The Pons Function

Pons is an essential part of the brain located above the medulla. It falls in the category of the hindbrain. Pons is a very important part of the brain for the regulation and control of a number of vital functions. It not only acts as a control center but also contains nuclei of some important cranial nerves. We can appreciate the importance of pons from the fact that any damage or trauma to pons can cause the immediate death of the person.

In this educational article, we will talk about the structure and location of the pons, its important components, blood supply and development of pons, and its functions. We will also discuss some important lesions of pons along with the diseases caused.

Anatomy of Pons

Location

Pons is a part of the hindbrain, which also includes the medulla and cerebellum. Pons is present in the uppermost part of the brainstem, just above the medulla and in front of the cerebellum. Inferiorly, it is continuous with the medulla, and superiorly, with the midbrain.

External Features

When viewed externally, the anterior surface of the pons is convex and shows transverse fibers that run from side to side. These fibers join on each side from the middle cerebellar peduncles.

A shallow groove is present in the middle of these fibers, called the basilar groove. The basilar groove lodges the basilar artery, which is the main artery for the blood supply of the entire brain.

The posterior surface of pons contributes to the upper half of the roof of the fourth ventricle. It is divided into symmetrical halves by median sulcus.

Internal Features

The internal anatomical features of pons can be viewed by studying the transverse section of the pons. It appears as a pear-shaped structure with pontine fibers running transversely in the anterior part. The cavity of the pons is formed by the fourth ventricle, present in the posterior part.

The transverse section of the pons is studied at two levels:

At the level of Facial Colliculus

At this level, the most important structure is the facial colliculus. It is formed by the winding of facial nerve (CN VII) fibers around the nucleus of the abducent nerve (CN VI). This part contains the nucleus of the facial nerve, abducent nucleus, spinal nucleus of the trigeminal nerve (CN V), and pontine and trapezoid nuclei.

Important motor fibers passing through this part of the pons include corticospinal and corticonuclear tracts, transverse pontine fibers, and medial longitudinal fasciculus.

The important sensory fibers include lateral, spinal, and medial lemnisci and the spinal tract of the trigeminal nerve.

At the level of Trigeminal Nucleus

The most important structures in this part are the motor and main sensory nuclei of the trigeminal nerve. Pontine and trapezoid nuclei are also present.

It gives passage to the same fibers as present at the level of facial colliculus, except the spinal tract of the trigeminal nerve. The spinal tract of the trigeminal nerve is not present at this level.

Nerves emerging from the Pons

Following four cranial nerves emerge from the pons:

• Trigeminal nerve: from the middle of ventrolateral aspect
• Abducent nerve: from the junction between pons and pyramids
• Facial nerve: from the cerebellopontine angle (angle between pons and cerebellum)
• Vestibulocochlear nerve: from the cerebellopontine angle

Blood supply

The blood supply of pons is mainly derived from the pontine branches of the basilar artery. Other arteries that supply blood to pons include:

• Anterior cerebellar artery
• Inferior cerebellar artery
• Superior cerebellar artery

After passing through the pons, blood mainly drains into the inferior petrosal sinus and basilar plexus of veins.

Physiology of Pons

In this section, we will focus on some important functions of the pons. As said earlier, it plays an important role in controlling and regulating vital body functions. Some of the important functions of the pons are given below.

Respiratory control

Respiratory control is regarded as the most important function of the pons. It controls the rate and depth of involuntary respiration or breathing. There are two centers present in pons for respiratory control:

Apneustic center

The apneustic center is present in the lower part of the pons. Its stimulation produces apneustic breathing or apneuses. Apneusis is an abnormal breathing pattern with prolonged inspiratory gasps. These inspiratory gasps are followed by a brief period expiratory period.

Stimulation of the neurons in this center sends signals to the inspiratory center in the medulla, causing the increased activation of the phrenic nerve. The phrenic nerve, in turn, causes increased inspiration.

Pneumotaxic center

The pneumotaxic center is present in the upper part of the pons. The stimulation of neurons present in this part of pons causes inhibition of inspiration. It inhibits the signaling from the apneustic center and limits the activity of the phrenic nerve. In this way, it decreases the tidal volume and helps to control the rate of respiration.

Relay of Signals

The nuclei present in the pons are essential to relay signals from the cerebrum to the cerebellum.

The cerebellopontine fibers terminate on the pontine nuclei present in the anterior part of the pons. The efferent fibers emerge from these pontine nuclei and form the transverse fibers. These transverse fibers join to form middle cerebellar peduncles on both sides of the pons. In this way, signals arising from the cerebrum are relayed to the cerebellum.

Nuclei of cranial nerve

The nuclei of important cranial nerves are present in pons. These cranial nerves, arising from the pons have important functions listed below.

Abducent nerve

The nucleus of the abducent nerve is present in the caudal part of the pons. It controls the movements of the eyeball. This nerve provides motor supply to the lateral rectus muscle of the eyeball. This muscle causes lateral rotation and abduction of the eyeball.

Trigeminal nerve

The motor and sensory nuclei of the trigeminal nerve are present in pons. This nerve provides sensory supply to the entire face, part of the neck, and scalp. This nerve is also important for the process of chewing, swallowing, and biting as it provides motor supply to the muscles of mastication.

Facial nerve

The nucleus of the facial nerve is also present in pons. It provides motor signals to the muscles of facial expressions. Thus, it is majorly responsible for controlling facial expressions. It also carries taste sensations from the anterior two-thirds of the tongue.

It also provides innervation to the salivary glands and thus controls the process of salivation.

The facial nerve is also involved in the corneal reflex arc.

Passage to fibers

Pons also provide passage to fibers to and from the brain and spinal cord. It acts as a relay center for these fibers. The fibers passing through the pons include corticonuclear fibers, corticospinal fibers, and the three lemnisci; medial, lateral, and spinal lemniscus.

Clinical Significance of Pons

Following are the clinically important pathologies of pons:

Tumors of Pons

The most common tumor of the brain stem is the astrocytoma of the pons. It causes weakness of facial muscles on the same side, weakness of lateral rectus muscles on one or both sides, and weakness of muscles involved in mastication.

It also causes impairment in hearing due to damage to the cochlear nerve. Other peripheral symptoms include contralateral hemiparesis, quadriparesis, and sensory defects of the trunk and limbs.

Pontine Hemorrhage

The pons is supplied by the basilar artery and superior, inferior, and anterior cerebellar arteries. The hemorrhage of any of these arteries will result in facial paralysis on the side of the hemorrhage and paralysis of limbs on the opposite side. There will also be paralysis of the lateral rectus muscle on the side of the lesion.

Extensive bilateral hemorrhage causes pinpoint pupil and bilateral paralysis of the face and limbs.

Infarction of Pons

Usually, the infarction of pons is due to the thrombus or embolus in the basilar artery or its branches. The infarct of pons can be paramedian or lateral.

The paramedian infarct causes damage to corticospinal tracts, pontine nuclei, and fibers passing to the middle cerebellar peduncle.

The lateral infarct will cause damage to the three lemnisci as well as corticospinal fibers to the lower limb.

Conclusion/Summary

Pons is a part of the hindbrain present just above the medulla oblongata in the brain stem.

On gross examination, pons has a convex anterior surface with a shallow groove in the middle for the basilar artery. It is on the sides by the middle cerebellar peduncles and forms the roof of the fourth ventricle.

The transverse sections of pons appear pear-shaped. It is studied at two levels; at the level of facial colliculus, and at the level of trigeminal nuclei.

Four cranial nerves emerge from pons including trigeminal nerve (CN V), abducent nerve (CN VI), facial nerve (CN VII), and vestibulocochlear nerve (CN VIII).

The arterial blood is supplied to pons by branches of the basilar artery as well as branches of anterior, superior, and inferior cerebellar arteries.

The important functions performed by pons include;

• Respiratory control via Pnuemotaxic center and Apneustic center
• Relay of fibers from the cerebrum to cerebellum
• Nuclei of four cranial nerves
• Passage to corticonuclear and corticospinal tracts

The important pathologies that can impair the functions of pons include:

• Pontine tumors
• Pontine hemorrhage
• Infraction

References

1. Henry Gray (1862). Anatomy, descriptive and surgical. Blanchard and Lea. pp. 514–. Retrieved 10 November 2010
2. Saladin Kenneth S.(2007) Anatomy & physiology the unity of form and function. Dubuque, IA: McGraw-Hill
3. Carpenter, M (1985). Core text of neuroanatomy (3rd ed.). Williams & Wilkins. p. 42. ISBN 0683014552
4. Pritchard, TE & Alloway, D (1999). Medical neuroscience. Hayes Barton Press. ISBN 978-1-59377-200-0
5. "BrainInfo". braininfo.rprc.washington.edu