Brain Stem (Truncus Encephali)

The brainstem or Truncus encephali in Latin is a brain structure located between the medulla and the spinal cord (1). It is the base of the brain. It consists of the midbrain, the medulla oblongata or the long medulla, the Varoli’s bridge, and the spinal cord.

The function of the brainstem is to receive, process, and adjust certain functions related to attention, vision, sleep, hearing, arousal, and temperature control.

Finally, brain stem or Truncus encephali has an important role in maintaining body position and controlling muscle tension (1).

Structure of the brain stem

Parts of the brainstem are:

  • The Midbrain (Mesencephalon)
  • The Varoli’s bridge (Pons)
  • The Medulla (medulla oblongata) (1)
  • The spinal cord (Medulla spinalis) – the brain stem does not contain it, but it is continuous to it.

The Midbrain (Mesencephalon)

The midbrain is part of the brain stem that has a complex structure. Its key peculiarity is the presence of both gray and white mass which connect the fourth to the third brain chamber (2).

This is part of the brain stem that has many autonomic nervous system centers. It also consists of two pairs of nodules. Those are the lower ones and the upper ones. The first ones have an auditory role, while the latter has a role in visual information processing. More precisely, they receive information from the retina and help adjust the eye position. 

The Varoli’s Bridge

The Varoli’s Bridge is the largest part of the brainstem. It is also called Pons. It is located above the medulla oblongata. Also, its position is below the midbrain. Varoli’s bridge connects the cerebellum and the midbrain to build a brain stem. That’s why it is called pons (“bridge”).

Its function is maintaining balance and position of the head and body in space. It receives impulses from the receptors important for maintaining the balance of the middle ear, adjusts facial mimicry, receives stimuli from the skin on our face, ears, and teeth.

It is important for chewing and sucking, as well as defense reflex centers for tears and blinking. It also houses a center that interrupts spontaneous inhalation and thus regulates breathing frequency.

The Medulla Oblongata

Medulla oblongata connects the brain and the spinal cord and forms part of the central nervous system. Its function is important because it houses the centers that control reflex functions such as breathing, digestion, blood flow, blood pressure, coughing, swallowing, etc.

The medulla oblongata is colloquially called “node of life”. It is one of the key parts of the central nervous system. Together with the Varoli’s Bridge and the midbrain, it forms the brain stem. It extends to the spinal cord and connects it to the Varoli bridge.

The structure of the elongated medulla is very similar to that of the spinal cord. However, due to the crossing of the pyramidal fibers, the gray mass does not have a butterfly shape that is characteristic of the spinal cord.

The medulla oblongata has a conical shape. As a result, its width decreases, while it inferiorly extends. When it comes to its dimensions, it measures 3cm x 2cm (length x largest width).

The medulla oblongata is often referred to as the node of life because it contains automatic centers that regulate the respiratory centers, the diameter of blood vessels (and therefore the blood pressure itself), and heart function.

These centers are housed in the evolutionarily oldest part of the brainstem, a reticulated structure that consists of complex “nets” of nerve cells.

All automatic centers receive information from the periphery of the body and, accordingly, change the breathing rate, heart rate per minute, and blood pressure. In addition to these centers in the extended brain, there are centers for swallowing, secretion of saliva, vomiting, sneezing, and coughing.

The Spinal Cord

The spinal cord is one of the most important parts of our body responsible for its proper functioning. Namely, it transfers the nerve impulses from our body to the brain and the other way around. As a result, it heavily affects our muscles, movements, etc.

Even though the spinal cord is not a part of the brain stem, we want to elaborate on this part of the CNS in this article, as these two structures are connected and represent a continuation of one another.

The spinal cord is surrounded by the CSF (cerebrospinal fluid), which acts as a protective factor that protects the delicate nerve tissues from damage that causes the spinal cord to collapse. 

The anatomy of the spinal cord itself is made up of millions of nerve fibers that transmit electrical information from the brain to the limbs, torso, and organs, and vice versa.

The nerves that exit the spinal cord on the upper part, inside the neck, control the hand on that side. Nerves coming out of the spinal cord in the middle and lower back control the torso and arms, as well as the bladder, womb, and sexual functions.

The nerves that transmit information from the brain to the muscles are called neurons. The nerves that transfer information from the body back to the brain are called sensory neurons. Sensory neurons transmit information to the brain regarding skin temperature, touch, pain, and joint position.

The brain and spinal cord are also called the CNS (central nervous system), while the nerves that connect the spinal cord to the body are called PNS (peripheral nervous system).

Functions of the Brain Stem

As already mentioned, the brainstem is an extremely important part of the central nervous system. Consequently, it has different functions. There are three main groups of brain stem roles. Those are:

  1. The link between the spinal cord and higher centers in the brain
  2. Houses reflex centers (as a result, it controls consciousness, respiration, and cardiovascular system)
  3. Houses the cranial nerve nuclei (from III to XII).

Consequently, this part of our brain plays an important role in the regulation and monitoring of the following functions:

  • Breathing
  • Alertness
  • Arousal
  • Digestion
  • Pupil dilation and contraction
  • Perspiration
  • salivation
  • Blood pressure
  • Attention
  • Heart rate
  • Urination.

Moreover, the brain stem is important because it sends information between the spinal cord and the peripheral nerves, and connects them to the brain.

Lesions and Damages of the Brain Stem

Even minor damages and lesions on the brain stem result in serious consequences. The reason is rather logical. Namely, the neural structures that are found in this part of the brain are of the utmost importance.

That’s why some brain stem injuries can jeopardize the patient’s life. Moreover, different speech disorders and dysphagia are often caused by damage to the brain stem. Similarly, vestibular and respiratory disturbance occurs due to a brain stem lesion.

Abnormal consciousness can also be a result of a tumor, a trauma, or infection of this part of the brain. Besides, doctors suspect a stroke that affected the brain stem or a condition called demyelination.

Depending on the affected area, be it the medulla oblongata, or the pons, etc., different disorders take place. Those vary from breathing issues that can lead to death as well as heart problems that can also be lethal.

Brain stem compression can result from swelling and this condition can lead to a stroke. In addition, these conditions often result in the above-mentioned disorders, including sleep apnea, swallowing difficulties, and speech impairment.

Again, depending on the damaged area, patients can experience memory loss or even personality changes after an injury or infection in a certain part of the brain stem.

Coma, paralysis and a vegetative status are some of the most difficult outcomes of a brain stem injury. Most noteworthy, many experts consider that total loss of brainstem function equals to brain death.

Conclusion

The brainstem is positioned in the posterior part of the brain. It is closely related to the spinal cord (medulla spinalis) as it structurally continues on this segment of our CNS. It contains the midbrain (mesencephalon), the pons (Varoli’s bridge) and the medulla oblongata.

The brain stem is a small structure. However, it has many different, important roles in our nervous system and is directly responsible for hundreds of every day, crucial functions our body performs.

Its key role is in ensuring the communication between the nerves, the spinal cord, our body, and our brain. It houses the connections between the sensory and the motor systems.

It thus allows an impeccable communication between the CNS and the peripheral nervous system. Most importantly, it regulates respiratory and cardiac function. It dictates our consciousness and sleeping.

That’s why any injury of the brain stem can lead to serious consequences, including death. Lesions, infections, physical injuries, and strokes that affect the parts of the brain stem structures can lead to speech impairment, coma, vegetative states, paralysis, or memory loss.

References:

1. Beissner F, Baudrexel S. Investigating the human brainstem with structural and functional MRI. Front Hum Neurosci. 2014 Feb 28;8:116. doi: 10.3389/fnhum.2014.00116. PMID: 24616692; PMCID: PMC3937611.  Found online at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3937611/

2. Batista-García-Ramó K, Fernández-Verdecia CI. What We Know About the Brain Structure-Function Relationship. Behav Sci (Basel). 2018 Apr 18;8(4):39. doi: 10.3390/bs8040039. PMID: 29670045; PMCID: PMC5946098.  Found online at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5946098/