In neuroscience, the term fiber describes a bundle of axons projecting from one group of neurons in a specific area to another. Mossy fiber actually refers to 2 such axonal pathways in the brain; they are both a projection between cells within the hippocampus, and also a major input pathway to the cerebellum. In spite of the common name, the projections share little in the way of similarity.

Hippocampus

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In the hippocampus, granule cells of the dentate gyrus form distinctive unmyelinated axons that project along the mossy fiber pathway to the CA3 region. The axons emerge from the basal portions of the granule cells and pass through the hilus (or polymorphic cell layer) of the dentate gyrus before entering the stratum lucidum of CA3. Granule cells tend to be glutamatergic, and hence "excitatory," although some show immunoreactivity for opiate peptides such as dynorphin and enkaphalin.

The pathway was so named by Ramon y Cajal because the axons display varicosities all along their lengths, giving them a "mossy" appearance. They are technically known as thorny excrescences, and are located at 140-um intervals. They are spine-like branchings of the axon that provide innervation to a cell while the axon continues forward to innervate more cells. It has since been shown that the axons of granule cells synapse with a wide variety of inhibitory GABA interneurons in the hilar region of the dentate gyrus before continuing on to innervate pyramidal cells in the CA3 region. A single mossy fiber projection may make as many as 37 contacts with a single pyramidal cell, but innervates only about a dozen different pyramidal cells. In contrast, a single CA3 pyramidal cell receives input from about 50 different granule cells.

Cerebellum

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Mossy fibers are also the name given to one of the major inputs to cerebellum. There are many sources of this pathway, the largest of which is the cortex, which sends input to the cerebellum via the pontocerebellar pathway. Other contributors include the vestibular nerve and nuclei, the spinal cord, the reticular formation, and feedback from cerebellar nuclei. Axons enter the cerebellum via the middle and rostral cerebellar peduncles, where some branch to make contact with deep cerebellar nuclei. They ascend into the white matter of the cerebellum , where each axon branches to innervate granule cells in several cerebellar folia.

In this case, the pathway is so named for a different type of unique synapse formed by its projections, the mossy fiber rosette. Fine branches of the mossy fiber axons twist through the granule cell layer, and slight enlargements giving a knotted appearance indicate synaptic contacts. These contacts have the appearance of a classic Gray's type 1 synapse, indicating they are glutamatergic and excitatory. Sensory information relayed from the pons through the mossy fibers to the granule cells is then sent along the parallel fibers to the Purkinje cells for processing. Extensive branching in white matter and synapses to granular cells ensures that input from a single mossy fiber axon will influence processing in a very large number of Purkinje cells.

References

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Purves D, et al. Neuroscience. Sunderland, MA: Sinauer Associates, Inc. 2001.

Shepherd, GM. The Synaptic Organization of the Brain. New York: Oxford University Press. 1998.

Nervous system