What Is the Brainstem?

The brainstem is the part of the brain that connects the front part of the brain (cerebrum) to the spinal cord and the cerebellum. It is made of the midbrain, pons, and medulla.

1. What Does the Brainstem Do?

The brainstem serves as a communication point between the body, the spinal cord, and the brain through ascending and descending sensory and motor pathways [1].

Ascending pathways are groups of nerve fibers that bring sensory and motor information from the body (peripheral nervous system) to the brain through the spinal cord, while descending pathways work in the opposite way.

Ascending and descending pathways are also known as ascending and descending tracts of the spinal cord.

The brainstem also contains most cranial nerve nuclei (group of neurons), and therefore, is involved in the processing of efferent (coming from) information of the head.  

As the brainstem is made of the midbrain, pons, and medulla, it performs its functions through these structures.

2. What Does the Midbrain Do?

Midbrain or mesencephalon is the most rostral part (top) of the brainstem, found just below the cerebral cortex and above the pons. It is involved in vision, hearing, sleep, wakefulness, temperature regulation, alertness (arousal), and motor control [2]. 

It is composed of the tectum, the tegmentum, and cerebral peduncles, and receives blood supply from the basilar artery, superior cerebellar artery, and posterior cerebellar artery.

The tectum is divided into the superior colliculi that contain nuclei (group of neurons) involved in visual (vision) reflexes and the inferior colliculi involved in auditory (hearing) reflexes.

The tegmentum is divided into the substantia Negra where dopamine is released to the basal ganglia, and the red nucleus, which is involved, together with the cerebellum, in movement coordination.

The cerebral peduncles are paired structures separated by the interpeduncular cistern (Filled with cerebrospinal fluid) that connects the brainstem to the thalami (plural of the thalamus).

Lesions affecting the midbrain such as infarct, tumor, hemorrhage, or infections, can result in neurological disorders, including Benedikt syndrome, Claude syndrome, Weber syndrome, or Nothnagel syndrome.

These neurological disorders are characterized by palsies (paralysis) and ataxia (movement incoordination).

3. What Does the Pons Do?

The pons or pons Varolii is the structure of the brainstem located between the midbrain and the medulla (Medulla oblongata) and serves as a relay between the cerebral hemispheres and the cerebellum [3].

It is divided into the basilar part of the pons (Basis pontis) and the pontine tegmentum and receives blood supply from branches of the superior cerebellar artery and the pontine branches of the basilar artery.

The basilar part of the pons contains the pontine nuclei (cluster of neurons) that relay motor information coming from the motor cortex in the frontal lobe to the cerebellum. This activity is mediated by the corticofugal fibers.

The pontine tegmentum contains nuclei involved in sleep and wakefulness, breathing, and alertness.

It also contains nuclei of the cranial nerves, trigeminal (V), abducens (VI), facial (VII), and vestibulocochlear (VIII), and therefore, is also involved in the functions of these trigeminal nerves.

Lesions affecting the pons such as infarct, hemorrhage, tumors, or infections, can lead to the following neurological disorders:

• Gasperini Syndrome
• Facial Colliculus Syndrome
• Millard-Gubler Syndrome
• Brissaud-Sicard Syndrome
• Raymond Syndrome
• Foville Syndrome
• Marie-Foix Syndrome
• Locked-In Syndrome
• Tuberculosis Rhombencephalis

These neurological disorders are characterized by palsies (paralysis), ataxia, and symptoms associated with alterations in the function of some cranial nerves, such as loss of taste, loss of pain, temperature sensations on the face, or hearing loss.

4. What Does the Medulla Do?

The medulla (Medulla oblongata) is the part of the brainstem located between the pons and the spinal cord. It is responsible for the body’s involuntary functions (autonomous nervous system) such as cardiovascular function, heart rate, breathing, sleep-wake cycle, vomiting, sneezing, and swallowing [4].

The medulla is composed of the ventral medulla and the medulla tegmentum and receives its blood supply from the posterior inferior cerebellar artery (PICA), the anterior spinal artery, and branches of the distal vertebral artery.

The ventral medulla contains the olive, the pyramidal tracts, and the rootlets (nerve fibers) of the cranial nerve IX (Glossopharyngeal nerve) and XII (Hypoglossal nerve).

The olive is involved in movement coordination and in sound perception, while the pyramidal tracts are involved in the movement of muscles, and the rootlets of cranial nerve IX and XII carry away information from the central nervous system (brain and spinal cord) to the tongue, pharynx, larynx, parotid gland, and sinus.

The medulla tegmentum contains the inferior olivary nuclei and the nuclei of the cranial nerves IX to XII.

Lesions affecting the medulla oblongata such as infarct, hemorrhage, tumors, or degeneration, lead to the following neurological disorders:

• Babinski-Nageotte Syndrome
• Opalski Syndrome
• Wallenberg Syndrome
• Déjerine syndrome
• Wallerian Degeneration
• Demyelinating lesions
• Hypertrophic Olivary Degeneration
• Brainstem Glioma
• Syringobulbia

These neurological disorders manifest with symptoms such hemiparesis/hemiplegia, paralysis of parts of the body (e.g., tongue, soft palate, the pupil of the eye), ataxia, sensory deficits, and slurred speech.

5. What Are the Developmental Anomalies of the Brainstem?

Developmental anomalies of the brainstem are disorders that are originated during embryonic development of the brainstem [4].

The main known developmental disorders of the brainstem are Möbius syndrome (congenital facial diplegia syndrome) and axonal guidance disorders.

5.1. What Is Möbius syndrome?

Möbius syndrome or congenital facial diplegia syndrome is due to the underdevelopment or absence of the nuclei of the abducens cranial nerve (VI) and the facial cranial nerve (VII) resulting in paralysis of one side (unilateral) or both sides (bilateral) of the face muscles [5].

5.2. What Are Axonal Guidance Disorders of the Brainstem?

This group of disorders is associated with abnormal white matter tracts in the brainstem such as ectopic white matter tracts or anomalies in their location [6].

These disorders result in disorders such as horizontal gaze palsy, Joubert syndrome, progressive scoliosis, and pontine tegmental cap dysplasia [4].

Conclusion

The brainstem is made of the midbrain, pons, and the medulla oblongata and plays an essential role in communicating information between the body, the spinal cord, the cerebellum, and the brain.

This information includes instruction related to vision, hearing, sleep, wakefulness, temperature regulation, alertness (arousal), motor control, and the autonomous nervous system.

Lesions such as those caused by trauma, hemorrhage, tumors, or infections can affect the different parts of the brainstem (midbrain, pons, and the medulla oblongata) resulting in paralysis of parts of the body, ataxia, and sensory deficits.