Brainstem

Surface anatomy

Ventral surface

The ventral surface of the medulla is marked by a midline depression known as the anterior median fissure. This is a cranial continuation of the anterior median fissure of the spinal cord, which divides the medulla into symmetrical halves. The most distal end of the anterior median fissure is covered by crisscrossing fibers known as the decussation of the pyramids. 

The pyramids contain the corticospinal fibers that start from the motor cortex, pass through the internal capsule, and contribute to the formation of the cerebral peduncles. The fibers then descend to the spinal cord, passing through the pons and the medulla. Their role is to regulate muscle movement. Most of these fibers cross to the opposite side at the level of the caudal medulla and they form the aforementioned decussation of pyramids. 

Lateral to the medullary pyramids is a shallow depression that represents the cranial continuation of the ventrolateral sulcus of the spinal cord. It separates the pyramids from another prominent bulge, the olive, which corresponds to the location of the inferior olivary nucleus. Posterolateral to the olives are the inferior cerebellar peduncles. Although these structures lie above the upper limit of the medulla, they are often labeled as medullary structures owing to the fact that they carry fibers from the medulla to the cerebellum.

The lower four cranial nerves also emerge from the anterior surface of the medulla. The hypoglossal nerve (CN XII) rootlets emerge between the pyramids and the olives before they unite to form the trunk of the nerve. The glossopharyngeal nerve (CN IX) rootlets arise between the inferior cerebellar peduncle and the olive (close to the choroid plexus of the fourth ventricle). Caudal to CN IX are the rootlets of the vagus nerve (CN X). The most inferior of the cranial nerves is the accessory nerve (CN XI) whose rootlets ascend the lateral aspect of the medulla to unite as the nerve trunk.

Dorsal surface

The posterior surface of the medulla can be divided into an upper open area and a lower closed area. The open area is also known as the floor of the fourth ventricle or rhomboid fossa. It has unique raised areas that mark the location of underlying structures. These include the hypoglossal and vagal trigones that are present on either side of the posterior (dorsal) median sulcus. The hypoglossal trigone (location of the nucleus of CN XII) is superior to the vagal trigone (location of CN X nucleus). The vagal trigone is above the obex, which is the most caudal aspect of the fourth ventricle. 

The closed part of the posterior surface (outside of the fourth ventricle) of the medulla is also divided into symmetrical halves by the caudal continuation of the dorsal median sulcus. On either side of the sulcus is a raised structure known as the gracile tubercle. Deep to the tubercle is the gracile nucleus, which is the point of termination of the gracile fasciculus of the dorsal column of the spinal cord. 

The paired cuneate tubercles, which are raised areas over the cuneate nucleus also marks the point of termination of the cuneate fasciculus. It is laterally related to the gracile tubercle. The cuneate fasciculus is medially related to the lateral funiculus which carries contralateral corticospinal and spinothalamic tracts. Similarly, the trigeminal tubercle is laterally related to the cuneate tubercle. It represents the location of the spinal nucleus of the trigeminal nerve (CN V).

The inferior cerebellar peduncles also form part of the posterior surface of the medulla. It is best seen when the cerebellum is removed. This paired tract carries efferent and afferent impulses between the cerebellum and vestibular nuclei.

There are unique openings (one on either posterolateral surface and one in the posterior midline) on the medulla that facilitate communication between the fourth ventricle and the surrounding cisterns. Those on the posterolateral surfaces of the medulla are called the lateral apertures of the fourth ventricle (lateral apertures of Luschka). The midline opening is called the median aperture of the fourth ventricle (foramen of Magendie). 

Read more in our study unit and then test how much you know about the subarachnoid cisterns by trying our custom quiz below.

Cross-section

The layout of tracts and nuclei varies from one level of the brainstem to the other. As a result, the cross-sectional layout of the nuclei and tracts within the medulla may be slightly different going from the decussation of the pyramids to the level of the olives, for example.

The two main cross-sectional points that will be discussed in the medulla is at the level of the vagus (CN X) and hypoglossal (CN XII) nerves. Note that the nuclei and tracts that are discussed are paired structures present in both the left and right halves of the brainstem.

The nuclei and tracts at the level of the vagus nerve (CN X) can be grouped according to their position on the cross-sectional view. At the dorsomedial part of the medulla is the inferior olivary nucleus, which is medial to the hypoglossal nucleus. The medial longitudinal fasciculus (MLF) and the raphe nuclei are two other midline tracts that are ventral to the inferior olivary nucleus. The dorsal and medial accessory olivary nuclei are laterally and ventrolaterally related to the raphe nucleus, respectively. The most ventral midline structure at this level is the dense corticospinal tracts that form the pyramidal tract. 

The nuclei at the dorsolateral part of the medulla have (arranged from medial to lateral) the posterior (dorsal) vagal, solitary, medial vestibular, and cuneate nuclei. The cuneate nucleus is situated within the inferior cerebellar peduncle. Both the spinal trigeminal tract and nucleus are ventral to the cuneate nucleus at this level. The lateral aspect of the medulla is occupied by the lateral reticular nucleus. It is ventral to the spinal trigeminal tract and nucleus, lateral to the nucleus ambiguus and dorsal accessory olivary nucleus, and dorsal to the spinothalamic and spinocerebellar tracts. The reticular formation occupies a substantial amount of space in the dorsal half of the medulla. It is surrounded by the midline (medially), dorsomedial, dorsolateral, and lateral groups of nuclei and tracts mentioned above.

The medial lemniscus occupies the ventral midline of the medulla. It is dorsally related to the pyramids and medial to the inferior and medial accessory olivary nuclei.

The nuclei and tracts found at the level of the hypoglossal nucleus (CN XII) can also be grouped based on their position on the cross-sectional view. The nuclei in the dorsomedial group (from medial to lateral) are the hypoglossal, dorsal vagal, the nucleus of the solitary tract, and gracile nuclei. The actual solitary tract is ventrolateral to its nucleus. Note also that the inferior olivary nucleus is no longer visible at this level. The cuneate nucleus is much larger at this level and is still found in the dorsolateral position. The medial longitudinal fasciculus and the medial lemniscus pathway maintain their midline location. The medial lemniscus pathway is the most ventral of the two structures and has the internal arcuate fibers extending laterally from it. 

The spinal trigeminal tract and nucleus are located in the lateral aspect of the medulla at this level. Both structures are found dorsal to the lateral reticular nucleus, which is lateral to the nucleus ambiguus and posterior accessory olivary nucleus. There is a central tegmental tract that separates the lateral reticular nucleus from the olivary nucleus. Both of these structures are located toward the ventrolateral aspect of the medulla. Furthermore, they are medially related to the superficial arcuate fibers. 

The fibers of CN XII leave the medulla between the olivary nucleus and the pyramidal tracts. The pyramidal tracts, along with the arcuate nucleus are found in the ventromedial part of the medulla. The arcuate nucleus is the most ventral of the two structures. 

The table below will provide general information about the nuclei found in the medulla. More information about the ascending and descending tracts can be found here. 

Now you can reinforce and challenge your memory! Read more about in our articles and then see how many new terms you have added to your anatomy bank by taking these quizzes on the cross-section of the medulla.

Surface anatomy

When viewed from the ventral aspect, the pons resembles a dome-like structure with numerous striations across its surface. It is widest in the middle and tapers toward the lateral extremities. The surface of the pons is otherwise unremarkable, with the exception of a shallow depression that runs along its vertical axis. This is called the basilar groove, which accommodates the basilar artery – a major part of the posterior blood supply of the brain. 

The function of the pons is to house the pontine nuclei and to facilitate corticopontocerebellar communication. It also enables communication between the left and right hemispheres of the cerebellum. Interestingly enough, it is the corticopontocerebellar fibers that are responsible for the striated appearance of the pons. 

The pons is limited superiorly by the superior pontine sulcus and ends inferiorly at the pontomedullary junction or inferior pontine sulcus. The motor and sensory fibers of the trigeminal nerve (CN V) emerge from the lateral surface of the pons. Additionally, there are three cranial nerves that emerge from the inferior pontine sulcus. These nerves are (listed from medial to lateral) the:

• Abducens nerve (CN VI)
• Facial (CN VII) and intermediate nerves
• Vestibulocochlear nerve (CN VIII)

The posterior surface of the pons is formed by the upper part of the rhomboid fossa, which is best appreciated when the cerebellum has been resected. This process reveals the most lateral boundary, which is formed by the middle cerebellar peduncles. These are the largest of the peduncles and contain massive input from the pons to the cerebellum (pontocerebellar fibers). The superior limit of the posterior surface of the pons is the superior medullary velum, while the inferior border is formed by the striae medullaris (fibers of the arcuate nucleus).

A pair of raised bodies known as the medial eminences, separated by a dorsal median sulcus, is the hallmark feature of the posterior surface of the pons. The caudal extent of the medial eminence is slightly more pronounced than the cranial segment of the structure. This portion is called the facial colliculus. It is a result of the presence of the genu of the facial nerve (CN VII), which curves around the nucleus of the abducens nerve (CN VI). On either side of the facial colliculus is another slightly raised area that corresponds to the location of the vestibular nucleus (CN VIII) known as the vestibular area. 

Cross-section

The pons can be subdivided into ventral (basal) and dorsal (tegmental) portions on the cross-section for easier discussion. The basal pons or basis pontis is the larger of the two parts of the pons. It is made up of corticospinal and corticobulbar tracts traversing a craniocaudal pathway. Additionally, it also has the pontine nuclei and horizontal pontine fibers that eventually form the middle cerebellar peduncle. 

The smaller pontine tegmentum contains the cranial nerve nuclei for CN V - CN VIII as well as several white matter tracts (medial longitudinal fasciculus, medial and lateral lemnisci, and others) and the fourth ventricle.

The table below summarizes the major nuclei found in the pons. For more details regarding the other tracts found in this area, refer to the article on the midbrain and pons.

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Surface anatomy

The superior boundary of the midbrain is indicated by an imaginary horizontal line extending from the posterior commissure (below the pineal gland and above the superior colliculi) to the mammillary bodies. Inferiorly, the midbrain ends at the superior pontine sulcus (anteriorly) and the superior medullary velum (posteriorly). The superior pontine sulcus is a shallow, horizontal groove between the upper border of the pons and the cerebral peduncles. The superior medullary velum is a thin sheet of neural tissue that forms the roof of the fourth ventricle.

Externally, the midbrain is characterized by two stalks known as the cerebral peduncles (pedunculus cerebri) along the ventral surface. They represent the numerous ascending and descending tracts that connect the cerebral cortex to the pons and spinal cord. There is a relatively shallow depression between the cerebral peduncles known as the interpeduncular fossa. The posterior perforated substance (which accommodates small blood vessels to the midbrain) forms the floor of the fossa. The oculomotor nerve (CN III), the terminal bifurcation of the basilar artery (the posterior cerebral arteries) and the mammillary bodies of the hypothalamus can all be observed in the interpeduncular fossa. The cerebral peduncles then converge toward the midline as they meet the pons. 

On either side of the midbrain, the optic tract course around the lateral surfaces. They eventually terminate at the lateral geniculate body (which is posterolateral to the thalamus). The medial geniculate body and the uncus of the temporal lobe are also laterally related to the midbrain. The posterior surface of the midbrain has two pairs of raised, round protrusions that are known collectively as the corpora quadrigemina (tectal plate or quadrigeminal plate). This complex consists of the superior and inferior colliculi, which house their respective nuclei. Each colliculus is separated from the contralateral counterpart by the frenulum of the superior medullary velum.

The fibers of each trochlear nerve (CN IV) nucleus crosses the midline and emerge on the opposite side of the frenulum of the superior medullary velum, just below the inferior colliculi. The trochlear nerves eventually become inferolaterally, then anteroinferiorly related to the cerebral peduncles as they course around the midbrain on its way to its target organ.

The superior cerebellar peduncles are also considered as part of the midbrain. It is best appreciated when the cerebellum is resected. This paired structure contains mainly cerebellar output and connects the cerebellum with the midbrain (the red nucleus specifically), the thalamus and the motor cortex.

Cross-section

The midbrain cross-section views reveal more information about the structure and organization of this portion of the brain. This brainstem segment is divided into a tectum (Latin word meaning roof), tegmentum (Latin word for covering) and the crus cerebri. 

The tectum is the part of the midbrain that is dorsal to the cerebral aqueduct of Sylvius (a conduit that connects the third and fourth ventricles). The tegmentum, on the other hand, is ventral to the aqueduct of Sylvius. The crus cerebri are large collections of ascending and descending tracts traveling to and from the brain. Each crus is separated from the tegmentum by the substantia nigra. This is a pigmented lamina that is made up of dopaminergic and GABAergic neurons. These are cells that produce specific neurotransmitters for which they are named. It is important to note that while each crus cerebri is separate from each other, the tegmentum is continuous across the midline of the midbrain. Some texts refer to the crus cerebri and tegmentum together as the cerebral peduncles. 

Another prominent structure seen on cross-section is the red nucleus. This is a well-defined collection of cell bodies that appear bright red on a freshly sectioned specimen. The paired structure is located along the midline of the tegmentum, posteromedial to the substantia nigra. Additionally, there is a circumferential area of gray matter surrounding the cerebral aqueduct known as the periaqueductal gray matter. 

There are several important nuclei and tracts located throughout the midbrain. This article will only give an overview of the major nuclei. Further details about the internal composition of the brainstem can be found in articles about the nuclei and tracts of the midbrain and pons.