Comparison between number and morphological features of neurons of cerebellum and caudate nucleus
Abstract
Different structures of brain vary in their structure and functions. The cerebellum and caudate nucleus have a range of functions although they share the common function of motor movement. The two structures may show an overlap when their neuronal morphology is compared. In this study two sectional slides from both the structures of the brain will be studied to learn their count and morphology so that comparisons can be drawn between them. This report will draw comparison between the neurons of cerebellum and caudate nucleus in terms of number and morphology. It is found that that the findings of the study support the hypothesis as both the regions show certain similarities in their neuronal morphology. Therefore, it can be derived that a set of morphological features of neuron may correspond to a particular functional group which in the case of neurons of caudate nucleus and cerebellum is the function of motor movement.
Introduction
Human brain has a range of structures which are responsible for performing various functions. Cerebellum and caudate nucleus of the striata of the basal nucleus are two such structures. This report will draw comparison between the neurons of cerebellum and caudate nucleus in terms of number and morphology. Dorsal striatum of the basal ganglia is made by many structures including caudate nucleus (Yager, Garcia, Wunsch, & Feguson, 2015). Cerebellum is a chief structure in the hindbrain. Basal ganglia in general performs the task of sensorimotor coordination which includes selection and activation of response. But it is seen in research that different areas of the basal ganglia are functionally demarcated along corticostriatal lines. Caudate nucleus plays a role in behavior by exciting accurate action plans and choosing suitable objectives on the basis of analysis of action-outcomes. Caudate nucleus is linked with motor activities because of its contribution in Parkinson’s disease (Malenka, Nestler, & Hyman, 2009). In addition, it contributes in several non-motor functions such as procedural and associative learnings (Anderson, et al., 2017) and regulation of action and many more. Moreover, caudate nucleus is involved in composing the reward system and serves as component of the cortico–basal ganglia–thalamic loop. While the cerebellum is involved in motor regulation. Evidence also suggests cerebellum’s involvement in cognitive functions like attention and language and also in controlling fear and pleasure reactions (Wolf, Rapoport, & Schweizer, 2009). Strong evidences in favor of cerebellum’s movement-linked functions are available. It does not start movement, but plays a huge role in synchronization, accuracy, and exact timing. Cerebellum gets input signals from sensory structures of the spinal cord and the other regions of the brain. These input signals are then integrated to refined motor activity. Damage to cerebellum leads to disruptions in movement, equilibrium, posture, and motor learning.
One of the studies that investigated the developmental and adult morphology of the projection neurons (spiny and aspiny neurons) in the caudate nucleus of dog identified three forms of each of spiny and aspiny neurons based on the morphology of the dendrites and cell soma size. The findings corresponded to the previous studies on the caudate nuclei of the rat, cat, and monkey as they described the neurons in the same way. Evidence suggests that neurons of the cerebellar nuclei assimilate the sensory and motor data and generate the ultimate output of the cerebellum. It indicates that as opposed to specific neurons and interneurons in cortical areas, neurons which are present in the nuclei translate and assimilate complex information that is presumably manifested in a huge differentiation of intrinsic membrane characteristics and assimilative capabilities of separate neurons. A study determined whether this huge differentiation in characteristics is indicated in a mixed physiological cell population of neurons of cerebellar nucleus with properly or inadequately defined cell types. Particularly, the study compared the in vivo features of 144 neurons of cerebellar nuclei in adult mice. It was found regularly firing and spontaneously bursting neurons. Subtypes of neurons were also identified based on the whole-cell parameters along with morphological neasures revealed by intracellular labelling with neurobiotin which enabled electrophysiological recognition of neurons. Large and small cerebellar nuclei neurons were identified with substantial variations in properties of the membrane. The large cells demonstrated lower membrane resistance and a shorter spike and tend to have more capacitance. So, it could be said that the neurons of the cerebellar nuclei seem to have wide range of physiological properties which is opposite to the neurons in several cortical regions like cerebral and cerebellar cortex. In the cerebellar cortex different types of neurons function in a limited area of electrophysiological parameter region. The cellular complexity of the neurons of cerebellar nuclei may grant the nucleus to do the convoluted calculations needed for sensorimotor coordination (Canto, Witter, & Zeeuw, 2016).
Another study was conducted to determine the link between the function of structures which are changed during the development and the behavioral changes of people with autism. Study used stereological techniques to determine the volume of cytoarchitectonic sections, density of neurons, and number of neurons per area in people with autism and age-corresponding control group. It was found that the autism group and control group had considerable differences but only in few brain areas which were found to be cerebellum and certain striatum and amygdala subdivisions. The autism group demonstrated reduced total number and numerical density of Purkinje cells in the by 25% and 24%, respectively. They also showed an enhancement in the volume of the caudate nucleus by 22%. So, it can be said that there is regional selectivity for developmental changes in the brain (Wegiel, et al., 2014).
The cerebellum and caudate nucleus have a range of functions but both the regions are involved in motor movement, therefore, it is vital to understand the morphology of the neurons of the two structures. Based on the above data it is proposed that the two structures of brain, caudate nucleus and cerebellum may show an overlap when their neuronal morphology is compared. In this study two sectional slides from both the structures of the brain will be studied to learn their count and morphology so that comparisons can be drawn between them.
Method
Appropriate slides containing sections of the two brain structures were collected. Then both the slides were observed using a microscope. A picture of the desired brain area was taken through a smartphone to record the data.
The point of the experiment was to compare the morphology and the approximate density and distribution of neurons in two regions of the brain that both deal with motor movement, the cerebellum and the caudate nucleus.
The two images (Fig. 1 and Fig. 2) are the two regions which were observed.
No lab manual was used for the experiment.
Fig. 1
Fig. 2
Result
On comparing the slides of section of caudate nucleus and cerebellum, it was found that the number of neurons in cerebellum slide was much higher than the caudate nucleus slide. It was seen that the cerebellar cortex comprises of three differentiated layers. First layer is the molecular layer which has two major kinds of neurons which are the stellate cells and basket cells. They have been distributed in Dendritic ramifications and various thin axons. Next layer which is called the Purkinje cell layer was seen as a layer of a single row of large Purkinje cells. They were seen as pear-shaped bodies. Purkinje cells’ axon is seen as the sole efferent pathway to the deep nuclei of the cerebellum. So, the Purkinje cells is the only output of all motor coordination in the cortex of the cerebellum. This layer had densely packed neuronal cells including granule cells which had dark staining nucleus with scarce cytoplasm and large interneurons. The macroneurons of cerebellum which includes Purkinje cells differentiate in long-term monolayer cultures. The intracellular record tells that the neurons have the capability of doing the activities as they are showing the rapid reaction and electrical activities and the synaptic activity as well. The type of cell is evaluated through light and electron microscopic study and it is done only after the intracellular iontophoresis of horseradish peroxidise. The Purkinje neurons have been evaluated with the help of dendrite arborisation and different spines of dendrites. Cortical granule cells and macroneurons arising from the deep nuclei can also be showed. The caudate cells which are there have been considered to be the spiny neurons whose task is defined from the excitatory inputs from the cortex, thalamus, global pallidus and the brainstem. Two forms of caudate nucleus were seen as per the dendritic field area, small and large neurons. It shows that spiny neurons in the caudate nucleus vary in their morphology.
Discussion
The study found that cerebellum has more number of neurons than caudate nucleus. Cerebellum has several small granule cells, therefore it has larger number of neurons than the entire from the remaining brain, despite its fairly small volume. Cerebellum has large number of neurons which is proportional to the number of neurons in the neocortex. The proportion of neurons in cerebellum to the number of neurons in cerebellum is 3.6 times (Herculano-Houzel, 2010). As described above, that cerebellum is involved in comparatively more number of diverse functions in humans yet every Purkinje cell axons from the cortex of cerebellum meet on a relatively less number of neurons in the deep cerebellar nucleus and vestibular nuclei.
Purkinje cells are one of the most differentiated neurons of the brain. These cells can be differentiated by their dendritic tree’s shape. Dendrites have abundant branches, but they are very compressed in a plane at 90 degrees to the folds of cerebellum. The dendrites has been filled it up with the dendritic spines which gets the synaptic input coming from the parallel fibre. The large cell bodies have been packed into narrow layer of the cerebellar cortex which is said to be the Purkinje fibre. The number of synaptic inputs that Purkinje cells receive is more than all the other types of cell in the brain. It is estimated that the number of spines on one Purkinje cell in human go as more as 200,000. Further, the big and round bodies of Purkinje cells are bundled into a thin single cell thick layer of the cerebellar cortex which is known as Purkinje layer. Purkinje cells make use of GABA for the process of neurotransmission and so it they have inhibition impact on the targets. Studies have suggested that a particular identifiable characteristic of Purkinje neurons is the demonstration through calbindin. It is found that calbindin staining of the brain of rat post unilateral chronic sciatic nerve damage indicates that Purkinje neurons could be newly synthesized in the adult brain, activating the structuring of new cerebellar lobules (Rusanescu & Mao, 2017). But the granule cells of cerebellum are one of the smallest neurons in the brain as opposed to the Purkinje cells. In addition, the granule cells are also the most abundant neurons in the brain. It is estimated that average adult human have nearly 50 billion granule cells are found. It shows that around 75% of the neurons of the brain are the granule cells of the cerebellum. The cell bodies of the granule cells are bundled in a thick layer which is located at the lowermost area of the cerebellar cortex. These cells give off just four to five dendrites each. These dendrites end in an extension known as the dendritic claw. Dendritic claw are the place where excitatory signal from mossy fibers and inhibitory signal from Golgi cells are received. It is also seen that the deep nuclei of the cerebellum are collections of gray matter which lie inside the white matter at the center of the cerebellum. Most of the neurons in the deep nuclei have huge cell bodies and round dendritic trees which have a diameter of around 800 μm. These make use of glutamate for the process of neurotransmission (Strick, Dum, & Fiez, 2009).
The caudate nucleus has two kinds of neurons, projection neurons which comprise the majority of the volume and interneurons. Projection neurons are of medium size which are spiny. They are multipolar neurons which have minor to average cellular somata usually with 20 µm diameter. Also, in these neurons the processes of dendrites are enclosed dendritic spines. All the projection neurons in the caudate nucleus are inhibitory in nature that utilize GABA for the process of neurotransmission. The projection neurons are classified as per their projection foci. One category are the neurons which innervate the nucleus of external globus pallidus and the second category is the one which projects into the output nuclei of internal Globus pallidus and pars reticulata. Apart from the spiny projection neurons, the caudate nucleus also comprises of various distinctive types of interneurons. All the interneurons exhibit smooth dendrites. The interneurons in the caudate nucleus are frequently grouped in four categories on the basis of their neurochemical features and morphological properties. The group of interneurons which exists most abundantly includes the large aspiny cholinergic neurons which utilize acetylcholine for the process of neurotransmission (Lanciego, Luquin, & Obeso, 2012).
It is found that that the findings of the study support the hypothesis as both the regions show certain similarities in their neuronal morphology. Therefore, it can be derived that a set of morphological features of neuron may correspond to a particular functional group which in the case of neurons of caudate nucleus and cerebellum is the function of motor movement. In future study can be conducted to compare the behavioural symptoms of brain disorders when the similar set of neurons in each region is affected.
Conclusion
The cerebellum and caudate nucleus have a range of functions although they share the common function of motor movement. On studying both the structures microscopically, it can be concluded that the number and morphology of the neurons could correspond with the functional similarities and differences. Subtypes of the neurons Therefore, it can be understood that a set of morphological features of neuron may correspond to a particular functional group which in the case of neurons of caudate nucleus and cerebellum is the function of motor movement. The findings of the normal human brain pave the way for conducting research on the brain which have disorders in which these areas are affected.
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