp sensation from the face.
Nucleus ambiguusIpsilateral paralysis of muscles of palate, larynx and pharynx
Vestibular NucleusVertigo
Inferior cerebellar peduncleIpsilateral ataxia
Descending sympathetic fibers in reticular formationIpsilateral Horner’s syndrome

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