Cerebellum

The cerebellum (from Latin “small brain” ; PNA : cerebellum ) is a region of the brain whose main function is to integrate the sensory pathways and the motor pathways . There are a large number of Nerve bundles that connect the cerebellum with other brain structures and with the spinal cord. The cerebellum integrates all the information received to specify and control the orders that the cerebral cortex sends to the locomotor system through the motor pathways.

For this reason, lesions at the cerebellum level do not usually cause paralysis but disorders related to the execution of precise movements, maintenance of balance and posture and motor learning do. The first studies carried out by physiologists in the 18th century indicated that those patients with cerebellar damage showed problems with motor coordination and movement. During the 19th century the first functional experiments began to be carried out, causing cerebellar lesions or ablations in Animals. Physiologists observed that such injuries generated strange and awkward movements, incoordination and muscle weakness. These observations and studies led to the conclusion that the cerebellum was an organ in charge of motor control. [1] However, modern research has shown that the cerebellum has a broader role, thus being related to certain cognitive functions such as Attention and processing of Language , Music , learning and other temporal sensory stimuli.

It was first described by Herophilus in the 4th century BC. C.

Summary

[ hide ]

  • 1 General characteristics
  • 2 Embryological development
  • 3 Phylogenetic evolution
  • 4 Anatomy
    • 1 External description
      • 1.1 Top face
      • 1.2 Bottom face
      • 1.3 Anterior face
    • 2 Divisions
      • 2.1 Morphological
      • 2.2 Phylogenetics
      • 2.3 Functions
    • 3 Topographical representation of the body
    • 4 Internal structure
      • 4.1 Cerebellar cortex
        • 4.1.1 Layers of the crust
        • 4.1.2 Neural types
        • 4.1.3 Extrinsic fibers
        • 4.1.4 Glia
      • 4.2 Deep nuclei
      • 4.3 White matter
    • 5 Cerebellar connections
      • 5.1 Vestibule Conferences
      • 5.2 Cemetery lobby lobby
      • 5.3 Spinocerebellus input
      • 5.4 Spinocerebellar Eferences
      • 5.5 Cerebrocerebral input
      • 5.6 Cerebrocerebral Eferences
      • 5.7 Contributions from monoaminergic systems
    • 6 Peduncles
      • 6.1 Lower cerebellar peduncles
      • 6.2 Middle cerebellar peduncles
      • 6.3 Superior cerebellar peduncles
    • 7 Arterial irrigation
      • 7.1 Superior cerebellar artery
      • 7.2 Inferior cerebellar artery
      • 7.3 Lower posterior cerebellar artery
    • 8 Venous drainage
  • 5 Neural circuits
    • 1 Neural circuits of the deep nuclei: main arc
    • 2 Neuronal circuits of the cerebellar cortex: secondary arch
      • 2.1 Exciter circuits
      • 2.2 Inhibitor circuits
    • 3 Exit signs
    • 4 Long-term depression of Purkinje cells: motor learning
  • 6 Theories of cerebellar function
    • 1 Modeling of cerebellar function
  • 7 Pathology
    • 1 Cerebellar syndrome
      • 1.1 Cerebellar vermis syndrome
      • 1.2 Hemispherical cerebellar syndrome
      • 1.3 Etiology of the cerebellar syndrome
    • 8 See also
    • 9 External links
    • 10 References
    • 11 For more information

General characteristics

Cerebellum

The cerebellum is an odd and a half organ, located in the posterior cranial fossa, dorsal to the brainstem and inferior to the occipital lobe. It has a central and odd portion, the vermis, and two other much larger portions that extend on both sides, the hemispheres.

The cellular organization of the cerebellar cortex is very uniform, with neurons arranged in three well-defined layers or strata. This very uniform organization makes nerve connections relatively easy to study. To get a general idea of ​​the neural connections that occur in the cerebellar cortex, imagine a row of trees with cables connecting the branches of each with those of the next.

Due to the high number of granular cells it has, the cerebellum contains about 50% of all brain neurons, but it only represents 10% of its volume. The cerebellum receives about 200 million afferent fibers. In comparison, the optic nerve is made up of a million fibers.

Embryological development

Like the rest of the central nervous system and the skin , the cerebellum derives from the ectodermal layer of the trilaminar germ disc.

During the earliest stages of embryonic development, the cephalic third of the neural tube has three dilations ( brain vesicles primary ) allowing us to divide it into three distinct segments: forebrain , midbrain and Rhombencephalon . The hindbrain is the most caudal segment, and when the embryo is 5 weeks old it is divided into two parts: the midbrain and the mylencephalon. The Metencéfalo is the most cephalic portion and will give rise to the protuberance (bridge) and the cerebellum, whereas of the Mielencéfalo will originate the medulla oblongata (medulla oblongata). The boundary between these two portions is marked by theProtuberance curvature .

Like all structures deriving from the neural tube, the metencephalon is made up of basal and wing plates separated by the limiting groove . The wing plates containing sensory nuclei that are divided into three groups: group afferent lateral somatic, visceral afferent special group and visceral afferent group general. The Basal Plates contain motor nuclei that are divided into three groups: the medial somatic efferent group, the special visceral efferent group, and the general visceral efferent group.

The dorsolateral portions of the wing plates are curved medially to form the rhombic lips . In the caudal portion of the midbrain, the rhombic lips are widely separated, but in the cephalic portion they approach the midline. As the protuberance fold deepens, the rhombic lips compress in the cephalo-caudal direction and form the cerebellar plate . At 12 weeks of development, the cerebellar plate shows the existence of three parts: the Vermis , in the midline, and two hemispheres, on both sides. Before long, a transverse fissure separates the nodule from the rest of the vermis and the flocs from the rest of the hemispheres.

Initially, the cerebellar plate is composed of three layers, which are deep to superficial: neuroepithelial layer , mantle layer and marginal layer . At approximately 12 weeks of development, some cells originating from the neuroepithelial layer migrate to the most superficial area of ​​the marginal layer. These cells retain the ability to divide and begin to proliferate on the surface where they end up forming the outer grainy layer.. In the 6-month-old embryo, the outer grainy layer begins to differentiate into various cell types that migrate inward to pass between Purkinje cells and give rise to the inner granular layer. The outer grainy layer ends up running out of cells and gives rise to the molecular layer. Basket cells and stellate cells come from cells that proliferate in the White Matter (marginal layer).

The deep cerebellar nuclei, such as the dentate nucleus , are in their final position before birth, while the cortex of the cerebellum reaches its full development after birth.

Phylogenetic evolution

Cerebellum

The cerebellum appears in all Vertebrates but with a different degree of development: very reduced in Fish , Amphibians and Birds , it reaches its maximum size in Primates, especially in man.

Anatomy

The cerebellum is attached to the posterior wall of the brain stem and is included within an osteofibrous case -the cerebellar or subtentorial cell- formed by an upper and a lower wall. The upper wall is made up of an extension of the dura called the Cerebellum Store and the lower wall is formed by the cerebellar fossae of the occipital bone covered by the dura . Normally, the cerebellum of an adult male weighs about 150 g and is 10 cm wide, 5 cm high, and 6 cm antero-posterior. In children the ratio between the cerebellum and brain volume is 1 to 20, while in adults it is 1 to 8.

External Description

The isolated cerebellum is shaped like a truncated cone flattened in the superior-inferior direction, in which three faces can be distinguished: superior, inferior and anterior.

Top face

The upper face is shaped like a roof with two lateral slopes and is in contact with the cerebellum tent. In the central part, it presents an elongated elevation in the antero-posterior direction that is called the upper vermis. On both sides of the upper vermis there are two sloping and almost flat surfaces that constitute the upper faces of the cerebellar hemispheres. The superior aspect is separated from the inferior aspect by the circumferential edge of the cerebellum. In a superior view, the circumferential edge presents two notches: one anterior in relation to the brainstem, and another posterior in relation to the Sickle of the cerebellum. The circumferential edge of the cerebellum is traversed longitudinally by a deep fissure called the horizontal fissure or circumferential groove.

Underside

The lower face is directly supported by the dura that covers the cerebellar fossae. It shows a wide groove in the midline called the vallecula or middle fissure that houses the Sickle of the cerebellum and at the bottom of which is the lower vermis, which is the continuation of the upper one. Laterally to the middle fissure, the inferior faces of the cerebellar hemispheres are located, which are convex downwards. In the most anterior part and on both sides of the lower vermis, the cerebellar hemispheres present an ovoid prominence called the cerebellar amygdala . These tonsils are closely related to the medulla oblongata.

Previous face

The anterior face is closely related to the posterior aspect of the brainstem and in order to see it, it is necessary to section the three pairs of peduncles that join it. It has a central depression that corresponds to the roof of the [IV ventricle] and is delimited by the peduncles on both sides and by the superior and inferior medullary veils. Above this depression, the anterior end of the upper vermis or lingula appears , and below it the anterior end of the lower vermis or nodule is seen . On both sides of the nodule, and below the inferior cerebellar peduncles, there are some prominences called Flocs. The nodule and flocs are linked together by the floc peduncle, which runs in part over the inferior medullary veil.

Divisions

There are three different ways to divide the cerebellum: morphologically, phylogenetically, and functionally.

Morphological

Classically, a morphological division is performed that is merely descriptive of the surface of the cerebellum, and has no functional or ontogenetic basis or any application in clinical practice.

The surface of the cerebellum is furrowed by many transverse fissures more or less parallel to each other. Among them there are two that stand out for being the deepest and serve to divide it into lobes. One is the primary or primary fissure that runs through the upper face and divides it into approximately two equal halves, and the other is the posterolateral or dorsolateral fissure that is located on the anterior side in a caudal position with respect to the nodule and the flocs.

These fissures delimit the three lobes of the cerebellum: the anterior, the posterior, and the flocculonodular. Each of these lobes includes a portion that is part of the vermis and another that is part of the cerebral hemispheres. The portion of the vermis that corresponds to each lobe is subdivided into segments that are generally associated with a pair of lobules located in the cerebellar hemispheres. The subdivision within each of the lobes is determined by the existence of other shallower transverse fissures.

The anterior lobe is located in front of the raw fissure and covers part of the anterior aspect and part of the superior aspect. It is subdivided into:

  • Lingula(I), which is the most anterior portion of the vermis and joins the superior medullary veil .
  • Central lobule(II and III), which is located just above the lingula and is extended on both sides by the wings of the central lobule (H II and H III). The fissure that separates it from the lingula is called the precentral fissure.
  • Culmen(IV and V), which is the most cranial portion of the entire vermis and is associated laterally with the anterior portion of the quadrangular lobules (H IV and HV). The fissure that separates it from the central lobule is called postcentral.

The posterior lobe is located between the prima and posterolateral fissures and covers part of the upper face and part of the lower face. It is subdivided into:

  • Decline(VI), which descends from the culm to the back and is associated laterally with the simple lobule or lower posterior portion of the quadrangular lobule (H VI).
  • Foliumor leaf of the vermis (VII-A), which is a narrow connecting sheet between the left and right upper semilunar lobes (or anseriformes; H VII-A).
  • Tuberor vermis tubercle (VII-B), which is associated laterally with the lower semilunar lobules (H VII-A) and the graceful lobules (thin or paramedian; H VII-B), and is located just below the fissure horizontal that separates it from the folium.
  • Vermis pyramid(VIII), which is located in front of the tuber and is associated with the left and right digastric lobules (H VIII-A and B). The fissure that separates it from the tuber is called prepyramidal and the fissure that separates it from the uvula is called postpyramidal or secondary.
  • Uvula of the vermis(IX), which is located between the two cerebellar friends (H IX) just above the pyramid.

The flocculonodular lobe is located in front of the posterolateral fissure and, as its name indicates, is formed by the nodule (X) -which corresponds to the vermis- and the flocs (HX) -which correspond to the hemispheres-, joined by the peduncle of the floc.

The term body of the cerebellum is used to refer to the entire cerebellum, with the exception of the flocculonodular lobe.

The upper vermis is made up of the lingula, the central lobule, the culm, the slope and the folium. The lower vermis is made up of the tuber, the pyramid, the uvula and the nodule.

Some authors instead of distinguishing three lobes distinguish four: the anterior, the middle, the posterior and the flocculonodular. The difference is that they divide the posterior lobe in two by means of the prepyramidal fissure, in such a way that the middle lobe extends above it and the posterior lobe below.

Phylogenetics

From the phylogenetic point of view, the cerebellum can be divided into three parts: archaeocerebellum, paleocerebellum and neocerebellum. This division is of great interest because each of the portions has a certain functional and clinical identity.

The archeocerebellum . It is the oldest phylogenetically portion and corresponds to the flocculonodular lobe. It arises during phylogenetic development at the same time as the vestibular apparatus of the inner ear. Most of the input he receives comes from the vestibular nuclei and corresponds largely to the vestibule cerebellum.

The paleocerebellum . It is more modern than the archaeocerebellum and is made up of the pyramid, the uvula, the central lobule with wings, the summit and the quadrangular lobule. Most of the input he receives comes from the spinal cord and has some correspondence with the spinocerebellum.

The neocerebellum . It is the most modern part and is made up of the entire posterior lobe with the exception of the pyramid and the uvula. Most of the input it receives comes from the cerebral cortex through the nuclei of the bridge and is identified with the cerebrocerebellum.

Features

Depending on the main function they perform and the connections they establish, 3 different regions can be identified in the cerebellum: vestibulocerebellum, spinocerebellum and cerebrocerebellum.

The vestibulocerebellum is formed by the flocculonodular lobe. It receives input from the semicircular canals and the macules through the vestibular nuclei, and from the visual cortex through the nuclei of the bridge. The efferences it sends go directly to the vestibular nuclei without previously going through any deep nucleus of the cerebellum. It is capable of modulating the activity of the tracts that descend from the vestibular nuclei to the spinal cord and of the α motor neurons that innervate the extrinsic muscles of the eyeball. Thanks to this, the vestibulocerebellum is in charge of controlling and regulating body balance and eye movements.

The spinocerebellum is made up of two portions of the cerebellar cortex: the vermian band and the paravermian bands.

The vermian band is an odd and middle band that corresponds to the upper and lower vermis (not including the nodule). It receives vestibular, visual, and acoustic input. He sends his efferences through the nucleus of hassle.

The paraverm bands are a pair of longitudinal stripes that are arranged on both sides of the verm band, in the most medial part of the cerebellar hemispheres. It receives somatosensory input from the spinal cord and the sensory nucleus of the trigeminal nerve. It sends its efferences through the intervening nucleus (emboliform + globose).

Starting from the intervening nucleus and the nucleus of the fastigus, the spinocerebellum modulates the activity of the descending motor pathways that start from the cerebral cortex and the brain stem and reach the spinal cord. Due to this, its main function is to regulate the movements of the extremities and the trunk. In the vermian band, muscle movements of the trunk, neck and proximal extremities are controlled. In paraverm bands, they control the distal portions of the upper and lower extremities, especially the hands, feet, and fingers.

The cerebrocerebellum is formed by the lateral portion of the cortex of the cerebellar hemispheres. It receives input from most of the neocortex through the nuclei of the bridge, which is why it is also known as pontocerebellum. It sends efferences that reach the thalamus through the dentate nucleus, and from the thalamus reach the cerebral cortex. It performs cognitive functions (visuospatial perception, linguistic processing, and emotion modulation), general planning of sequenced motor activities, and motor learning.

Topographical representation of the body

In the same way that the somatosensory cortex , the motor cortex , the basal ganglia, the red nuclei and the reticular formation have a topographic representation of the different parts of the body, this also happens in the case of the cerebellar cortex. The trunk and neck as well as the proximal portions of the extremities are located in the region belonging to the vermis. Instead, the facial regions and the distal portions of the extremities are located in the paraverm bands. The lateral portions of the cerebellar hemispheres (cerebrocerebellum), as well as the floculonodular lobe (vestibulocerebellum), do not have a topographic representation of the body.

These topographic representations receive input from all the respective portions of the body and also from the corresponding motor areas in the cerebral cortex and the brainstem. In turn, they return motor signals to the same respective areas of the motor cortex and also to the appropriate topographic regions of the red nucleus and reticular formation in the brainstem.

Internal structure

In a similar way to the brain, the cerebellum can be divided into Gray Matter and White Matter . The gray matter is arranged on the surface, where it forms the cerebellar cortex, and inside, where it constitutes the deep nuclei. The white matter is located in the internal part, completely enveloping the deep nuclei.

Cerebellar cortex

The cerebellar cortex has a very large surface, about 500 cm² thanks to the numerous predominantly transverse folds or convolutions ( folia cerebelli ) that increase its area about three times. Abundant grooves and fissures give the cerebellar surface a characteristic rough appearance.

The cortex is made up of a multitude of histofunctional units known as cerebellar lamellae. In a sagittal section of a cerebellum gyrus seen under a microscope, it can be seen that it is made up of a multitude of microcircuits. These microcircuits are the cerebellar lamellae, which are made up of a thin sheet of white matter covered with gray matter.

The peripheral gray matter of the cerebellar lamella is about 1 mm thick. It has a histological structure, homogeneous in all its regions, made up of three layers in which seven fundamental types of neurons are distinguished. Like the rest of the nervous system , the cerebellar cortex also has glial cells and blood vessels .

Layers of the crust

In the cerebellar cortex, from deep to superficial, the following layers can be distinguished: granular cell layer, middle layer or Purkinje cell layer and molecular or plexiform layer.

The granular layer is the deepest layer of the cerebellar cortex and it limits in its internal zone with the white matter. It owes its name to the fact that a type of small intrinsic neurons called grains or granular cells of the cerebellum predominate. Due to the dyeing characteristics of the nuclei of these cells, the granular layer has a lymphocytoid ( Basophilic ) appearance , although from time to time small eosinophilic acellular spaces can be seen, called protoplasmic islets. It has a variable width of 500 in the convexity to 100 μm in the groove, being the thickest layer of the cerebellar cortex.

The Purkinje cell layer is made up of the purkinje cell somas arranged in a single cell layer. At a few increases it presents a higher cell density in the convexity of the lamella than in the grooves. Some authors do not consider Purkinje cells to form a defined layer and divide the cerebellar cortex into only two layers: granular and molecular.

The molecular layer gets its name because it mainly contains cell extensions and few neuronal somas. It has an eosinophilic dyeing character (it acquires a pinkish color in the sections stained with hematoxylin-eosin). Its approximate thickness is about 300 to 400 μm and its surface is covered by the Piamadre .

Neural types

The neurons of the cerebellar cortex are classified into: main or projection neurons and intrinsic or interneurons. The main ones are those whose axons leave the cortex to reach the deep cerebellar nuclei or vestibular nuclei. The intrinsic ones are those that extend their axons exclusively through the cortex. We also have to take into account the extrinsic afferent fibers that reach the cortex, among which mossy and climbing fibers stand out.

The main neurons are the Purkinje Cellswhose disposition, shape and size are homogeneous throughout the cerebellar cortex. It has been estimated that there are about 30 million of these neurons in the human cerebellum. Its soma has a diameter of between 40 and 80 μm. A thick dendritic trunk starts from the upper part of the neuronal body, which branches out profusely into branches of the first, second and third order, so that they constitute a dense dendritic tree characteristic of these neurons. This dendritic tree extends over the entire thickness of the molecular layer, with the particularity that it is wooded in practically a single plane, perpendicular to the transverse axis of the lamella. In this way, in parasagittal sections, the ramifications of these neurons are fully appreciated, while in cross-sections its treeing is observed as a few narrow vertical branches. Dendrites are covered in spines, so it has been calculated that each Purkinje cell can have 30,000 to 60,000 spines. The axon originates from the lower part of the soma, which, near its origin, myelinates, crosses the layer of granular cells and, after emitting collaterals, enters the white matter. From here the axons of the Purkinje cells go to the cerebellar and vestibular nuclei where they end. The recurrent axons return to the Purkinje cell layer in the vicinity of which they are arborized forming the supraganglionic and infraganglionic plexuses. Ultrastructurally, Purkinje cells are characterized in that their soma shows abundant Dendrites are covered in spines, so it has been calculated that each Purkinje cell can have 30,000 to 60,000 spines. The axon originates from the lower part of the soma, which, near its origin, myelinates, crosses the layer of granular cells and, after emitting collaterals, enters the white matter. From here the axons of the Purkinje cells go to the cerebellar and vestibular nuclei where they end. The recurrent axons return to the Purkinje cell layer in the vicinity of which they are arborized forming the supraganglionic and infraganglionic plexuses. Ultrastructurally, Purkinje cells are characterized in that their soma shows abundant Dendrites are covered in spines, so it has been calculated that each Purkinje cell can have 30,000 to 60,000 spines. The axon originates from the lower part of the soma, which, near its origin, myelinates, crosses the layer of granular cells and, after emitting collaterals, enters the white matter. From here the axons of the Purkinje cells go to the cerebellar and vestibular nuclei where they end. The recurrent axons return to the Purkinje cell layer in the vicinity of which they are arborized forming the supraganglionic and infraganglionic plexuses. Ultrastructurally, Purkinje cells are characterized in that their soma shows abundant The axon originates from the lower part of the soma, which, near its origin, myelinates, crosses the layer of granular cells and, after emitting collaterals, enters the white matter. From here the axons of the Purkinje cells go to the cerebellar and vestibular nuclei where they end. The recurrent axons return to the Purkinje cell layer in the vicinity of which they are arborized forming the supraganglionic and infraganglionic plexuses. Ultrastructurally, Purkinje cells are characterized in that their soma shows abundant The axon originates from the lower part of the soma, which, near its origin, myelinates, crosses the layer of granular cells and, after emitting collaterals, enters the white matter. From here the axons of the Purkinje cells go to the cerebellar and vestibular nuclei where they end. The recurrent axons return to the Purkinje cell layer in the vicinity of which they are arborized forming the supraganglionic and infraganglionic plexuses. Ultrastructurally, Purkinje cells are characterized in that their soma shows abundant The recurrent axons return to the Purkinje cell layer in the vicinity of which they grow trees, forming the supraganglionic and infraganglionic plexuses. Ultrastructurally, Purkinje cells are characterized in that their soma shows abundant The recurrent axons return to the Purkinje cell layer in the vicinity of which they are arborized forming the supraganglionic and infraganglionic plexuses. Ultrastructurally, Purkinje cells are characterized in that their soma shows abundantRough endoplasmic reticulum and a highly developed Golgi apparatus . Both in the soma and in the dendrites and the axon, flattened membranous cisterns belonging to the smooth endoplasmic reticulum just below the membranes (hypolemnal cisterns) frequently appear . These hypolemnal cisterns are characteristic of this cell type, although some of them can be found in other types of large neurons.

The intrinsic neurons are distributed by the granular and molecular layers. Three types of cells are found in the granular layer: granular cells, large stellate cells – Golgi and Lugaro cells – and tufted monodendritic or monopolar cells. In the molecular layer are small stellate cells – stellate cells and basket cells.

The granule cells or grains of the cerebellum are the smallest neurons in the entire human nervous system and their soma is 5 to 8 µm in diameter. They are densely packed in the granular layer. They are very numerous, calculating that in the human cerebellum there are about 50,000 million of these neurons. The soma has hardly any Nissl lumps and is almost completely occupied by the nucleus, which has dense Chromatin , which causes a great Chromophiliaand it is responsible for the lymphocytoid appearance of the cell. The neuronal bodies are not covered with glia and are located very close to each other but without presenting synapses. Four to six short dendrites, about 30 μm in length, leave the soma, with a somewhat flexuous and unbranched path, with Neurotubules and Neurofilaments inside.. These dendrites end in several dilatations that are reminiscent of the fingers of one hand, which converge on the protoplasmic islets and through which it synapses with the mossy fibers. From the soma, or one of its dendrites, the axon starts, unmyelinated throughout its path, which ascends through the molecular layer following a slightly curved path. Once the surface of the molecular layer is reached, the axon branches into a T branch giving rise to two fibers called parallel fibers. These parallel fibers have a transverse path, that is, parallel to the lamella axis and perpendicular to the dendritic arborization of the Purkinje cells. The parallel fibers grow to 2 to 3 mm in length, which is extraordinary for a neuron with such a small soma. Usually, the deepest grains are those with the thickest axons and giving rise to the deepest parallel fibers. By means of the parallel fibers, the granular cells synapse “in passant” with the dendritic spines of the Purkinje cells, so that a single granular cell can contact a variable number (50 to 100) of Purkinje cells and, to in turn, each of these receives impulses from some 200,000 to 300,000 parallel fibers. This arrangement is reminiscent of that of the posts (dendritic tree) and cables (parallel fibers) of a power line. In addition, the parallel fibers also synapse “in passant” on the dendrites of the Golgi cells, the basket cells and the stellate ones. Granular cells receive their input from the rosettes of mossy fibers and from the axons of Golgi cells. Both types of terminals synapse on the digitiform varicosities of the granule cells, forming together what is called the cerebellar glomerulus. The cerebellar glomeruli are located on the protoplasmic islets of the granular layer.

Under the name of large stellate cells are included all those neurons, other than grains and tufted monodendritic cells, which are located in the granular layer.

The cells of Golgithey are somewhat smaller in size than Purkinje cells and are similar in number to the latter neurons. Its soma is star-shaped and is preferably located in the superficial zone of the granular cell layer. It contains abundant Nissl clumps and neurofibrils, and a smooth endoplasmic reticulum and Golgi cell almost as rich as those of the Purkinje cell; on the other hand, hypolemnal cisterns are very rare. It has a low-cut nucleus, with loose chromatin and a prominent eccentric nucleolus. Its dendrites, numbering four or five, start in a horizontal or descending direction, bend and dichotamise, adopting together the shape of a not very dense bouquet that projects into the molecular layer. Dendritic spines are not very abundant. As we move away from the soma,Organellesand in the most distal regions there are only bundles of neurotubules and some smooth endoplasmic reticulum. Unlike the Purkinje cell, the dendritic field of the Golgi cell is arranged in all three dimensions and covers a wide territory spanning an area of ​​about 20 Purkinje cells. From the basal region of the cell or one of the main dendritic trunks emerges an extremely dense branched plexus axon located in the granular cell layer. The axonal plexus of the Golgi cells presents three basic types of arborization with a perfect functional correspondence. In the first type, the axonal plexus would cover a field similar to the dendritic field; in the second type, the axon would extend much more but without leaving the lamella; in the third type, two plexuses originate, one in the lamella itself and one in the neighboring one. The axonal plexus ends in numerous clusters of clustered terminations that converge on the protoplasmic islets and synapse with the dendrites of the granule cells. Golgi cells receive their input from mossy fibers and climbing fibers and, to a lesser extent, from other neurons such as granule cells. A characteristic type of synapse is the axo-somatic synapse formed by a dilation of mossy fibers that is embedded in the body of a Golgi cell, being almost enveloped by its cytoplasm. Golgi cells receive their input from mossy fibers and climbing fibers and, to a lesser extent, from other neurons such as granule cells. A characteristic type of synapse is the axo-somatic synapse formed by a dilation of mossy fibers that is embedded in the body of a Golgi cell, being almost enveloped by its cytoplasm. Golgi cells receive their input from mossy fibers and climbing fibers and, to a lesser extent, from other neurons such as granule cells. A characteristic type of synapse is the axo-somatic synapse formed by a dilation of mossy fibers that is embedded in the body of a Golgi cell, being almost enveloped by its cytoplasm.

The cells Lugaro are not as well known or are as studied as other neuronal types of the cerebellum. They are characterized by having a large fusiform soma located just below the Purkinje cell layer. They have long rectilinear or fan-shaped opposite polar dendrites, which extend along a transverse plane and cover a field that houses 1 or 2 complete rows of Purkinje cells. Its axon bifurcates into a broad, beaded plexus that extends from the top of the granular layer to the surface of the molecular layer, arranged in a sagittal plane.

Apart from Golgi and Lugaro cells, there are other types of cells that are also large stellate cells. These are aberrant elements and, therefore, very infrequent and with little functional meaning. They are Golgi cells, Purkinje cells and projection neurons of the deep nuclei, in an ectopic situation.

The monodendríticas tufted cells are a new type cell recently described. They are found in the granular layer, they have a spherical soma and a single dendritic trunk that ends in a short tuft of trees.

Small stellate cells can be superficial (stellate cells) or deep (basket cells).

The basket cells son un tipo especial de células estrelladas pequeñas a las que Cajal denominó “pequeñas estrelladas profundas”. En el cerebelo humano, hay alrededor de 90 millones de células en cesta. Se caracterizan porque su soma tiene forma triangular o estrellada con unos 10 a 20 μm de diámetro y se sitúa en la mitad interna de la capa molecular justo por encima de las células de Purkinje. Tiene una núcleo lobulado y excéntrico, y su citoplasma posee unas pocas organelas concentradas en el polo opuesto al núcleo. Los grumos de Nissl y las cisternas hipolemnales son escasas, y el aparato de Golgi y el retículo endoplásmico liso están poco desarrollados. Sus dendritas pueden ser descendentes aunque lo normal es que asciendan hasta el tercio superior de la capa molecular, miden entre 100 y 200 μm de longitud, y se orientan en el mismo plano, aproximadamente, que las células de Purkinje. Las dendritas son rectilíneas, casi sin ramificaciones y con espinas, aunque mucho menos abundantes y más groseras que las de las células de Purkinje. Tienen abundantes neurotúbulos, neurofilamentos y retículo endoplásmico liso hasta en sus porciones más distales, y mitocondrias, retículo endoplásmico rugoso y aparato de Golgi en los principales tronco dendríticos. El axón, que puede alcanzar 1 mm de longitud, tras recorrer un trayecto horizontal en el plano sagital, aumenta de calibre, emite colaterales a la capa molecular y finaliza en una serie de terminales que rodean los somas de las células de Purkinje estableciendo numerosos contactos sinápticos. Estos terminales axónicos forman una especie de cesta -por lo que estas neuronas reciben su característico nombre- confluyendo sus extremos en la base del soma de la célula de Purkinje donde forman un pincel que rodea el segmento inicial del axón. Cada axón de una célula en cesta puede dar origen a unas diez cestas perisomáticas, mientras que varias células en cesta contribuyen a formar los nidos pericelulares de una célula de Purkinje. En contraposición a las otras neuronas del cerebelo, los campos axónicos de las células en cesta presentan una notable superposición. Las aferencias de las células en cesta provienen principalmente de las fibras trepadoras y paralelas, así como de células estrelladas, de colaterales del plexo supragangliónico de las células de Purkinje y de otras células en cesta.

Inside the stellate cellsseveral different types are distinguished, although their general morphology is essentially similar in all of them. Its soma is starry or polygonal and is located on the outside of the molecular layer. It has a nucleus with lax chromatin and a cytoplasm with few organelles. Its axon, after an initial stretch of 5 to 6 μm in length, branches near the soma forming a plexus that ends up synapsing on different areas of the Purkinje cell and on other interneurons. Its dendrites originate from five or six main trunks and branch in the transverse plane forming a varicose plexus with spines that extends through the molecular layer receiving synapses of the parallel and climbing fibers as well as other stellate cells and basket cells.

Extrinsic fibers

Extrinsic fibers are the afferent myelinic axons that reach the cerebellar cortex from other regions of the central nervous system. The most important are the mossy and climbing fibers.

The mossy fibersThey are thick myelin fibers that come from numerous areas of the nervous system such as the ganglion and vestibular nuclei, the spinal cord, the reticular formation and the nuclei of the bridge. Through these fibers, the cerebellum receives information from practically the entire nervous system, including the cerebral cortex. They enter mainly through the middle and upper cerebellar peduncles, and provide collaterals for the deep nuclei, then being distributed throughout the cerebellar cortex. When reaching the granular layer, the mossy fibers follow a tortuous path and are divided into several branches that present mossy-like tree-like varicose and rosette dilation. Each mossy fiber gives rise to about 20 rosettes that are located both in the course of the fiber and in its terminations and bifurcations. These rosettes synapse on the digitiform dilations of the granule cells and the axons of the Golgi cells, forming the so-called cerebellar glomeruli. They also synapse with the Goloma cell soma.

Mossy fibers are thick, with abundant neurotubules, neurofilaments, and mitochondria. They are wrapped in a thick myelin sheath at whose Ranvier nodes the rosettes are located.

The climbing fibersthey are the axons of the projection neurons of the inferior olive grove from where they enter the cerebellum through the inferior peduncle. A single neuron in the lower olive nucleus gives rise to about ten climbing fibers. They are smaller in diameter than the mossy ones. Upon reaching the cerebellum, these fibers provide collaterals for the deep nuclei and then distribute throughout the cerebellar cortex where they lose myelin. They penetrate the granular layer in a straight line and without varicosities, giving one or two collaterals. Reach the Purkinje cell layer where each fiber overlaps several Purkinje cells by ascending over them while branching. There is one climbing fiber for every 5 to 10 Purkinje cells that makes about 300 synapses with each neuron.

The climbing fibers in their most distal portion become thin and unmyelinated, with some neurofilaments, few mitochondria, and abundant synapses “in passant” with the dendrites of the Purkinje cells. They also have very dense buttons and full of rounded vesicles that demonstrate the existence of synapses between these fibers and the dendrites of stellate cells and basket cells.

In addition to the mossy and climbing ones, the cerebellar cortex receives other afferent nerve fibers, among which those coming from locus caeruleus, which are noradrenergic and are distributed throughout the three layers, and those that originate in the raphe nuclei, which send serotonin to the granular cell layer and the molecular layer.

Glia

Protoplasmic astrocytes predominate in the cerebellar cortex, among which a peculiar type of astrocyte called Bergmann’s glia stands out. The soma of this cell is irregular in shape and is located between the Purkinje cells, from where two to three extensions start with thick protoplasmic excrescences that extend throughout the molecular layer and reach the pia mater. Once the pia mater is reached, they are attached to it by means of widenings that form the limiting layer of Cajal. Another special type of astrocytes are the cells of Fañanás whose somas are located in the molecular layer and their expansions do not reach the pia mater. Both Fañanás cells and Bergmann’s glia do not show any ultrastructural pecularity, both expressing positivity for the Antibody of theacid gliofibrilar protein (GFAP).

Protoplasmic astrocytes that do not isolate all neurons and that appear to form circles around the cerebellar glomeruli can be seen in the granular layer. Likewise, oligodendrocytes exist in the molecular layer but not in the granular layer.

Deep cores

Inside the white matter we can find 4 pairs of gray matter nuclei, which from medial to lateral are: the nucleus of the fastigio (or the roof), the globose, the emboliform and the dentate. The emboliform and the globose are functionally closely related and together they form the intervening nucleus. The vestibular nuclei of the medulla oblongata also function in certain respects as if they were deep cerebellar nuclei due to their direct connections to the cortex of the floculonodular lobe.

The nucleus of the fastigus is a thick, comet-shaped mass located almost in the midline, just above the roof of the IV ventricle from which it is separated by a thin layer of white matter. The globose nucleus is elongated anteroposteriorly and is located between the nucleus of the fastigus and the emboliform. The emboliform nucleus is comma-shaped, with the thick part directed forward and located next to the hilum of the dentate nucleus.

The dentate nucleus is the largest one and it has been calculated that it has about 250,000 neurons. It is yellowish gray in color and is shaped like a bag with folds open towards the front and towards the midline. The opening is called the Hilium of the dentate nucleus and most of the fibers that form the superior cerebellar peduncle come out of it. In the dentate nucleus, at least two types of neurons are distinguished: the large or projection neurons and the small or interneurons. But the synaptic circuits of this nucleus are not clearly established. Both the projection neurons and the interneurons have not very numerous, long and poorly branched extensions, which give them a generally starry appearance.

The dentate nucleus, like the rest of the cerebellar nuclei, in addition to receiving collaterals from fibers that reach the cerebellum from other nerve centers, receive axons from Purkinje cells. Each of these axons ends in a dilated terminal plexus on about 30 neurons in the cerebellar nuclei. The axons of the projection neurons are directed through the peduncles to specific nerve centers. There are no direct connections from the cerebellar cortex to the exterior, except for some axons that directly reach the vestibular nuclei.

White substance

In a sagittal section of the cerebellum, the white matter adopts an arborescent arrangement, which is why it is sometimes known as the Tree of Life of the cerebellum or arbor vitae . It is made up of a bulky central mass, called the body or medullary center, from which extensions start towards the cerebellum convolutions called white sheets. The medullary body is continued forward directly with the peduncles, which are also made of white matter.

From a histological point of view, the white matter of the cerebellum is made up of axons together with fibrous astrocytes and abundant oligodendrocytes.producers of the myelin sheath. The axons of the white matter are both efferent and afferent fibers and intrinsic fibers that connect different cortical areas to each other. The afferent fibers of the cortex correspond to axons of the Purkinje cells, while those of the deep nuclei correspond to axons of the projection neurons of these nuclei. The inputs correspond to the mossy fibers, the climbers and those that come from the noradrenergic and serotonergic systems. Among the intrinsic or own fibers, two types are distinguished: commissural fibers and arcuate or association fibers. The commissurals cross the midline and connect the opposite halves of the cerebellum while the arches connect adjacent cerebellar circumvaluations to each other.

Cerebellar connections

The cerebellum receives input from all motor and sensory pathways except olfactory, and from it outputs leave to control all descending motor pathways. The efferences do not usually synapse directly on the motor neurons of the common final pathway except those of the extrinsic muscles of the eyeball. The efferences normally act on the motor nuclei of the brain stem. The number of cerebellar afferent fibers is more than 40 times greater than that of efferent fibers. All connections of the cerebellum pass through the peduncles.

Next, the main connections established by the cerebellum will be explained, arranged according to their functional division. It must be taken into account that the afferent fibers, unlike the efferent ones, do not end up on the cerebellar cortex, strictly following the functional division.

Vestibule references

They mainly come from the vestibular system through two tracts: the direct or Edinger vestibulocerebellar and the indirect vestibulocerebellar. It also receives some fibers from the cerebellar corticoponic tract that come from the visual cortex of the occipital lobe (occipitoponotic cerebellar fibers).

The direct vestibulocerebellar or Edinger’s tract is formed by the axons of the neurons located in the vestibular or Scarpa ganglion, which preferably reach the nodule and some to the vermian band. It does not pass through the vestibular nuclei, it does not break on its path and it enters directly through the lower peduncle. It transmits information about the position of the head and the linear and angular accelerations suffered by the body.

The indirect vestibulocerebellar tract is formed by the axons of the neurons settled in the superior and medial vestibular nuclei, which will end in the flocs and, to a lesser extent, in the vermian band. It does not break on its path and enters the lower peduncle. It transmits information about the position of the head and the linear and angular accelerations suffered by the body.

Hallway Ceremony

The main fiber tracts that start from the vestibule-cerebellum are: the cerebellum-vestibular, the floculo-occulomotor and Russell’s uncinate.

The cerebellovestibular tract is made up of direct and crossed fibers that originate in the flocs and leave the cerebellum through the lower peduncle to reach the medial and lateral vestibular nuclei. Regulates the activity of the medial and lateral vestibulospinal tracts.

The floculooculomotor tract originates from the flocs, it is pronounced in the cerebellum, it leaves the superior peduncle and ascends through the brain stem until reaching the nucleus of the oculomotor nerves (or common ocular motor). Control the movements of the eyeball.

The uncinate tract Russell originates in the flocs, intersects and is directed cranially to the superior cerebellar peduncle. But before reaching that peduncle, it abruptly changes direction forming a kind of hook and ends up going out through the bottom. It ends in the vestibular nuclei. On its way in the cerebellum it emits collaterals that exit through the upper peduncle and reach the nuclei of the ocular motor nerves, the reticular formation and the hypothalamus. It controls the movements of the eyeball and the activity of the vestibulespinales.

Spinocerebellus input

The contributions of the spinocerebellum come from three areas of the Neuroaxis : the spinal cord, the medulla oblongata and the midbrain.

At the level of the spinal cord, the inputs come through the posterior and anterior spinocerebellar tracts. These tracts are capable of transmitting nerve impulses faster than any other CNS pathway, reaching a speed of 120 m / s. This speed is necessary for information about changes in peripheral muscle groups to reach the cerebellum and to coordinate them in time.

The anterior spinocerebellar (ventral) or Gowers’ tractit originates in the medulla, in neurons that settle in the lateral zone of the base of the posterior horn, between the last lumbar segments and the sacrococcygeus. Some of its fibers cross the gray corner to ascend through the lateral cord on the opposite side, where it is located close to the medullary surface. The few fibers that do not cross ascend through the side chord on the same side. All its fibers cross the bulb and the bridge, reaching the most caudal area of ​​the midbrain where they change direction abruptly to enter the cerebellum through the upper peduncle. It reaches the vermis and the paraverm bands on both sides. Transmits unconscious proprioceptive and exterioceptive information from the lower limb.

The posterior spinocerebellar (dorsal) or Flechsing tract is made up of axons of neurons whose soma is located in the thoracic spine or Stilling-Clarke nucleus . It ascends through the lateral cord attached to the surface and just behind the anterior spinocerebellar tract. Upon reaching the bulb, it enters the cerebellum through the lower peduncle and reaches the vermis and the paraverm band on the same side as its origin. Transmits unconscious and exteroceptive proprioceptive information from the trunk and lower limb.

At the level of the medulla oblongata, the inputs come through the cuneocerebellar, olive-cerebellar and reticulocerebellar tracts.

The cuneocerebellar tract is formed by the axons of the neurons that settle in the accessory cuneiform nucleus (posterior external arcuate fibers). It ascends through the medulla oblongata without delay and mixed with the posterior spinocerebellar tract. It enters through the inferior cerebellar peduncle and ends at the vermis and the paraverm band on the same side. It transmits the unconscious and exteroceptive proprioceptive sensitivity of the upper half of the body.

The oligocerebellar tract is the most important connection established between the medulla oblongata and the cerebellum. It is made up of axons from the neurons of the lower olive nucleus and from the accessory olive nuclei . These nuclei receive somatoesthetic, visual and cerebral cortex information in addition to receiving vestibular input and the cerebellum itself. Shortly after it originates, the oligocerebellar tract is fully cleared and enters the cerebellum through the lower peduncle. It ends up providing climbing fibers for the entire cerebellar cortex. It transmits to the cerebellum the information received by the olive groves.

The reticulocerebellar tract is made up of axons of neurons located in the bulbar and pontic reticular formation. Part of the fibers cross and another part goes straight. It enters through the inferior cerebellar peduncle and mainly reaches the spinocerebellum, although it also sends some fibers to the cerebrocerebellum. It transmits complex information, both from the periphery and from the cerebral cortex and other parts of the central nervous system .

At the midbrain level, the inputs come through the tectocerebellar, trigeminocerebellar and rubrocerebellar tracts.

The tectocerebellar tract is formed by the axons of the neurons of the superior and inferior quadrigeminal tubercles. They enter the cerebellum through the upper peduncle on the same side and end in the middle of the vermis. It transmits visual and acoustic information from the cerebral cortex.

The trigeminocerebellar tract is made up of axons of neurons from the mesencephalic nucleus of the trigeminal nerve that enter the cerebellum through the superior peduncle without getting away along the way. They end up in the vermis and in the vermiana band on the same side of their origin. It transmits proprioceptive information from the Craniofacial Massif .

The rubrocerebellar tract is made up of axons of settled neurons in the parvocellular portion of the Red Nucleus, which are completely resolved before reaching the cerebellum by the superior peduncle.

Spinocerebellar Eferences

The main references that start from the spinocerebellum are: the interpost-reticular tract, the interpost-olive grove, the interpost-tectal tract and the interpost-rubric tract.

The interpuestorreticular tract originates from the interposed nucleus, its fibers partially decuse and leave the cerebellum through the lower peduncles to reach the nuclei of the reticular formation.

The interpositional- olive tract exits through the superior cerebellar peduncle, is entirely decimated at the midbrain level, and descends through the brainstem to reach the inferior olive grove.

The interpuestotectal tract is partially decoupled before exiting the superior cerebellar peduncle and ascending the brainstem until reaching the superior and inferior quadrigeminal tubercles.

The interpuestorrubic tract is the most important discharge from the spinocerebellum and the main route of discharge from the interposed nucleus. The fibers that make it up leave the cerebellum through the upper peduncle, they are completely resolved in the midbrain and reach the contralateral red nucleus. Axons depart from the red nucleus towards the intermediate ventral nucleus of the thalamus, which, in turn, sends axons to the sensory and motor cerebral cortex. It controls the activity of the motor pathways that descend to the spinal cord.

Cerebrocerebral input

All inputs received by the cerebrum are part of the cerebellar corticoponic tract . This tract originates from a wide area of ​​the cerebral cortex that includes the frontal, parietal, occipital and temporal lobes, and before entering the cerebellum it synapses in the nuclei of the bridge.

Most of the fibers that go from the cortex to the nuclei of the bridge are collateral axons that go to other areas of the brain or to the spinal cord and whose neuronal body is located in layer V of the cerebral cortex. These fibers can be divided, according to their origin, into: Fronto-Pontic, Parietopontic, Occipitopontic and Temporopontic.

The frontopontic fibers originate in the motor and premotor cortices, and pass through the anterior arm of the internal capsule . In the midbrain, they run through the base of the cerebral peduncles medially to the Corticonuclear Tract . They end at the most medial bridge cores.

The parietopónticas fibers originate in the primary and secondary somatosensory areas and visual areas. They pass through the posterior arm of the internal capsule and then through the base of the cerebral peduncles laterally to the corticospinal tract . They end in the most lateral bridge cores.

The occipitopontic fibers originate in secondary areas related to the processing of visual stimuli of movement (magnocellular current of the Optical Pathway ). They pass through the retrolenticular portion of the internal capsule and then through the base of the cerebral peduncles laterally to the corticospinal tract. They end in the most lateral bridge cores.

The temporopontic fibers pass through the sublenticular portion of the internal capsule and at the midbrain level are placed laterally to the corticospinal tract. It ends in the most lateral bridge cores.

The fibers that go from the nuclei of the bridge to the cerebellum (pontocerebellar fibers) follow a horizontal path through the protuberance, they decuse and enter through the middle peduncle. They end in the cortex of the hemispheres and in the globose nucleus.

Cerebralcerebral Eferences

Most of the cerebrospinal brain output leaves through the dentadotallamic tract . This tract is formed by the axons of the neurons located in the dentate nucleus, which leave the cerebellum through the upper peduncle. They decuse in the caudal portion of the midbrain (Wernekink decusation) and end in the intermediate ventral nucleus of the thalamus. From the thalamus, thalamocortical fibers start, reaching the same areas of the cerebral cortex from which the corticoponicocerebellar input originated.

There is a group of fibers called dentadorrubricas , which, starting from the dentate nucleus, come out through the superior cerebellar peduncle, decuse and reach the contralateral red nucleus.

Contributions from monoaminergic systems

The cerebellum, like other parts of the CNS, receives fibers from the modulating neurochemical systems. Specifically two of the monoaminergic systems: the noradrenergic and the serotonergic.

The noradrenergic system sends the caeruleocerebellar tract from group A6 (which coincides with the locus caeruleus) to the cerebellum. This tract penetrates the upper peduncle and ends up distributed throughout the nuclei and the cortex. Its fibers do not behave as mossy fibers or as climbers but as diffuse projections.

The cerebellar serotonergic tract originates in groups B5 and B6, enters the middle peduncle, and ends up distributed throughout the nuclei and cortex. Its fibers end in diffuse projections.

Peduncles

The cerebellum is attached to the posterior aspect of the brain stem using 3 pairs of peduncles through which all the nerve fibers that enter and leave it run. There are two lower peduncles, two middle peduncles, and two upper peduncles.

Lower cerebellar peduncles

The inferior cerebellar peduncles or restiform bodies connect the cerebellum with the upper part of the medulla oblongata. Between them extends the inferior medullary veil. Through them enter the fibers of the dorsal spinocerebellar tract, those of the cuneocerebellar tract, those of the vestibulocerebellar tracts, those of the reticulocerebellar tract and the climbing fibers from the lower olive nucleus and accessories (olivocerebellar tract). Through them come the fibers of the cerebelovestibular tract, those of the Russell uncinate tract, and those of the interpost-reticular tract.

Middle cerebellar peduncles

The middle or pontine cerebellar peduncles connect the cerebellum with the pons or bridge. They are the largest and are separated from the upper peduncles by the interpeduncular sulcus. They constitute the lateral faces of the protuberance. Through them enter the fibers of the corticopontocerebellar tract and those of the cerebellar serotonergic tract. Through them important afferent fibers do not come out.

The fibers of the middle peduncles are organized into three fascicles: upper, lower and deep.

The upper , most superficial fascicle is derived from the superior transverse fibers of the pons. It is directed dorsally and laterally, superficially crossing the other two fascicles. It is mainly distributed by the lobules of the inferior aspect of the cerebellar hemispheres and by the adjacent portions of the superior aspect.

The lower bundle is made up of the lower transverse fibers of the pons. It passes deep into the upper bundle and continues back and down more or less parallel to it. It is distributed by the lobules of the inferior face in the portions near the vermis.

The deep bundle includes most of the deep transverse fibers of the pons. In its first sections it is covered by the inferior and superior fascicles, but ends up crossing obliquely and appears on the medial side of the superior fascicle, from which it receives a bundle of fibers. Its fibers disintegrate and end in the lobules of the anterior part of the upper face. The fibers of this fascicle cover those of the restiform body.

Superior cerebellar peduncles

The superior cerebellar peduncles connect the cerebellum with the midbrain. Between these two peduncles the superior medullary veil extends. Through them enter the fibers of the ventral spinocerebellar tract, those of the tectocerebellar tract, those of the trigeminocerebellar tract, those of the rubrocerebellar tract and those of the caeruleocerebellar tract. Through them come the fibers of the floculo-occulomotor tract, those of the interpositional olive, those of the interpuestorubrhubic, those of the interpostotectal, those of the dentadotallamic tract, the dentadorrubrics and the collaterals of Russell’s uncinate.

Arterial irrigation

There are three main pairs of arteries supplying the cerebellum: the superior cerebellar arteries (SCA), the inferoanterior cerebellar arteries (AICA), and the inferoposterior cerebellar arteries (PICA).

Superior cerebellar artery

It originates from the basilar artery just below the place where it is divided into its two terminal branches. It is directed laterally and backwards, contouring the corresponding cerebellar peduncle, at the level of the pontomesencephalic sulcus. It passes immediately below the Common Ocular Motor Nerve (III) and passes through the Cisterna ambiens accompanying the Trochlear Nerve (IV). Its terminal branches run through the Piamadre , between the cerebellum tent and the superior aspect of the cerebellum. It anastomoses with the inferior cerebellar arteries. It irrigates the cerebellar cortex of the superior aspect and the deep nuclei, as well as the superior and middle cerebellar peduncles.

When it outlines the midbrain, the superior cerebellar artery provides the rhomboidal artery that follows the superior cerebellar peduncle and penetrates into the cerebellum to irrigate the deep nuclei. It also gives several collateral branches that reach the pienal gland , the superior medullary veil and the choroidal fabric of the III ventricle .

Inferior cerebellar artery

It originates from the basilar artery just above the place where it is formed by the union of the two vertebral arteries . It is directed laterally and backward, outlining the lateral aspect of the bridge just below the apparent origin of the trigeminal nerve (V). It follows its path along the lower edge of the middle cerebellar peduncle. It supplies the anterior portion of the inferior aspect of the cerebellum, as well as the facial (VII) and vestibulocochlear (VIII) nerves . Its terminal branches are anastomosed with those of the inferoposterior and superior cerebellar arteries.

In some people, the inferior cerebellar artery emits the internal auditory or labyrinth artery (in other people, the labyrinth artery originates from the basilar artery). This branch accompanies the vestibulocochlear nerve (VIII) through the internal auditory canal until reaching the middle ear .

Lower posterior cerebellar artery

It originates from the vertebral arteries just below the place where they meet to form the basilar artery. It is directed backwards, surrounding the upper part of the medulla oblongata and passing between the origin of the Vagus Nerve (X) and the Accessory Nerve (XI). It follows its path over the inferior cerebellar peduncle and when it reaches the inferior aspect of the cerebellum it divides into two terminal branches: one medial and the other lateral. The medial branch is continued backward through the middle fissure, between the two cerebellar hemispheres. The lateral branch is distributed along the lower surface of the hemispheres until it reaches the circumferential edge, where it anastomoses with the inferior and superior cerebellar arteries.

It supplies the posterior aspect of the inferior aspect of the cerebellum, the inferior cerebellar peduncle, the ambiguous nucleus , the motor nucleus of the vagus nerve , the spinal nucleus of the trigeminal nerve , the solitary nucleus , the vestibular nuclei, and the cochlear nuclei .

Its most important collateral branches are the choroidal branch of the IV ventricle and the medial and lateral bulbar branches. The first contributes to the choroid plexus of the IV ventricle, and the other two supply the medulla oblongata and the inferior cerebellar peduncle.

Venous drainage

The main veins that drain blood from the cerebellum are: the superior veins of the cerebellum, the superior vein of the vermis, the precentral vein of the cerebellum, the inferior veins of the cerebellum, the inferior vein of the vermis, and the petrous veins. All of them end up sending the blood to the venous sinuses of the dura.

The superior veins of the cerebellum collect blood from the lateral portion of the superior aspect of the cerebellar hemispheres and normally empty into the transverse sinus .

The superior vein of the vermis collects the blood from the superior vermis and empties into the rectus sinus through the internal cerebral vein or the great cerebral vein (Galen’s vein).

The precentral vein of the cerebellum collects blood from the lingula and the central lobule, and empties into the great cerebral vein.

The inferior veins of the cerebellum collect blood from the lateral portion of the inferior aspect of the cerebellar hemispheres and flow into the transverse, occipital, and superior petrous sinuses.

The inferior vein of the vermis collects the blood from the inferior vermis and empties directly into the rectum .

The petrous veins collect blood from the floc region and empty into the inferior or superior petrous sinus.

systematization of the cerebellar faces

1 upper occipital lobe

2 anterior brain stem

3 posterior internal occipital protuberance lateral edges

4 lower cerebellar fossa

5 spinal thalamic hawthorn lingual proprioceptive unconscious of pain arms and legs

Neural circuits

Together, the neural connections of the cerebellum can be divided into: afferent axons, which transmit information from other parts of the CNS to the cerebellum; Intrinsic cerebellar circuits – cortical and nuclear – that integrate and process information; and efferent axons, which transmit the processed information to other parts of the CNS.

The axons or afferent fibers reach the cerebellar cortex after giving collaterals for the deep cerebellar nuclei or for the vestibular nuclei. In turn, the information is processed in the intrinsic circuits of the cerebellar cortex, and the result, in the form of nerve impulses, is sent by the axons of the Purkinje cells to the deep nuclei. In these nuclei the information is also processed and from them the efferent fibers of the cerebellum depart both in an ascending direction, towards the thalamus and cortex, and descending towards the spinal cord.

In this way, the basic functional circuit of the cerebellum is made up of two arches: one main or excitatory, which passes through the deep nuclei, and others secondary or inhibitory, which passes through the cortex and regulates the anterior one. This circuit is repeated about 30 million times throughout the cerebellum and consists of a single Purkinje cell and the corresponding projection nuclear neuron plus related interneurons.

The basic functional circuit and the cellular elements that make it up are identical in all parts of the cerebellum, for this reason information is considered to be processed in a similar way throughout the cerebellum.

Neural circuits of the deep nuclei: main arch

The main arch is made up of the collateral branches of the mossy and climbing fibers, which end in the neurons of the deep nuclei. The axons of the projection neurons of the deep nuclei exit the cerebellum through the peduncles to end in different nuclei of the brain stem and in the thalamus.

In the deep nuclei are mainly axodendritic and some axosomatic synapses, although there are also more complex arrangements such as serial synapses and triads. The most frequent synapse is the excitatory axodendritic synapse that is established between a terminal of the axonal collaterals of the mossy or climbing fibers -as a presynaptic element- and a dendrite of a projection neuron or an interneuron of the deep nuclei -posynaptic elemeto-. The collaterals of the mossy fibers and the climbing fibers use glutamate as the main neurotransmitter, although they can also use other neurotransmitters (spatially the mossy fibers). The synaptic circuits that take place between the neurons of the deep nuclei are poorly understood.

From a functional point of view, the deep nuclei of the cerebellum have two basic types of projection neuron: some gabaergic (inhibitory) and small neurons that send their axon towards the inferior olive grove, and other glutaminergic (excitatory) neurons that send their axons to other nerve centers.

Projection neurons in deep nuclei under normal conditions permanently fire action potentials at a rate of more than 100 per second. This frequency can be modulated up or down depending on the excitatory and inhibitory signals reaching the neuron. The excitatory signals come mainly from the axonal collaterals of the mossy and climbing fibers, while the inhibitory signals come from the axons of the Purkinje cells, which are part of the secondary arch. The balance between these two effects is slightly favorable to arousal, which explains why the firing neuron firing rate remains relatively constant at a moderate level of continued stimulation.

Neural circuits of the cerebellar cortex: secondary arch

The secondary arch passes through the cerebellar cortex and is built around a fundamental neural piece: the Purkinje cell. Two types of circuits end in the Purkinje cell: the excitatory or main circuits, which are the ones that stimulate it, and the inhibitory circuits, formed by inhibitory interneurons. Finally, the axons of the Purkinje cells project onto the neurons of the cerebellar and vestibular nuclei, exerting an inhibitory action on them by means of gabaergic synapses. In this way the main exciter arc is modulated and regulated.

To all this we must add that the noradrenergic endings that reach the cerebellum release a neurotransmitter in a diffuse way that produces a hyperpolarization of the Purkinje cells.

Exciting circuits

Purkinje cells can be stimulated in two different ways: by climbing fibers (direct route) or by mossy fibers (indirect route).

The climbing fibers, when ending on the soma and the dendritic tree of the Purkinje cells, produce a direct and very specific stimulation by means of Gray type I synapses that use glutamate as a neurotransmitter. By forming multiple contacts with each Purkinje cell, a single climbing fiber produces a much more effective exciting action than mossy fibers.

The mossy fibers do not act directly on the Purkinje cells, but do so through excitatory interneurons, the granule cells. The presence of excitatory interneurons is very rare in the nervous system and is characteristic of the cerebellar cortex. At the level of the cerebellar glomerulus, the mossy fibers synapse Gray type I (excitators) on the dendrites of the granule cells and the impulses are carried by the parallel fibers until they reach the dendrites of the Purkinje cells. Parallel fibers present synapses containing spherical vesicles with glutamate and type I Gray conformation, which is consistent with its excitatory character. Together, the mossy fibers act on Purkinje cells with much convergence and divergence,

Purkinje cells do not comply with the principle that all the action potentials produced by a neuron are the same because they have two different types of action potentials depending on the path by which they are stimulated. If stimulated directly through the climbing fibers, they generate prolonged depolarization and a complex Peak Action Potential with a discharge rate of 3 or 4 Hertz . When stimulated by the indirect route through the mossy fibers, they generate a short action potential called a single peak, with a discharge frequency of 100 to 200 Hertz.. To generate a single peak, the temporal and spatial sum of the stimulation produced by several parallel fibers is necessary. All this shows that the information provided by the two types of extrinsic fibers that reach the cerebellum is different and is processed differently.

Inhibitor circuits

Inhibitory circuits are made up of the three fundamental types of inhibitory interneurons: Golgi cells, stellate cells, and basket cells. They can act directly on Purkinje cells – as stellate cells and basket cells do – or indirectly through granular cells – as Golgi’s cells do. All of these interneurons use GABA as an inhibitory neurotransmitter.

Stellate cells and basket cells are stimulated by the parallel fibers of the grains, which have previously been stimulated by mossy fibers, and are responsible for modulating the activation of Purkinje cells by climbing fibers, producing a phenomenon of inhibition. lateral . This lateral inhibition makes the signal reaching Purkinje cells more accurate in the same way that other lateral inhibitory mechanisms enhance signal contrast in many other neural circuits of the nervous system.

Golgi cells receive excitatory stimuli from parallel fibers and, to a lesser extent, from climbing and mossy fibers. They act at the level of the cerebellar glomeruli making Gray type II synapses (inhibitory) on the dendrites of the grains. By means of these syntheses they modulate the activation of the granular cells by the mossy fibers and, consequently, regulate the activity of the Purkinje cells. In this way, the Golgi cells create a negative feedback loop for the granule cells.

Exit signs

Long-term depression of Purkinje cells: motor learning

Cerebellar function theories

Cerebellar function modeling

Pathology

Classically, cerebellar lesions are clinically manifested by:

  • Hypotonia: Characterized by decreased resistance to palpation or passive manipulation of the muscles; it is usually accompanied by decreased pendulum-like osteotendinous reflexes , together with a striking rebound phenomenon in the Stewart-Holmes test.
  • Ataxia or lack of coordination of voluntary movements: Altered coordination of voluntary movements leads to the appearance of hypermetry, assynergy, dyschronometry and adiadochokinesia. In cerebellar tests (finger-nose or heel-knee), the speed and the start of movement are not affected, but when the finger or heel approaches the nose or knee, they exceed their destination or excessively correct the maneuver (hypermetry). Assynergy consists of a decomposition of movement into its constituent parts.

All of these disorders are best seen the faster the maneuvers are executed. Adiadochokinesia indicates a difficulty or inability to execute quick alternate movements (puppet test).

  • Alteration of balance and gait: The alteration of the static causes instability in orthostatism, so the patient must expand his base of support (separate the feet); When standing and walking, his body fluctuates frequently. Unlike vestibular disorders, these alterations are not modified when closing the eyes. The gait is characteristic and resembles that of a drunk (drunken gait), hesitant, with his feet apart and deviating to the side of the injury.
  • Intentional tremor: Thick and evident when trying to move (intentional or action tremor).
  • Others: Expanded, explosive word, nystagmus, fatigability,

etc.

Cerebellar syndrome

The disease or injury of all or a large part of the cerebellum is what is known as cerebellar syndrome. Selective lesions of the cerebellum are extremely rare.

Vermis cerebellar syndrome

The most common cause is vermis medulloblastoma in children. Engagement of the flocculonodular lobe produces signs and symptoms related to the vestibular system. Since the vermis is unique and influences midline structures, muscle mismatch affects the head and trunk, not the limbs. There is a tendency to fall forward or backward, as well as difficulty keeping the head still and upright. There may also be difficulty in keeping the trunk erect.

Hemispheric cerebellar syndrome

The cause of this syndrome may be a tumor or ischemia in one cerebellar hemisphere. In general, the symptoms and signs are unilateral and affect the ipsilateral muscles of the diseased cerebellar hemisphere. Limb movements are altered, especially of the arms and legs, where hypermetry and decomposition of movement are very evident. Oscillation and fall to the side of the injury often occurs. Dysarthria and nystagmus are also frequent findings.

Cerebellar syndrome aetiology

The most frequent etiologies of cerebellar syndromes are:

·         Vascular:

o    Vertebro-basilar insufficiency

o    Heart attacks

o    Hemorrhages

o    Thrombosis

·         Tumors:

o    Medulloblastoma (cerebellar vermis)

o    Cystic astrocytoma (cerebellar hemispheres)

o    Hemangioblastoma (cerebellar hemispheres)

o    Acoustic neurinoma (cerebellopontine angle)

o    Metastasis

o    Paraneoplastic (lung cancer)

·         Traumatic:

o    Contusion

o    Laceration

o    Bruising

·         Toxic:

o    Alcohol

o    Drugs

o    Hydantoinates

·         Infectious:

o    Virosic cerebelitis

o    Suppurative cerebelitis

o    Abscess

o    Tuberculomas

·         Degenerative:

o    Friedrich’s disease

o    Pierre-Marie disease

o    Multiple sclerosis

·         Malformations:

o    Arnold chiari

o    Dandy Walker Malformation

o    Vascular malformations

Cerebellar lesions produce ipsilateral neurological signs. The most common are

  • ATAXIA Imprecisionin speed, strength and distance in movements. § NISTAGMUS : involuntary rhythmic oscillation of the eyes (MOST FREQUENT ON ROUTES OF ENTRY). § TREMOR : involuntary oscillation of the movements or the trunk, when trying a precise reaching movement (MOST FREQUENT ON ROADS OF EXIT). § DYSMETRY: it exceeds or it is short in its movements. § DISDIADOCOCINECIA OR ADIADOCOCINECIA : Inability to perform alternative movements.

Other characteristics that can occur are due to damage to the different cerebellar lobes, these may be due to different causes and the manifestations are those described below:

Lobe Previous : Malnutrition / Alcoholism. For these causes loss of coordination can be seen, mainly in unstable gait: drunken gait, staggering with some rigidity.

Lobe Post : may be due to stroke, tumors, trauma, Enf degenerative them can demonstrate ataxic gait, loss of muscle tone, abnormal movements, inability of limbs to address a target, with oscillations perpendicular to the movement: QUAKE INTENTIONAL • DYSMETRY • DISDIADO COCINECIA OR ADIADO COCINECIA.

Flocular Nodular Lobe : can be given by, MEDULOBLASTOMAS. In this case, people would manifest:

Inability to control over axial muscles with gait with a broad support base§ Truncal Ataxia: Incoordination of paraxial muscles §

 

Leave a Comment