The Pons is the largest part of the brainstem, located above the medulla and below the midbrain. It is a group of nerves that function as a connection between the cerebrum and cerebellum (pons is Latin for bridge). The pons develops from the embryonic metencephalon (part of the hindbrain, developed from the rhombencephalon), alongside the cerebellum. In this article, we will look at the anatomy and functions of the pons - its location, structure, blood supply, and clinical relevance.
The pons is a horseshoe-shaped collection of nerve fibres located in the anterior part of the posterior cranial fossa. Its anatomical relations are as follows:
Anterior Surface: The anterior or ventral surface of the pons is marked by a bulging formed by the transverse pontocerebellar fibres. These fibres wrap around the otherwise vertically oriented brainstem. It measures around 2.5 cm in adults. The basilar groove demarcates the midline of the ventral surface and is where the basilar artery is located. The pontomedullary junction is an important anatomical landmark defined by the angle between the lower border of the pons and the superior border of the medulla.
Several cranial nerves originate from the ventral surface of the pons: Cranial nerve V – trigeminal – originates from the lateral aspect of mid pons; Cranial nerve VI – abducens – originates from the pontomedullary junction, close to the midline; Cranial nerve VII – facial – originates from the cerebellopontine angle, the more lateral aspect of the pontomedullary junction; Cranial nerve VIII – vestibulocochlear – originates laterally to the facial nerve.
The pons is intimately related to the cerebellum and is connected to it by the middle cerebellar peduncles. Removal of the cerebellum will reveal the underlying fourth ventricle. The floor of the fourth ventricle is composed of the dorsal surface of the pons and the medulla. There are some important anatomical landmarks here:
The pons is comprised of two major components - the ventral pons and the tegmentum. The ventral pons contains the pontine nuclei, which are responsible for coordinating movement. Fibres from the pontine nuclei cross the midline and form the middle cerebellar peduncles on their way to the cerebellum. The tegmentum is the evolutionarily older part of the pons which forms part of the reticular formation - a set of nuclei found throughout the brainstem that are responsible for arousal and attentiveness. Damage to this part of the pons may result in anosognosia for hemiplegia, where patients are unaware of their paralysis.
The rest of the pons is made up of tracts passing through the pons including descending corticospinal tracts - responsible for voluntary motor control of the body. Descending corticobulbar tracts – responsible for voluntary motor control of face, head and neck. Ascending medial lemniscus tracts – responsible for fine touch, vibration and proprioception. Ascending spinothalamic tracts – responsible for pain and temperature sensation. The main sensory nucleus and the trigeminal motor nucleus are located in the midpons – at the level where the fibres originate from the lateral aspect of the pons.
The pons is supplied by a number of arteries, many of which arise from the basilar artery, which runs along the basilar groove on the ventral surface of the pons. These branches of the basilar artery include the anterior inferior cerebellar artery (AICA), the superior cerebellar artery (SCA), the posterior inferior cerebellar artery (PICA) and the medullary arteries. An additional artery arises from the vertebrobasilar system and supplies the dorsolateral pons, known as the pontine artery.
The basal artery has many anastomoses (connections) with other arteries in this region, which makes it a reliable source of collateral circulation if necessary. For instance, if the AICA artery is blocked, other arteries may take over its blood supply, like the SCA.
The anatomy of the pons is relevant to many medical conditions. For example, a stroke involving the pons can result in difficulty in speaking, hearing, and controlling facial movements. It can also lead to paralysis on one side of the body (hemiplegia), a condition known as “locked-in syndrome”. Damage to the reticular formation may lead to a profound decrease in alertness and consciousness.
In addition to strokes, other medical conditions can involve the pons. These include tumors, multiple sclerosis, and trauma. The blood supply of the pons is also relevant in surgery, where it is often necessary to access the pons in order to perform a procedure or to remove a tumor.
The anatomy of the pons is complex, with many important structures and pathways located here. Its location in the anterior part of the posterior cranial fossa means it is well protected and also intimate related to the cerebellum. As described, it is the connection point for many cranial nerves and has many significant tracts that pass through it.
The pons is thus an important part of the nervous system, and its functions and relevance are important to understand for a variety of medical conditions. It is a complex and fascinating structure in the brain, and further study of its anatomy can help us to better understand its significance and role in the body.
The pons is a structure located in the brainstem, located posteriorly to the midbrain and superiorly to the medulla oblongata. It measures around 2.5 cm in adults and is related to multiple cranial nerves (CN V, VI, VII, and VIII) and several important blood vessels. In this article, we will discuss the anatomy and clinical implications of the pons.
In addition to the trigeminal nerve, two other nuclei also receive sensory information from the trigeminal nerve – the Spinal trigeminal nucleus, which extends caudally towards the medulla, and the Mesencephalic nucleus, which extends all the way to the midbrain. The abducens nucleus, which controls the abducens nerve (which innervates the ipsilateral lateral rectus muscle), is located in the caudal pons on the medial aspect of its dorsal surface. The facial nucleus, which controls the muscles of facial expression, is located more anteriorly and laterally at the same level as the abducens nucleus. Its fibres take an unusual course and loop around the abducens nucleus before exiting the brainstem through its ventrolateral surface. The cochlear and vestibular nuclei sit dorsolaterally from the inferior pons to the superior medulla.
The blood supply to the pons consists of branches of the vertebrobasilar system - most of the pons is supplied by the pontine arteries, branches of the basilar artery. A smaller part of its blood supply comes from the anterior inferior cerebellar artery and the superior cerebellar artery (AICA and SCA). The venous drainage of the pons consists of the anterior pontomesencephalic vein, which drains superiorly into the basal vein and then into the cerebral veins. Inferiorly, the pons drains into the inferior petrosal sinus, which drains into the internal jugular veins.
Anatomically, the posterior of the pons is connected to the cerebellum and separated by the fourth ventricle, while the inferior of the pons is connected to the medulla oblongata. The midbrain lies immediately above the pons, while the anterior or ventral surface of the pons is marked by a bulging formed by the transverse pontocerebellar fibres. The basilar groove demarcates the midline of the ventral surface and is where the basilar artery is located. The pontomedullary junction is an important anatomical landmark defined by the angle between the lower border of the pons and the superior border of the medulla.
Several cranial nerves originate from the ventral surface of the pons:
The posterior surface of the pons is intimately related to the cerebellum and is connected to it by the middle cerebellar peduncles. Removal of the cerebellum will reveal the underlying fourth ventricle. The floor of the fourth ventricle is composed of the dorsal surface of the pons and the medulla.
The cerebellopontine angle is clinically important since it is often the place of intracranial growths, which can lead to impairment of nearby structures. Symptoms start with mild impairment of the cranial nerve in the area (CN VIII). Vestibular schwannomas (also called Acoustic Neuromas, although this terminology is incorrect since these tumours originate in Schwann cells, not in neurons, and usually on the vestibular component of the nerve) are the most common growth in the area. Similar to most primary intracranial tumours, they are benign, and most complications arise from compression of nearby structures. They are slowly growing and are frequently related to slowly progressing ipsilateral hearing loss.
Although vestibular schwannomas are responsible for roughly 80% of the cases, other causes can range from different tumours (e.g. meningiomas) to vascular abnormalities. Most common symptoms are ipsilateral hearing loss and tinnitus – due to compression of the cochlear component of CN VIII, dysequilibrium, and more rarely vertigo – due to the compression of the vestibular component of CN VIII, nystagmus – due compression of cerebellar flocculus, facial paralysis – due to compression of CN VII, reduced corneal reflex – due to involvement of both CN V and CN VII, and trigeminal palsy – due to compression of CN V. Untreated, large tumours will lead to compression of additional structures with more serious complications.
Surgical resection of these growths guarantees a very good outcome in most cases, with the risk of complications increasing as the size of the tumour increases. As such, it is important for healthcare professionals to be aware of the clinical implications associated with these intracranial growths in order to provide timely and effective treatment.
The pons is an important anatomical structure located at the base of the brain. It is the bridge between the midbrain and the medulla oblongata, and it contains several important nerve tracts that pass through it.
The pons has several important anatomical landmarks. The medial eminence marks the midline of the floor, while the facial colliculus is a bulging formed by the fibres of the facial nerve looping around the abducens nucleus. The stria medullaris of the fourth ventricle is a bundle of nerve fibres crossing transversely from the lateral aspect into the midline, and marks the posterior border between the pons and the medulla. The angle formed at the junction of the pons, medulla, and cerebellum is another anatomical landmark and is known as the cerebellopontine angle. Here, the cerebellar flocculus, the ventricular choroid plexus and the emerging CNs VII and VIII surround the lateral apertures of the fourth ventricle, or the foramen of Luschka.
The pons is comprised of two major components – the ventral pons and the tegmentum. The ventral pons contains the pontine nuclei, which are responsible for coordinating movement. Fibres from the pontine nuclei cross the midline and form the middle cerebellar peduncles on their way to the cerebellum. The tegmentum is the evolutionarily older part of the pons which forms part of the reticular formation – a set of nuclei found throughout the brainstem that are responsible for arousal and attentiveness. Damage to this part of the pons may result in anosognosia for hemiplegia, where patients are unaware of their paralysis.
The pons is also made up of several tracts that pass through it. These include the descending corticospinal tracts, which are responsible for voluntary motor control of the body; the descending corticobulbar tracts, which are responsible for voluntary motor control of face, head and neck; the ascending medial lemniscus tracts, which are responsible for fine touch, vibration and proprioception; and the ascending spinothalamic tracts, which are responsible for pain and temperature sensation.
The pons also houses several important cranial nerve nuclei. The main sensory nucleus and the trigeminal motor nucleus are located in the midpons, at the level where the fibres originate from the lateral aspect of the pons. The main sensory nucleus receives somatosensory information from the face, and there are two other nuclei that receive sensory information from the trigeminal nerve: the spinal trigeminal nucleus, which extends caudally towards the medulla; and the mesencephalic nucleus, which extends rostrally all the way to the midbrain. The abducens nucleus, which is located in the caudal pons on the medial aspect of its dorsal surface, controls the abducens nerve, which innervates the ipsilateral lateral rectus muscle. The facial nucleus, which is located more anteriorly and laterally at the same level as the abducens nucleus, controls the muscles of facial expression.
The cochlear and vestibular nuclei sit dorsolaterally from the inferior pons to the superior medulla. The blood supply of the pons is formed by branches of the vertebrobasilar system, with most of the pons being supplied by the pontine arteries, branches of the basilar artery, and a smaller part of its blood supply coming from the anterior inferior cerebellar artery and the superior cerebellar artery (AICA and SCA). The venous drainage of the pons consists of the anterior pontomesencephalic vein, which drains superiorly into the basal vein, and then into the cerebral veins. Inferiorly, the pons drains into the inferior petrosal sinus, which drains into the internal jugular veins.
The cerebellopontine angle is clinically important since it is often the place of intracranial growths, which can lead to impairment of nearby structures. Symptoms start with mild impairment of the cranial nerve in the area (CN VIII). Vestibular schwannomas (also called Acoustic Neuromas, although this terminology is incorrect since these tumours originate in Schwann cells, not in neurons, and usually on the vestibular component of the nerve) are the most common growth in the area. Similar to most primary intracranial tumours, they are benign, and most complications arise from compression of nearby structures. They are slowly growing and are frequently related to slowly progressing ipsilateral hearing loss.
In summary, the pons is an important anatomical structure located at the base of the brain. It is the bridge between the midbrain and the medulla oblongata, and is composed of two main components: the ventral pons and the tegmentum. It contains several important nerve tracts that pass through it, as well as several important cranial nerve nuclei. Clinically, the cerebellopontine angle is important, since it is often the place of intracranial growths, such as benign vestibular schwannomas, which can lead to compression of nearby structures.
Vestibular schwannomas, which account for roughly 80% of all neurotological disorders, are a type of tumour which can cause a variety of symptoms. Other causes of these disorders can range from different tumours, such as meningiomas, to vascular abnormalities.
The most common symptoms of vestibular schwannomas and other causes of these disorders include:
If left untreated, large tumours of this type can lead to the compression of additional structures in the body, which can lead to more serious complications. Fortunately, surgical resection can generally provide an excellent outcome for the majority of patients, although there is a risk of complications which may increase in accordance with the size of the tumour.
