Granule cells: characteristics and functions of these neurons

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

This group of neurons share a single characteristic, and that is their particular smallness. The different functions they carry out vary depending on the brain area in which they are located, and are involved in auditory, olfactory processes, memory or motor learning.

In this article we explain what granule cells are, where they are located, what their structure is, and what type of functions they perform.

• Related article: "Types of neurons: characteristics and functions"

Granule cells: definition and anatomical location

The term granule cell is used to define different types of neurons., whose only characteristic in common is that they all have very small cell bodies. We can find granule neurons within the granular layer of the cerebellum, in the dentate gyrus of the hippocampus, in the superficial layer of the dorsal cochlear nucleus, in the olfactory bulb, and in the cerebral cortex.

The vast majority of neurons in the brain are granule cells and Almost half of the cells of the nervous system are part of the cerebellum. The cerebellar granule cells receive excitatory inputs (using glutamate as a neurotransmitter) from the mossy fibers that They originate in the pontine nuclei, located in the ventral pons and related to activity and skill learning. motors.

In turn, the cerebellar granule cells also send parallel fibers upward through the Purkinje layer to the molecular layer where they branch and They extend by branches to the dendrites of the cells that bear the name of said layer, the Purkinje cells, large neurons that function through the neurotransmitter GABA and whose dendrites are capable of releasing endocannabinoids that reduce the potential of synapses, whether they are excitatory or inhibitory.

On the other hand, the axons of the granule cells of the dentate gyrus of the hippocampus are transformed into mossy fibers that connect exclusively with the foot of the hippocampus. Additionally, studies have observed that hippocampal granule neurons They are one of the few cells capable of regenerating throughout the life cycle, mainly in certain mammals (although it is being studied whether the same 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, the granule neurons of the olfactory bulb function with the neurotransmitter GABA and do not have axons, while the same cells in the dentate gyrus of the hippocampus have projection axons that function with glutamate.

The granule cell nuclei of the olfactory bulb and dentate gyrus are the only groups of neurons that undergo adult neurogenesis, contrary to what happens with those of the cerebellum and cortical. On the other hand, all granulosa cells (except the olfactory bulb) They have a typical structure consisting of a neuron with dendrites, a soma or cell body and an axon.

The granule cells of the cerebellum have a tightly packed round nucleus and synaptic glomeruli formed by neurons. granulosa cells, Golgi cells and mossy fibers (one of the main contributions to the cerebellum from the cerebral cortex and other regions). Those located in the dentate gyrus of the hippocampus, for their part, have an elliptical soma 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-shaped dendrites, which act as inhibitory interneurons. These form glomeruli through which the mossy fibers pass, similar to what occurs in the cerebellum.

Regarding the structure of the granule neurons of the olfactory bulb, it should be noted that they lack a main axon (and accessory), and each cell has several short dendrites in its central part and a single long dendrite with a tip at the end. The branches project towards the plexiform outer layer of the olfactory tract.

• You may be interested: "Parts of the human brain (and functions)"

Features

Granule cells have different functions depending on the structure where they are located.

1. granule cells in the cerebellum

It has been suggested that granule cells located in the cerebellar cortex receive a few excitatory inputs that come from the mossy fibers, and whose function would be to encode different combinations of the inputs of the latter cells. Another type of fibers, climbing fibers, would be responsible for sending specific signals to Purkinje cells. to modify the strength of synaptic connections of parallel fibers.

This last explanation is part of the well-known theory of the cerebellum by neuroscientist David Marr, known among other things for his work on computational theories of the cerebellum, neocortex, and hippocampus. However, these are statements that have not been corroborated, so even more research is necessary in this regard.

• You may be interested: "Purkinje neurons: their functions and characteristics"

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 granule cells in this brain area would play an important role in the formation of spatial memories.

Furthermore, it appears that granule cells born in adults would be highly active during the first weeks after functional integration into the neuronal network. What animal studies have been able to verify is that, As granule cells age in adults, their function changes and they go from being specialized in the separation of patterns (formation of different memories of similar episodes when generating distinct 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 relay of the primary auditory pathway and receive the axons of the ganglion cells of the auditory nerve, whose function is to decode auditory information (duration, intensity and frequency).

The granule cells of the ventral area of ​​the cochlear nucleus receive projections from the primary auditory cortex and the signals they receive Neurons contain information about parameters such as head position, which allows correct auditory orientation to occur. The granule cells of this brain structure would also be involved in the perception and orientation response to environmental sound stimuli.

4. granule cells of the olfactory bulb

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

In addition, the granule neurons located in the olfactory bulb would also have an essential function in the formation of memories, as well as in the selection and discrimination of the most significant odors, discarding the least important ones so that the brain focuses only on the most prominent part of the stimulus olfactory.

Bibliographic 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 granule cell layer of adult rat dentate gyrus. Brain Res. 1017: 21 - 31.
• Balu, R., Pressler, R. T., & Strowbridge, B. W. (2007). Multiple modes of synaptic excitation of olfactory bulb granule cells. Journal of Neuroscience, 27(21), 5621 - 5632.
• Weedman, D. L., & Ryugo, D. K. (1996). Projections from auditory cortex to the cochlear nucleus in rats: synapses on granule cell dendrites. Journal of Comparative Neurology, 371(2), 311-324.