Il Cervelletto, Dispense di Anatomia

Scarica Il Cervelletto e più Dispense in PDF di Anatomia solo su Docsity! THE CEREBELLUM The cerebellum is a structure of the CNS involved in the control of movement during its execution. The control happens at different levels, at the level of the cerebral cortex and of the motor neurons of the spinal cord. In fact, the cerebellum is connected to all this structure. ANATOMICAL LOCATION The cerebellum is located at the back of the brain, immediately inferior to the occipital and temporal lobes, and within the posterior cranial fossa. It is separated from these lobes by the tentorium cerebelli, a tough layer of dura mater. The tentorium cerebelli attaches here separating the superior part of the posterior cranial fossa from the inferior part which contains the cerebellum. (By removing part of the tentorium cerebelli, there is the area where are placed the occipital lobes of the brain and inferiorly the area (lodge) where is placed the cerebellum). It lies at the same level of and posterior to the pons, from which it is separated by the fourth ventricle. The small region of the cerebellum in close relation, posteriorly, with the medulla oblongata is called tonsil, connected to the uvula (a portion of the vermis) by a ridge of gray matter. The sulcus (or groove) of the transverse sinus, which divides the latter in a superior and inferior lodge. The cerebellum forms the roof of the fourth ventricle. Whereas the floor is formed by the pons but also the medulla oblongata and here there are two structures, the superior and inferior medullary velum. On the roof of this superior medullary velum, we have a specialization of the connective and glial system which takes the name of Vala Coroidea Or Coroid Lamina where there are some blood vessels that are going to form the Coroid Plexus of the fourth ventricle, attached to the lateral angles of the 4th ventricle. It remains inferior to the origin of the middle and inferior cerebellar peduncles and antero-inferior to the tonsils. Its role is to produce the CSF. We can functionally divide the cerebellum in three segments, and in 3 ways: Anatomical, functional and phylogenetic. Anatomical means to distinguish the parts of the cerebellum, functional means to distinguish the regions with specific part and connections and phylogenetic means to distinguish 3 regions with different origins. They don’t always correspond except one case. ANATOMICAL SUBDIVISION The cerebellum consists of two hemispheres which are connected by the vermis, a narrow midline area. Like other structures in the central nervous system, the cerebellum consists of grey matter and white matter: ▪ Grey matter – located on the surface of the cerebellum. It is tightly folded, forming the cerebellar cortex. ▪ White matter – located underneath the cerebellar cortex. Embedded in the white matter are the four cerebellar nuclei (the dentate, emboliform, globose, and fastigi nuclei). The cerebellum can be subdivided in anatomical lobes and zones. ANATOMICAL LOBES There are three anatomical lobes that can be distinguished in the cerebellum; - the anterior lobe, - the posterior lobe - the flocculo-nodular lobe (made of 2 floccules and 1 nodule in the middle, that is a portion of the vermis) These lobes are divided by two fissures: - the primary fissure (divides the anterior lobe and posterior lobe) - the posterolateral fissure (divides the two lobes of the flocculo-nodular lobe) Zones There are three cerebellar zones. In the midline of the cerebellum is the vermis. - Either side of the vermis is the intermediate zone. - Lateral to the intermediate zone are the lateral hemispheres. There is no difference in gross structure between the lateral hemispheres and intermediate zones The cerebellum are tons of lines distinguishable called “folia”. In the inferior surface, they form with the vermis different lobules and each of them has a specific name (ala of the central lobe, quadrangular lobe and so on- don’t memorize these names). FUNCTIONAL SUBDIVISION ▪ Cerebrocerebellum or ponto cerebellum or cortico-ponto cerebellum – the largest division, formed by the lateral hemispheres (main part of the hemispheres so belong to this area). It is involved in planning movements and motor learning. It receives inputs from the cerebral cortex and pontine nuclei which in turn are connected with the cerebral motor cortex and so it can be involved in control of the movement at cortical level and so you have a double way of control of the movement; a spinal and cortical level, receiving proprioception and receiving projection from the pontine nuclei. It sends outputs to the thalamus and red nucleus. This area also regulates coordination of muscle activation and is important in visually guided movements. ▪ Spinocerebellum – comprised of the vermis (anterior portion) and intermediate zone of the cerebellar hemispheres (posterior medial part according to the professor). It is involved in regulating body movements by allowing for error correction. It also receives proprioceptive information from the proprioceptive pathways. ▪ Vestibulocerebellum – the functional equivalent to the flocculonodular lobe. It is involved in controlling balance and ocular reflexes, mainly fixation on a target. It receives inputs from the vestibular system, and sends outputs back to the vestibular nuclei. In the case of the flocculo-nodular lobe the anatomical segmentation and functional segmentation correspond since the flocculo- nodular lobe represents the vestibular cerebellum, it means that it is connected to vestibular nuclei and is involved in control axial movement and equilibrium. The structure of the ponto-cerebellar cortex is made up of 3 layers: - Granular layer - Purkinje cell layer - Molecular layer. The purkinje cells are very big and easily distinguishable are very important cells of the cerebellum those giving the output of the cerebllar cortex to the deep nuclei of the cerebellum. How the cerebellum basically works: The fibers arriving from the cerebral cortex, red nucles and spinal cord (so proprioception) and vestibular nuclei reach the cerebellar cortex. At this point, the information is elaborated and is transmitted by the purkinje cells to the deep nuclei of the cerebellum. From here the info will reach again the areas from which it has originated. IMP: the output will always be gaba-ergic. This picture represents the cerebellar cortex. When the cerebellum receives the information each part of it (spino-cerebellum, vestibulo-cerebellum and ponto- cereblum) receives information from one region of the brain. - Vestibulo-cerebellum receives it from vestibular nuclei, - spino-cerebellum receives it from proprioceptorsin the spinal cord - corticoponto-cerebellum from the cerebral cortex. Each of them is involved in the regulation movement on a different level, so - vestibulo-cerebellum balance and gate; - regulation of movement during the execution at the spinal level by the spino-cerebellumand - regulation of movement during the execution at the level of the cerebral cortex. by the corticoponto-cerebellum All these projections arrive into the cerebellum and all these informations arrive into the cerebral cortex. All the input arrives to the cerebellum as mossy fibers, that are going to be in contact with the granular layer that represents the receptive part of the cerebellar cortex 8so this fibers are reaching the cerebellar cortex). The mossy fibers will excite the granular cells, that will give origin to an axon that will go in the molecular layer and will create the parallel fibers. From the granular layer the axons will create a long fiber, the parallel fiber (they have this name because they are parallel to the folia), that arrive to the molecular layer. The parallel fibers excitate the Purkinje cells, because their dendrites are in the molecular layer. Their axons, are going to make synapses into the deep cerebellar nuclei. Each cerebellar nucleus are going to project back to the areas form which the cerbellum has received the inputs. So, the basic circuit is the following: cerebral cortex >spinal cord>vestibular nuclei (the three inputs to the cerebellum, each of them will reach a specific functional area of the cerebellum) - The mossy fibers are excitatory fibers and glutamatergic, as well as the granular cells. - The Purkinje cells and the deep cerebellar nuclei are both inhibitory GABAurgic neurons. IMP: the final output is excitatory (-x-=+). The most important complication is the presence of the climbing fibers, additional projections of the whole cerebellar cortex. They come from the inferior olive in the medulla oblongata. They are called climbing fibers because they climb the Purkinje cells bodies. These cells are excitatory, but the neurotransmitter is ASPARTATE. By this circuit we are able to control the movement time by time. - Into the molecule layer we have also basket cells and stellate cells, which are other inhibitory interneurons. - In the granular layer we find another important cell, the Golgi cell. This on the left is a Purkinje cell stained with Immunoflorescence. You can see all the arborization, that is characterized by spines, as the spiny neurons that are usually those of the cerebral cortex. This means that the branches of the Purkinje cells are huge connections. The picture on the right is showing the cerebellar cortex with the same staining method. We can distinguish the white matter, the molecular layer, the Purkinje cells layer with the red cells and the granular layer with the granular cells. The Purkinje cells are characterized by a high system of intracellular cisterns and Golgi apparatus, because they are characterized by a huge calcium released. So, they are strongly activated by action potential of the parallel fibers. They are very active neurons, and this makes them very sensible to oxidative stress and to alcohol. THE CEREBELLAR GLOMEROLUS It is one of the most important structure into the cerebellar cortex, which is in the granular layer, the cerebellar glomerulus. The cerebellar glomerulus is characterized by mossy fibers, which take contact with different granular cells and with the axon of Golgi cells. So, in it you can have in inhibitory and excitatory inputs which arrive to the Golgi cell. IMP: All these Golgi cells, together with the parallel fibers and the connected Purkinje cells create modules, and infant the cerebellar cortex can be divided in modules. The modular organization of the cerebellar cortex is crucial, because to each module correspond or a motor-planning (ponto-cerebellum) or a group of muscles (spino-cerebellum or vestibulo- cerebellum). So you can control specific motor segments also from a cellular level (where it starts). IMP: Basket cells inhibit the surrounding purkinje cells. This allows us to activate only the module we need that controls only a specific group of muscles or a specific motor-plan. KEY POINTS OF THE MODULAR ORGANIZATION OF THE CEREBRAL CORTEX: - Parallel fibers, activated by the Mossy fibers, can activate more the 400 Purkinje cells, belonging to a “microzone” or module. - Basket cells and Stellate cells (At the periphery of the microzone) inhibit the Purkinje cells, creating a column of activated Purkinje cells - The column of activated Purkinje cells activates, in turn, a specific group of neurons in the deep cerebellar nuclei. - The Golgi cells, by acting on the glomerulus reduce the firing of the parallel fibers, terminating the activation of the module. - At the end, you are going to activate the output from the cerebellum. The output of the cortex are the axons of the Purkinje cells, which are going to make e synapses on the nuclei, which are going to going back to the area from which they received the projections. CLIMBING FIBERS: motor learning The climbing fibers, from the inferior olive, reach the dendrites and cell body of the Purkinje cells. - The potential below is considered a normal potential, - The potential above is considered as a complex potential. When you have a double activation of a module by mossy fibers and climbing fibers, you have a double activation which creates a complex potential. This is important for motor learning, because time by time you learn how to perform a specific movement in a correct way. Who activates the olivary complex? The mossy fibers, each time that, by all the structures I told you, are going to reach the cerebellum they make a branch for the olivary complex.