Neuropil

Last updated
Neuropil
Details
System Nervous system
Identifiers
Latin neuropilus
MeSH D019581
TH H2.00.06.2.02005
Anatomical terms of microanatomy

Neuropil (or "neuropile") is any area in the nervous system composed of mostly unmyelinated axons, dendrites and glial cell processes that forms a synaptically dense region containing a relatively low number of cell bodies. The most prevalent anatomical region of neuropil is the brain which, although not completely composed of neuropil, does have the largest and highest synaptically-concentrated areas of neuropil in the body. For example, the neocortex and olfactory bulb both contain neuropil. [1]

Contents

White matter, which is mostly composed of myelinated axons (hence its white color) and glial cells, is generally not considered to be a part of the neuropil.[ citation needed ]

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Neuropil (pl. neuropils) comes from the Greek: neuro, meaning "tendon, sinew; nerve" and pilos, meaning "felt". [2] The term's origin can be traced back to the late 19th century. [3]

Location

Neuropil has been found in the following regions: outer neocortex layer, barrel cortex, inner plexiform layer and outer plexiform layer, posterior pituitary, and glomeruli of the cerebellum. These are all found in humans, with the exception of the barrel cortex, but many species have counterparts similar to our own regions of neuropil. However, the degree of similarity depends upon the composition of neuropil being compared. The concentrations of neuropil within certain regions are important to determine because simply using the proportions of the different postsynaptic elements does not verify the necessary, conclusive evidence. Comparing the concentrations can determine whether or not proportions of different postsynaptic elements contacted a particular axonal pathway. Relative concentrations could signify a reflection of different postsynaptic elements in the neuropil or show that axons sought out and formed synapses only with specific postsynaptic elements. [4]

Function

Since neuropils have a diverse role in the nervous system, it is difficult to define a certain overarching function for all neuropils. For instance, the olfactory glomeruli function as sorts of way-stations for the information flowing from the olfactory receptor neurons to the olfactory cortex. The inner plexiform layer of the retina is a little more complex. The bipolar cells post-synaptic to either rods or cones are either depolarized or hyperpolarized depending on whether the bipolar cells have sign-inverting synapses or a sign-conserving synapses. [1]

Efficiency in the brain

Neurons are necessary for all connections made in the brain, and thus can be thought of as the "wires" of the brain. As in computing, an entity is most efficient when its wires are optimized; therefore, a brain which has undergone millions of years of natural selection would be expected to have optimized neural circuitry. To have an optimized neural system it must balance four variables—it must "minimize conduction delays in axons, passive cable attenuation in dendrites, and the length of 'wire' used to construct circuits" as well as "maximize the density of synapses", [5] essentially optimizing the neuropil. Researchers at Cold Spring Harbor Laboratory formulated the optimal balance of the four variables and calculated the optimal ratio of axon plus dendrite volume (i.e. the "wire" volume or neuropil volume) to total volume of grey matter. The formula predicted an optimal brain with 3/5 (60%) of its volume occupied by neuropil. Experimental evidence taken from three mouse brains agrees with this result. The "fraction of wire is 0.59 ± 0.036 for layer IV of visual cortex, 0.62 ± 0.055 for layer Ib of piriform cortex, and 0.54 ± 0.035 for the stratum radiatum of hippocampal field CA1. The overall average is 0.585 ± 0.043; these values are not statistically different from the optimal 3/5." [5]

Clinical significance

Schizophrenia

It has been shown that a certain protein[ which? ] is lost in people with schizophrenia that causes dendrites and spines to deteriorate in the dorsolateral prefrontal cortex, a part of the neocortex, which plays a key role in information processing, attention, memory, orderly thinking and planning which are all functions that deteriorate in people with schizophrenia. The deterioration of the neuropil in this cortex has been proposed as the cause of schizophrenia. [6]

Alzheimer's disease

Alzheimer's is a neuropathological disease that is hypothesized to result from the loss of dendritic spines and/or deformation of these spines in the patient's frontal and temporal cortices. Researchers have tied the disease to a decrease in the expression of drebrin, a protein thought to play a role in long-term potentiation, meaning the neurons would lose plasticity and have trouble forming new connections. This malfunction presents itself in the form of helical filaments that tangle together in the neuropil. This same phenomenon seems to occur in the elderly as well. [7] [8]

Other animals

Other mammals

A significant non-human area of neuropil is the barrel cortex found in mammals with whiskers (e.g. cats, dogs and rodents); each "barrel" in the cortex is a region of neuropil where the input from a single whisker terminates. [9]

Significance of neuropil difference in chimpanzees and humans

Neuropil have been hypothesized to be a key factor in differentiating human cognitive capacity from that of other animals. In one study comparing chimpanzee and human frontopolar cortex and the frontoinsular cortex neuropil, it was found that humans exhibit a significantly higher neuropil fraction than the other areas of their brain. This suggests that as we evolved our prefrontal cortex developed denser neuropil which translates to more neural connections. In chimpanzees these prefrontal regions did not display significantly more neuropil. [6]

Arthropods

The optic lobe of arthropods and the ganglia of the arthropod brain as well as the ganglia in the ventral nerve cord are unmyelinated and therefore belong to the class of neuropils.

Research

Research has focused on where neuropil is found in many different species in order to unveil the range of significance it has and possible functions.

Recent studies

In chimpanzees and humans the neuropil provides a proxy measure of total connectivity within a local region because it is composed mostly of dendrites, axons, and synapses. [10]

In insects the central complex plays an important role in higher-order brain function. The neuropil in Drosophila Ellipsoid is composed of four substructures. Each section has been observed in several insects as well as the influence it has on behavior, however the exact function of this neuropil has proven elusive. Abnormal walking behavior and flight behavior are controlled primarily by the central complex and genetic mutations that disrupt the structure support the hypothesis that the central complex neuropil is a site of behavioral control. However, only specific components of the behavior were affected with the genetic mutations. For example, basic leg coordination of walking was normal, whereas speed, activity, and turning were affected. These observations suggest that the central complex not only plays a role in locomotor behavior, but fine tuning as well. There is also additional evidence that the neuropil may function in olfactory associative learning and memory. [11]

In humans, schizophrenia may be caused by deterioration of neuropil, with much evidence specifically pointing to dysfunction in the dorsolateral prefrontal cortex (DLPFC). [6] Research has shown reduced neuropil in area 9 of schizophrenics, [12] as well as consistent findings of reduced spine density in layer III pyramidal neurons of the temporal and frontal cortices. Since neuropil is the location of most cortical synapses it is likely that the deterioration greatly affects processing and produces the symptoms schizophrenics exhibit. [6]

See also

Related Research Articles

Dendrite Small projection on a neuron that receive signals

Dendrites, 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 neuron from which the dendrites project. Electrical stimulation is transmitted onto dendrites by upstream neurons via synapses which are located at various points throughout the dendritic tree. Dendrites play a critical role in integrating these synaptic inputs and in determining the extent to which action potentials are produced by the neuron. Dendritic arborization, also known as dendritic branching, is a multi-step biological process by which neurons form new dendritic trees and branches to create new synapses. The morphology of dendrites such as branch density and grouping patterns are highly correlated to the function of the neuron. Malformation of dendrites is also tightly correlated to impaired nervous system function. Some disorders that are associated with the malformation of dendrites are autism, depression, schizophrenia, Down syndrome and anxiety.

Neuron Electrically excitable cell that communicates via synapses

A neuron, neurone or nerve cell, is an electrically excitable cell that communicates with other cells via specialized connections called synapses. It is the main component of nervous tissue in all animals except sponges and placozoa. Plants and fungi do not have nerve cells.

Neurotransmitter Chemical substance that enables neurotransmission

Neurotransmitters are endogenous chemicals acting as signaling molecules that enable neurotransmission. They are a type of chemical messenger which transmits signals across a chemical synapse from one neuron to another 'target' neuron, to a muscle cell, or to a gland cell. Neurotransmitters are released from synaptic vesicles in synapses into the synaptic cleft, where they are received by neurotransmitter receptors on the target cell. Many neurotransmitters are synthesized from simple and plentiful precursors such as amino acids, which are readily available and only require a small number of biosynthetic steps for conversion. Neurotransmitters are essential to the function of complex neural systems. The exact number of unique neurotransmitters in humans is unknown, but more than 200 have been identified.

Nervous system Highly complex part of an animal that coordinates actions and sensory information by transmitting signals between different parts of the body

The nervous system is a highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. The nervous system detects environmental changes that impact the body, then works in tandem with the endocrine system to respond to such events. Nervous tissue first arose in wormlike organisms about 550 to 600 million years ago. In vertebrates it consists of two main parts, the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and spinal cord. The PNS consists mainly of nerves, which are enclosed bundles of the long fibers or axons, that connect the CNS to every other part of the body. Nerves that transmit signals from the brain are called motor or efferent nerves, while those nerves that transmit information from the body to the CNS are called sensory or afferent. Spinal nerves serve both functions and are called mixed nerves. The PNS is divided into three separate subsystems, the somatic, autonomic, and enteric nervous systems. Somatic nerves mediate voluntary movement. The autonomic nervous system is further subdivided into the sympathetic and the parasympathetic nervous systems. The sympathetic nervous system is activated in cases of emergencies to mobilize energy, while the parasympathetic nervous system is activated when organisms are in a relaxed state. The enteric nervous system functions to control the gastrointestinal system. Both autonomic and enteric nervous systems function involuntarily. Nerves that exit from the cranium are called cranial nerves while those exiting from the spinal cord are called spinal nerves.

Chemical synapse

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 system. They are crucial to the biological computations that underlie perception and thought. They allow the nervous system to connect to and control other systems of the body.

Dendritic spine Small protrusion on a dendrite that receives input from a single axon

A dendritic spine is a small membranous protrusion from a neuron's dendrite that typically receives input from a single axon at the synapse. Dendritic spines serve as a storage site for synaptic strength and help transmit electrical signals to the neuron's cell body. Most spines have a bulbous head, and a thin neck that connects the head of the spine to the shaft of the dendrite. The dendrites of a single neuron can contain hundreds to thousands of spines. In addition to spines providing an anatomical substrate for memory storage and synaptic transmission, they may also serve to increase the number of possible contacts between neurons.

Olfactory bulb neural structure of the vertebrate forebrain involved in olfaction, which sends olfactory information to be further processed in the amygdala, the orbitofrontal cortex and the hippocampus where it plays a role in emotion, memory and learning

The olfactory bulb is a neural structure of the vertebrate forebrain involved in olfaction, the sense of smell. It sends olfactory information to be further processed in the amygdala, the orbitofrontal cortex (OFC) and the hippocampus where it plays a role in emotion, memory and learning. The bulb is divided into two distinct structures: the main olfactory bulb and the accessory olfactory bulb. The main olfactory bulb connects to the amygdala via the piriform cortex of the primary olfactory cortex and directly projects from the main olfactory bulb to specific amygdala areas. The accessory olfactory bulb resides on the dorsal-posterior region of the main olfactory bulb and forms a parallel pathway. Destruction of the olfactory bulb results in ipsilateral anosmia, while irritative lesions of the uncus can result in olfactory and gustatory hallucinations.

Pyramidal cell projection neurons in the cerebral cortex and hippocampus

Pyramidal cells, or pyramidal neurons, are a type of multipolar neuron found in areas of the brain including the cerebral cortex, the hippocampus, and the amygdala. Pyramidal neurons are the primary excitation units of the mammalian prefrontal cortex and the corticospinal tract. Pyramidal neurons are also one of two cell types where the characteristic sign, Negri bodies, are found in post-mortem rabies infection. Pyramidal neurons were first discovered and studied by Santiago Ramón y Cajal. Since then, studies on pyramidal neurons have focused on topics ranging from neuroplasticity to cognition.

Synaptogenesis is the formation of synapses between neurons in the nervous system. Although it occurs throughout a healthy person's lifespan, an explosion of synapse formation occurs during early brain development, known as exuberant synaptogenesis. Synaptogenesis is particularly important during an individual's critical period, during which there is a certain degree of synaptic pruning due to competition for neural growth factors by neurons and synapses. Processes that are not used, or inhibited during their critical period will fail to develop normally later on in life.

Neural circuit Network or circuit of neurons

A neural circuit is a population of neurons interconnected by synapses to carry out a specific function when activated. Neural circuits interconnect to one another to form large scale brain networks. Biological neural networks have inspired the design of artificial neural networks, but artificial neural networks are usually not strict copies of their biological counterparts.

An apical dendrite is a dendrite that emerges from the apex of a pyramidal cell. Apical dendrites are one of two primary categories of dendrites, and they distinguish the pyramidal cells from spiny stellate cells in the cortices. Pyramidal cells are found in the prefrontal cortex, the hippocampus, the entorhinal cortex, the olfactory cortex, and other areas. Dendrite arbors formed by apical dendrites are the means by which synaptic inputs into a cell are integrated. The apical dendrites in these regions contribute significantly to memory, learning, and sensory associations by modulating the excitatory and inhibitory signals received by the pyramidal cells.

Mitral cell cell type

Mitral cells are neurons that are part of the olfactory system. They are located in the olfactory bulb in the mammalian central nervous system. They receive information from the axons of olfactory receptor neurons, forming synapses in neuropils called glomeruli. Axons of the mitral cells transfer information to a number of areas in the brain, including the piriform cortex, entorhinal cortex, and amygdala. Mitral cells receive excitatory input from olfactory sensory neurons and external tufted cells on their primary dendrites, whereas inhibitory input arises either from granule cells onto their lateral dendrites and soma or from periglomerular cells onto their dendritic tuft. Mitral cells together with tufted cells form an obligatory relay for all olfactory information entering from the olfactory nerve. Mitral cell output is not a passive reflection of their input from the olfactory nerve. In mice, each mitral cell sends a single primary dendrite into a glomerulus receiving input from a population of olfactory sensory neurons expressing identical olfactory receptor proteins, yet the odor responsiveness of the 20-40 mitral cells connected to a single glomerulus is not identical to the tuning curve of the input cells, and also differs between sister mitral cells. Odorant response properties of individual neurons in an olfactory glomerular module. The exact type of processing that mitral cells perform with their inputs is still a matter of controversy. One prominent hypothesis is that mitral cells encode the strength of an olfactory input into their firing phases relative to the sniff cycle. A second hypothesis is that the olfactory bulb network acts as a dynamical system that decorrelates to differentiate between representations of highly similar odorants over time. Support for the second hypothesis comes primarily from research in zebrafish.

Synapse Junction between two neurons or a neuron and another cell

In the nervous system, a synapse is a structure that permits a neuron to pass an electrical or chemical signal to another neuron or to the target effector cell.

Mossy fiber (hippocampus)

In the hippocampus, the mossy fiber pathway consists of unmyelinated axons projecting from granule cells in the dentate gyrus that terminate on modulatory hilar mossy cells and in Cornu Ammonis area 3 (CA3), a region involved in encoding short-term memory. These axons were first described as mossy fibers by Santiago Ramón y Cajal as they displayed varicosities along their lengths that gave them a mossy appearance. The axons that make up the pathway emerge from the basal portions of the granule cells and pass through the hilus of the dentate gyrus before entering the stratum lucidum of CA3. Granule cell synapses tend to be glutamatergic, though immunohistological data has indicated that some synapses contain neuropeptidergic elements including opiate peptides such as dynorphin and enkephalin. There is also evidence for co-localization of both GABAergic and glutamatergic neurotransmitters within mossy fiber terminals. GABAergic and glutamatergic co-localization in mossy fiber boutons has been observed primarily in the developing hippocampus, but in adulthood, evidence suggests that mossy fiber synapses may alternate which neurotransmitter is released through activity-dependent regulation.

Axon terminal Terminal inflated portion of the axon, containing the specialized apparatus necessary to release neurotransmitters. The axon terminus is considered to be the whole region of thickening and the terminal bouton is a specialized region of it.

Axon terminals 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 conducts electrical impulses called action potentials away from the neuron's cell body, or soma, in order to transmit those impulses to other neurons, muscle cells or glands.

Cellular neuroscience is a branch of neuroscience concerned with the study of neurons at a cellular level. This includes morphology and physiological properties of single neurons. Several techniques such as intracellular recording, patch-clamp, and voltage-clamp technique, pharmacology, confocal imaging, molecular biology, two photon laser scanning microscopy and Ca2+ imaging have been used to study activity at the cellular level. Cellular neuroscience examines the various types of neurons, the functions of different neurons, the influence of neurons upon each other, and how neurons work together.

Granule cell Cerebellar neuron

The name granule cell has been used for a number of different types of neuron 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 the hippocampus, the superficial layer of the dorsal cochlear nucleus, the olfactory bulb, and the cerebral cortex.

Network of human nervous system comprises nodes that are connected by links. The connectivity may be viewed anatomically, functionally, or electrophysiologically. These are presented in several Wikipedia articles that include Connectionism, Biological neural network, Artificial neural network, Computational neuroscience, as well as in several books by Ascoli, G. A. (2002), Sterratt, D., Graham, B., Gillies, A., & Willshaw, D. (2011), Gerstner, W., & Kistler, W. (2002), and Rumelhart, J. L., McClelland, J. L., and PDP Research Group (1986) among others. The focus of this article is a comprehensive view of modeling a neural network. Once an approach based on the perspective and connectivity is chosen, the models are developed at microscopic, mesoscopic, or macroscopic (system) levels. Computational modeling refers to models that are developed using computing tools.

Dendrodendritic synapses are connections between the dendrites of two different neurons. This is in contrast to the more common axodendritic synapse (chemical synapse) where the axon sends signals and the dendrite receives them. Dendrodendritic synapses are activated in a similar fashion to axodendritic synapses in respects to using a chemical synapse. These chemical synapses receive a depolarizing signal from an incoming action potential which results in an influx of calcium ions that permit release of Neurotransmitters to propagate the signal the post synaptic cell. There is also evidence of bi-directionality in signaling at dendrodendritic synapses. Ordinarily, one of the dendrites will display inhibitory effects while the other will display excitatory effects. The actual signaling mechanism utilizes Na+ and Ca2+ pumps in a similar manner to those found in axodendritic synapses.

An axo-axonic synapse is a type of synapse, formed by one neuron projecting its axon terminals onto another neuron’s axon.

References

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