Neuron (Encyclopedia of Psychology)
Technical term for nerve cell.
Neurons are the basic working unit of the nervous system, sending, receiving, and storing signals through a unique blend of electricity and chemistry. The human brain has more than 100 billion neurons.
Neurons that receive information and transmit it to the spinal cord or brain are classified as afferent or sensory; those that carry information from the brain or spinal cord to the muscles or glands are classified as efferent or motor. The third type of neuron connects the vast network of neurons and may be referred to as interneuron, association neuron, internuncial neuron, connector neuron, and adjustor neuron.
Although neurons come in many sizes and shapes, they all have certain features in common. Each neuron has a cell body where the components necessary to keep the neuron alive are centered. Additionally, each neuron has two types of fiber. The axon is a large tentacle and is often quite long. (For example, the axons connecting the toes with the spinal cord are more than a meter in length.) The function of the axon is to conduct nerve impulses to other neurons or to muscles and glands. The signals transmitted by the axon are received by other neurons through the second type of fiber, the dendrites....
(The entire section is 669 words.)
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Neurons (Encyclopedia of Nursing & Allied Health)
A neuron is a specialized cell of the nervous system designed to rapidly communicate with other neurons and organs by sending chemical and electrical signals.
The nervous system contains two major types of cells, neurons and glia. Neurons are specialized cells of the central and peripheral nervous systems that play key roles in transmitting and propagating information from one neuron to another. The role of glial cells is less clear, but they are involved in supporting the functions of the neuron. There are many different types of neurons, such as motor neurons, sensory neurons, and interneurons. Each class of neuron is specially designed to perform certain functions, and therefore neuronal populations differ in structure and chemical composition. Most neurons are polarized, which means that fibers extend from the cell in a certain direction or orientation. Polarization is determined by the direction and length of structures unique to neurons, which are axons, dendrites, and the cell body.
Neurons are similar to other types of cells in that they contain all the basic cell organelles such as a nucleus, mitochondria, ribosomes, lysosomes, endoplasmic reticulum, and Golgi apparatus. However, neurons differentiate into polarized cells that contain three basic structural components: cell body (soma), axon, and dendrites. The cell body contains the nucleus and other cellular organelles and is the major place where protein synthesis occurs.
Dendrites are branched fibers extending from the cell body. The number and organization of dendrites is unique to each neural population and most neurons extend multiple dendrites that are relatively short processes. Dendrites contain small protrusions called spines. These spines express protein receptors on the surface
that are capable of responding to chemical neurotransmitters such as acetylcholine. The dendritic spines contact axon terminals of other neurons at a connection point called a synapse. The dendrites send this chemical information to the cell body. The cell body integrates the chemical signal from all the dendrites and generates an electrical signal called an axon potential that is sent down the length of the axon to signal the next neuron.
The axon is a fiber neurite process that extends from the cell body and can be up to a meter in length. The axon protrudes from a bulge at the base of the cell body at a region called the axon hillock. Most neurons have only one axon but may have hundreds of dendrites. The axon is specially designed to send electrical signals known as action potentials down the length of the axon to the axon terminals. The axon terminal releases chemical neurotransmitters in response to the action potential onto the dendrite of another neuron. The place where the axon terminal of one neuron meets the dendrite of another neuron is called a synapse. The axon contains a cytoskeletal structure designed to transport proteins and other molecules down the length of the axon to the axon terminal and from the axon terminal back up to the cell
body. This cytoskeletal structure is composed of actin filaments, neurofilaments, and microtubules.
Neurons are specially designed to communicate with other neurons by converting chemical signals into electrical ones. This is accomplished by the axon. A covering called myelin insulates the outside of the axon. Myelin is a sheath of stacked membranes and is very high in lipid. Axon myleination is conduction by the glia, oligodenrocytes, and Schwann cells. There are periodic interruptions in the myelin at the nodes of Ranvier. Electrical signals referred to as action potentials are rapidly transmitted down the axon by jumping from one node of Ranvier to the next. The action potential induces the release of chemical neurotransmitters from the axon terminal. The axon terminal contains vesicles containing packaged neurotransmitters. The action potential triggers the release of neurotransmitters onto the next neuron that then generates an axon potential to propagate the signal for cell-cell communication. This process allows signaling to occur over very long distances within milliseconds.
Common diseases and disorders
Neurons are implicated in numerous nervous system diseases from Alzheimer's disease to Huntington's disease to certain types of brain cancer. In many neural diseases, neurons degenerate due to abnormalities in basic cellular function. Populations of neurons can also become cancerous, such as in neuroblastomas.
Axon fiber process extended from the neuronal cell body that carries action potentials.
Dendrite branch-like projection from the cell body of a neuron.
Synapse meeting place between the axon terminal of one neuron and the dendrite of another neuron.
Behbehani, Michael M. "Biology of Neurons." In Cell Physiology. Edited by Nicholas Sperelakis. San Diego, CA: Academic Press, 1998, pp. 429-434.
Zigmond M.J., F.E. Bloom, S.C. Landis, J.L. Roberts, and L.R. Squire. Fundamental Neuroscience. San Diego, CA: Academic Press, 1999.
Susan M. Mockus, Ph.D.
Neuron (Encyclopedia of Drugs, Alcohol, and Addictive Behavior)
The gross anatomy of the central nervous systemhe brain and spinal cordas studied in some detail during the seventeenth and eighteenth centuries, but not until the nineteenth century did scientists begin to appreciate that the central nervous system (CNS) was composed of many millions of separate cells, the neurons (also called nerve cells). This discovery had to await technical improvements in the microscope and the development of specialized stains that permitted scientists to observe the microscopic anatomy of the nervous system.
In the 1870s, the Italian anatomist Camillo Golgi developed such a special staining technique, and he and other scientists were then able to observe, under the microscope, the fine structures of the cells of the nervous system. Yet Golgi may not have fully appreciated that what seemed to be an extended network of nerve tissue, in reality, were millions of distinct neurons with fine fibrils touching each other. It was the Spanish scientist, Santiago Ramón y Cajal, who was credited with expounding the neuron theory. In 1906, Golgi and Ramón y Cajal shared the Nobel prize in physiology/medicine for their discoveries on the nature of the nervous system.
Even after the concept of separate neurons was generally accepted, there was controversy for many years about how the separate neurons communicated with each other. At the end of the nineteenth century, many scientists believed they did so by means of electric impulses. Others believed there was a chemical messenger that allowed neurons to influence each other. Around 1920, ACETYLCHOLINE was discovered, the first of many nerve messengers that would be discovered during the subsequent decades.
The neuron is the basic functional cellular unit of nervous system operations; it is the principal investigational target of research into the actions of addictive drugs and ALCOHOL. An essential feature of the cellular composition of the brain is the high density of extremely varied, heterogeneously shaped neuron groups (see Figure 1). To understand the specialized aspects of neurons and their
Neurons share many cellular properties that distinguish them significantly from other cell types in other tissues; those changes within the cell's regulatory processes of greatest interest to researchers of addictive drugs, however, depend on features that form distinctions within the class of cells called neurons. Furthermore, the assembly of individual neurons into functional systems, through highly precise circuitry employing highly specified forms of chemical interneuronal transmission, allows for the sensitivity of a brain to addictive drugs.
In some organs of the bodyuch as the liver, kidney, or muscleach cell of the tissue is generally similar in shape and function. Within that tissue, all perform in highly redundant fashion to convert their incoming raw material into, respectively, nutrients, urine, or contractions, which establishes the function of the specific tissue. In the nervous system, the variously (heterogeneously) shaped neurons (see Figure 2), supported by an even larger class of similarly (homogeneously) shaped non-neuronal cells, termed neuroglia, convert information from external, or from internal, sources into information ultimately integrated into programs for the initiation and regulation of behavior.
This integrative conversion of sensory information into behavioral programs results from the rich interconnections between neurons, and it depends on the extremely differentiated features of neuronsheir size and shape; their extended cell-surface cytoplasmic processes (dendrites and axons); and their resultant interconnections that establish the sources of their incoming (afferent) information
As cells, neurons share some features in common with cells in all other organ systems (see Figure 3). They have a plasma membrane acting as an external cell wall to form a distinct boundary between the environment inside (intracellular) and outside (extracellular) the cells. The intracellular material enclosed by the plasma membrane is termed the cytoplasm. Like all other cells (except red blood cells), neurons have numerous specialized intracellular organelles, which permit them to maintain their vitality while performing their specialized functions.
Thus, neurons have mitochondria (singular, mitochondrion), by which they convert sugar and oxygen into intracellular energy molecules, which then fuel other metabolic reactions. Neurons have abundant microtubules, thin intracellular tubular struts, by which they form and maintain their often highly irregular cell structure. Neurons are also rich in a network of intracellular membranous channels, the endoplasmic reticulum, through which they distribute the energy molecules, membrane components, and other synthesized products required for functioning. Like other cells that must secrete some of their synthesized products for functioning, as neurons do with their neurotransmitters, some parts of the endoplasmic reticulum, the smooth endoplasmic reticulum, are specialized for the packaging of secretion products into storage particles, which in neurons are termed synaptic vesicles. At the center of the pool of cell material, the cytoplasm, neurons possess a nucleus, which, as in other nucleated cells, contains the full array of the genetic information characteristic of the individual organism. From this nucleus, selected subsets of genetic information are expressed to provide for the general shared and the specific unshared features of the cell. The nucleus of the neuron cell is enclosed within a membranous envelope that, as in
The plasma membrane of neurons differs from that of non-neuronal cells in that it contains special proteins, termed voltage-sensitive ion channels. Such channels are conceptually small tubular proteins embedded in the membrane of the neuron, which, when activated under specific conditions, allow positively charged ions of sodium, potassium, and calcium to enter the neuron. The existence of such electrically sensitive channels permits the neuron to become electrically excitable. The expression and selective distribution (compartmentalization) of such electrically excitable channels along its efferent processes, the axons, permit neurons to conduct signals efficiently for long distances; this also accounts for the bioelectrical activity of the brain assessed by electroencephalography (EEG). Similarly, the distribution of such electrically excitable ion channels along the receptive surfaces of the neurons (its dendrites and cell body [soma]) allows them to conduct and integrate signals from all over the extended shape of the neuron.
The smooth endoplasmic reticulum of the neuron is somewhat more elaborate and extensive than other cells that secrete their products; this specialized and extensive smooth endoplasmic system is termed the Golgi complex (or Golgi apparatus). Discovered accidentally, it was a useful marker for staining the nervous system to distinguish neurons from other cells of the brain when under inspection by microscope.
The nucleus of neurons is often highly elaborated, with multiple creases or infoldings, exhibiting complex configurations, within which are typically dense accumulations of cytoplasmic organelles, and almost always a very distinctive intranuclear clustering of genetic material, the nucleolus. Differentiated neuronseurons whose developmental stage is past the step at which celltype
The most distinctive cellular feature of neurons is the degree to which they express unique patterns of size and shape. In mammals, all neurons have highly irregular shapes; such shape variations are categorized in terms of the number of cell surface extensions, or neuronal processes, that the neuronal subset expresses, as in Figure 2.
Some neurons have only one cellular process extending from the surface of a round or nearly round cell body; this form of neuron, a unipolar neuron, is typical of invertebrate nervous systems. Typical unipolar neurons are the cells of the dorsal root ganglia, in which a single efferent axon conducts information toward or away from the cell body through a branched axon.
Most neurons of the central nervous system of mammals are multipolar. That is, in addition to the efferent axon, which may also have many subsets of secondary axons, called collateral branches, that stem from the main efferent process axon, elaborations may also be expressed from the cell body surface. The latter elaborations are termed dendrites, because their shape resembles the limbs of trees. Dendrites protrude from the cell body, and they, as well as the cell body, constitute the receptive surfaces of the target neuron onto which the afferent connections make their synaptic connections.
Since neurons come in so many shapes and sizes, early investigators of the brain sought to make distinctions among them, based in part on their locations, their sizes and shapes, and the connections they could be shown to receive or emit. Every scientist who worked in the formative era of brain research sought to describe a unique subset of neurons that were forever after named for their initial describer or the unique property defined. Thus, we have Betz neurons, large layer V-VI neurons of the motor cortex, and Purkinje neurons, the major output neurons of the cerebellar cortex, as well as neurons named for their shapes and appearancei>pyramidal neurons of the cerebral and hippocampal cortices, mitral and tufted neurons of the olfactory bulb, and granule cell neurons of the cerebellar, hippocampal, and olfactory cortices. The last mentioned have relatively compact cell bodies, densely packed together, giving the brain a granular appearance by optical microscopy.
Dendrites and axons exhibit highly distinctive morphological patterns. The surfaces of dendrites and axons can be distinctive in the shapes of their branches. This permits fine discrimination among neurons (stellar, or star-shaped, neurons; chandelier neurons; or mossy or climbing axon fibers). Some neurons exhibit dendrites whose surfaces are smooth (aspiny); others are highly elaborated (spiny), which may serve to enlarge the receptive surfaces and enhance the degree to which such neurons may integrate afferent information.
Similarly, the morphology and stability of the axons may also be highly variable. Some neurons direct their axons to highly constrained targets in a more or less direct route; others may be highly branched, with multiple collateral branches to integrate communications from one cell cluster to many divergent targets. To provide the essential support of anabolic and secretory materials within these highly elaborated cellular structures, neurons have evolved an efficient form of intracellular transport, an energy-dependent, microtubule-guided, centripetal and centrifugal process by which organelles are dispensed to and returned from the distal processes (as well as probable macromolecular signals sensed by pinocytotic-like [fluid uptake] incorporation of such signals by distal dendrites and axons). Such signals may serve as local growth-regulatory factors, allowing even the nondividing neurons to alter their shape and connections in response to activity and signals received from their afferent sources.
An individual neuron may be referred to on the basis of its size (magnocellular, parvicellular). A layer or "nuclear" cluster of neurons may be referred to by shape (pyramidal, mitral), the morphology of its axon terminals (i.e., basket cells, whose axon terminals make basket-shaped terminations on their targets), and its position in a sensory or motor circuit. In the latter classification scheme, those neurons closest to the incoming sensory event or to the outgoing motor-control event are termed primary sensory or motor neurons, respectively, whereas neurons at more distal positions of circuitry from the primary incoming or outgoing event are termed secondary, tertiary, and so on, depending on their position in that hierarchy.
In addition to these morphological qualities, neurons may also be separately distinguished on the basis of the functional systems to which they are connected (visual, auditory, somatosensory, proprioceptive, attentional, reinforcing, etc.) and on the basis of the neurotransmitters they employ to communicate with the neurons to which they are connected (cholinergic, adrenergic, GABA-ergic, etc.). Each of those features provides for a multidimensional definition of virtually every neuron in the brain.
(SEE ALSO: Brain Structures and Drugs; Neurotransmission; Neurotransmitters; ; Drug; Reward Pathways and Drugs)
CORSI, P. (ED.). (1991). The enchanted loom: Chapters in the history of neuroscience. New York: Oxford University Press.