Mossy fiber (hippocampus)
Encyclopedia
In the hippocampus
, granule cells of the dentate gyrus
form distinctive unmyelinated axon
s 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 cell synapses tend to be glutamatergic
(e.g. excitatory), though recent immunohistological data has indicated that
some synapse contain neuropeptidergic elements including opiate
peptide
s such as dynorphin
and enkephalin
. Moreover, there is growing evidence for co-localization of GABAergic and glutamatergic neurotransmitters within mossy fiber terminals, though the functional consequence of this colocalization is unclear.
The pathway was so named by Ramon y Cajal because the axons display varicosities all along their lengths, giving them a "mossy" appearance. These mossy fibers form multiple synapses with the elaborate dendritic spines of CA3 pyramidal cells in stratum lucidum. These complex spines are known as "thorny excrescences". It has also been shown that the axons of granule cells synapse
with a wide variety of inhibitory GABA
interneuron
s in the hilar region of the dentate gyrus before continuing on to innervate pyramidal cell
s 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. It has been shown in rodents that the size of the mossy fiber projections can show large interindividual variations, which are to a large part heritable. In addition, these variations show strong correlations with different types of behavior, mainly, but not exclusively, spatial learning.
, which shares little similarity with its hippocampal namesake.
Hippocampus
The hippocampus is a major component of the brains of humans and other vertebrates. It belongs to the limbic system and plays important roles in the consolidation of information from short-term memory to long-term memory and spatial navigation. Humans and other mammals have two hippocampi, one in...
, granule cells of the dentate gyrus
Dentate gyrus
The dentate gyrus is part of the hippocampal formation. It is thought to contribute to new memories as well as other functional roles. It is notable as being one of a select few brain structures currently known to have high rates of neurogenesis in adult rats, .The dentate gyrus cells receive...
form distinctive unmyelinated axon
Axon
An axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma....
s 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
Dentate gyrus
The dentate gyrus is part of the hippocampal formation. It is thought to contribute to new memories as well as other functional roles. It is notable as being one of a select few brain structures currently known to have high rates of neurogenesis in adult rats, .The dentate gyrus cells receive...
before entering the stratum lucidum
Stratum lucidum of hippocampus
The stratum lucidum of the hippocampus is also a layer of the hippocampus between the stratum pyramidale and the stratum radiatum. It is the tract of the mossy fiber projections, both inhibitory and excitatory from the granule cells of the dentate gyrus. One mossy fiber may make up to 37...
of CA3.
Granule cell synapses tend to be glutamatergic
Glutamic acid
Glutamic acid is one of the 20 proteinogenic amino acids, and its codons are GAA and GAG. It is a non-essential amino acid. The carboxylate anions and salts of glutamic acid are known as glutamates...
(e.g. excitatory), though recent immunohistological data has indicated that
some synapse contain neuropeptidergic elements including opiate
Opiate
In medicine, the term opiate describes any of the narcotic opioid alkaloids found as natural products in the opium poppy plant.-Overview:Opiates are so named because they are constituents or derivatives of constituents found in opium, which is processed from the latex sap of the opium poppy,...
peptide
Peptide
Peptides are short polymers of amino acid monomers linked by peptide bonds. They are distinguished from proteins on the basis of size, typically containing less than 50 monomer units. The shortest peptides are dipeptides, consisting of two amino acids joined by a single peptide bond...
s such as dynorphin
Dynorphin
Dynorphins are a class of opioid peptides that arise from the precursor protein prodynorphin. When prodynorphin is cleaved during processing by proprotein convertase 2 , multiple active peptides are released: dynorphin A, dynorphin B, and α/β-neo-endorphin...
and enkephalin
Endorphin
Endorphins are endogenous opioid peptides that function as neurotransmitters. They are produced by the pituitary gland and the hypothalamus in vertebrates during exercise, excitement, pain, consumption of spicy food, love and orgasm, and they resemble the opiates in their abilities to produce...
. Moreover, there is growing evidence for co-localization of GABAergic and glutamatergic neurotransmitters within mossy fiber terminals, though the functional consequence of this colocalization is unclear.
The pathway was so named by Ramon y Cajal because the axons display varicosities all along their lengths, giving them a "mossy" appearance. These mossy fibers form multiple synapses with the elaborate dendritic spines of CA3 pyramidal cells in stratum lucidum. These complex spines are known as "thorny excrescences". It has also been shown that the axons of granule cells synapse
Synapse
In the nervous system, a synapse is a structure that permits a neuron to pass an electrical or chemical signal to another cell...
with a wide variety of inhibitory GABA
Gabâ
Gabâ or gabaa, for the people in many parts of the Philippines), is the concept of a non-human and non-divine, imminent retribution. A sort of negative karma, it is generally seen as an evil effect on a person because of their wrongdoings or transgressions...
interneuron
Interneuron
An interneuron is a multipolar neuron which connects afferent neurons and efferent neurons in neural pathways...
s in the hilar region of the dentate gyrus before continuing on to innervate pyramidal cell
Pyramidal cell
Pyramidal neurons are a type of neuron found in areas of the brain including cerebral cortex, the hippocampus, and in the amygdala. Pyramidal neurons are the primary excitation units of the mammalian prefrontal cortex and the corticospinal tract. Pyramidal neurons were first discovered and...
s in the CA3 region. A single mossy fiber projection may make as many as 37 contacts with a single pyramidal cell
Pyramidal cell
Pyramidal neurons are a type of neuron found in areas of the brain including cerebral cortex, the hippocampus, and in the amygdala. Pyramidal neurons are the primary excitation units of the mammalian prefrontal cortex and the corticospinal tract. Pyramidal neurons were first discovered and...
, but innervates only about a dozen different pyramidal cells. In contrast, a single CA3 pyramidal cell receives input from about 50 different granule cells. It has been shown in rodents that the size of the mossy fiber projections can show large interindividual variations, which are to a large part heritable. In addition, these variations show strong correlations with different types of behavior, mainly, but not exclusively, spatial learning.
See also
Mossy fiber (cerebellum)Mossy fiber (cerebellum)
Mossy fibers are one of the major inputs to cerebellum. There are many sources of this pathway, the largest of which is the cerebral cortex, which sends input to the cerebellum via the pontocerebellar pathway. Other contributors include the vestibular nerve and nuclei, the spinal cord, the...
, which shares little similarity with its hippocampal namesake.