Cerebellum Gross Anatomy
There are over 206 bones in the human body that are interconnected by a myriad of ligaments. They provide support for over 650 skeletal muscles that facilitate the process of locomotion. While these skeletal muscles are mostly under the influence of the somatic nervous system, there are involuntary impulses that help in the maintenance of muscle tone, balance and posture. These impulses are generated and regulated at the level of the cerebellum. In addition to the aforementioned tasks, the cerebellum is also responsible for the coordination of volitional activities such as the acrobatics performed while rushing to get dressed at 7:59 am for that 8:00 am anatomy class. This article will be centred on the general structure and organization of the cerebellum and its blood supply.
The cerebellum arises from the rhombencephalon or hindbrain. More specifically, it arises from the rhombic lips or alar plates (dorsal thickening of the neural tube that forms the sensory areas of the nervous system) of the metencephalon (spans between the pontine flexure and the rhombencephalic isthmus). It is an ovoid structure that resides in the posterior cranial fossa, inferior to the tentorium cerebelli. It has an outer grey matter cortex and white matter internally.
When viewed from the superior (tentorial) surface, it is easy to appreciate that the cerebellum has a central, vertically protruding structure that runs anteroposteriorly known as the superior vermis. The superior vermis commences at the anterior cerebellar notch, and then curves 180o at the posterior cerebellar notch and continues along the midline of the inferior (occipital) surface as the inferior vermis. Bilateral to the vermis (both superior and inferior) are the cerebellar hemispheres.
There are several horizontal grooves along both surfaces of the cerebellum that gives it a stratified appearance. However, some are more prominent than others and provide ideal demarcations for the lobes and lobules. One such groove is the horizontal fissure, which arcs around the cerebellum along its lateral and posterior margins to the point of the posterolateral fissure (inferior to the flocculonodular lobe). Additionally, there is the postlunate fissure that arcs from left to right across the tentorial cerebellar surface. This is posterior to the primary fissure that forms the main demarcation between the anterior and posterior lobes. Both the primary and postlunate fissures continue along the inferior surface of the cerebellum. Finally, there is a retrotonsillar fissure that grooves behind each cerebellar tonsil.
There are three main lobes, nine lobules along the vermis, and five hemispheric lobules. The anterior lobe extends from the level of the cerebellar peduncles anteriorly and includes the anterior two-thirds of the superior vermis along with the anterior third of each hemisphere. This lobe terminates at the primary fissure. From this point posteriorly and laterally and continuing along the inferior surface to the posterolateral fissure is the larger posterior lobe. The smallest of the lobes is the flocculonodular lobe. It is a flattened lobe that lies between the posterolateral fissure (inferiorly) and the inferior medullary velum and the cerebellar peduncles (superiorly).
The vermal lobules are assigned Roman Numerals I – X. They can be better appreciated when the cerebellum is divided along the midline of the vermis. In this section, the cerebellum has a cauliflower appearance and the lobules are numbered in a clockwise manner, with the apex of the first lobule at the 10:00 position. The lobules can be better remembered with the aid of the following mnemonic: “Loving Caring Children Donate Food To Poor Unfed Needy”.
- L – Lungula (I) [10:00]
- C – Central lobule (II & III)
- C – Culmen (IV & V) [11:00 – 1:00]
- D – Declive (VI) [2:00]
- F – Folium (VII A) [3:00]
- T – Tuber (VII B) [4:00]
- P – Pyramid (VIII) [5:00]
- U – Uvula (IX) [7:00]
- N – Nodule (X) [9:00]
Lobules I – V are a part of the anterior lobe along the superior vermis. Lobules VI – VII A are also along the superior vermis, but in the posterior lobe. Lobules VII B – IX are also in the posterior lobe but on the inferior vermis. Lobule X is on the inferior vermis, but is a part of the flocculonodular lobe. The entire vermis gains its afferents from the spinocerebellar tracts and sends its information to the fastigial nucleus.
In addition to the central vermal lobules, there are horizontal lobules that are also assigned Roman Numerals, but with the letter “H” before it, denoting it is a part of the hemispheres. Anterior to the primary fissure is the quadrangular lobule (H IV – V). Between the primary and the postlunate fissures is the simple lobule (H VI). The region from the postlunate fissure to the horizontal fissure is the superior semilunar lobule (H VII A). The inferior semilunar lobule (H VII B) continues from the horizontal fissure to the postlunate fissure on the inferior surface of the cerebellum. The retrotonsillar fissure (anteriorly) and the postlunate fissure on the inferior surface (posteriorly) forms the border of the biventer lobule (H VIII). Finally, as characteristic feature of the inferior surface of the posterior lobe is the cerebellar tonsils. They are triangular structures that border the distal inferior vermis.
There are three foot processes that not only anchor the cerebellum to the brainstem, but also provide a pathway for neuronal tracts to travel to and from the cerebellum. These structures are the superior, middle and inferior cerebellar peduncles.
Connections to brainstem
- to midbrain: superior cerebellar peduncles
- to pons: middle cerebellar peduncles
- to medulla: inferior cerebellar peduncles
The superior cerebellar peduncle provides a pathway for nerve fibers of the cerebellar nuclei to leave for their destination in the thalamus and for the fastigial fibers to send information to the vestibular nuclei. Additionally, impulses enter the cerebellum by this pathway from the ventral spinocerebellar tract (proprioception), trigeminal nucleus and the locus coeruleus.
The middle cerebellar peduncle provides a pathway for afferent pontocerebellar fibers to enter the neocerebellum (bilateral to the paravermal zone, which is bilateral to the vermal zone).
Thirdly, the inferior cerebellar peduncle carries afferent and efferent fibers by two pathways. The first, which carries dorsal spinocerebellar, olivocerebellar and cuneocerebellar fibers, is the restiform body that carries only afferent fibers. The second is the juxtarestiform body that carries both afferent (vestibulocerebellar) and efferent (cerebellovestibular) fibers.
As previously mentioned, the cerebellum is located in the posterior cranial fossa, inferior to the tentorium cerebelli. The great cerebral vein of Galen is located anterosuperior to the cerebellum. It continues into the straight sinus (in the midline of the tentorium cerebelli), which has a direct superior relation to the cerebellum. The lingual gyrus of the occipital lobe is also superior to the cerebellum. There are several relevant anterior relations to the cerebellum. These include the brainstem, and superior and inferior medullary vela (s. velum), which covers the fourth ventricle. The distal end of the cerebral aqueduct of Sylvius, and the corpora quadrigemini and its respective cistern is anterosuperior with respect to the cerebellum, while the posterior cerebellomedullary cistern, the foramen of Magendie, medulla oblongata and the foramen magnum are all located anteroinferiorly. It should also be noted that like the rest of the brain, the cerebellum is bathed in cerebrospinal fluid that enter the cisterns from the aforementioned foramen of Magendie and foramina of Luschka. Posterior and lateral to the cerebellum are the occipital bone and its overlying layer of dura mater. The sigmoid sinuses are also found inferiorly, and the occipital sinus (if present) and the confluence of sinuses are both posteriorly related.
The vertebrobasilar system arises from the first part of the subclavian arteries and travel cranially through the transverse foramina of the upper 6 cervical vertebrae. The left and right vertebral arteries unite after they enter the cranial vault through the foramen magnum at the pontomedullary junction. Here it forms the basilar artery. Four main branches provide vascular supply to the cerebellum, two of which arise from the basilar component of the system and one arises from each vertebral branch of the system. The first branch from the basilar artery is the superior cerebellar artery. It arises bilaterally and inferior to CN III. It provides perfusion for the superior cerebellar region along with the superior medullary velum, pineal gland and pons. The anterior inferior cerebellar artery (AICA) arises inferior to CN VI at the pontomedullary junction at the proximal part of the basilar artery. It then courses posterolateral to supply the inferior and anterior cerebellar regions. Thirdly, a posterior inferior cerebellar artery (PICA) arises from each vertebral artery, just superior to the rootlet of CN XI. It provides arterial blood to the cerebellar nuclei and its inferior surface (including the vermis).