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Pons is an essential part of the brain located above the medulla. It falls in the category of the hindbrain. Pons is very important part of 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 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
Pons is a part of the hindbrain, which also includes medulla and cerebellum. Pons is present in the uppermost part of brainstem, just above the medulla and in front of the cerebellum. Inferiorly, it is continuous with the medulla, and superiorly, with the midbrain.
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 to from 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 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.
The internal anatomical features of pons can be viewed by studying the transverse section of pons. It appears as a pear-shaped structure with pontine fibers running transversely in the anterior part. The cavity of pons is formed by the fourth ventricle, present in the posterior part.
The transverse section of 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 facial nerve, abducent nucleus, spinal nucleus of trigeminal nerve (CN V), and pontine and trapezoid nuclei.
Important motor fibers passing through this part of pons include corticospinal and corticonuclear tracts, transverse pontine fibers and medial longitudinal fasciculus.
The important sensory fibers include lateral, spinal and medial lemnisci, and spinal tract of 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 trigeminal nerve. The spinal tract of 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
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 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 is regarded as the most important function of pons. It controls the rate and depth of involuntary respiration or breathing. There are two centers present in pons for the respiratory control:
Apneustic center is present in the lower part of pons. Its stimulation produces apneustic breathing or apneuses. Apneusis is an abnormal breathing pattern with prolonged inspiratory gasps. These inspiratory gasps are followed by brief period expiratory period.
Stimulation of the neurons in this center sends signals to inspiratory center in medulla, causing the increased activation of phrenic nerve. Phrenic nerve, in turn, causes increased inspiration.
The pneumotaxic center is present in the upper part of 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 rate of respiration.
Relay of Signals
The nuclei present in pons are essential to relay signals from the cerebrum to the cerebellum.
The cerebropontine fibers terminate on the pontine nuclei present in the anterior part of 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 pons. In this way, signals arising from the cerebrum are relayed to 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.
The nucleus of the abducent nerve is present in the caudal part of the pons. It controls the movements of eyeball. This nerve provides motor supply to the lateral rectus muscle of eyeball. This muscle causes lateral rotation and abduction of eyeball.
The motor and sensory nuclei of the trigeminal nerve are present in pons. This nerve provides sensory supply to the entire face, part of 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.
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.
Facial nerve is also involved in 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 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 cochlear nerve. Other peripheral symptoms include contralateral hemiparesis, quadriparesis and sensory defects of trunk and limbs.
Pons are supplied by 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 hemorrhage and paralysis of limbs on the opposite side. There will also be paralysis of lateral rectus muscle on the side of lesion.
Extensive bilateral hemorrhage causes pinpoint pupil, and bilateral paralysis of 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.
Pons is a part of the hindbrain present just above the medulla oblongata in the brain stem.
On gross examination, pons has convex anterior surface with shallow groove in the middle for the basilar artery. It is on 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 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
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