Overview of the cerebellum and the brainstem

Brainstem

As the name suggests, the brainstem is a structure situated at the base of the brain connecting the subcortical structures with the spinal cord. It is associated with various vital functions, such as the sleep-wake cycle, consciousness, and respiratory and cardiovascular control. It also houses the majority of the cranial nerve nuclei and facilitates communication between the cerebrum, spinal cord, and cerebellum by relaying neural tracts.

The brainstem is formed of three parts: medulla oblongata, pons, and midbrain.

Medulla oblongata

The medulla oblongata is the most inferior portion of the brain stem, sitting in the posterior cranial fossa. It is continuous with the spinal cord from below and the pons above. The medulla oblongata is responsible for various autonomic functions and contains the cardiac, respiratory, reflex, and vasomotor centers.

Its anterior surface has several emerging cranial nerves as well unique bumps or protuberances formed by the various tracts and nuclei coursing through them. These are the anterior median fissure, pyramids (corticospinal tracts), olives (inferior olivary nuclei), and the hypoglossal (CN XII), glossopharyngeal (CN IX), and vagus nerves (CN X).

The medulla oblongata also contains several anatomical features on its posterior surface. These include the posterior medial sulcus, cuneate and gracile fasciculi, cuneate and gracile tubercles (respective nuclei), trigeminal tubercle (spinal nucleus of the trigeminal nerve), lateral funiculus (lateral white matter fibers), inferior half of the rhomboid fossa (floor of the fourth ventricle), and the obex. The arterial supply of the medulla oblongata is provided by the anterior and posterior inferior cerebellar arteries, together with the anterior spinal artery.

Pons

Continuing further up, we meet the pons. This is the middle portion of the brainstem, bridging the medulla oblongata and the midbrain. The pons' function is varied, mainly involving sleep, respiration, swallowing, hearing, bladder control, equilibrium and taste, as well as various other motor functions. 

The anterior surface of the pons appears striated containing many parallel lines created by the corticopontocerebellar fibers running horizontally through them. The superior and inferior pontine sulci (grooves) separate it from the neighboring brainstem parts, while the trigeminal (CN V), abducens (CN VI), facial (CN VII), and vestibulocochlear (CN VIII) cranial nerves exit from its anterior face. There is also a basilar groove running along it that houses the basilar artery. 

The posterior surface of the pons is intimately connected with the fourth ventricle and the cerebellum. The associated structures and protuberances include the superior half of the rhomboid fossa, median eminence, posterior median sulcus, facial colliculus (facial nerve), striae medullaris (fibers of the arcuate nucleus), locus coeruleus (part of reticular formation), and the vestibular areas (vestibular nuclei). The pons is supplied by pontine branches from the basilar artery.

Midbrain

Last but not least, we have reached the third part of the brainstem called the midbrain or mesencephalon. It is the most superior portion, lying between the pons found inferiorly and the thalamus superiorly. It plays an important role in motor eye movement, visual and auditory processing, alertness and temperature regulation.

First of all, the midbrain is divided into two halves by the cerebral aqueduct contained within. The anterior part is known as the tegmentum and the posterior as the tectum. 

The anterior surface of the midbrain contains two cerebral peduncles and the red nucleus. Each peduncle contains the substantia nigra and the crus cerebri (cerebral crus) that carries the fibers of the corticospinal tracts, which are part of the pyramidal tract. Situated between the peduncles is the interpenduncular fossa that contains the posterior perforated substance through which the perforating arteries supplying the midbrain pass through. The red nucleus is a round and highly vascularised mass of grey matter where the rubral afferent (corticobulbar, cerebellorubral) and efferent (rubrospinal, rubro-olivary) tracts end and begin, respectively. Additional structures on the anterior surface of the midbrain include the optic tracts and the emerging oculomotor nerve (CN III). 

Going around the midbrain, we come across several features visible on its posterior surface. The most prominent one is the quadrigeminal plate (corpora quadrigemina) which appears as four swellings called colliculi. Each superior colliculus represents the relay station for visual reflexes, while each inferior colliculus is a relay station of the auditory pathway. Corresponding colliculi are separated medially by the frenulum of the superior medullary velum. There, the trochlear nerve (CN IV) emerges. 

Learning about the brainstem can be quite difficult and time-consuming due to its complicated structure, so use the following resources to simplify it as much as possible.

Cranial nerve nuclei

Now that we’ve clarified the external structure of the brainstem, let’s take a closer look at its internal structure. The brainstem houses the majority of the cranial nerve nuclei, except those involved with olfaction (olfactory nerve (CN I)) and vision (optic nerve (CN II)). Originating from these nuclei are either efferent or afferent cranial nerves. The efferent ones provide motor functions to the structures of the head, neck, and internal organs, while the afferent transmit sensory information.

Furthermore, both efferent and afferent cranial nerves can be: 

• Special, responsible for innervating special senses
• General, transmitting impulses to/from everywhere else other than the senses
• Somatic, travelling to/from the skin and skeletal muscles
• Visceral, carrying information related to internal organs

The brainstem also consists of important nuclei associated with parasympathetic and sympathetic functions.

Memorizing all the cranial nerve nuclei can be daunting, especially at first.

However, you can find some study units below that can help you.

Reticular formation

The reticular formation is a collection of neurons interspersed between the cranial nuclei and the tracts of the brainstem. This region of grey matter extends from the spinal cord to the thalamus and is responsible for keeping you conscious during the day and for waking you up from sleeping.

The nuclei of the reticular formation are located deep within the brainstem and are divided into median, medial, and lateral groups. The afferent and efferent pathways associated with the reticular formation are the spinothalamic, dorsal column-medial lemniscus, reticulobulbar, and reticulospinal tracts.

Cerebellum

The cerebellum is situated in the posterior cranial fossa behind the pons and medulla oblongata, separated from them by the fourth ventricle. It plays an important role in the coordination and precision of motor functions, as well as in motor learning. Thanks to your cerebellum function, you can walk without thinking about each step. Unfortunately, cerebellar degeneration can lead to a variety of disorders such as walking difficulties, body tremors, and limb jerkiness. 

The cerebellum consists of two large hemispheres united in the middle by the vermis. Numerous transverse fissures divide the cerebellum into three lobes (anterior, posterior, and flocculonodular) and many lobules. The flocculonodular lobe consists of a flocculus and a nodule. The blood supply of the cerebellum is provided by the superior cerebellar, anterior inferior cerebellar, and posterior inferior cerebellar arteries.

The cerebellum is connected to the brainstem by three pairs of cerebellar peduncles: the superior peduncle with the midbrain, the middle peduncle with the pons, and the inferior peduncle with the medulla oblongata. Afferent and efferent connections that run between the cerebellum, brainstem and spinal cord, travel through the cerebellar peduncles.

The cerebellum is comprised of a grey cerebellar cortex, a medullary core of white matter and four pairs of intrinsic nuclei. The cerebellar cortex consists of numerous narrow leaf-like laminae known as the cerebellar folia. The medullary core of white matter has a complex branching system, and together with the tree-like appearance of the laminae and folia, it has been given the descriptive term ‘arbor vitae’, the tree of life. The deep cerebellar nuclei are called fastigial, globose, emboliform, and dentate nuclei.

To master the external and internal structure of the cerebellum, take a look at the following videos. In addition, be sure to check out our custom quiz about the cerebellum and brainstem and learn more about the anatomy of these structures!
 

Related articles

Articles within this topic:

• Afferent and efferent pathways of the cerebellum
• Brainstem
• Cerebellar ataxia
• Cerebellar degeneration
• Cerebellar nuclei and tracts
• Cerebellum gross anatomy
• Corpora quadrigemina
• Cranial nerve nuclei
• Cross section of the medulla oblangata
• Inferior colliculus
• Medulla oblongata
• Midbrain (Mesencephalon)
• Pons
• Red nucleus
• Solitary tract and nucleus
• Spinocerebellar tract
• Tectum and tegmentum