Varoli’s Bridge

The brainstem also called the Truncus encephali is a brain part located in between the medulla and the spinal cord. It is divided into four different parts namely the medulla oblongata, the midbrain, the spinal cord and the varoli's bridge (2).

The varoli’s bridge is the largest section of the brain stem and is also called Pons Varolii in Latin. The two words Pons Varolii are derived from “Pons” which means bridge in Latin and “Varoli” is the name of the Italian surgeon Costanzo Varolio who discovered this part of the brain between the years 1543 and 1575 (1).

According to Costanzo Varolio an Italian anatomist and surgeon, the varoli's bridge are found inferior to the midbrain, superior to the medulla oblongata and anterior to the cerebellum.

Moreover, it is called a “bridge” because it links the cerebellum to the midbrain to form a brain stem (1). The main function of the varoli's bridge is to maintain the balance and position of the head and body. Besides, Varoli's Bridge contains parts like neural pathways and tracts.

The neural pathway conveys signals from the brain to the medulla and cerebellum, while the tracts transport the sensory waves to the thalamus.

Structure of the Varoli’s Bridge

The pons is found in the brainstem located in the middle of the midbrain and the medulla oblongata, just in front of the cerebellum. The inferior pontine sulcus is the groove that separates the varoli’s bridge and the medulla (2). On the other hand, the superior pontine sulcus detaches the Valori's bridge from the midbrain.

According
to scientists, the varoli’s bridge is divided into two main parts, the basilar
part also known as the ventral pons and the pontine tegmentum commonly called
the dorsal pons.

1. Basilar part of the varoli’s bridge

It is the ventral part of the pons and any form of tissue damage in this part may result in impairment of motor functioning. Furthermore, the ventral pons has the corticospinal tract that runs craniocaudally and is considered the rostral extension of the ventral medulla oblongata (2).

The difference between the basilar and ventral medulla is that basilar contains more transverse pontine fibers that form the middle cerebellar peduncle.

2. Pontine tegmentum

It is the dorsal part of the pons and together with the dorsal medulla, they form the rhomboid fossa which is the floor of the fourth ventricle. Also, the dorsal pons contains nuclei of the cranial nerves which link it to a wide range of functions such as sensory and motor functions, controlling sleep, respiration, and level of arousal and vigilance (2).

The basilar sulcus runs down to the midline of the varoli’s bridge on its front surface. This part is a groove of the basilar artery which is one of the blood vessels that supply oxygenated blood to the brain. Pontine arteries are connected to basilar artery and they supply oxygen-rich blood to most parts of the pons. Also, the anterior and posterior inferior cerebellar arteries circulate oxygenated blood to some parts of the varoli's bridge.

In humans, the varoli’s bridges measures close to 2.5 centimeters in length and a huge part of it appears as a broad anterior bump on top of the medulla (2). At the back, it has two pairs of thick stalks known as the cerebellar peduncles which link the cerebellum to the varoli's bridge which is the middle cerebellar peduncle and the midbrain which is the superior cerebellar peduncle.  

Embryonic growth

During embryonic growth, the rhombencephalon gives rise to the embryonic metencephalon. This forms two structures namely; the varoli’s bridge and cerebellum. The special visceral afferent (SVA), solitary nucleus, cochlear and the vestibular nuclei are formed from the sensory neuroblasts which are developed by the alar plate.

Moreover, the general somatic afferent column (GSA) of the trigeminal nerve and the pontine nuclei are formed from spinal nerve nuclei and the trigeminal nerve nuclei (1). The abducens nucleus is developed from the basal plate.

This creates the general somatic efferent fibers (GSE), the special visceral efferent (SVE) which includes the superior salivatory nucleus, the fascial and motor trigeminal nuclei.

Nuclei

Cranial nerve nuclei are a collection of neurons found in the brain stem that is associated with one or more cranial nerves. The varoli’s bridge has several cranial nerve nuclei which enables it to perform many different functions. Some of the common cranial nerve nucleus found in pons include;

1.  The trigeminal nerve sensory nucleus (V)

These
nuclei are found in the trigeminal nerve. Derived from the words “tri” which
means three and “geminus” which means two, trigeminal nerve is the chief
cranial nerve.  Furthermore, it is the
fifth cranial nerve and its responsible for facial sensation and motor
functions like biting and chewing (2). It has 3 major branches namely;

1. Ophthalmic
   nerve (V1),
2. Maxillary
   nerve (V2) and
3. Mandibular
   nerve (V3).

Both
ophthalmic and maxillary nerves are for sensory purposes while the mandibular
nerve performs motor and sensory functions.

2.  The abducens nucleus (VI)

It originates from the abducens nerve (VI) and it is found below the fourth ventricle in the caudal portion of the varoli's bridge, medial to the sulcus limitans.

Together with the internal genu, they form the facial colliculus (1). The wiring nature of the abducens nucleus enables coordinated movements of the eyes.

3.    The facial nerve nucleus (VII)

These are a collection of neurons that belong to the facial nerve. The facial nerves energize the facial muscles of expression and the stapedius.

Facial muscles of expression enable you to make a different facial expression that shows happiness, anxiety, sadness and anger (1). The stapedius muscles stabilize the stapes which are the smallest bones in the body.

4. The vestibulocochlear nuclei (VII)

It is found in the vestibulocochlear nerve which consists of bipolar neurons divided into the cochlear nerve and the vestibular nerve (2). According to surgeons, the cochlear nerve conveys auditory sensory information from the inner ear to the brain while the vestibular nerve transmits spatial orientation information from the semicircular canals to the brain.

In general, the vestibulocochlear nerve is known as the auditory vestibular nerve and it is the eighth cranial nerve. The main purpose of this nerve is to transmit sound and balance information from the inner ear to the brain (2).

Functions of the
varoli’s bridge

Based on scientific facts, the varoli’s bridge through the help of various nerves plays an important role which enables us to perform different functions.  Some of the major functions of the varoli’s bridge include;

1. To maintain balance

The pons helps in maintaining the balance and position of the head and body in space.  The varoli’s bridge through the vestibulocochlear nerve receives and transmits signals which are essential for maintaining the balance of the middle ear (1).

2. Responsible for our facial expression

Facial mimicry such as happiness, anger, and anxiety are facilitated by the pons via the facial nerves. This enables people to display their feelings through different facial expressions. 

3.    Motor functions

Different motor functions such as walking, blinking, chewing and biting are made possible by the varoli’s bridge via the trigeminal nerve (2). To be able to do different motor activities the pods must coordinate different body parts.

For example, to focus on one spot, the varoli’s bridge must coordinate both eyes see the same spot.

4. Respiratory activities  

The varoli’s bridge houses a center that interrupts automatic inhalation and hence controls the breathing pattern. The two parts that control respiration are;

Apneustic center- it
is a lower part of the pons that sends signals to the dorsal respiratory center
and controls the intensity of breathing.

Pneumotaxic center-
it is the upper part of the pons, also called the pontine respiratory group
(PRG) (2). It
is responsible for producing abnormal inspiratory gasps which control the
amount of air a person takes in when breathing.  

These
two parts help you to inhale and exhale freely and easily.

Clinical
significance

Based on different research and studies, the varoli’s bridge is subjectable to one major disease which is the central pontine myelinolysis.

Central pontine
myelinolysis (CPM)

It is a demyelinating disease that makes it difficult for one to balance, walk, touch, swallow and speak. CPM is a result of severe damage to the myelin sheath of nerve cells in the pons (2). Besides, the signs and symptoms of this disease depend on which part of the brain is affected.

According to doctors, the major cause of central pontine myelinolysis is a rapid correction of low blood sodium levels which is called hyponatremia (1). Other than that, persons suffering from alcoholism, HIV/AIDS, hyperemesis gravidarum, severe burns, and hypokalemia also risk getting CPM.

CPM
can be properly diagnosed through MRI. However, to minimize the risk of
severing from central pontine myelinolysis, hyponatremia should be corrected at
a speed not exceeding 10 mmol/L/24hrs. Moreover, alcoholic patients should be
supplemented with vitamins and formal analysis of the nutrition should be done
frequently.

It is important to note that if CPM is not diagnosed early it can result in death or locked-in Syndrome (2). Locked-in Syndrome is also familiar as pseudocoma and it is one kind of condition whereby a patient remains conscious but can’t communicate or move verbally as a result of paralysis of all the voluntary muscles except for the vertical eye movement and blinking. 

References

1. Swanson, L. (2014). Neuroanatomical Terminology: A Lexicon of Classical Origins and Historical Foundations. Cary: Oxford University Press. Found online at: https://books.google.co.ke › books
2. Carter, R., Aldridge, S., Page, M., & Parker, S. (2014). The human brain book. Found online at: https://books.google.co.ke › books