 Cerebular circuitry consists of three layers in cerebral cortex then there are four deep cerebellar nuclei and there are five types of cells which are present in the cerebellar cortex. So let us see what are these layers nuclei and cells and how they are interconnected. So first of all let us see the layers of the cerebellar cortex. So there is outer molecular layer, okay, outer molecular layer then there is middle perkinje layer, this middle layer is perkinje layer and innermost there is granular layer. So three layers of the cerebellar cortex and these different layers have different cells. So how to remember that is that granular layer has cells which start by the alphabet G. So there is a granule cells, there are granule cells and there are Golgi cells. Inner granular layer has granule cells and Golgi cells, middle perkinje layer as a name suggest perkinje it has perkinje cells, perkinje cells and then the outer molecular layer has again two cells that is the basket cells and stellate cells, basket cells and stellate cells. Then deep within the cerebellum we have a deep cerebellar nuclei and there are four cerebellar nuclei and what are these? These are pastigial nuclei, emboliform nuclei, globos nuclei and there is tentate nuclei. So they are presented deep within the cerebellum. Now let us see what are the connections in the cerebellum circuit. So first of all there are two types of inputs which are coming into the cerebellum. These inputs include mossy fibers and the other one is the climbing fibers. And these climbing fibers are basically from inferior olivary nucleus and they are carrying the error signal rest all the other input is coming from the mossy fibers. So all the different tracks which we say that they enter into the cerebellum they form the mossy fibers and the error signal from the inferior olivary nucleus is being carried by the climbing fibers. Now these mossy and climbing fibers they act on deep cerebellar nuclei in two ways. One is directly they are going to excite deep cerebellar nuclei. So see I am putting excitatory connection between the mossy fibers and climbing fibers with the deep cerebellar nuclei. Then they act on purkinje cells also but you see purkinje cells actually inhibit deep cerebellar nuclei. So these climbing and mossy fibers are excitatory. They excite deep cerebellar nuclei also and they excite purkinje cells also but purkinje cells are inhibitory. So how they excite? Actually purkinje cells have dendrites which extend into the outer molecular layer and these climbing fibers they ascent and make excitatory connections with the dendrites of the purkinje cells. So again they excite the purkinje cells. Mossy fibers on the other hand they excite the granules cells okay and via granules cells they excite the purkinje cells but they excite number of purkinje cells okay. So actually the exons of the granules cells see mossy fibers make connection with the dendrites of the granules cells and the exons of the granules cell this kind of divides okay and this forms the parallel fibers in the outer molecular layer okay. And then these parallel fibers make connections with many purkinje cells. So here also other purkinje cells will be there. So climbing fibers make connections with the multiple dendrites of a single purkinje cell. So the granules cells make connections with many purkinje cells and what it will be again it will be excitatory. So simple remember mossy fibers and climbing fibers they excite deep cerebellar nuclei and they excite purkinje cells also. Climbing fibers excite directly while mossy fibers excite via granules cells the purkinje cells. But you see purkinje cell is actually inhibiting the deep cerebellar nuclei right. So what is this known as? This is known as inner excitatory loop, inner excitatory loop because the input is directly exciting the deep cerebellar nuclei and this part is known as outer inhibitory loop, outer inhibitory loop by inhibitory because it is indirectly inhibiting the deep cerebellar nuclei by exciting the purkinje cells. So that is known as outer inhibitory loop. So this is the fundamental basic circuitry how the input fibers are acting on the cerebellar nuclei. But we said that there are other cells also there are Golgi cells which are presented in a granular layer then there are basket and cellate cells which are present in outer molecular layer. What is their role? Well granule cells you see they are the excitatory cells they are going to excite all the cells. So they have excited the purkinje cells they will also excite the basket cells and they are also going to excite the cellate cells plus also the Golgi cells. So granule cells are exciting all the cells purkinje cells, basket cells, cellate cells and Golgi cells. But these three cells that is the basket cell, cellate and Golgi cells are going to inhibit the other cells. So let me just draw it what happens actually this Golgi cell inhibits granule cells. So what is this? This is a feed pack inhibition of granule cell okay feed pack inhibition occurring of the granule cell. Then this basket and cellate cells they also inhibit the granule cells okay so again there is feed pack inhibition of the granule cells. So this basket and cellate cells they inhibit the purkinje cells also okay they are going to inhibit the purkinje cells also and this is known as feed forward inhibition, feed forward inhibition why? You see Golgi cell is exciting the purkinje cell but why a basket and cellate cell it is inhibiting the purkinje cell. So initially by the mossy pulpers why are the granule cells purkinje cells will be excited but then there will be inhibition of the purkinje cells because of this basket cell and cellate cell action and because of this connection we get the damping action of the cerebellum damping action that means the movement will be initiated but it will be dampened within a short period of time so that is the damping action. So this is the basic about this cerebellum circuit just quickly we will revise 3 layers outer molecular layer with basket and cellate cells, middle purkinje layer with purkinje cells and inner granule layer that is with granule and Golgi cells then 4 nuclei are there that is the deep cerebellum nuclei which I have presented deep within the cerebellum and 5 cells what are these 5 cells granule cells Golgi cells purkinje cells basket and cellate cells then input is brought about by mossy and clamping fibers which directly excite deep cerebellum nuclei so that is forming the inner excitatory loop and indirectly they inhibit the deep cerebellum nuclei via the purkinje cell. So this is basically an MCQ the purkinje cell output is inhibitory while the deep cerebellum output is always excitatory so how much excitatory input is going to go from the deep cerebellum nuclei that is being decided by the amount of the excitation of the deep cerebellum nuclei then we have further complications here granule cell is excitatory always excitatory to all the cells but it is itself inhibited by the Golgi cells basket cells and cellate cells that is the feedback inhibition and basket cell and cellate cells further cause inhibition of the purkinje cells that is the feed forward inhibition and this circuitry though complex it is very important for Mdbs first year exams also and very important for MCQs also so this is one MCQ that deep cerebellum output is excitatory this is one MCQ okay then feedback inhibition is also an MCQ both these feedback inhibition is MCQ granule cell is excitatory again an MCQ okay and then feed forward inhibition again this is an MCQ then yes here also parallel fibers are formed by Vixen again this is an MCQ it is the granule cell exons which form the parallel fibers so that's all for cerebellum circuit thanks for watching the video if you liked it to press the like button share the video with others and don't forget to subscribe to the channel physiology open thank you