Medulla Oblongata

We arrive at everyone’s favorite part of the brain, the medulla oblongata! This infamous part of the hindbrain controls various autonomic nervous system functions including respiration, digestion, heart rate, functions related to the blood vessels, swallowing, and sneezing.

Being a part of the brainstem, the medulla oblongata is essential to communications between the peripheral and central nervous systems as nearly all neuronal signals are sent through this organ.

The medulla oblongata is the most inferior part of the brainstem (no offense, “inferior” in this context means “lowest”), anterior to (in front of) the cerebellum. It’s one of three main regions of the brainstem, the other two being the pons and midbrain, in ascending order.

Within the medulla are vital ascending and descending nerve tracts along with important brainstem nuclei. The medulla is comprised of both myelinated (white matter) and unmyelinated (gray matter) nerve fibers.

Anatomy of the Medulla Oblongata

The position of the medulla oblongata enables it to be the primary connection of the central nervous system to the peripheral nervous system as it is continuous with the spinal cord. The medulla oblongata and spinal cord merge at the opening located at the base of the skull, the foramen magnum. Within the medulla oblongata are both white matter and gray matter.

The nerves of the white matter are myelinated, meaning that they are covered in a myelin sheath made up of lipids and proteins. These are differentiated from the gray matter, since the gray matter is unmyelinated, having no protective sheath.

The function of this covering is to insulate the axons and enhances the conductivity of neuronal impulses traveling along the neurons and across synapses. However, despite the glamorous enhancements that are available for the white matter, the gray matter gets to serve as the physical connection of the brainstem to four cranial nerves.

What is special about this combination in the medulla oblongata is that the two come together to produce a part of the reticular formation, a network of nerve fibers that can be found in the pons. This network plays an integral role in the transmission of motor and sensory signals – the ones in the medulla oblongata specifically are involved in controlling autonomic nervous system functions.

The medulla’s connection to the reticular formation is how it controls respiration, heart rate, and digestive functions. There are also numerous neurons in the medulla oblongata that control somatosensory signals and are a part of the sleep-wake cycle, or Circadian rhythm.

The upper region of the medulla oblongata comprises the fourth ventricle, one of the structures of the ventricular system that holds cerebrospinal fluid (CSF). The fourth ventricle is continuous with the cerebral aqueduct to allow the flow of CSF from the anterior and superior ventricles and cisterns into the spinal cord.

The lower region of the medulla narrows to partially form the central canal of the spinal cord. This central canal fills is a hollow tube that runs along the length of the spinal cord to fill with CSF. The function of CSF is to protect the brain and spinal cord by providing a buffer to protect these vital organs of the nervous system from the shock of sudden impacts or movements.

Miscellaneous Anatomical Features of the Medulla Oblongata

There are a few more key anatomical features that may help you to identify the medulla oblongata:

  • Median fissures: This can also be referred to as the midline fissure. These are shallow grooves that are found on the anterior and posterior parts of the medulla oblongata. The posterior median fissure is known as the posterior median sulcus (sulcus meaning “shallow groove”). This sulcus begins at the end of the fourth ventricle and continues down into the sulcus of the spinal cord. On either side are the fasciculus cuneatus and the fasciculus gracilis*. The one on the anterior side is simply referred to as the median fissure. The median fissure is continuous with the median sulcus of the spinal cord.
  • Olive: The olive, more formally called the olivary body, is a pair of oval structures on the surface of the medulla oblongata that contains nerve fibers that connect the medulla to the pons and cerebellum. The olive plays a role in the control of motor functions, learning, and auditory sensation. More specifically, the inferior portion of the olives is involved with motor activity and learning, and the superior portion is a part of the control of auditory sensation.
  • Pyramid: These are yet another two round masses of nerve fibers located on either side of the median fissure (the anterior one) The pyramid connects the medulla to the spinal cord, pons, and cerebral cortex. They extend down the length of the medulla and appear as ridges. The pyramids are composed of corticospinal fibers each having about 1,000,000 nerves! These ultimately cross in a process called the decussation of the pyramids. They then move transition into the white matter of the spinal cord and become the lateral corticospinal tract.

*The fasciculus gracilis (also referred to as the gracile fasciculus) is a bundle of nerve fibers that connect the brainstem to the sacral, lumbar, and lower thoracic vertebrae, transmitting information on tactile and proprioception (awareness of the position of particular body parts). The fasciculus cuneatus is another collection of nerve fibers that connects the brainstem to vertebrae, however, these nerves connect to the upper thoracic and cervical vertebrae.

Another bonus of being a part of the brainstem is the medulla’s connections with cranial nerves. The cranial nerves associated with the medulla oblongata are the glossopharyngeal, vagus, accessory, and hypoglossal nerves.

Glossopharyngeal Nerve

The glossopharyngeal nerve is the 9th of the twelve pairs of cranial nerves. Its functions are quite diverse, as it takes part in the transmission of sensory information, parasympathetic nervous activity, and motor activities.

Regarding sensory functions, the glossopharyngeal nerve innervates (supplies with nerves) the oropharynx (portion of the pharynx that is located between the soft palate and hyoid bone), the carotid body (cluster of chemoreceptor cells in the fork of the carotid artery), sinus, posterior third of the tongue, middle ear cavity and the Eustachian tube. (Phew, that was a long list! Take a breather – now let’s get back to it.)

This nerve controls the parasympathetic nervous system by supplying nerve fibers to the parotid gland (salivary glands), and motor activity by innervating the stylopharyngeus muscle (a muscle of the pharynx).

Vagus Nerve

This nerve comes immediately after the glossopharyngeal nerve, being the 10th of twelve cranial nerves. It is associated with the pharyngeal arches and is also involved in many diverse functions.

The vagus nerve is involved in sensory processes including the reception of sensory signals via the external acoustic meatus and internal surfaces of the larynx. It also provides visceral sensation to the heart and abdominal viscera, and taste to the epiglottis and tongue.

Regarding motor functions, the vagus nerve innervates the muscles of the pharynx, soft palate, and larynx. For parasympathetic nervous functions, this nerve innervates the muscles of the trachea, bronchi, gastrointestinal tract, and heart.

Accessory Nerve

The accessory nerve falls in line behind the vagus nerve as the 11th nerve of the twelve cranial nerves. It is involved in the control of somatic motor activity by innervating the sternocleidomastoid and trapezius muscles.

Hypoglossal Nerve

The hypoglossal nerve is the 12th of the twelve pairs of cranial nerves. This nerve’s function is focused on somatic motor activity, specifically controlling all muscles of the tongue.

(Something to help you remember the function of this nerve is the breakdown of its name: hypo means “under” and glossal means “tongue.”)

Functions of the Medulla Oblongata

The medulla regulates many of the vital functions of the human body including respiration, heart rate, and blood pressure. Regarding its role in the circulation of blood throughout the body, the medulla oblongata works with the nucleus of the solitary tract.

The nucleus of the solitary tract – also known as the solitary nucleus, nucleus tractus solitarii (NTS) – is located in the dorsomedial medulla and is the first site through which sensory information related to taste and visceral afferent fibers (nerves going to the internal organs) pass.

This structure allows the medulla to control cardiovascular, respiratory, and gastrointestinal functions. It also receives signals from baroreceptors located in the walls of blood vessels.

Baroreceptors enable the NTS, and therefore, the medulla, to detect the constriction and expansion of the blood vessels which informs the regulation of blood pressure.

The medulla also controls the reflex of vomiting through the area postrema, a paired structure of the medulla which has been known to play a part in vomiting for over 40 years.

This chemoreceptor is not protected by the blood-brain barrier, which is exactly what allows it to be able to detect toxic substances in the bloodstream (as you know, the goal of vomiting is to rid the body of toxic or otherwise harmful substances).

The medulla oblongata is a great example of the efficiency and dynamic capabilities of the nervous system, and especially of those in the brainstem – both individually and collectively.

References

  1. Bailey, R. (2019, May 23). What is the medulla oblongata?. Retrieved October 30, 2019, from https://www.thoughtco.com/medulla-oblongata-anatomy-373222
  2. BioTutorials. (n.d.). Spinal cord – Central canal. Retrieved from https://psych.athabascau.ca/html/Psych402/Biotutorials/15/canal.shtml
  3. Cutsforth-Gregory, J. K., & Benarroch, E. E. (2017). Nucleus of the solitary tract, medullary reflexes, and clinical implications. Neurology, 88(12), 1187-1196. doi:10.1212/wnl.0000000000003751
  4. Healthline’s Medical Network. (2015, March 4). Medulla oblongata function, definition & location | Body maps. Retrieved October 30, 2019, from https://www.healthline.com/human-body-maps/medulla-oblongata#1
  5. Healthline’s Medical Network. (2015, March 4). Olivary nucleus function, anatomy & diagram | Body maps. Retrieved October 30, 2019, from https://www.healthline.com/human-body-maps/olivary-nucleus#1
  6. Healthline’s Medical Network. (2015, March 4). Pyramid. Retrieved October 30, 2019, from https://www.healthline.com/human-body-maps/pyramid#2
  7. Jalan, M. (2019, October 21). Medulla oblongata: Definition, structure and functions. Retrieved October 30, 2019, from https://www.scienceabc.com/humans/medulla-oblongata-definition-structure-and-functions.html
  8. Leanage, N. (2019, January 28). The vagus nerve (CN X). Retrieved October 30, 2019, from https://teachmeanatomy.info/head/cranial-nerves/vagus-nerve-cn-x/
  9. Miller, A. D., & Leslie, R. A. (1994). The area postrema and vomiting. Frontiers in Neuroendocrinology, 15(4), 310-320. Retrieved from https://www.ncbi.nlm.nih.gov/pubmed/7895890
  10. Neurosci. (n.d.). Proprioception – definition | Neuroscientifically challenged. Retrieved October 30, 2019, from https://www.neuroscientificallychallenged.com/glossary/proprioception
  11. Radhakrishnan, A. (2019, May 18). The accessory nerve (CN XI). Retrieved October 30, 2019, from https://teachmeanatomy.info/head/cranial-nerves/accessory/
  12. Radhakrishnan. (2019, September 18). The hypoglossal nerve (CN XII). Retrieved October 30, 2019, from https://teachmeanatomy.info/head/cranial-nerves/hypoglossal/
  13. ScienceDirect. (n.d.). Gracile fasciculus. Retrieved October 30, 2019, from https://www.sciencedirect.com/topics/neuroscience/gracile-fasciculus