Pons and Midbrain: Anatomy, Cranial Nerve Origins, and Vascular Structures

 

PONS and Midbrain: Anatomy, Cranial Nerve Origins

The medulla oblongata and the midbrain are connected by the pons, which is located anterior to the cerebellum. Its name comes from the appearance on the anterior surface, which resembles a bridge joining the left and right cerebellar hemispheres. It is around 1 inch (2.5 cm) long.

Cranial Nerve Origins and Vascular Structures:

The middle cerebellar peduncle is formed by the convergence of several transverse fibers on each side of the anterior surface, which is convex from side to side. The basilar artery is lodged in the basilar groove, a shallow groove in the midline. The trigeminal nerve arises on both sides of the pons' anterolateral surface. Every neuron is made up of two parts: the larger, lateral sensory root and the smaller, medial motor root. The abducens, facial, and vestibulocochlear nerves emerge in the groove that runs from medial to lateral in the medulla oblongata, between the pons and the latter.

Posterior Surface and Cerebellar Relations:

The cerebellum obscures the posterior surface of the pons. It is triangular in shape and makes up the upper portion of the fourth ventricle's floor. The posterior surface is split into symmetrical halves by a median sulcus and is restricted laterally by the superior cerebellar peduncles. An extended elevation, the medial eminence, lies lateral to this sulcus and is bordered laterally by the sulcus limitans.

Facial Colliculus and Substantia Ferruginea in the Brainstem:

The facial colliculus, which is formed by the root of the facial nerve looping around the abducens nerve nucleus, is the somewhat enlarged inferior end of the medial eminence. The bluish gray substantia ferruginea, the floor of the superior part of the sulcus limitans, gets its color from a collection of strongly pigmented nerve cells. The vestibuli generated by the underlying vestibular nuclei are located lateral to the sulcus limitans.

Internal Structure:

For descriptive reasons, the transversely running fibers of the trapezoid body typically divide the pons into an anterior basal part and a posterior part called the tegmentum.
A transverse section through the caudal part of the pons, traveling through the facial colliculus, and a transverse section through the cranial part, going through the trigeminal nuclei, can be used to study its structure at two different levels.
the primary structures found at each of the pons' two levels.

Transverse Section Through the Caudal Part:

When the medial lemniscus moves from the medulla to the pons, it rotates. With its long axis extending transversely, it is located in the most anterior region of the tegmentum. The spinal and lateral lemnisci accompany the medial lemniscus.
The lateral portion of the medial lemniscus is located anterior to the facial nucleus. The face colliculus is created when the facial nerve's fibers coil around the abducens nerve's nucleus. The facial nerve's fibers then travel anteriorly between the trigeminal nerve's upper end and the face nucleus. This is the unique nerve, the spinal tract.

Medial Longitudinal Fasciculus:

On either side of the midline, just below the floor of the fourth ventricle, is where the medial longitudinal fasciculus is located. The primary route that links the nuclei governing the extraocular muscles (oculomotor, trochlear, and abducens nuclei) with the vestibular and cochlear nuclei is the medial longitudinal fasciculus.

Anatomical Relationships of the Inferior Cerebellar Peduncle:

The inferior cerebellar peduncle and the medial vestibular nucleus are closely related. It is located lateral to the abducens nucleus. At this level are located the superior section of the lateral and the inferior part of the superior vestibular nucleus. At this level, the anterior and posterior cochlear nuclei are also present.

The anteromedial side of the inferior cerebellar peduncle houses the spinal nucleus and tract of the trigeminal nerve.

The fibers that comprise the trapezoid body are generated from both the cochlear and trapezoid body nuclei. They are located in the anterior region of the tegmentum and run transversely.

Connections Between Cerebral Cortex and Cerebellum:

These little masses of nerve cells, known as pontine nuclei, are found in the basilar region of the pons. Pontine nuclei are where the corticopontine fibers of the midbrain's crus cerebri terminate. These cells' axons give rise to the pons' transverse fibers, which split the corticospinal and corticonuclear tracts into little bundles as they cross the midline. The transverse pons fibers reach the cerebellar hemisphere after entering the middle cerebellar peduncle. The primary route connecting the cerebral cortex and the cerebellum is formed by this connection.

Transverse Section Through the Cranial Part:

The motor and main sensory nuclei of the trigeminal nerve are now located in the cranial portion of the pons, which has an internal structure comparable to that observed at the caudal level. Within the reticular formation, the motor nucleus of the trigeminal nerve is located beneath the lateral portion of the fourth ventricle. Emerging motor fibers leave the pons on its anterior surface after passing through its material anteriorly.

Primary Sensory Nucleus of the Trigeminal Nerve:

Located inferiorly to the spinal tract nucleus, the primary sensory nucleus of the trigeminal nerve is placed on the lateral side of the motor nucleus. The sensory fibers that enter pass through the pons' material and position themselves laterally relative to the motor fibers.

Superior Cerebellar Peduncle and Medial Lemniscus:

The trigeminal nerve's motor nucleus is posterolateral to the superior cerebellar peduncle. The anterior spinocerebellar tract connects it. The location of the medial lemniscus and the trapezoid body is the same as it was in the preceding section. At the lateral extremities of the medial lemniscus are the lateral and spinal lemnisci.

MIDBRAIN

The midbrain, which connects the cerebellum and pons to the forebrain, is roughly 0.8 in (2 cm) long. Its long axis slopes anteriorly when it passes through the tentorium cerebelli aperture and ascends. Cerebrospinal fluid-filled cerebral aqueduct, a thin conduit, runs through the midbrain.

Structure and Function of Trochlear Nerve Pathway:

There are four colliculi (corpora quadrigemina) on the posterior surface. These are spherical eminences with a transverse and vertical groove dividing them into superior and inferior pairs. The lower auditory centers are called inferior colliculi, and the visual reflex centers are called superior colliculi. Trochlear nerves sprout in the midline, below the inferior colliculi. These are small-diameter neurons that penetrate the cavernous sinus lateral wall by winding around the lateral face of the midbrain.

Lateral Surface of the Midbrain:

The superior and inferior brachia climb anterolaterally on the lateral surface of the midbrain. The lateral geniculate body, the optic tract, and the superior colliculus are all reached by the superior brachium. The inferior geniculate body and inferior colliculus are joined by the inferior brachium.

Interpeduncular Fossa and Oculomotor Nerve:

The interpeduncular fossa is a deep depression in the midline on the anterior portion of the midbrain, bordered on both sides by the crus cerebri. The bottom of the interpeduncular fossa is punctured by numerous small blood veins; this area is known as the posterior perforated material. The oculomotor nerve travels forward in the cavernous sinus's lateral wall after emerging from a groove on the medial side of the crus cerebri.

Internal Structure:

The substantia nigra, a pigmented strip of gray matter, divides each of the two lateral portions of the midbrain, known as the cerebral peduncles, into an anterior section called the crus cerebri and a posterior part called the tegmentum. The cerebral aqueduct, which links the third and fourth ventricles, is a thin hollow located in the midbrain. The tectum is the region of the midbrain located posterior to the cerebral aqueduct. 

Superior and Inferior Colliculi:

The two superior and two inferior colliculi are the four tiny surface swellings that were previously mentioned. The central gray matter encircles the cerebral aqueduct, which is bordered by ependyma. The tegmentum is continuous across the median plane, but the interpeduncular fossa separates the crura cerebri on transverse slices of the midbrain.

Transverse Section of the Midbrain at the Level of the Inferior Colliculi:

Part of the auditory system, the inferior colliculus is located beneath the equivalent surface elevation and consists of a huge nucleus of gray matter. Many of the lateral lemniscus's terminal fibers are received by it. After there, the route goes to the medial geniculate body via the inferior brachium.

Trochlear nucleus:

The trochlear nucleus is located slightly posterior to the medial longitudinal fasciculus in the central gray matter, near the median plane. The trochlear nucleus's growing fibers exit the midbrain immediately below the inferior colliculi after circling the central gray matter laterally and posteriorly. The trochlear nerve fibers now fully decussate in the superior medullary velum.

Tegmentum Structures, and Reticular Formation:

The cerebral aqueduct is lateral to the trigeminal nerve's mesencephalic nuclei. The middle region of the tegmentum prior to the cerebral aqueduct is occupied by the decussation of the superior cerebellar peduncles. The reticular formation is located lateral to the decussation and is smaller than the pons.

Medial and Lateral Lemnisci:

The spinal and trigeminal lemnisci are located lateral to the medial lemniscus, which ascends posterior to the substantia nigra. After the trigeminal lemniscus is where you'll find the lateral lemniscus. Throughout the midbrain, the substantia nigra is a sizable motor nucleus that is located between the tegmentum and the crus cerebri. The medium-sized multipolar neurons that make up the nucleus contain inclusion granules of melanin pigment in their cytoplasm.

Role of the Substantia Nigra:

The cerebral cortex, spinal cord, hypothalamus, and basal nuclei are all related to the substantia nigra, which regulates muscle tone. Important descending pathways are located in the crus cerebri, which is divided from the tegmentum by the substantia nigra. The middle two thirds of the crus are occupied by the corticospinal and corticonuclear fibers. The medial and lateral regions of the crus are occupied by the frontopontine and temporopontine fibers, respectively. These descending routes link the CN nuclei, the pons, the cerebellum, and the anterior gray column cells of the spinal cord to the cerebral cortex.

Cross-sectional View of the Midbrain at the Superior Colliculi Level:

Part of the visual reflexes is the superior colliculus, a large gray matter nucleus located under the equivalent surface elevation. The superior brachium connects it to the lateral geniculate body. Afferent fibers from the optic nerve, visual cortex, and spinotectal tract are received by it. The tectospinal and tectobulbar tracts, which are made up of efferent fibers, are most likely in charge of the reflex movements of the head, neck, and eyes in reaction to visual stimuli. The pretectal nucleus is where the afferent route for the light reflex finishes. This little cluster of neurons is located in close proximity to the superior colliculus' lateral region.

Pathway to Oculomotor Nucleus:

The fibers go to the Edinger-Westphal nucleus, which is the parasympathetic nucleus of the oculomotor nerve, after relaying in the pretectal nucleus. The oculomotor nerve is then reached by the developing fibers. Located slightly posterior to the medial longitudinal fasciculus, in the central gray matter along the median plane, is the oculomotor nucleus.

Organization of Oculomotor Nucleus:

The oculomotor nucleus fibers emerge on the medial side of the crus cerebri in the interpeduncular fossa after passing anteriorly through the red nucleus. The oculomotor nerve's nucleus can be divided into several cell types. The lateral lemniscus does not extend superiorly to the substantia nigra; instead, the medial, spinal, and trigeminal lemnisci form a curving band posterior to this level.

Connections of Red Nucleus in Motor Pathways:

The substantia nigra and the cerebral aqueduct are separated by the rounded gray matter mass known as the red nucleus. Its vascularity and the presence of an iron-containing pigment in the cytoplasm of many neurons give it a crimson color when observed in fresh specimens. Afferent fibers travel via the corticospinal fibers from

(1) the cerebral cortex to the red nucleus.

(2) the cerebellum via the superior cerebellar peduncle;

(3) the spinal cord, substantia nigra, subthalamic and hypothalamus nuclei, and lentiform nucleus. After exiting the red nucleus, efferent fibers travel via the rubrospinal tract to the spinal cord, the rubroreticular tract to the reticular formation, the thalamus, and the substantia nigra. As the tract descends, it decusses.

Relationship Between Reticular Formation, Red Nucleus, and Descending Fiber Tracts:

The reticular formation is located behind the red nucleus and to the lateral side of the tegmen-tum. The corticospinal, cortico nuclear, and corticopontine fibers that are present at the level of the inferior colliculus are the same essential descending tracts as are found in the crus cerebri. Diagrammatic representation of the continuity of the distinct CN nuclei across the various brainstem regions is shown.