Neuropil (or "neuropile") is any area in the

nervous system In biology, the nervous system is the 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 ...

composed of mostly unmyelinated

axons 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 p ...

dendrite 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 ...

s 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 A brain is an organ (biology), organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as Visual perception, vision. I ...

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 The neocortex, also called the neopallium, isocortex, or the six-layered cortex, is a set of layers of the mammalian cerebral cortex involved in higher-order brain functions such as sensory perception, cognition, generation of motor commands, sp ...

and

olfactory bulb The olfactory bulb (Latin: ''bulbus olfactorius'') 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 ...

both contain neuropil.

White matter White matter refers to areas of the central nervous system (CNS) that are mainly made up of myelinated axons, also called tracts. Long thought to be passive tissue, white matter affects learning and brain functions, modulating the distribu ...

, 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. Gray881

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

Location

Neuropil has been found in the following regions: outer neocortex layer,

barrel cortex The barrel cortex is a region of the somatosensory cortex that is identifiable in some species of rodents and species of at least two other orders and contains the barrel field. The 'barrels' of the barrel field are regions within cortical lay ...

, inner plexiform layer and outer plexiform layer,

posterior pituitary The posterior pituitary (or neurohypophysis) is the posterior lobe of the pituitary gland which is part of the endocrine system. The posterior pituitary is not glandular as is the anterior pituitary. Instead, it is largely a collection of axonal ...

, and glomeruli of the

cerebellum 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 cerebe ...

. 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.

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 neuron An olfactory receptor neuron (ORN), also called an olfactory sensory neuron (OSN), is a sensory neuron within the olfactory system. Structure Humans have between 10 and 20 million olfactory receptor neurons (ORNs). In vertebrates, ORNs are ...

s 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.

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", 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."

Clinical significance

Schizophrenia

It has been shown that a certain protein

synaptophysin Synaptophysin, also known as the major synaptic vesicle protein p38, is a protein that in humans is encoded by the ''SYP'' gene. Genomics The gene is located on the short arm of X chromosome (Xp11.23-p11.22). It is 12,406 bases in length a ...

is lost in people with

schizophrenia Schizophrenia is a mental disorder characterized by continuous or relapsing episodes of psychosis. Major symptoms include hallucinations (typically hearing voices), delusions, and disorganized thinking. Other symptoms include social w ...

that causes dendrites and spines to deteriorate in the

dorsolateral prefrontal cortex The dorsolateral prefrontal cortex (DLPFC or DL-PFC) is an area in the prefrontal cortex of the primate brain. It is one of the most recently derived parts of the human brain. It undergoes a prolonged period of maturation which lasts until adult ...

, 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 a contributor to schizophrenia pathophysiology.

Alzheimer's disease

Alzheimer's Alzheimer's disease (AD) is a neurodegenerative disease that usually starts slowly and progressively worsens. It is the cause of 60–70% of cases of dementia. The most common early symptom is difficulty in remembering recent events. As t ...

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 In neuroscience, long-term potentiation (LTP) is a persistent strengthening of synapses based on recent patterns of activity. These are patterns of synaptic activity that produce a long-lasting increase in signal transmission between two neurons ...

, 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.

Other animals

Other mammals

A significant non-human area of neuropil is the

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.

Arthropods

The optic lobe of

arthropods Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chitin, ...

and the

ganglia A ganglion is a group of neuron cell bodies in the peripheral nervous system. In the somatic nervous system this includes dorsal root ganglia and trigeminal ganglia among a few others. In the autonomic nervous system there are both sympathe ...

of the arthropod brain as well as the ganglia in the

ventral nerve cord The ventral nerve cord is a major structure of the invertebrate central nervous system. It is the functional equivalent of the vertebrate spinal cord. The ventral nerve cord coordinates neural signaling from the brain to the body and vice versa, i ...

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. 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 Behavior (American English) or behaviour (British English) is the range of actions and mannerisms made by individuals, organisms, systems or artificial entities in some environment. These systems can include other systems or organisms as we ...

, 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 In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, m ...

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 The sense of smell, or olfaction, is the special sense through which smells (or odors) are perceived. The sense of smell has many functions, including detecting desirable foods, hazards, and pheromones, and plays a role in taste. In humans, ...

associative learning and

memory Memory is the faculty of the mind by which data or information is encoded, stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If past events could not be remember ...

. Research has shown reduced neuropil in area 9 of schizophrenics, as well as consistent findings of reduced spine density in layer III pyramidal neurons of the temporal and frontal cortices.

Citations

Sources

* ''Neuropil: Roche Encyclopedia of Medicine'', Dictionary Barn. * * *

External links

* —"Nervous Tissue and Neuromuscular Junction: spinal cord, cell bodies of anterior horn cells"
Neuropil—Cell Centered Database
*
"Neuropil—Neural Circuits"
��''Neuroscience''. 2nd edition, Editors: Dale Purves, George J Augustine, David Fitzpatrick, Lawrence C Katz, Anthony-Samuel LaMantia, James O McNamara, and S Mark Williams. Sunderland (MA): Sinauer Associates; 2001. . * https://www.ncbi.nlm.nih.gov/books/NBK11158/ {{Authority control Neuroanatomy