Pons

Ventral surface

Ventrally, the protruding basilar pons resembles a bridge that appears to traverse the longitudinally oriented brainstem. The transverse curvature of the basilar pons bears several surface ridges that contain bundles of transverse fibers from various nuclei within the pons. This curvature continues laterally into the large middle cerebellar peduncle, which connects the pons with the cerebellum. This establishes a neural pathway between the basilar pons, cerebrum and cerebellum.

The surface of the basal pons bears a shallow midline groove called the basilar sulcus through which the basilar artery runs. Emerging directly from the anterolateral aspect of the pons at the mid-pontine level is the trigeminal nerve (CN V). The nerve exits the pons as a large sensory root which lies inferior and lateral to a small motor root. The site where the trigeminal nerve emerges marks the transition from the basilar pons to the middle cerebellar peduncle.

A transverse sulcus on the ventral aspect of the brainstem, the inferior pontine sulcus, delineates the pontomedullary junction. Three cranial nerves emerge within this sulcus: the abducens nerve (CN VI), which is the most medial, the facial (CN VII) and the vestibulocochlear (VIII) nerves, which lie laterally.

Dorsal surface

The dorsal surface of the pons forms the rostral half of the rhomboid fossa, a diamond shaped depression that represents the floor of the fourth ventricle. Its roof is formed by the superior medullary velum, which is continuous with the superior cerebellar peduncles on either side of the midline.

The dorsal surface of the pons bears a shallow midline groove, the dorsal median sulcus, which is continuous with that of the medulla and the spinal cord. On each side of this sulcus is a vertical prominence called the medial eminence. The inferior end of the medial eminence forms an elevation, the facial colliculus, that runs parallel to the sulcus. The facial colliculus is formed by the motor fibers of the facial nerve (CN VII) looping around the abducens (CN VI) nucleus. Just lateral to the facial colliculus is the sulcus limitans and lateral to this sulcus is the vestibular area, which contains the vestibular nuclei.

The dorsal surface of the pons is delineated from that of the medulla by the stria medullaris, a bundle of nerve fibers that course transversely along the floor of the fourth ventricle and forms the caudal border of the pons posteriorly.

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Ventral (basilar) pons

The basilar pons forms the prominent, bulbous ventral segment of the pons. It typically extends rostrally and sits anteriorly at the level of the tectum of the midbrain and the pineal gland. The basilar pons receives three large bundles of axons (descending pathways) from the crus cerebri of the midbrain rostrally, which are transmitted caudally into the pyramids of the medulla oblongata. These longitudinal fibres, collectively called the longitudinal pontine fibers include the corticospinal, corticonuclear and corticopontine tracts. The basilar pons contains various small nuclei scattered among bundles of longitudinally and transversely oriented nerve fibers, collectively referred to as the pontine nuclei. The pontine nuclei give rise to transverse pontine fibers that become the middle cerebellar peduncle. Additionally, the basilar pons contains axonal tracts of pontine cranial nerve nuclei.

Pontine tegmentum

The pontine tegmentum is the region of the pons that lies dorsal to the basilar pons and anterior to the fourth ventricle, forming part of its floor. It is continuous with the tegmentum of the medulla caudally and the tegmentum of the midbrain rostrally. The pontine tegmentum contains several grey matter nuclei including nuclei of cranial nerves linked with the pons and their associated tracts, the pontine reticular formation and the mesopontine cholinergic system which consists of the pedunculopontine nucleus and the laterodorsal tegmental nucleus.

The pontine tegmentum also contains the respiratory center, which consists of the pneumotaxic and apneustic center. Additionally, it contains multiple ascending and descending white matter tracts; major tracts include medial lemniscus, lateral lemniscus, medial longitudinal fasciculus, anterolateral system (spinothalamic tract), and trigeminothalamic fibers.

Nuclei of the pons

The nuclei of the pons are involved in many important roles within the body. These nuclei include the cranial nerve nuclei, pontine nuclei, solitary nucleus, nucleus of lateral lemniscus, nuclei of reticular formation, pedunculopontine nucleus, laterodorsal tegmental nucleus and locus coeruleus.

Cranial nerve nuclei

Within the pontine tegmentum are motor and sensory nuclei of four cranial nerves: trigeminal (CN V), abducens (CN VI), facial (CN VII), and vestibulocochlear nerve (CN VIII). It also contains the inferior salivatory nucleus of the glossopharyngeal nerve (CN IX).

• The sensory nuclei of trigeminal nerve within the pons include the principal (main) sensory nucleus, the ascending fibers of the mesencephalic nucleus, and the subnucleus oralis part of spinal trigeminal nucleus. The principal sensory nucleus lies lateral to the motor nucleus of the trigeminal nerve, within the upper pons, and plays an important role in the perception of touch. The mesencephalic nucleus ascends from the rostral pons into the midbrain at the level of the superior colliculus. This nucleus carries general somatic afferent fibers and is responsible for receiving proprioceptive information from muscles of mastication and facial muscles. The spinal trigeminal nucleus extends from the lower pons caudally to the level of C2 or C3 of the spinal cord. This nucleus receives general visceral afferent fibers from the glossopharyngeal (CN IX) and vagus (CN X) nerves and is involved in pain and temperature sensation.
• The motor nucleus of trigeminal nerve is located in the upper pontine tegmentum, below the superolateral aspect of the rhomboid fossa, and medial to the principal sensory nucleus. This motor nucleus receives afferent nerve fibers from the sensory nuclei of trigeminal nerve as well as other surrounding nuclei and tracts. It provides motor neurons for the innervation of the muscles of mastication, tensor tympani muscle, tensor veli palatini, and the anterior belly of digastric muscle.
• The nucleus of abducens nerve is a small motor nucleus that is located in the lower pontine tegmentum, deep to the facial colliculus, which is formed as a result of the facial nerve fibers looping around the abducens nucleus. It lies ventral and medial to the medial longitudinal fasciculus and is adjacent to the midline. Together with the trochlear, oculomotor and hypoglossal nuclei, the nucleus of abducens nerve forms the somatic motor column. This nucleus provides motor fibers that innervate the lateral rectus muscle.
• The nucleus of facial nerve is a motor nucleus located in the lower pontine tegmentum,ventromedial to the spinal nucleus and spinal tract of the trigeminal nerve. This nucleus provides efferent fibers that innervate the facial muscles, the posterior belly of digastric muscle, the stapedius and stylohyoid muscles.
• The superior salivatory (salivary) nucleus is located in the lower pontine tegmentum, dorsal and lateral to the motor facial nerve nucleus. This visceromotor cranial nerve nucleus gives off some efferent fibers to the facial nerve that provide parasympathetic innervation to the submandibular and sublingual salivary glands, the lacrimal gland and the mucosa of the nasal and oral cavities.
• The inferior salivatory (salivary) nucleus lies just caudal to the superior salivatory nucleus in the lower pontine tegmentum, above the pontomedullary junction. This nucleus serves as the origin of the general visceral efferent fibers of the glossopharyngeal nerve, providing parasympathetic secretomotor innervation to the parotid gland.
• The vestibular nuclei consist of a group of four sensory nuclei: superior, inferior, medial and lateral nuclei. These nuclei lie within the vestibular area of the lower pontine tegmentum and extend into the rostral medulla oblongata. The vestibular nuclei receive input on position, movement and balance from semicircular canals of the inner ear via the vestibular branch of vestibulocochlear nerve and other areas, including the spinal cord, contralateral vestibular nuclei and the reticular formation. In turn, these nuclei relay the received information to areas of the brainstem such as the reticular formation, oculomotor nuclei, thalamus and cerebellum.
• The cochlear nuclei consist of a group of two sensory nuclei, the ventral (anterior) and dorsal (posterior) nuclei, located at the pontomedullary junction. The cochlear nuclei lie posterolaterally to the restiform body of the inferior cerebellar peduncle and are separated by a thin bundle of nerve fibers of the inferior cerebellar peduncle. The cochlear nuclei receive afferents from the organ of Corti in the inner ear and relay auditory information to areas of the brainstem.

Pontine nucle

The pontine nuclei are a collection of small nuclei located in the basilar pons that surround the corticospinal fibers. These nuclei receive afferent fibers from the cerebral cortex and project efferent fibers (pontocerebellar fibers) to the contralateral cerebellar cortex via the middle cerebellar peduncle.

Solitary nucleus

The caudal pontine tegmentum contains the rostral tip of the solitary nucleus (nucleus solitarius). This is a sensory nucleus that extends into the dorsolateral aspect of the medulla oblongata, where it lies ventral and lateral to the dorsal nucleus of the vagus nerve.

The solitary nucleus receives special sensory information of taste as well as general sensory input from the tongue, palate and pharynxvia the facial, glossopharyngeal and vagus nerves.

Nucleus of the lateral lemniscus

The nucleus of the lateral lemniscus is located in the rostral pons, medial to the tract of the lateral lemniscus. It consists mainly of a larger anterior (ventral) nucleus and a smaller posterior (dorsal) nucleus. It receives fibers from the cochlear nuclei and the superior olivary complex and projects efferents to the superior and inferior colliculi. The nucleus of the lateral lemniscus is involved in the coordination of auditory and visual responses.

Nuclei of reticular formation

The nuclei of the reticular formation span the entire brainstem and are grouped into longitudinal columns, the lateral, medial and median (raphe) column. The pons contains a number of these nuclei from all three groups, including the parabrachial nuclei and Kölliker-Fuse nucleus, which play a role in involuntary respiratory control. It also contains the rostral pontine nucleus, the caudal pontine reticular nucleus and the pontine raphe nucleus. These nuclei are involved in the modulation of eye movements and contribute to the regulation of consciousness, pain and cardiovascular and respiratory control.

Pedunculopontine nucleus

The pedunculopontine nucleus is located in the dorsolateral aspect of the upper pontine tegmentum and extends into the caudal part of the tegmentum of the midbrain. It lies between the superior cerebellar peduncle medially, and the medial lemniscus laterally. This nucleus is divided into two subnuclei:

• The rostral pars dissipatus, which contains both cholinergic and glutamatergic nerves.
• The caudal pars compacta, which contains mostly cholinergic neurons.

These neurons form connections with nuclei in various regions, including the reticular formation of the brain stem, thalamus, basal ganglia and the substantia nigra. These connections serve to initiate and modulate gait and other repetitive body movements.

Laterodorsal tegmental nucleus

The laterodorsal tegmental nucleus extends from the tegmentum of the midbrain to the tegmentum of the pons. Along with the pedunculopontine nucleus, the laterodorsal tegmental nucleus provides cholinergic afferents to various brain regions such as the pontine reticular formation, thalamus and limbic system, and is thought to play a role in processes such as arousal, eye movements and sleep-wake cycle.

Locus coeruleus

The locus coeruleus is a pigmented nucleus located in the rostral pons. It lies in the lateral aspect of the floor of the fourth ventricle and contains neurons that synthesize norepinephrine.

The locus coeruleus projects fibers to various regions of the central nervous system, including the cerebral cortex, diencephalon, limbic system, brainstem, cerebellum and spinal cord. It plays a role in the wake-sleep cycle, attention and stress response.

Tracts of the pons

The tracts within the pons are usually grouped as either transverse or longitudinal. The transverse tracts consist of fibers of the cochlear nucleus and the pontocerebellar tracts.

On the other hand, the longitudinal tracts are further subgrouped into ascending and descending fibers.

• The ascending fibers consist of the medial lemniscus, lateral lemniscus, spinothalamic tract (spinal lemniscus), trigeminal lemniscus (trigeminothalamic tract), and the superior cerebellar peduncle.
• The descending tracts, which travel through the basilar pons, include the corticospinal, corticobulbar (corticonuclear) and corticopontine tracts.

Also found within the dorsal pons are the medial longitudinal fasciculus and the central tegmental tract, which contain both ascending and descending fiber tracts from various sources including the surrounding nuclei.

Fibers of the cochlear nuclei

The cochlear nuclei comprise the dorsal (posterior) and ventral (anterior) group. Arising from the dorsal cochlear nucleus is the dorsal acoustic stria which traverses the midline, providing fibers to the contralateral lateral lemniscus which ascends to the inferior colliculus.

On the other hand, the ventral (anterior) cochlear nucleus gives rise to the trapezoid body, a bundle of transverse fibers that run between the basilar pons and the pontine tegmentum. These fibers comprise the acoustic pathway that crosses to the contralateral side towards the superior olivary complex, the nucleus of the lateral lemniscus, or the inferior colliculus.

Transverse pontine (pontocerebellar) fibers

The pontocerebellar fibers are transverse fibers that originate from the pontine nuclei of the basilar pons. These fibers are second-order neurons of the cerebropontocerebellar pathway that project into the contralateral cerebellar cortex via the middle cerebellar peduncles. The pontocerebellar fibers play a role in the interaction between the cerebrum and the cerebellum in modulating motor function.

Ascending pathways

The pons conveys various ascending pathways which include the following:

• The medial lemniscus is a sensory tract that runs posterior to the pyramidal tract (corticospinal and corticobulbar tracts). The medial lemniscus is formed by fibers from the gracile and cuneate nuclei within the dorsal medulla. It transmits proprioception, vibration and fine touch sensations from the dorsal column of the spinal cord to the thalamus, and ultimately to the primary somatosensory cortex.
• The lateral lemniscus runs lateral to the spinothalamic tract in the ventral aspect of the pontine tegmentum. It forms part of the auditory pathway and its fibers terminate in the inferior colliculus. The lateral lemniscus receives fibers from various nuclei, including the cochlear nuclei, superior olivary nucleus and the nucleus of the lateral lemniscus.
• The spinothalamic tract (spinal lemniscus) ascends within the ventral aspect of the pontine tegmentum between the medial and lateral lemnisci. This tract is formed from the combination of the anterior and lateral spinothalamic tracts. The spinothalamic tract conveys coarse touch, pain and temperature information from the contralateral side of the body to the thalamus and to the primary somatosensory cortex.
• The trigeminal lemniscus (trigeminothalamic tract) ascends on the lateral aspect of the tegmentum. It receives fibers from the principal sensory nucleus as well as the spinal nucleus of the trigeminal nerve. This tract conveys fine touch, pain and temperature information from the contralateral face to the thalamus and then to the primary somatosensory cortex.
• The fibers of the superior cerebellar peduncle ascend from the cerebellum into the midbrain at the level of the inferior colliculus. It conveys efferent fibers from various nuclei in the cerebellum to the contralateral red nucleus of the midbrain and thalamus.

Descending pathways

The pons provides passage for the following descending pathways:

• The corticospinal tract is a motor tract that arises from the primary motor cortex and various other regions of the cerebral cortex and descends into the spinal cord through the brainstem. Within the pons, this tract appears as several separated bundles compared to a compact bundle in the medulla. A large number of these fibers cross over within the medulla oblongata, forming the pyramidal decussation, after which they continue on the contralateral side as the lateral corticospinal tract. The remaining fibers that do not cross over remain ipsilateral and continue caudally as the anterior corticospinal tract. The fibers of the corticospinal tract synapse with anterior horn motor neurons of the spinal cord and are responsible for voluntary movement of muscles of the trunk and limbs.
• The corticonuclear (corticobulbar) tract is a motor pathway that arises from the motor cortex of the brain and descends to the motor nuclei of cranial nerves within the brainstem. This tract is responsible for voluntary motor control of the face, head and neck.
• The corticopontine tract is a large group of fibers that originate from almost all regions of the cerebral cortex and terminate on neurons in the ipsilateral pontine nuclei. These nuclei in turn project pontocerebellar fibers to the contralateral cerebellum via the middle cerebellar peduncle, establishing the corticopontocerebellar system.

Medial longitudinal fasciculus

The medial longitudinal fasciculus is located near the midline of the pyramidal fossa at the level of the facial colliculus, lying medial to the abducens nucleus. This pathway is made up of both ascending and descending fibers that establish connections with various nuclei including the oculomotor, trochlear, abducens, Edinger-Westphal, vestibular, reticular and spinal accessory nuclei. Through these connections, the medial longitudinal fasciculus plays a role in coordinating conjugate eye movements and associated movements of the head and neck.

Central tegmental tract

The central tegmental tract is a complex nerve bundle that spans the entire brainstem terminating in the inferior olivary nucleus. In the pons, it courses within the central portion of the pontine tegmentum. The central tegmental tract contains descending fibers from the red nucleus that terminate in the ipsilateral inferior olivary nucleus, and the ascending gustatory fibers from the nucleus solitarius that terminate in the cortical taste area. The central tegmental tract also carries ascending and descending fibers of the brainstem reticular formation, and serves as a pathway for connections between the midbrain to the cerebellum.

Cranial nerves

The pons houses several cranial nerve nuclei that control the following bodily functions:

• Via the trigeminal nerve (CN V): Motor innervation to muscles of mastication, mylohyoid, anterior belly of digastric, tensor veli palatini, and tensor tympani; general sensation, pain and temperature from the face and proprioception from the muscles of mastication and facial muscles.
• Via the abducens nerve (CN VI): Motor innervation to lateral rectus muscle.
• Via the facial nerve (CN VII): Motor innervation to facial muscles, stapedius muscle, stylohyoid, and posterior belly of digastric.
• Via the vestibulocochlear nerve (CN VIII): Vestibular sensation from semicircular ducts, utricle, and saccule related to position and movements of head; hearing from organ of Corti.

In addition to cranial nerves, the pons contains several other important structures that perform various functions:

• The pons transmits the major descending pathways of the cerebral cortex, the corticospinal and corticobulbar tracts, that are involved with voluntary movement of the head, neck and body.
• It contains the pontine nuclei that receive the corticopontine tract and project pontocerebellar fibers, thus, linking the cerebellum to the cerebral cortex to allow for coordination and refining of movement.
• It carries the main ascending pathways from the spinal cord, the medial, lateral, spinal, and trigeminal lemnisci which convey sensory stimuli including tactile, proprioceptive, pain and temperature sensations from the body and head and neck, to the brain.
• It contains the pontine portion of the reticular formation, which is involved in various functions including control of pain, sleep cycle, mood, and breathing.
• The pons serves as a conduit for the medial longitudinal fasciculus, which plays an important role in the coordination of eye, head and neck movements.
• It contains the locus coeruleus, which is the main source of norepinephrine in the brain and is involved in various functions including arousal, wakefulness, response to stress and attention.
• The pons houses the respiratory center which plays a key role in the involuntary control of breathing.