Brainstem

Location and parts

The brainstem is a stalk-like projection that extends caudally from the diencephalon into the cranial fossa, to connect distally with the spinal cord and posteriorly with the cerebellum. The brainstem begins at the level of the cerebral peduncles (anteriorly) and the superior colliculi of the corpora quadrigemina (posteriorly). It continues along a slight posteroinferior course until it ends at the decussation of the pyramids (at the level of the foramen magnum of the skull).

In the superior-inferior direction, the brainstem can be divided into three parts:

• the midbrain is the superior segment
• the pons is the middle segment and extends anteriorly
• the medulla oblongata is the inferior segment and it is the narrowest

In the anterior-posterior direction, the brainstem can be divided into three parts:

• the basilar part is the most anterior region; most descending pathways travel through this region
• the tegmentum is posterior to the basilar part and ventral to the cerebral aqueduct and fourth ventricle; most ascending pathways travel through this region
• the tectum is dorsal to the cerebral aqueduct and fourth ventricle, primarily in the midbrain.

Review the anatomy of the central nervous system and find out more about the anatomy of the brainstem in these study units.

Midbrain

The midbrain is the superior segment of the brainstem. It extends caudally from the base of the thalamus to the superior roof of the fourth ventricle. The superior boundary of the midbrain is an imaginary line extending approximately from the posterior commissure to the mammillary bodies. Inferiorly, the midbrain ends at the superior pontine sulcus (anteriorly) and the superior medullary velum (posteriorly). The midbrain arises from the mesencephalon.

Two stalks known as the cerebral peduncles extend from the ventral surface of the midbrain to connect numerous ascending and descending tracts between the cerebral cortex and the brainstem. The anterior part of the midbrain is occupied by the crus cerebri, which contains motor fibers of the corticospinal, corticonuclear and corticopontine tracts.

The tegmentum of the midbrain contains several structures, including the nuclei of the oculomotor nerve (CN III) and the trochlear nerve (CN IV), the red nucleus, the substantia nigra, and ascending pathways like the medial lemniscus and the spinothalamic tract.

The tectum of the midbrain contains two pairs of relay nuclei, the superior colliculi and the inferior colliculi, involved in visual and auditory pathways.

Learn how the midbrain relates to its surrounding structures in this study unit.

Pons

The pons is located between the midbrain and the medulla oblongata. It is limited superiorly by the superior pontine sulcus, and inferiorly by the medullopontine sulcus. It develops from the metencephalon, which is a secondary brain vesicle that is formed from the hindbrain (rhombencephalon).

The ventral (basal) part of the pons houses fibers of the corticospinal and corticobulbar tracts and the pontine nuclei, which relay corticopontocerebellar communication through the middle cerebellar peduncles.

The smaller pontine tegmentum contains the nuclei of the trigeminal nerve (CN V), abducens nerve (CN VI), facial nerve (CN VII) and vestibulocochlear nerve (CN VIII), as well as several white matter tracts like the medial longitudinal fasciculus, medial and lateral lemnisci, and others.

Need to review the structures involved in the ascending and descending pathways? We’ve got you covered with this study unit!

Medulla oblongata

The medulla oblongata (or medulla) is the narrowest and most caudal part of the brainstem. It connects with the pons at the medullopontine sulcus, and inferiorly is continuous with the spinal cord. The medulla develops from the myelencephalon, which is a secondary brain vesicle that arises from the rhombencephalon (the hindbrain).

Important structures of the ventral medulla include the pyramids, the inferior olivary nucleus, and the rootlets of the glossopharyngeal nerve (CN IX), vagus nerve (CN X), accessory nerve (CN XI) and hypoglossal nerve (CN XII). Fibers of the corticospinal tract cross over at the decussation of pyramids in the lower medulla oblongata.

The dorsolateral part of the superior medulla contains the dorsal vagal, solitary, medial vestibular, and cuneate nuclei; in the lower section, the hypoglossal and the gracile nuclei can also be found. The lateral aspect of the medulla is occupied by the lateral reticular nucleus and by the spinal trigeminal tract and nucleus. Several ascending tracts also travel through the posterior medulla, including the medial lemniscus which decussates in the arcuate fibers.

Interested in knowing more about the medulla oblongata? Here is a study unit for you.

Blood supply

The brainstem is primarily supplied by branches of the vertebrobasilar system, including the vertebral and basilar arteries. The mesencephalic (midbrain) branches originate from the superior cerebellar, collicular, posteromedial choroidal, posterior cerebral and anterior choroidal arteries. The pontine branches arise from the basilar, anteroinferior cerebellar and superior cerebellar arteries. The medullary branches originate from the vertebral, anterior spinal, basilar, posterior inferior and anterior inferior cerebellar arteries.

Drainage of the brainstem is achieved by a deep plexus of veins that drain into a number of veins, including the veins of the spinal cord, the basal vein and the great cerebral vein of Galen.

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Function

The primary functions of the brainstem include:

• Regulation of the functions of involuntary vital systems (e.g. heart rate, breathing etc)
• Conduction of motor and sensory information between the cerebral cortex, cerebellum and spinal cord
• Functions related to the cranial nerves

Specific functions are localized to distinct areas of the brainstem. The midbrain, for example, contributes to the motor control of eye movements and the processing of visual and auditory information (via cranial nerves III and IV). A main function of the pons is to facilitate the communication between the cerebrum and the cerebellum, and between hemispheres. Finally, functions of the medulla oblongata include the regulation of respiratory function, cardiovasculatory functions, as well as gastrointestinal functions. All the parts of the brainstem contribute to cranial nerve functions and conduct sensory and motor information via white matter tracts.

Consciousness and arousal are functions regulated by the reticular formation, a vast network of neurons that extends throughout the brainstem. The reticular formation is also the origin of the reticulospinal and reticulobulbar descending pathways, which regulate the involuntary activation of muscles involved in tasks like breathing, facial expression and balance.

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Related articles

Articles within this topic:

• Corpora quadrigemina
• Cranial nerve nuclei
• Cross section of the medulla oblangata
• Inferior colliculus
• Medulla oblongata
• Midbrain (Mesencephalon)
• Obex
• Pons
• Red nucleus
• Solitary tract and nucleus
• Tectum and tegmentum

Related videos

Videos within this topic:

• Anterior view of the brainstem
• Basal view of the brain
• Brainstem
• Cranial nerve nuclei
• Medulla oblongata: Vagus nerve level
• Ventricles of the brain