Neuroanatomy

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Neuroanatomy is the study of the anatomy and organisation of the nervous system. Pictured here is a cross-section showing the gross anatomy of the human brain Sobo 1909 624.png
Neuroanatomy is the study of the anatomy and organisation of the nervous system. Pictured here is a cross-section showing the gross anatomy of the human brain

Neuroanatomy is the study of the structure and organization of the nervous system. In contrast to animals with radial symmetry, whose nervous system consists of a distributed network of cells, animals with bilateral symmetry have segregated, defined nervous systems. Their neuroanatomy is therefore better understood. In vertebrates, the nervous system is segregated into the internal structure of the brain and spinal cord (together called the central nervous system, or CNS) and the routes of the nerves that connect to the rest of the body (known as the peripheral nervous system, or PNS). The delineation of distinct structures and regions of the nervous system has been critical in investigating how it works. For example, much of what neuroscientists have learned comes from observing how damage or "lesions" to specific brain areas affects behavior or other neural functions.

Contents

For information about the composition of non-human animal nervous systems, see nervous system. For information about the typical structure of the Homo sapiens nervous system, see human brain or peripheral nervous system. This article discusses information pertinent to the study of neuroanatomy.

History

J. M. Bourgery's anatomy of the brain, brainstem, and upper spinal column Bourgery Neuroanatomie.jpg
J. M. Bourgery's anatomy of the brain, brainstem, and upper spinal column

The first known written record of a study of the anatomy of the human brain is an ancient Egyptian document, the Edwin Smith Papyrus. [1] The next major development in neuroanatomy came from the Greek Alcmaeon, who determined that the brain and not the heart ruled the body, and that the senses were dependent on the brain. [2]

After Alcmaeon’s findings, many scientists, philosophers, and physicians from around the world continued to contribute to the understanding of neuroanatomy, notably: Galen, Herophilus, Rhazes and Erasistratus. Herophilus and Erasistratus of Alexandria were perhaps the most influential Greek neuroscientists with their studies involving dissecting brains. [2] For several hundred years afterward, with the cultural taboo of dissection, no major progress occurred in neuroscience. However, Pope Sixtus IV effectively revitalized the study of neuroanatomy by altering the papal policy and allowing human dissection. This resulted in a boom of research in neuroanatomy by artists and scientists of the Renaissance. [3]

In 1664, Thomas Willis, a physician and professor at Oxford University, coined the term neurology when he published his text Cerebri anatome which is considered the foundation of neuroanatomy. [4] The subsequent three hundred and fifty some years has produced a great deal of documentation and study of the neural system.

Composition

At the tissue level, the nervous system is composed of neurons, glial cells, and extracellular matrix. Both neurons and glial cells come in many types (see, for example, the nervous system section of the list 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 across synapses (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, produce myelin (oligodendrocytes), and provide support and protection for the brain's neurons. Some glial cells (astrocytes) can even propagate intercellular calcium waves over long distances in response to stimulation, and release gliotransmitters in response to changes in calcium concentration. Wound scars in the brain largely contain astrocytes. The extracellular 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 the hippocampus in mammals or the mushroom bodies of the fruit fly. [5] 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 efferent nerves, 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 the axons 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 into nerve fascicles.

The vertebrate nervous system is divided into the central and peripheral nervous systems. The central nervous system (CNS) consists of the brain, retina, and spinal cord, while the peripheral 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. The somatic 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. The autonomic nervous system can work with or without the control of the CNS (that's why it's called 'autonomous'), and also has two subdivisions, called sympathetic and parasympathetic, 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).

Orientation in neuroanatomy

Para-sagittal MRI of the head in a patient with benign familial macrocephaly. Parasagittal MRI of human head in patient with benign familial macrocephaly prior to brain injury (ANIMATED).gif
Para-sagittal MRI of the head in a patient with benign familial macrocephaly.

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 (see Anatomical 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.

Tools

Modern developments in neuroanatomy are directly correlated to the technologies used to perform research. Therefore, it is necessary to discuss the various tools that are available. Many of the histological 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.

Cell staining

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 the rough endoplasmic reticulum, which is abundant in neurons. This allows researchers to distinguish between different cell types (such as neurons and glia), and neuronal shapes and sizes, in various regions of the nervous system cytoarchitecture.

The classic Golgi stain uses potassium dichromate and silver 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

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 and glia) 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.

Genetically encoded markers

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 with fluorescence 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.

Non-invasive brain imaging

Magnetic resonance imaging has been used extensively to investigate brain structure and function non-invasively in healthy human subjects. An important example is diffusion 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.

Viral-based methods

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. [6] Two tracer viruses which replicate and spread transneuronal/transsynaptic are the Herpes simplex virus type1 (HSV) [7] and the Rhabdoviruses. [8] 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. [9] Another study injected herpes simplex virus into the eye, thus allowing the visualization of the optical pathway from the retina into the visual system. [10] An example of a tracer virus which replicates from the synapse to the soma is the pseudorabies virus. [11] By using pseudorabies viruses with different fluorescent reporters, dual infection models can parse complex synaptic architecture. [12]

Dye-based methods

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 transported anterogradely (from soma to axon terminals) or retrogradely (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.

Connectomics

Serial section electron microscopy has been extensively developed for use in studying nervous systems. For example, the first application of serial block-face scanning electron microscopy was on rodent cortical tissue. [13] Circuit reconstruction from data produced by this high-throughput method is challenging, and the Citizen science game EyeWire has been developed to aid research in that area.

Computational neuroanatomy

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.

Model systems

Aside from the human brain, there are many other animals whose brains and nervous systems have received extensive study as model systems, including mice, zebrafish, [14] fruit fly, [15] and a species of roundworm called C. elegans. Each of these has its own advantages and disadvantages as a model system. For example, the C. elegans nervous system is extremely stereotyped from one individual worm to the next. This has allowed researchers using electron microscopy to map the paths and connections of all of the approximately 300 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-layered cortex, yet its genes can be easily modified and its reproductive cycle is relatively fast).

Caenorhabditis elegans

Nervous system of a generic bilaterian animal, in the form of a nerve cord with segmental enlargements, and a "brain" at the front Bilaterian-plan.svg
Nervous system of a generic bilaterian animal, in the form of a nerve cord with segmental enlargements, and a "brain" at the front

The brain is small and simple in some species, such as the nematode 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 (a ganglion) for each body segment, with an especially large ganglion at the front, called the brain. The nematode Caenorhabditis elegans has been studied because of its importance in genetics. [16] 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 the hermaphrodite contains exactly 302 neurons, always in the same places, making identical synaptic connections in every worm. [17] 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 complete connectome of the nematode. [18] 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. [19]

Drosophila melanogaster

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 central brain with three divisions and large optical 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 the Drosophila brain is dedicated to visual processing.

Thomas Hunt Morgan started to work with Drosophila 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 studying Drosophila genetics, they have been a natural subject for studying the role of genes in the nervous system. [20] 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 of Drosophila neurogenetics have turned out to be relevant to humans. For instance, the first biological clock genes were identified by examining Drosophila mutants that showed disrupted daily activity cycles. [21]

See also

Related Research Articles

Central nervous system Brain and spinal cord

The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because it integrates the received information and coordinates and influences the activity of all parts of the bodies of bilaterally symmetric animals—i.e., all multicellular animals except sponges and radially symmetric animals such as jellyfish—and it contains the majority of the nervous system. The CNS also includes the retina and the optic nerve, as well as the olfactory nerves and olfactory epithelium as parts of the CNS, synapsing directly on brain tissue without intermediate ganglia. As such, the olfactory epithelium is the only central nervous tissue in direct contact with the environment, which opens up for therapeutic treatments. The CNS is contained within the dorsal body cavity, with the brain housed in the cranial cavity and the spinal cord in the spinal canal. In vertebrates, the brain is protected by the skull, while the spinal cord is protected by the vertebrae. The brain and spinal cord are both enclosed in the meninges. Within the CNS, the interneuronal space is filled with a large amount of supporting non-nervous cells called neuroglia or glia from the Greek for "glue".

Nerve Enclosed, cable-like bundle of axons in the peripheral nervous system

A nerve is an enclosed, cable-like bundle of nerve fibres called axons, in the peripheral nervous system. A nerve transmits electrical impulses and is the basic unit of the peripheral nervous system. A nerve provides a common pathway for the electrochemical nerve impulses called action potentials that are transmitted along each of the axons to peripheral organs or, in the case of sensory nerves, from the periphery back to the central nervous system. Each axon within the nerve is an extension of an individual neuron, along with other supportive cells such as some Schwann cells that coat the axons in myelin.

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.

Motor neuron Neuron whose cell body is located in the motor cortex, brainstem or the spinal cord, and whose axon projects to the spinal cord or outside of the spinal cord to directly or indirectly control effector organs, mainly muscles and glands

A motor neuron is a neuron whose cell body is located in the motor cortex, brainstem or the spinal cord, and whose axon (fiber) projects to the spinal cord or outside of the spinal cord to directly or indirectly control effector organs, mainly muscles and glands. There are two types of motor neuron – upper motor neurons and lower motor neurons. Axons from upper motor neurons synapse onto interneurons in the spinal cord and occasionally directly onto lower motor neurons. The axons from the lower motor neurons are efferent nerve fibers that carry signals from the spinal cord to the effectors. Types of lower motor neurons are alpha motor neurons, beta motor neurons, and gamma motor neurons.

Brainstem posterior part of the brain, adjoining and structurally continuous

The brainstem is the posterior part of the brain, continuous with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is continuous with the thalamus of the diencephalon through the tentorial notch, and sometimes the diencephalon is included in the brainstem.

Somatic nervous system Part of the peripheral nervous system

The somatic nervous system is the part of the peripheral nervous system associated with the voluntary control of body movements via skeletal muscles.

Afferent nerve fiber

Afferent nerve fibers refer to axonal projections that arrive at a particular brain region, as opposed to efferent projections that exit the region. These terms have a slightly different meaning in the context of the peripheral nervous system (PNS) and central nervous system (CNS).

Spinothalamic tract Sensory pathway from the skin to the thalamus

The spinothalamic tract is a sensory pathway to the thalamus. From the ventral posterolateral nucleus in the thalamus, sensory information is relayed upward to the somatosensory cortex of the postcentral gyrus.

Dorsal column–medial lemniscus pathway sensory spinal pathway

The dorsal column–medial lemniscus pathway (DCML) is a sensory pathway of the central nervous system that conveys sensations of fine touch, vibration, two-point discrimination, and proprioception (position) from the skin and joints. It transmits information from the body to the primary somatosensory cortex in the postcentral gyrus of the parietal lobe of the brain. The pathway receives information from sensory receptors throughout the body, and carries this in nerve tracts in the white matter of the dorsal columns of the spinal cord to the medulla, where it is continued in the medial lemniscus, on to the thalamus and relayed from there through the internal capsule and transmitted to the somatosensory cortex. The name dorsal-column medial lemniscus comes from the two structures that carry the sensory information: the dorsal columns of the spinal cord, and the medial lemniscus in the brainstem.

Neuromeres are morphologically or molecularly defined transient segments of the early developing brain. Rhombomeres are such segments that make up the rhombencephalon or hindbrain. More controversially, some argue that there exist early developmental segments that give rise to structures of the midbrain (mesomeres) and forebrain (prosomeres).

Spinocerebellar tract set of axonal fibers originating in the spinal cord and terminating in the ipsilateral cerebellum

The spinocerebellar tract is a nerve tract originating in the spinal cord and terminating in the same side (ipsilateral) of the cerebellum.

Lateral corticospinal tract largest part of the corticospinal tract

The lateral corticospinal tract is the largest part of the corticospinal tract. It extends throughout the entire length of the spinal cord, and on transverse section appears as an oval area in front of the posterior column and medial to the posterior spinocerebellar tract.

Floor plate

The floor plate is a structure integral to the developing nervous system of vertebrate organisms. Located on the ventral midline of the embryonic neural tube, the floor plate is a specialized glial structure that spans the anteroposterior axis from the midbrain to the tail regions. It has been shown that the floor plate is conserved among vertebrates, such as zebrafish and mice, with homologous structures in invertebrates such as the fruit fly Drosophila and the nematode C. elegans. Functionally, the structure serves as an organizer to ventralize tissues in the embryo as well as to guide neuronal positioning and differentiation along the dorsoventral axis of the neural tube.

Alpha motor neuron

Alpha (α) motor neurons (also called alpha motoneurons), are large, multipolar lower motor neurons of the brainstem and spinal cord. They innervate extrafusal muscle fibers of skeletal muscle and are directly responsible for initiating their contraction. Alpha motor neurons are distinct from gamma motor neurons, which innervate intrafusal muscle fibers of muscle spindles.

Spinal cord Long, tubular central nervous system structure in the vertebral column

The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column. It encloses the central canal of the spinal cord, which contains cerebrospinal fluid. The brain and spinal cord together make up the central nervous system (CNS). In humans, the spinal cord begins at the occipital bone, passing through the foramen magnum and entering the spinal canal at the beginning of the cervical vertebrae. The spinal cord extends down to between the first and second lumbar vertebrae, where it ends. The enclosing bony vertebral column protects the relatively shorter spinal cord. It is around 45 cm (18 in) in men and around 43 cm (17 in) long in women. The diameter of the spinal cord ranges from 13 mm in the cervical and lumbar regions to 6.4 mm in the thoracic area.

Anatomical plane plane used to transect the human body, in order to describe the location of structures or the direction of movements

An anatomical plane is a hypothetical plane used to transect the body, in order to describe the location of structures or the direction of movements. In human and animal anatomy, three principal planes are used:

Anatomical terms of neuroanatomy Wikimedia list article

This article describes anatomical terminology that is used to describe the central and peripheral nervous systems - including the brain, brainstem, spinal cord, and nerves.

Outline of the human nervous system Overview of and topical guide to the human nervous system

The following Diagram is provided as an overview of and topical guide to the human nervous system:

Segmentation is the physical characteristic by which the human body is divided into repeating subunits called segments arranged along a longitudinal axis. In humans, the segmentation characteristic observed in the nervous system is of biological and evolutionary significance. Segmentation is a crucial developmental process involved in the patterning and segregation of groups of cells with different features, generating regional properties for such cell groups and organizing them both within the tissues as well as along the embryonic axis.

Flexure (embryology) Part of the embryonic neural tube

Three flexures form in the part of the embryonic neural tube that develops into the brain. At four weeks gestational age in the human embryo the neural tube has developed at the cranial end into three swellings – the primary brain vesicles. The space into which the cranial part of the neural tube is developing is limited. This limitation causes the neural tube to bend, or flex, at two ventral flexures – the rostral cephalic flexure, and the caudal cervical flexure. It also bends dorsally into the pontine flexure. These flexures have formed by the time that the primary brain vesicles have developed into five secondary brain vesicles in the fifth week.

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