Fig. 4.1
Tentorial and suboccipital surface view of cerebellum
The cerebellum has fissures that divide the organ into series of layers or leaves. The largest and deepest fissure is the horizontal sulcus. The horizontal sulcus divides the semilunar lobule into inferior and superior semilunar lobules.
The vermis is positioned between the two hemispheres and is an important structure in the transvermian approach as it connects both hemispheres. Culmen represents the most apical part of the vermis at the tentorial surface. From posterior to anterior, the subdivisions of the superior vermis are represented by lingula, central lobule, monticulus, and folium vermis. Vermal parts of the posterior lobe are, in the following order, the declive, folium, tuber, pyramid, and uvula. This attachment area of the pyramid is a landmark for the dentate nucleus in cadaveric dissections. The lobus centralis is a small square lobule situated in the anterior cerebellar notch (Fig. 4.2).
Fig.4.2
(a) On the tentorium surface the primary fissure is located in the DN area. The simple and quadrangular lobules have been retracted to expose the lateral part of the central lobule. Culmen is medial and above the DN from the tentorium surface. Its posteroinferior attachment area is approximately the beginning of the DN from the tentorium surface. If dissection begins from the primary fissure, after retraction of the simple and quadrangular lobules laterally, inferior cerebellar peduncle fibers can be followed from lateral to medial. (b) Inferior cerebellar peduncle has fibers crossing to the contralateral side of the cerebellum that pass anterior and inferior to the nodule. The hilus of the DN is approximately at this level. (c) Superiolateral side of the cerebellum, after removing quadrangular lobule on both sides. Superior medullary velum is dissected. After retraction of the superior cerebellar triangle laterally, the superiolateral side of the choroid plexus and the fourth ventricle floor is observed with the facial colliculus and median eminence. (d) Closure view of image C with the retraction of the superior cerebellar peduncle. Some middle cerebellar peduncle fibers have been removed to expose the border of the DN. CC corticocerebellar, DN dentate nucleus, MCP middle cerebellar peduncle, SCP superior cerebellar peduncle
The folium vermis is a short, narrow, and concealed band-like structure at the posterior extremity of the vermis. Laterally, it expands in either hemisphere into a sizeable lobule, the superior semilunar lobule which occupies the posterior third of the upper surface of the hemisphere and connected below by the horizontal sulcus. The superior semilunar lobule receives fibers mainly from middle cerebellar peduncles. These are known as pontocerebellar and corticoccerebellar fibers.
4.1 Lobes of the Cerebellum
The cerebellum consists of three parts, a median and two lateral, which are continuous with each other, and are substantially the same in structure. The median portion is constricted and is called the vermis. The hemispheres are separated below and behind by a deep notch, the posterior cerebellar notch, and in front by a broader shallower notch, the anterior cerebellar notch. The anterior notch lies close to the pons and upper part of the medullas, and its superior edge encircles the inferior colliculi and superior cerebellar peduncle. The posterior notch contains the upper part of the falx cerebelli, fold of dura mater. The cerebellum is characterized by a laminated or foliated appearance; it is marked by deep, somewhat curved fissures and divides into a series of layers or leaves. The largest and deepest fissure is named horizontal sulcus. Several secondary but deep fissures separate the cerebellum into lobes, and these are further subdivided by shallower sulci, which separate the individual folia, though differing in appearance from the convolutions of the cerebrum, are analogous to them, in as much as they consist of central white substance covered by gray substance (Figs. 4.2 and 4.3).
Fig. 4.3
Larger view of lateral appearance of cerebellum
The upper surface of the cerebellum is elevated in the middle and sloped toward the circumference of the hemisphere being connected together by the superior vermis. The superior vermis is mainly subdivided into four parts: lingula, the lobus centralis, the monticulus, and folium vermis. These are continuous with corresponding parts of cerebellar hemisphere, except lingua (Fig. 4.3).
The lingula is a small tongue-shaped process consisting of four or five folia. Anteriorly it rests on the dorsal surface of the anterior medullary velum and its white substance is continuous with that of the velum. Anteriorly it overlaps the lobulus centralis from which it is separated by the postcentral fissure. Laterally it is continuous with the quadrangular lobule in the hemispheres.
4.1.1 Superior Cerebellar Peduncle
On sagittal dissections, the superior cerebellar peduncle located approximately 5 mm from the midline can be identified lateral to superior medullary velum, tracking obliquely from the cerebellum to the midbrain. The superior cerebellar peduncle is the largest cerebellar efferent bundle. The superior cerebellar peduncles emerge from the upper and medial part of the white substance of the hemispheres, and they project in parallel and are placed undercover of the superior and inferior colliculi. They are joined by the superior medullary velum, can be followed up to the inferior colliculi, and disappear in the red nucleus.
The superior cerebellar peduncle is the largest efferent group of fibers of the DN. They form the upper lateral boundaries of the fourth ventricle, but as they ascend, they converge on the dorsal aspect of the ventricle and thus assist in the formation of its roof. The dentate-rubro-thalamic tract (dentatothalamic tract) is the major tract of the superior cerebellar peduncle. It ascends to the red nucleus, decussates, and projects to the ventrolateral side of the thalamus.
After exposure of the superior medullary velum, the nuclei of the abducens, the vestibular, and cochlear nerves can be readily seen in the fourth ventricle floor in anatomical specimens (Fig. 4.4).
Fig. 4.4
Tractographic reconstruction (a) showing an oblique posterior view of cerebellum and cerebellar peduncles with dentate nucleus (arrow) and dentate nucleus connectivity on the right side (light blue). SCP is only shown on the left side. SCP (superior cerebellar peduncle, green), ICP (inferior cerebellar peduncle, red, bilateral), MCP (middle cerebellar peduncle, yellow, bilateral), LM (lemniscus medialis, blue). (b) Enlarged view of the cerebellum with dissection of the tentorial surface. Middle cerebellar peduncle fibers are making a curve around the dentate nucleus. Inferior cerebellar peduncle fibers are passing anterior to the DN and crossing to other side anterior to the nodule. Dentate nucleus has projection fibers to posterior part of the inferior semilunar lobule
4.1.2 Middle Cerebellar Peduncle
There are two groups of middle cerebellar peduncle fibers passing to the cerebellum. The first group of fibers is parallel to the midline and is essentially called the corticocerebellar fibers. The other group projects parallel to the DN, toward the posterior cerebellum to reach the superior and inferior semilunar lobules. This group is known as pontocerebellar fibers. Middle cerebellar peduncles radiate to entire cerebellar locations. Superior fibers of the middle cerebellar peduncle make a curve around the DN. These fibers cover the DN from medial to lateral direction. Middle cerebellar peduncle fibers wrap the DN superiorly and inferiorly. Fibers on the inferior side are named as the middle cerebellar inferior fibers and they project to the posterior cerebellum passing superiorly over the tonsils. These fibers constitute the pontocerebellar fibers. Fibers located superiorly are referred to as the middle cerebellar superior fibers and some of those fibers project mainly ipsilaterally while others project contralaterally. Middle cerebellar peduncle fibers curve over the DN and project to the contralateral side from the posterior of the nodule (Fig. 4.5).
Fig. 4.5
On selective tracking, the images show very clearly the projection of the SCP. The importance of images showing the SCP suggests that these tracts correspond mainly to efferent cerebellar pathways. Nevertheless, there are images showing also fibers corresponding to the middle cerebellar peduncle (a). This is not surprising due to the fact that these nuclei are enveloped by the terminal fibers of this peduncle. The section corresponding to the anatomic specimen (b) shows very clearly the SCP with fibers from the dentate which cross the midline below the level of the red nucleus. The dentate nucleus is very outlined and could be traced with surrounding fibers. At the midbrain level, the ascending lateral lemniscus, the more oblique SCP, and the transverse crossing fibers will be easily distinguished by their different colors when they are analyzed on DTI. (c) The commissural vermian system was apparent in red on the midline. The other anteroposterior or posteroanterior tracts appeared in green, and these correspond to the projections of the cerebellar hemispheres to the dentate. However, their more anterior components were intermixed with the afferent white matter projections following the MCP. (d) The same specimen with closure view; lateral lemniscus and trigeminothalamic tract dissected over the dissector. Trochlear nerve is passing over the lateral lemniscus. Superior cerebellar peduncle is passing medially and inferiorly to lateral lemniscus. (e) Inferior cerebellar peduncle passes other side of nodule. Middle cerebellar peduncle fibers are covering the dentate nucleus. (f) Lateral view of the same specimen trigeminal nucleus arising from the middle cerebellar peduncle. Flocculus is just lateral to the middle cerebellar peduncle fibers. Corticopontine cerebellar fibers are passing over the trigeminal nerve
4.1.3 Inferior Cerebellar Peduncle
The inferior cerebellar peduncle ascends from the medulla to the cerebellum conveying a number of fiber systems to the cerebellum. Inferior cerebellar peduncle fibers ascend to the cerebellum from the lateral pons and pass dorsally from the roof of the fourth ventricle. The inferior cerebellar peduncle fibers cross to the contralateral side of cerebellum over the nodule; however, some fibers follow an ipsilateral course from the lateral border of the nodule. They mainly carry the spinocerebellar, cuneocerebellar, olivocerebellar, and vestibulocerebellar pathways.
The nodule or the anterior part of the inferior vermis is located between the two DNs. It has a semilunar shape and its convex border is continuous with the white substance of the cerebellum, extending on either side up to the flocculus. On either side of the nodule, a thin layer of a white substance exists which it is referred to as the inferior medullary velum. It also has a semilunar form and its convex border is continuous with the white substance of the cerebellum. Inferior cerebellar peduncle fibers cross to the contralateral side superior to the nodule and some ipsilateral fibers project posteriorly through the superolateral side of the nodule.
While some of these fibers pass from the anterior surface of the DN projecting from lateral direction to the medial, some others follow a parallel course to the nodule and dorsally to the fourth ventricle roof. Dorsal spinocerebellar fibers ascend ipsilaterally in a tract that is located at the edge of the spinal cord and terminate ipsilaterally. The inferior cerebellar peduncle also carries the olivocerebellar pathway. This pathway is distributed to all parts of the cerebellum at the ipsilateral and contralateral sides. The cuneocerebellar tract conveys fibers from the ipsilateral accessory cuneate nucleus to the cerebellum (Fig. 4.6).
Fig. 4.6
Inferior view of the fourth ventricle
4.2 Cerebellar Surfaces
4.2.1 Tentorial Surface
The tentorial surface faces the tentorium and is retracted in the supracerebellar approach. The anteromedial part of the tentorial surface, the apex, formed by the anterior vermis, is the highest point on the cerebellum. Its surface slopes downward from its anteromedial to its posterolateral edge. The tentorial surface carries the inferior and middle cerebellar peduncle fibers from superioanterior to posterioinferior direction. The hemispheric part of the tentorial surface includes the quadrangular, simple, and superior semilunar lobules and the vermian division includes the culmen, declive, and folium.
This lobe receives corticocerebellar and pontocerebellar fibers from the middle cerebellar peduncle. Anterior and medial to these fibers lie the inferior cerebellar peduncle fibers.
The tentorial surface is divided into anterior and posterior sections by its major fissure, the tentorial fissure. This fissure, located between the quadrangular and the simple lobules on the hemisphere and the culmen and the declive on the vermis, has also been called the primary fissure. The postclival fissure separates the simple and the superior semilunar lobules.
The dissection of the tentorial surface from the simple lobe to the superior semilunar lobule is shown. The superior cerebellar peduncle and lateral lemniscus can be seen after dissection of the simple and quadrangular lobules. The inferior cerebellar peduncle fibers and superior cerebellar peduncle fibers can be observed after removing the simple and quadrangular lobules. After dissection of middle cerebellar peduncle fibers, a very thin external capsule is observed superior to the DN (Fig. 4.1).
4.2.2 Lateral Surface
The lateral recesses are narrow, curved pouches formed by the union of the roof and the floor. The rostral wall of each lateral recess is formed by the caudal margin of the cerebellar peduncles. The inferior cerebellar peduncle courses upward in the floor ventral to the lateral recess and turns posteriorly at the lower part of the pons to form the ventricular surface of the rostral wall. The inferior cerebellar peduncle ascends superomedial to the DN and courses between the superior and middle cerebellar peduncles. The biventral lobule that is inferomedial to the DN is dorsal to the lateral recess. The lateral lemniscus courses lateral and superior to the DN and just lateral to the inferior cerebellar peduncle, and it crosses the superior cerebellar peduncle. The trigeminothalamic tract passes lateral to the lateral lemniscus and reaches the ventral posteriomedial nucleus of the thalamus ipsilaterally. It arises from the sensory trigeminal nucleus that is located lateral to the locus cereus and vestibular fibers that join the medial longitudinal fasciculus (Fig. 4.5).
4.2.3 Suboccipital Surface
Operative approaches to the fourth ventricle and to most cerebellar tumors commonly involve this surface and its close proximity. The lateral walls of the incisura are formed by the medial part of the cerebellar hemispheres. Deep clefts, known as the vermohemispheric fissures, separate the vermis from the hemispheres. The vermian surface within the incisura has a diamond shape. The upper half of the diamond-shaped formation has a pyramidal shape, thus called the pyramid. The folium and the tuber, superior to the pyramid, form the apex of the suboccipital part of the vermis. These structures are posteromedial to the DN. The nodule, the lowermost subdivision of the vermis, is hidden deep to the uvula.
The hemispheric portion of the suboccipital surface is formed by the superior and inferior semilunar and biventral lobules and the tonsils. This portion is important for the supratonsillar and subtonsillar approaches.
The tonsils and biventral lobule have been separated and the white matter in between is known as the tonsillobiventral fissure. This is an important landmark for the supratonsillar approach. This white matter is formed mainly by the middle cerebellar peduncle fibers. On the suboccipital surface, the pyramid and uvula form the area where the DN is closest to the surface. The uvula and tonsil comprise the lobus uvula. The pyramid and biventral lobules constitute the lobus pyramidalis. The pyramid is a conical projection forming the largest prominence of the inferior vermis. The lateral border is separated from the inferior semilunar lobule by the postpyramidal fissure.
In the sagittal section through the center of the vermis, superior cerebellar peduncle fibers and superior medullary velum can be observed along with an arbor vitae appearance. After sharp dissection of the cerebellum from posterior to anterior, the fourth ventricle can be observed with the facial colliculus. Facial nucleus and its fibers have been dissected in the specimen as to demonstrate the relation of nucleus. When the cerebellum is sharply dissected step-by-step from posterior to anterior, no gray matter can be encountered in the first 2 cm of dissection. In the second 2 cm of dissection, symmetrical appearance of the DN can be observed. In the third 2 cm dissection, the DN disappears (Fig. 4.7).
Fig. 4.7
(a) The suboccipital surface. The vermis sits in a large median depression, the posterior cerebellar incisura, between the cerebellar hemispheres. The portions of the vermis within the incisura from above to below are the folium, tuber, pyramid, and uvula. The parts of the hemispheric surface from above to below are the superior and inferior semilunar and biventral lobules and the tonsils. These lobules extend beyond the suboccipital surface to the other surfaces of the cerebellum. The prebiventral fissures between the inferior semilunar and the biventral lobules separate the hemispheres into superior and inferior parts, and the prepyramidal fissure between the pyramid and tuber separates the vermis into superior and inferior parts. From below to above, the corresponding vermian and hemispheric parts are the uvula and the tonsils, the pyramid and the biventral lobules, the tuber and inferior semilunar lobules, and the folium and the superior semilunar lobules. The petrosal (horizontal) fissure, the most prominent fissure on the petrosal surface, extends onto the suboccipital surface and divides the superior half of the suboccipital surface between the superior and inferior semilunar lobules. Between the biventral and tonsillar lobules there is prebiventral fissure which is important for approaching the fourth ventricle via the supratonsillar approach. (b) Biventral lobule has been dissected. The pyramid and biventral lobules constitute the lobuspyramidis. The pyramid is a conical projection, forming the largest prominence of the inferior vermis. The lateral border is separated from the inferior semilunar lobule by the postpyramidal fissure. The pyramid has attachment to the DN laterally. The uvula has attachment at the base of the tonsils and the uvula and pyramid make an important landmark for the location of the DN. The base is directed forward, and is on a line with the anterior border of the tonsil, and is separated from the flocculus by the postnodular fissure. White matter in this area is formed with middle cerebellar peduncle fibers. (c) The pyramid attachment side to cerebellum is demonstrated. The folium and the tuber, superior to the pyramid, form the apex of the suboccipital part of the vermis. These structures are posteriomedial to the DN. The uvula is the medial part of DN. The tonsil is located anterior, inferior and medial to the DN. (d) The lower half of the diamond-shaped formation, the uvula, projects downward between the tonsils. The uvula and pyramid are very close to the DN rather than the other midline structures. (e) There are some fibers projecting posteriorly called the middle cerebellar peduncle inferior fibers. Middle cerebellar peduncle fibers can be distinguished laterally and medially. Note the DN at this level, which is medial to the trajectory of the supratonsillar approach. (f) Closure view of (e). The DN border is inferior to the tonsil attachment area, medially pyramidal attachment is 1.8 mm away from the medial border of the DN. CC corticocerebellar, DN dentate nucleus, ICP inferior cerebellar peduncle, ISL inferior semilunar lobule, LL lateral lemniscus, MCP middle cerebellar peduncle, N nodule, PC pontocerebellar, SL simple lobule, SCP superior cerebellar peduncle, SMV superior medullary velum, SSL superior semilunar lobule
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