Granulosa 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 unique 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 they are involved in auditory, olfactory, memory or motor learning processes.

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

Granule cells: definition and anatomical location

The term granulosa cell is used to define different types of neurons , whose only common characteristic is that they all possess very small cell bodies. Granular neurons can be found within the granular layer of the cerebellum, in the serrated convolution 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 the brain’s neurons are granular 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 the mossy fibers that originate in the pontine nuclei, located in the ventral lump and related to activity and learning of motor skills.

In turn, the granulosa cells of the cerebellum also send parallel fibres upwards, through the Purkinje layer, to the molecular layer where they branch out and extend by branching to the dendrites of the cells named after this 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 excitatory or inhibitory.

On the other hand, the axons of the granular cells of the hippocampal gyrus are transformed into mossy fibers that connect exclusively to the foot of the hippocampus. In addition, studies have shown that granulosic hippocampal neurons are among the few cells capable of regeneration throughout the life cycle, mainly in certain mammals (although whether the same is true in humans is being studied).

Structure

The granular cells in different regions of the brain 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 granular neurons of the olfactory bulb function with the neurotransmitter GABA and do not possess axons, while the same cells in the dentate gyrus of the hippocampus have projection axons that function with glutamate.

The nuclei of granular cells in the olfactory bulb and the gyrus dentate are the only groups of neurons that undergo adult neurogenesis, unlike those in the cerebellum and cortices. On the other hand, all granulosa cells (with the exception of the olfactory bulb) have a typical structure consisting of a neuron with dendrites, a soma or cell body and an axon .

The granulosa cells of the cerebellum have a tightly packed rounded nucleus with synaptic glomeruli made up of granular neurons, golgi cells and moss fibers (one of the main inputs 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-shaped soma and their dendrites project towards the molecular layer.

In the dorsal cochlear nucleus we can find small granular 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 the shape of the cerebellum.

As for the structure of the granular 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 outer plexiform layer of the olfactory tract.

Functions

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

1. Granular cells in the cerebellum

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

This last explanation is part of the well-known theory of the cerebellum of neuroscientist David Marr , known among other things for his work on computational theories of the cerebellum, neocortex and hippocampus. However, these claims have not been substantiated, so further research is needed.

2. Granular cells of the hippocampal gyrus

As is known, the jagged turn 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 granular cells of this brain area would play an important role in the formation of spatial memories.

In addition, it appears that adult born granulosa cells would be highly active during the first weeks after functional integration into the neural network. What animal studies have been able to prove is that, as adult granulosa cells age, their function is modified and they go from being specialized in pattern separation (formation of 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. Granular 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 granular cells in the ventral area of the cochlear nucleus receive projections from the primary auditory cortex and the signals these neurons receive contain information about parameters such as head position, allowing correct auditory orientation to occur. The granular cells of this brain structure would also be involved in the perception and orientation response to environmental sound stimuli.

4. Granular cells of the olfactory bulb

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

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

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.