Neuroanatomy is the study of the structure and organization of thenervous system. In contrast to animals withradial symmetry, whose nervous system consists of a distributed network of cells, animals withbilateral symmetry have segregated, defined nervous systems. Their neuroanatomy is therefore better understood. Invertebrates, the nervous system is segregated into the internal structure of thebrain andspinal cord (together called thecentral nervous system, or CNS) and the series of nerves that connect the CNS to the rest of the body (known as theperipheral nervous system, or PNS). Breaking down and identifying specific parts of the nervous system has been crucial for figuring out how it operates. For example, much of what neuroscientists have learned comes from observing how damage or "lesions" to specific brain areas affectsbehavior or other neural functions.
For information about the composition of non-human animal nervous systems, seenervous system. For information about the typical structure of the Homo sapiens nervous system, seehuman brain orperipheral nervous system. This article discusses information pertinent to thestudy of neuroanatomy.
The first known written record of a study of the anatomy of the human brain is anancient Egyptian document, theEdwin Smith Papyrus.[1] InAncient Greece, interest in the brain began with the work ofAlcmaeon, who appeared to have dissected the eye and related the brain to vision. He also suggested that the brain, not the heart, was the organ that ruled the body (what Stoics would call thehegemonikon) and that the senses were dependent on the brain.[2]
The debate regarding thehegemonikon persisted among ancient Greek philosophers and physicians for a very long time.[3] Those who argued for the brain often contributed to the understanding of neuroanatomy as well.Herophilus andErasistratus of Alexandria were perhaps the most influential with their studies involvingdissecting human brains, affirming the distinction between thecerebrum and thecerebellum, and identifying theventricles and thedura mater.[4][5] The Greek physician and philosopherGalen, likewise, argued strongly for the brain as the organ responsible forsensation andvoluntary motion, as evidenced by his research on the neuroanatomy ofoxen,Barbary apes, and other animals.[3][6]
The cultural taboo on human dissection continued for several hundred years afterward, which brought no major progress in the understanding of the anatomy of the brain or of the nervous system. However,Pope Sixtus IV effectively revitalized the study of neuroanatomy by altering the papal policy and allowing human dissection. This resulted in a flush of new activity by artists and scientists of the Renaissance,[7] such asMondino de Luzzi,Berengario da Carpi, andJacques Dubois, and culminating in the work ofAndreas Vesalius.[8][9]
In 1664,Thomas Willis, a physician and professor at Oxford University, coined the term neurology when he published his textCerebri Anatome which is considered the foundation of modern neuroanatomy.[10] The subsequent three hundred and fifty some years has produced a great deal of documentation and study of the neural system.
At the tissue level, the nervous system is composed ofneurons,glial cells, andextracellular matrix. Both neurons and glial cells come in many types (see, for example, the nervous system section of thelist of distinct cell types in the adult human body). Neurons are the information-processing cells of the nervous system: they sense our environment, communicate with each other via electrical signals and chemicals called neurotransmitters which generally act acrosssynapses (close contacts between two neurons, or between a neuron and a muscle cell; note also extrasynaptic effects are possible, as well as release of neurotransmitters into the neural extracellular space), and produce our memories, thoughts, and movements. Glial cells maintain homeostasis, producemyelin (oligodendrocytes, Schwann cells), and provide support and protection for the brain's neurons. Some glial cells (astrocytes) can even propagate intercellularcalcium waves over long distances in response to stimulation, and releasegliotransmitters in response to changes in calcium concentration. Wound scars in the brain largely contain astrocytes. Theextracellular matrix also provides support on the molecular level for the brain's cells, vehiculating substances to and from the blood vessels.
At the organ level, the nervous system is composed of brain regions, such as thehippocampus in mammals or themushroom bodies of thefruit fly.[11] These regions are often modular and serve a particular role within the general systemic pathways of the nervous system. For example, the hippocampus is critical for forming memories in connection with many other cerebral regions. The peripheral nervous system also contains afferent or efferentnerves, which are bundles of fibers that originate from the brain and spinal cord, or from sensory or motor sorts of peripheral ganglia, and branch repeatedly to innervate every part of the body. Nerves are made primarily of theaxons or dendrites of neurons (axons in case of efferent motor fibres, and dendrites in case of afferent sensory fibres of the nerves), along with a variety of membranes that wrap around and segregate them intonerve fascicles.
The vertebrate nervous system is divided into the central and peripheral nervous systems. Thecentral nervous system (CNS) consists of thebrain,retina, andspinal cord, while theperipheral nervous system (PNS) is made up of all the nerves and ganglia (packets of peripheral neurons) outside of the CNS that connect it to the rest of the body. The PNS is further subdivided into the somatic and autonomic nervous systems. Thesomatic nervous system is made up of "afferent" neurons, which bring sensory information from the somatic (body) sense organs to the CNS, and "efferent" neurons, which carry motor instructions out to the voluntary muscles of the body. Theautonomic nervous system can work with or without the control of the CNS (that's why it is called 'autonomous'), and also has two subdivisions, calledsympathetic andparasympathetic, which are important for transmitting motor orders to the body's basic internal organs, thus controlling functions such as heartbeat, breathing, digestion, and salivation. Autonomic nerves, unlike somatic nerves, contain only efferent fibers. Sensory signals coming from the viscera course into the CNS through the somatic sensory nerves (e.g., visceral pain), or through some particular cranial nerves (e.g., chemosensitive or mechanic signals).
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In anatomy in general and neuroanatomy in particular, several sets of topographic terms are used to denote orientation and location, which are generally referred to the body or brain axis (seeAnatomical terms of location). The axis of the CNS is often wrongly assumed to be more or less straight, but it actually shows always two ventral flexures (cervical and cephalic flexures) and a dorsal flexure (pontine flexure), all due to differential growth during embryogenesis. The pairs of terms used most commonly in neuroanatomy are:
Note that such descriptors (dorsal/ventral, rostral/caudal; medial/lateral) are relative rather than absolute (e.g., a lateral structure may be said to lie medial to something else that lies even more laterally).
Commonly used terms for planes of orientation or planes of section in neuroanatomy are "sagittal", "transverse" or "coronal", and "axial" or "horizontal". Again in this case, the situation is different for swimming, creeping or quadrupedal (prone) animals than for Man, or other erect species, due to the changed position of the axis. Due to the axial brain flexures, no section plane ever achieves a complete section series in a selected plane, because some sections inevitably result cut oblique or even perpendicular to it, as they pass through the flexures. Experience allows to discern the portions that result cut as desired.
According to these considerations, the three directions of space are represented precisely by the sagittal, transverse and horizontal planes, whereas coronal sections can be transverse, oblique or horizontal, depending on how they relate to the brain axis and its incurvations.
Modern developments in neuroanatomy are directly correlated to the technologies used to performresearch. Therefore, it is necessary to discuss the various tools that are available. Many of thehistological techniques used to study other tissues can be applied to the nervous system as well. However, there are some techniques that have been developed especially for the study of neuroanatomy.
In biological systems,staining is a technique used to enhance the contrast of particular features in microscopic images.
Nissl staining uses aniline basic dyes to intensely stain the acidic polyribosomes in therough endoplasmic reticulum, which is abundant in neurons. This allows researchers to distinguish between different cell types (such as neurons andglia), and neuronal shapes and sizes, in various regions of the nervous systemcytoarchitecture.
The classicGolgi stain usespotassium dichromate andsilver nitrate to fill selectively with a silver chromate precipitate a few neural cells (neurons or glia, but in principle, any cells can react similarly). This so-called silver chromate impregnation procedure stains entirely or partially the cell bodies and neurites of some neurons -dendrites,axon- in brown and black, allowing researchers to trace their paths up to their thinnest terminal branches in a slice of nervous tissue, thanks to the transparency consequent to the lack of staining in the majority of surrounding cells. Modernly, Golgi-impregnated material has been adapted for electron-microscopic visualization of the unstained elements surrounding the stained processes and cell bodies, thus adding further resolutive power.
Histochemistry uses knowledge about biochemical reaction properties of the chemical constituents of the brain (including notably enzymes) to apply selective methods of reaction to visualize where they occur in the brain and any functional or pathological changes. This applies importantly to molecules related to neurotransmitter production and metabolism, but applies likewise in many other directions chemoarchitecture, or chemical neuroanatomy.
Immunocytochemistry is a special case of histochemistry that uses selective antibodies against a variety of chemical epitopes of the nervous system to selectively stain particular cell types, axonal fascicles, neuropiles, glial processes or blood vessels, or specific intracytoplasmic or intranuclear proteins and other immunogenetic molecules, e.g., neurotransmitters. Immunoreacted transcription factor proteins reveal genomic readout in terms of translated protein. This immensely increases the capacity of researchers to distinguish between different cell types (such as neurons andglia) in various regions of the nervous system.
In situ hybridization uses synthetic RNA probes that attach (hybridize) selectively to complementary mRNA transcripts of DNA exons in the cytoplasm, to visualize genomic readout, that is, distinguish active gene expression, in terms of mRNA rather than protein. This allows identification histologically (in situ) of the cells involved in the production of genetically-coded molecules, which often represent differentiation or functional traits, as well as the molecular boundaries separating distinct brain domains or cell populations.
By expressing variable amounts of red, green, and blue fluorescent proteins in the brain, the so-called "brainbow" mutant mouse allows the combinatorial visualization of many different colors in neurons. This tags neurons with enough unique colors that they can often be distinguished from their neighbors withfluorescence microscopy, enabling researchers to map the local connections or mutual arrangement (tiling) between neurons.
Optogenetics uses transgenic constitutive and site-specific expression (normally in mice) of blocked markers that can be activated selectively by illumination with a light beam. This allows researchers to study axonal connectivity in the nervous system in a very discriminative way.
Magnetic resonance imaging has been used extensively to investigate brainstructure andfunction non-invasively in healthy human subjects. An important example isdiffusion tensor imaging, which relies on the restricted diffusion of water in tissue in order to produce axon images. In particular, water moves more quickly along the direction aligned with the axons, permitting the inference of their structure.
Certain viruses can replicate in brain cells and cross synapses. So, viruses modified to express markers (such as fluorescent proteins) can be used to trace connectivity between brain regions across multiple synapses.[12] Two tracer viruses which replicate and spread transneuronal/transsynaptic are theHerpes simplex virus type1 (HSV)[13] and theRhabdoviruses.[14] Herpes simplex virus was used to trace the connections between the brain and the stomach, in order to examine the brain areas involved in viscero-sensory processing.[15] Another study injected herpes simplex virus into the eye, thus allowing the visualization of theoptical pathway from theretina into thevisual system.[16] An example of a tracer virus which replicates from the synapse to the soma is thepseudorabies virus.[17] By using pseudorabies viruses with different fluorescent reporters, dual infection models can parse complex synaptic architecture.[18]
Axonal transport methods use a variety of dyes (horseradish peroxidase variants, fluorescent or radioactive markers, lectins, dextrans) that are more or less avidly absorbed by neurons or their processes. These molecules are selectively transportedanterogradely (from soma to axon terminals) orretrogradely (from axon terminals to soma), thus providing evidence of primary and collateral connections in the brain. These 'physiologic' methods (because properties of living, unlesioned cells are used) can be combined with other procedures, and have essentially superseded the earlier procedures studying degeneration of lesioned neurons or axons. Detailed synaptic connections can be determined by correlative electron microscopy.
Serial section electron microscopy has been extensively developed for use in studying nervous systems. For example, the first application ofserial block-face scanning electron microscopy was on rodent cortical tissue.[19] Circuit reconstruction from data produced by this high-throughput method is challenging, and the Citizen science gameEyeWire has been developed to aid research in that area.
Is a field that utilizes various imaging modalities and computational techniques to model and quantify the spatiotemporal dynamics of neuroanatomical structures in both normal and clinical populations.
Aside from thehuman brain, there are many other animals whose brains and nervous systems have received extensive study asmodel systems, including mice,zebrafish,[20]fruit fly,[21] and a species of roundworm calledC. elegans. Each of these has its own advantages and disadvantages as a model system. For example, theC. elegans nervous system is extremely stereotyped from one individual worm to the next. This has allowed researchers usingelectron microscopy to map the paths and connections of all of the 302 neurons in this species. The fruit fly is widely studied in part because its genetics is very well understood and easily manipulated. The mouse is used because, as a mammal, its brain is more similar in structure to our own (e.g., it has a six-layeredcortex, yet its genes can be easily modified and its reproductive cycle is relatively fast).
The brain is small and simple in some species, such as thenematode worm, where the body plan is quite simple: a tube with a hollow gut cavity running from the mouth to the anus, and a nerve cord with an enlargement (aganglion) for each body segment, with an especially large ganglion at the front, called the brain. The nematodeCaenorhabditis elegans has been studied because of its importance in genetics.[22] In the early 1970s,Sydney Brenner chose it as a model system for studying the way that genes control development, including neuronal development. One advantage of working with this worm is that the nervous system of thehermaphrodite contains exactly 302 neurons, always in the same places, making identical synaptic connections in every worm.[23] Brenner's team sliced worms into thousands of ultrathin sections and photographed every section under an electron microscope, then visually matched fibers from section to section, to map out every neuron and synapse in the entire body, to give a completeconnectome of the nematode.[24] Nothing approaching this level of detail is available for any other organism, and the information has been used to enable a multitude of studies that would not have been possible without it.[25]
Drosophila melanogaster is a popular experimental animal because it is easily cultured en masse from the wild, has a short generation time, and mutant animals are readily obtainable.
Arthropods have a centralbrain with three divisions and largeoptical lobes behind each eye for visual processing. The brain of a fruit fly contains several million synapses, compared to at least 100 billion in the human brain. Approximately two-thirds of theDrosophila brain is dedicated tovisual processing.
Thomas Hunt Morgan started to work withDrosophila in 1906, and this work earned him the 1933 Nobel Prize in Medicine for identifying chromosomes as the vector of inheritance for genes. Because of the large array of tools available for studyingDrosophila genetics, they have been a natural subject for studying the role of genes in the nervous system.[26] The genome has been sequenced and published in 2000. About 75% of known human disease genes have a recognizable match in the genome of fruit flies.Drosophila is being used as a genetic model for several human neurological diseases including the neurodegenerative disorders Parkinson's, Huntington's, spinocerebellar ataxia and Alzheimer's disease. In spite of the large evolutionary distance between insects and mammals, many basic aspects ofDrosophila neurogenetics have turned out to be relevant to humans. For instance, the first biological clock genes were identified by examiningDrosophila mutants that showed disrupted daily activity cycles.[27]
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