Granular cells: characteristics and functions of these neurons

Granule cells can be found in various brain structures, Such as the cerebellum, olfactory bulb or dentate gyrus of the hippocampus, among others.

This group of neurons share only one characteristic, and that is their special smallness. The different functions they perform vary depending on the area of ​​the brain in which they are located and are involved in auditory, olfactory, memory or motor learning processes.

In this article, we tell you what granule cells are, where they are, what they are structured and what kind of functions they perform.

    Granular cells: definition and anatomical location

    The term granular cell is used to define different types of neuronsThe only thing in common is that they all have very small cell bodies. Granular neurons are found in the granular layer of the cerebellum, in the dentate gyrus of the hippocampus, in the surface layer of the dorsal cochlear nucleus, in the olfactory bulb, and in the cerebral cortex.

    The vast majority of brain neurons are granule cells and almost half of the cells of the nervous system are part of the cerebellum. Cerebellar granulosa cells receive excitatory inputs (using glutamate as a neurotransmitter) from mossy fibers that originate from the pontine nuclei, located in the ventral bulge, and linked to the drivers of activity and skill learning.

    In turn, the granule cells of the cerebellum also send parallel fibers upward, through the Purkinje layer, to the molecular layer where they branch out and extend by branching into the dendrites of the cells. neurons that function through the neurotransmitter GABA and dendrites are able to release endocannabinoids that reduce the potential of synapses, whether they are excitatory or inhibitory.

    In contrast, the axons of the granule cells of the dentate gyrus of the hippocampus turn into mossy fibers that connect exclusively to the foot of the hippocampus. In addition, studies have observed that granular neurons in the hippocampus they are one of the few cells able to regenerate throughout the life cycle, mainly in some mammals (Although we are studying whether the same thing happens in humans).

    structure

    Granule cells in different regions of the brain they are functionally and anatomically diverse. As we mentioned at the beginning of the article, the only thing they have in common is their small size. For example, neurons in the granular olfactory bulb work with the neurotransmitter GABA and lack axons, while the cells themselves in the dentate gyrus of the hippocampus have projection axons that work with glutamate.

    The granular cell nuclei of the olfactory bulb and dentate gyrus are the only groups of neurons that undergo adult neurogenesis, as opposed to those in the cerebellum and cortex. In contrast, all granular cells (except the olfactory bulb) they have a typical structure consisting of a neuron with dendrites, a soma or cell body and an axon.

    Granule cells in the cerebellum have a tightly packed round nucleus with synaptic glomeruli formed by granular neurons, Golgi cells, and mossy fibers (one of the major contributions to the cerebellum from the cerebral cortex and other regions). Those located in the dentate gyrus of the hippocampus, on the other hand, have an elliptical shape and their dendrites project towards the molecular layer.

    In the dorsal cochlear nucleus, we can find small granule cells with two or three short claw-like dendrites, which act as inhibitory interneurons. These form glomeruli through which the mossy fibers pass, in the same way as in the cerebellum.

    Regarding the structure of the granular neurons of the olfactory bulb, it should be noted that they do not have a main axon. (And accessory), and each cell has several short dendrites in the central part and a single long dendrite with a spike at the end. The branches protrude towards the plexiform outer layer of the olfactory tract.

      the functions

      Granule cells have different functions depending on the structure in which they are found.

      1. Granular cells of the cerebellum

      It has been suggested that the granule cells located in the cerebellar cortex receive some excitatory inputs from the mossy fibers, and the function would be to encode different combinations of the inputs from the latter cells. Another type of fiber, climbers, is said to be responsible for sending specific signals to Purkinje cells. to change the strength of synaptic connections of parallel fibers.

      This last explanation is part of the well-known brain theory of neuroscientist David Marr, Known among others for his work on computational theories of the cerebellum, neocortex and hippocampus. However, these are claims that have not been corroborated, so more research is needed in this regard.

        2. Granule cells of the dentate gyrus of the hippocampus

        As is known, the dentate gyrus of the hippocampus is involved in processes that have to do with the formation and consolidation of episodic memory, navigation and spatial memory. Research suggests that the granule cells in this area of ​​the brain play an important role in the formation of spatial memories.

        In addition, granule cells born in adults appear to be very active during the first few weeks after functional integration into the neural network. What animal studies have been able to verify is that, as granule cells age in adults, their function changes and they move from specializing in pattern separation (forming different memories of similar episodes by generating different representations of the temporal and spatial relationships of events) to the rapid completion of those same patterns.

        3. Granule cells of the dorsal cochlear nucleus

        The cochlear nuclei are the first relief of the primary auditory pathway and receive axons from the ganglion cells of the auditory nerve, the function is to decode the auditory information (Duration, intensity and frequency).

        Granule cells in the ventral area of ​​the cochlear nucleus receive projections from the primary auditory cortex, and the signals received by these neurons contain information about parameters such as the position of the head, allowing for correct auditory orientation. The granular cells of this brain structure would also be involved in the perception and targeting of the response to environmental sound stimuli.

        4. Granular cells of the olfactory bulb

        Granule cells in the olfactory bulb receive input from deep areas of the brain involved in memory and cognition formation, and are responsible for inhibiting neurons that receive sensory input. In this way, the granule cells they allow the brain to interpret and shape olfactory experiences.

        In addition, the granular neurons located in the olfactory bulb would also play an essential role in the formation of memory, as well as in the selection and discrimination of the most important smells, rejecting the less important so that the brain can focus only on the smell. most important part. prominent of the olfactory stimulus.

        Bibliographical references:

        • Ambrogini, R., Lattanzi, D., Ciuffoli, S., Agostini, D., Bertini, L., Stocchi, V., Santi, S., et al. (2004). Morpho-functional characterization of neuronal cells at different stages of maturation in the granule cell layer of the dentate gyrus of adult rats. Nothing. Cerebral. 1017: 21-31.
        • Balu, R., Pressler, RT and Strowbridge, BW (2007). Several modes of synaptic excitation of granule cells of the olfactory bulb. Journal of Neuroscience, 27 (21), 5621-5632.
        • Weedman, DL and Ryugo, DK (1996). Projections from the auditory cortex to the cochlear nucleus in rats: synapses in granule cell dendrites. Journal of Comparative Neurology, 371 (2), 311-324.

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