The cerebellar glomerulus is a small, intertwined mass of

nerve fiber An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences), is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action po ...

terminals in the granular layer of the

cerebellar cortex The cerebellum (Latin for "little brain") is a major feature of the hindbrain of all vertebrates. Although usually smaller than the cerebrum, in some animals such as the mormyrid fishes it may be as large as or even larger. In humans, the cerebel ...

. It consists of

post-synaptic Chemical synapses are biological junctions through which neurons' signals can be sent to each other and to non-neuronal cells such as those in muscles or glands. Chemical synapses allow neurons to form circuits within the central nervous sys ...

granule cell A granule is a large particle or grain. It can refer to: * Granule (cell biology), any of several submicroscopic structures, some with explicable origins, others noted only as cell type-specific features of unknown function ** Azurophilic granul ...

dendrites Dendrites (from Greek δένδρον ''déndron'', "tree"), also dendrons, are branched protoplasmic extensions of a nerve cell that propagate the electrochemical stimulation received from other neural cells to the cell body, or soma, of the n ...

and pre-synaptic

Golgi cell In neuroscience, Golgi cells are inhibitory interneurons found within the granular layer of the cerebellum. They were first identified as inhibitory in 1964. It was also the first example of an inhibitory feedback network, where the inhibitory i ...

axon terminal Axon terminals (also called synaptic boutons, terminal boutons, or end-feet) are distal terminations of the telodendria (branches) of an axon. An axon, also called a nerve fiber, is a long, slender projection of a nerve cell, or neuron, that condu ...

s surrounding the pre-synaptic terminals of mossy fibers.

Function

The cerebellar glomeruli are the first "processing station" for

afferent nerve fiber Afferent nerve fibers are the axons (nerve fibers) carried by a sensory nerve that relay sensory information from sensory receptors to regions of the brain. Afferent projections ''arrive'' at a particular brain region. Efferent nerve fibers a ...

s entering the cerebellum. Input comes from the mossy fibers, which terminate there and synapse with the Golgi and granule cell fibers. The Golgi cells regulate the glomeruli with

inhibitory An inhibitory postsynaptic potential (IPSP) is a kind of synaptic potential that makes a postsynaptic neuron less likely to generate an action potential.Purves et al. Neuroscience. 4th ed. Sunderland (MA): Sinauer Associates, Incorporated; 2008. ...

signals, while information is passed on to the granule and Golgi cells from the mossy fiber.

Structure

A cerebellar glomerulus is about 2.5 um in diameter, and is wrapped by

glia Glia, also called glial cells (gliocytes) or neuroglia, are non-neuronal cells in the central nervous system (brain and spinal cord) and the peripheral nervous system that do not produce electrical impulses. They maintain homeostasis, form mye ...

l sheathing. Glomeruli are centered on the large axonal terminals of

glutamatergic Glutamatergic means "related to glutamate". A glutamatergic agent (or drug) is a chemical that directly modulates the excitatory amino acid (glutamate/ aspartate) system in the body or brain. Examples include excitatory amino acid receptor agonist ...

afferent mossy fibers. Each terminal comes into contact with dendrites from 50–60 different granule cells. The granule cells themselves each have a single or multiple dendrites, and each participates in a different glomerulus. Glomeruli also contain the

GABAergic In molecular biology and physiology, something is GABAergic or GABAnergic if it pertains to or affects the neurotransmitter GABA. For example, a synapse is GABAergic if it uses GABA as its neurotransmitter, and a GABAergic neuron produces GABA. A ...

(inhibitory) synapses of Golgi cells onto granule cells, and the glutamatergic (excitatory) synapses from mossy fibers onto Golgi cells. Each glomerulus contains approximately 50 granule cell dendrites, 210 total dendritic digits and 230 synaptic junctions.

Velate astrocytes

Velate astrocytes are glia that sheath the glomeruli. They are

protoplasmic Protoplasm (; ) is the living part of a cell that is surrounded by a plasma membrane. It is a mixture of small molecules such as ions, monosaccharides, amino acid, and macromolecules such as proteins, polysaccharides, lipids, etc. In some defini ...

astrocytes Astrocytes (from Ancient Greek , , "star" + , , "cavity", "cell"), also known collectively as astroglia, are characteristic star-shaped glial cells in the brain and spinal cord. They perform many functions, including biochemical control of endo ...

with extremely thin veil-like processes that spread out and overlap each other. Researchers

Sanford Palay Sanford Louis "Sandy" Palay (23 September 1918 in Cleveland, Ohio, United States – 5 August 2002 in Concord, Massachusetts, United States) was an American scientist and educator. Academic Background Palay received his bachelor's degree from Ober ...

and

Victoria Chan-Palay Victoria Lye-Hua Chan-Palay (born 9 October 1945) is a Singaporean-born neuroscientist who has worked in the United States and Switzerland. Early life and education Chan is the second daughter of noted Singaporean swimming coach Dr. Chan Ah Kow. ...

noted that the sheath does not penetrate into the deeper part of the glomeruli or come into contact with the mossy fiber. Instead it forms a capsule, through which the neural processes of the granule and Golgi cells penetrate. The purpose of the glial sheath is still unknown, though multiple functions have been proposed, including structural support,

electrophysiological Electrophysiology (from Greek , ''ēlektron'', "amber" etymology of "electron"">Electron#Etymology">etymology of "electron" , ''physis'', "nature, origin"; and , ''-logia'') is the branch of physiology that studies the electrical properties of bi ...

insulation, and chemical equilibrium maintenance in the

interstitial fluid In cell biology, extracellular fluid (ECF) denotes all body fluid outside the cells of any multicellular organism. Total body water in healthy adults is about 60% (range 45 to 75%) of total body weight; women and the obese typically have a lower ...

, while creating a chemical barrier to the further outgrowth of granule and Golgi cell fibers. Research conducted by

David Eagleman David Eagleman (born April 25, 1971) is an American neuroscientist, author, and science communicator. He teaches neuroscience at Stanford University and is CEO and co-founder of Neosensory, a company that develops devices for sensory substituti ...

suggests that the glial sheath limits the supply of

extracellular This glossary of biology terms is a list of definitions of fundamental terms and concepts used in biology, the study of life and of living organisms. It is intended as introductory material for novices; for more specific and technical definitions ...

calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to ...

to regulate signaling.Olivier, D. E. et al. (2001
Cerebellar glomeruli: Does limited extracellular calcium implement a sparse encoding strategy?
''Proceedings of the 8th Annual Joint Symposium on Neural Computation''

References

{{reflist Cerebellum