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 pot ...
terminals in the
granular layer of the
cerebellar cortex
The cerebellum (: cerebella or cerebellums; 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 it or e ...
. It consists of
post-synaptic granule cell
The name granule cell has been used for a number of different types of neurons whose only common feature is that they all have very small cell bodies. Granule cells are found within the granular layer of the cerebellum, the dentate gyrus of t ...
dendrites
A dendrite (from Greek δένδρον ''déndron'', "tree") or dendron is a branched cytoplasmic process that extends from a nerve cell that propagates the electrochemical stimulation received from other neural cells to the cell body, or soma ...
and pre-synaptic terminals of
mossy fibers.
Function
The cerebellar glomeruli are the first "processing station" for
afferent nerve fiber
Afferent nerve fibers are axons (nerve fibers) of sensory neurons that carry sensory information from sensory receptors to the central nervous system. Many afferent projections ''arrive'' at a particular brain region.
In the peripheral nerv ...
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 Chemical synapse, postsynaptic neuron less likely to generate an action potential.Purves et al. Neuroscience. 4th ed. Sunderland (MA): Sinauer Associates, Inc ...
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 (the brain and the spinal cord) and in the peripheral nervous system that do not produce electrical impulses. The neuroglia make up ...
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 to 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 gamma-aminobutyric acid (GABA). For example, a synapse is GABAergic if it uses GABA as its neurotransmitter, and a GABAergic ...
(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 part of a cell that is surrounded by a plasma membrane. It is a mixture of small molecules such as ions, monosaccharides, amino acids, and macromolecules such as proteins, polysaccharides, lipids, etc.
In some definitions, ...
astrocytes
Astrocytes (from Ancient Greek , , "star" and , , "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 end ...
with extremely thin veil-like processes that spread out and overlap each other. Researchers
Sanford Palay 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. A ...
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,
insulation, and chemical equilibrium maintenance in the
interstitial fluid
In cell biology, extracellular fluid (ECF) denotes all body fluid outside the Cell (biology), cells of any multicellular organism. Body water, Total body water in healthy adults is about 50–60% (range 45 to 75%) of total body weight; women ...
, 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 was CEO and co-founder of Neosensory, a now-defunct company that developed devices for senso ...
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; it has 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 it ...
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
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Cerebellum