The brain is almost always associated with a kind of oval with a rough surface full of folds, but underneath this cerebral cortex there is a multitude of very important structures.

In fact, if we were to consider the importance of each of the parts of the brain by judging them according to how relevant they are to our survival, we would conclude that the most fundamental structure is one that is neither cortex-shaped nor oval-shaped. S e is the brain stem, or encephalic trunk, located in the lowest part of the brain and in direct contact with the spinal cord.

What is the brain stem?

The brain stem, sometimes called the brain stem , is a cylindrical or elongated cone-shaped part of the brain that is located between the rest of the brain and the spinal cord . This means that the brain stem is aligned with the neural fibers that run along the lower spinal cord down the spine, specifically in front of the cerebellum.

Therefore, it is the part of the brain that is in an anatomically lower position and closer to the neck. In addition, most of the cranial pairs (or cranial nerves) come from the brain stem.

The brain stem is made up of both parts of white substance and some areas where grey matter predominates, which means that it runs through both connection areas and areas where the bodies of the neurons are concentrated forming control nuclei.

Brain stem functions

Although the brain stem is attached to the spinal cord and because of its shape can be confused with an extension of it, its main function is not to act as a simple bridge between the brain and the nerves that run through the human body .

The brain stem is the part of the human brain that houses the most primitive and ancient functions, and it appeared in our evolutionary line in species that looked nothing like human beings. It is part of what, according to Paul MacLean’s theory of the 3 brains, has been called the “reptilian brain” , precisely because it has been associated with ancient physiological processes (although MacLean’s ideas are not considered valid, among other things, because they are based on a very simplified view of the evolution of the human brain.

Thus, the brain stem is responsible for performing the most basic nervous system tasks for our survival, those that we can barely influence voluntarily and that have been automated over millions of years of evolution precisely so that our ill-advised decisions or distractions do not cost us our lives.

Maintaining vital signs

Among the functions in which the brain stem plays a fundamental role are the regulation and maintenance of heart rhythm and the automatic control of breathing . That is why the brain stem is composed of vital centres that when damaged can cause immediate death.

Other slightly less important but almost as primitive functions of the brain stem are the control of hiccups, sneezing and coughing, sucking, swallowing, vomiting and pain sensitivity . It also has a very important role in regulating the levels of arousal . Specifically, a network of neurons distributed in part by the brain stem called reticular formation intervenes both in the regulation of the circadian cycle (sleep-wake) and in the maintenance of consciousness.

A bridge to the spinal cord

In addition to all these functions, of course, the brain stem serves to communicate the cranial nerves and the spinal cord with the brain , thus being the communication pathway between the brain and the rest of the body in both the afferences and the efferences. This is a more passive role than the previous ones, but equally essential for the survival of the brain and the whole body in general.

Parts of the brain stem

The brain stem is composed of three main structures: the midbrain, the brain stem bridge and the spinal cord .

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Midbrain

The midbrain is the structure of the brain stem that is located in a higher position and therefore closer to structures located at the top, such as the thalamus. Like other parts of the brain stem, it is involved in such primitive functions as regulation of the sleep-wake cycle and body temperature, but it also plays a role in reacting quickly to visual and auditory stimuli in a reflex manner, as well as in controlling certain movements.

The two basic components of the midbrain are structures called tectum and tegmentum .

Trunk brain bridge

The trunk brain bridge , or Varolio bridge , is located just below the midbrain and above the spinal cord. On its posterior face (the one closest to the neck) is the cerebellum. This structure is the bulkiest part of the brain stem, and its front side sags outward like half an egg.

This part of the brain stem is involved in the control of breathing, in the transition between sleep phases and in the regulation of the level of consciousness , among other basic survival processes.

Spinal cord

The spinal cord (or medulla oblongata ) is located in the lower part of the brain stem. It controls all kinds of automatic processes that are absolutely necessary for survival, such as cardiac control or gastric secretion . In addition, it is the part that communicates with the spinal cord directly.

In addition, it is in this part of the brain stem that the decusation of the pyramids is found, that is, the point at which the nerve fibres change from the hemicorpus to the right to the left and vice versa (which explains that one half of the body is controlled by the opposite half of the brain).

If you want to know more about the spinal cord and its parts, this article may interest you: “Spinal cord: anatomical structure and functions”.