The medulla oblongata or simply medulla is a long stem-like structure which makes up the lower part of the brainstem. It is anterior and partially inferior to the cerebellum. It is a cone-shaped neuronal mass responsible for autonomic (involuntary) functions, ranging from vomiting to sneezing. The medulla contains the cardiovascular center, the respiratory center, vomiting and vasomotor centers, responsible for the autonomic functions of breathing, heart rate and blood pressure as well as the sleep–wake cycle.

  • 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 anterior spinal and posterior inferior cerebellar arteries, including the solitary nucleus and other sensory nuclei and fibers.
  • Posterior spinal artery: This supplies the dorsal column of the closed medulla containing fasciculus gracilis, gracile nucleus, fasciculus cuneatus, and cuneate nucleus.

Development

The medulla oblongata forms in prenatal development from the myelencephalon, a secondary brain vesicle derived from the rhombencephalon. The final differentiation of the medulla is observed around 20 weeks of 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.

Function

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 phrenical 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.

Clinical significance

A blood vessel blockage (such as in a stroke) will injure the pyramidal tract, medial lemniscus, and the hypoglossal nucleus. This causes a syndrome called medial medullary syndrome.

Lateral medullary syndrome can be caused by the blockage of either the posterior inferior cerebellar artery or of the vertebral arteries.

Progressive bulbar palsy (PBP) is a disease that attacks the nerves supplying the bulbar muscles. Infantile progressive bulbar palsy is progressive bulbar palsy in children.

Other animals

Both Lamprey and Hagfish possess a fully developed medulla oblongata. Since these are both very similar to early agnathans, it has been suggested that the medulla evolved in these early fish, approximately 505 million years ago. The status of the medulla as part of the primordial reptilian brain is confirmed by its disproportionate size in modern reptiles such as the crocodile, alligator, and monitor lizard.

Additional images

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File:Cerebrum lobes.svg|Lobes

File:Medulla Oblongata and Cerebellum.svg|Cross section of the medulla (in red) and surrounding tissues.

File:Gray679.png|Anteroinferior view of the medulla oblongata and pons.

File:Gray724.png|Base of brain.

File:Gray768.png|Diagram showing the positions of the three principal subarachnoid cisternæ.

File:Medulla oblongata - posterior - very low mag.jpg|Micrograph of the posterior portion of the open part of the medulla oblongata, showing the fourth ventricle (top of image) and the nuclei of CN XII (medial) and CN X (lateral). H&E-LFB stain.

</gallery>

References