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Neuron

Neuron

The neuron (nerve cell) is the fundamental unit of the nervous system. The basic purpose of a neuron is to receive incoming information and, based upon that information, send a signal to other neurons, muscles, or glands. Neurons are designed to rapidly send signals across physiologically long distances. They do this using electrical signals called nerve impulses or action potentials . When a nerve impulse reaches the end of a neuron, it triggers the release of a chemical, or neurotransmitter. The neurotransmitter travels rapidly across the short gap between cells (the synapse) and acts to signal the adjacent cell.

Functions and Classification

Communication by neurons can be divided into four major steps. First, a neuron receives information from the external environment or from other neurons. For example, one neuron in the human brain may receive input from as many as one hundred thousand other neurons. Second, the neuron integrates, or processes, the information from all of its inputs and determines whether or not to send an output signal. This integration takes place both in time (the duration of the input and the time between inputs) and in space (across the surface of the neuron). Third, the neuron propagates the signal along its length at high speed. The distance may be up to several meters (in a giraffe or whale), with rates up to 100 meters (328 feet) per second. Finally, the neuron converts this electrical signal to a chemical one and transmits it to another neuron or to an effector such as a muscle or gland.

When combined into networks, neurons allow the human body memory, emotion, and abstract thought as well as basic reflexes. The human brain contains an estimated one hundred billion neurons which relay, process, and store information. Neurons that lie entirely within the brain or spinal cord are referred to as interneurons and make up the central nervous system . Other neurons, receptors, and afferent (sensory) neurons are specialized to receive signals from within the body or from the external environment and to transmit that information to the central nervous system. Efferent neurons carry signals from the central nervous system to the effector organs (muscles and glands) of the body. If an efferent neuron is connected to a muscle, it is also called a motor neuron.

The ability of a neuron to carry out its function of integration and propagation depends both upon its structure and its ability to generate electrical and chemical signals. While different neurons have different shapes, all neurons share the same signaling abilities.

The Structure of a Typical Neuron

Neurons have many different shapes and sizes. However, a typical neuron in a vertebrate (such as a human) consists of four major regions: a cell body, dendrites, an axon , and synaptic terminals. Like all cells, the entire neuron is surrounded by a cell membrane. The cell body (soma) is the enlarged portion of a neuron that most closely resembles other cells. It contains the nucleus and other organelles (for example, the mitochondria and endoplasmic reticulum ) and it coordinates the metabolic activity of the neuron. The dendrites and axon are thin cytoplasmic extensions of the neuron. The dendrites, which branch out in treelike fashion from the cell body, are specialized to receive signals and transmit them toward the cell body. The single long axon carries signals away from the cell body.

In humans, a single axon may be as long as 1 meter (about 3 feet). Some neurons that have cell bodies in the spinal cord have axons that extend all the way down to the toes. Axons generally divide and redivide near their ends and each branch gives rise to a specialized ending called a synaptic knob (synaptic terminal). It is the synaptic terminals of a neuron that form connections either with the dendrites or cell body of another neuron or with effector cells in muscles or glands. Once an electrical signal has arrived at the end of an axon, the synaptic terminals release a chemical messenger called a neurotransmitter, which relays the signal across the synapse to the next neuron or to the effector cell.

Classifying Neurons by Shape

Neurons can be classified according to the number of processes that extend from the cell body. Multipolar neurons are the most common type. They have several dendrites and one axon extending from the cell body. Bipolar neurons have two processes extending from the cell body, an axon and a single dendrite. This type of neuron can be found in the retina. Unipolar neurons are generally sensory (afferent) neurons that have a single process, which then divides into two. One of the two processes extends outward to receive sensory information from various areas of the body, while the other process relays sensory information towards the spinal cord or brain.

Electrical Signals in Neurons

All living cells have a separation of charges across the cell membrane. This separation of charges gives rise to the resting membrane potential . Neurons and muscle cells both use brief changes in this resting membrane potential to quickly send signals from one end of the cell to the other. In neurons, electrical signals called action potentials propagate from the cell body down the axon to the synaptic terminals, where stored neurotransmitter is released. Action potentials are transient, all-or-none changes in resting membrane potential that travel along the axon at rates of 1 to 100 meters per second.

Myelin, a fatty insulating material derived from the cell membranes of glial cells, covers the axons of many vertebrate neurons and speeds the conduction of action potentials. The importance of this myelin covering to normal nervous system function is made painfully obvious in individuals with demyelinating diseases in which the myelin covering of the axons is destroyed. Among these diseases is multiple sclerosis, a demyelinating disease of the central nervous system that can have devastating consequences, including visual, sensory, and motor disturbances.


HYDE, IDA HENRIETTA (18571945)

American physiologist who invented the microelectrode, a tiny needle used to measure electrical activity in living cells. The microelectrode was fundamental to studies of nerve and muscle cells. Hyde was the first woman elected to the American Physiological Society and the first woman to conduct research at Harvard Medical School.


Although neurons share many of the features found in other cell types, they have some special characteristics. For example, neurons have a very high metabolic rate and must have a constant supply of oxygen and glucose to survive. Also, mature neurons lose the ability to divide by mitosis . Until the late twentieth century it was thought that no new neurons were produced in the adult human brain. However, there is evidence that, at least in some brain areas, new neurons are produced in adulthood. This finding suggests an exciting avenue for possible approaches to treating such common neurological diseases as Parkinson's disease and Alzheimer Disease, which are characterized by the loss of neurons in certain brain areas.

see also Autoimmune Disease; Brain; Central Nervous System; Chemoreception; Eye; Hearing; Muscle; Nervous Systems; Neurologic Diseases; Peripheral Nervous System; Psychoactive Drugs; Spinal Cord; Synaptic Transmission; Touch

Katja Hoehn

Bibliography

Kandel, Eric R., James H. Schwartz, and Thomas M. Jessell. Principles of Neural Science, 4th ed. New York: McGraw-Hill, 2000.

Kemperman, Gerd, and Fred H. Gage. "New Nerve Cells for the Adult Brain." Scientific American 280, no. 5 (1999): 4853.

Purves, Dale, et al. Neuroscience, 2nd ed. Sunderland, MA: Sinauer Associates, 2001.

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Neuron

Neuron

Neurons are highly specialized cells in both form and function. They contain the same suite of organelles as other cells, including the nucleus, endoplasmic reticulum, mitochondria , and bilipid membrane . Unlike many cells, however, neurons are polar cells, meaning that one side of the cell has a different form and function than the other side of the cell. The dendrites are located at one extremity, and the axon is at the other end. Dendrites are an extension of the neuronal membrane. This extension stretches out from the cell body like a tree with many branches. Each "twig" of the dendritic tree is in contact with another neuron, and the function of the dendrites is to receive information from these other neurons. It is not uncommon for thousands of neurons to contact a single dendritic arbor. The axon, at the opposite pole of the cell, is generally long and unbranched until its tip, where it may have several small branches. After the dendrites pass information through the cell body to the axon, the axon passes this information to the dendrites of other neurons.

Neurons must maintain a particular internal environment. They actively pump positively charged sodium molecules from their cytoplasm to their extracellular space, at the same time bringing positively charged potassium ions in. This is accomplished by the sodium/potassium pump, a molecular exchange protein in the membrane that creates different concentrations of ions outside and inside the neuron. The result is that the inside of the cell is negatively charged with respect to the outside of the cell. The difference in charge between the inside and outside of the cell membrane is known as the membrane potential. If the cell is depolarized, the inside of the cell contains more positive charge than normal. If the cell is hyperpolarized, the inside contains more negative charge than normal. If a neuron were disconnected from all other neurons, its membrane potential would remain constant, but when a neuron is in contact with other neurons, it receives many depolarizing signals at its dendrites. The depolarization is caused by allowing more sodium molecules to enter the cell, thereby making the inside more positively charged than normal. The depolarization begins at the tip of the dendrites and travels toward the cell body. If the depolarization is strong enough, it will not die off before reaching the cell body. If the depolarization is very strong, it will reach the axon at the other side of the cell body.

When depolarization reaches the axon, it causes an electrical chain reaction that reaches to the tip of the axon. This action potential , or spike, occurs as an active process by which specific ion channels open, allowing positively charged molecules into or out of the cell. First, the base of the axon becomes slightly depolarized from the dendritic signal. This causes specific sodium channels to open. Sodium then enters the axon, increasing the amount of depolarization. Soon the sodium channels fatigue and close, as potassium channels open, allowing positively charged potassium ions to leave the cell. The potassium ion flow cancels the depolarization and even hyperpolarizes the cell a little before potassium channels close and the membrane returns to its normal potential. This electrical event passes along the axon like a wave. The axon is covered by a number of specialized cells called glial cells. These cells wrap around the neuron and insulate it from ion exchange, except at small gaps called the nodes of Ranvier. Because ions can enter or leave the cell only at the nodes of Ranvier, the action potential jumps from node to node, thereby increasing its speed. Because the electrochemical signal moves so quickly through the neuron, the transmission of a signal along the axon is called firing.

The manner by which one neuron's axon stimulates another neuron's dendrite is through a signal molecule called a neurotransmitter. This occurs at the synapse, a specialized region that includes the tip of one neuron's axon and the conjoining region of another neuron's dendrite. Neurotransmitters are stored within the axon tip in pouches of membrane called vesicles. When an action potential travels down the axon and reaches the synapse, it triggers the release of neurotransmitter-containing vesicles into the synaptic clef, the region of space between the axon of one neuron and the dendrite of another. The neuron that releases the neurotransmitter from its axon is called the presynaptic cell and the neuron that receives the neurotransmitter at its dendrites is called the postsynaptic cell. The neurotransmitter diffuses across the synaptic clef, the space between the pre and postsynaptic cells, and binds to special neurotransmitter receptors in the dendrite of the postsynaptic neuron. These receptors open, allowing sodium ions to flow into the cell. This event is the origin of the dendritic depolarization. Neurotransmitters can be excitatory, meaning that they cause depolarization in the postsynaptic cell, or inhibitory, which means that they prevent depolarization in the postsynaptic cell. Inhibitory neurotransmitters cause a different set of receptors to open, allowing the entry of negatively charged ions such as chlorine. In this inhibition event, the negative charge hyperpolarizes the cell and decreases the probability that the postsynaptic neuron will be depolarized by excitatory presynaptic neurons.

see also Growth and Differentiation of the Nervous System; Nervous System; Sense Organs.

Rebecca M. Steinberg

Bibliography

Levitan, Irwin B., and Leonard K. Kaczmarek. The Neuron: Cell and Molecular Biology. New York: Oxford University Press, 1997.

Nicholls, John G. From Neuron to Brain, 4th ed. Sunderland, MA: Sinauer Associates, 2001.

Louis-Antoine Ranvier (1835-1922), a French histologist, described in 1878 the constriction in nerve fibers now known as nodes of Ranvier.

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Neuron

Neuron

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 dendrites are usually relatively short and have many branches to receive stimulation from other neurons. In many cases, the axon (but not the cell body or the dendrites) has a white, fatty covering called the myelin sheath. This covering is believed to increase the speed with which nerve impulses are sent down the axon.

An unstimulated neuron has a negative electrical charge. The introduction of a stimulus makes the charge a little less negative until a critical pointthe thresholdis reached. Then the membrane surrounding the neuron changes, opening channels briefly to allowing positively charged sodium ions to enter the cell. Thus, the inside of the neuron becomes positive in charge for a millisecond (thousandth of a second) or so. This brief change in electrical charge is the nerve impulse, or spike, after which the neuron is restored to its original resting charge.

This weak electrical impulse travels down the axon to the synapse . The synapse or synaptic gap forms the connection between neurons, and is actually a place where the neurons almost touch , but are separated by a gap no wider than a few billionths of an inch. At the synapses, information is passed from one neuron to another by chemicals known as neurotransmitters. The neurotransmitter then combines with specialized receptor molecules of the receiving cell.

Neurotransmitters either excite the receiving cell (that is, increase its tendency to fire nerve impulses) or inhibit it (decrease its tendency to fire impulses), and often both actions are required to accomplish the desired response. For example, the neurons controlling the muscles that pull your arm down (the triceps) must be inhibited when you are trying to reach up to your nose (biceps excited); if they are not, you will have difficulty bending your arm.

Physiological psychologists are interested in the involvement of the nervous system in behavior and experience. The chemistry and operation of the nervous system is a key component in the complex human puzzle. A number of chemical substances act as neurotransmitters at synapses in the nervous system and at the junction between nerves and muscles. These include acetylcholine, dopamine, epinephrine (adrenalin), and neuropeptides (enkephalins, endorphins, etc.). A decrease in acetylocholine has been noted in Alzheimer's disease which causes deterioration of the thought processes; shortage of dopamine has been linked to Parkinson's disease , whereas elevated dopamine has been observed in schizophrenics.

Drugs that affect behavior and experiencethe psychoactive drugs generally work on the nervous system by influencing the flow of information across synapses. For instance, they may interfere with one or several of the stages in synaptic transmission, or they may have actions like the natural neurotransmitters and excite or inhibit receiving cells. This is also true of the drugs which are used in the treatment of certain psychological disorders.

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neuron

neuron (neurone; nerve cell) An elongated branched cell that is the fundamental unit of the nervous system, being specialized for the conduction of impulses. A neuron consists of a cell body, containing the nucleus and Nissl granules; dendrites, which receive incoming impulses and pass them towards the cell body; and an axon, which conducts impulses away from the cell body, sometimes over long distances. Impulses are passed from one neuron to the next via synapses. Sensory neurons transmit information from receptors to the central nervous system. Motor neurons conduct information from the central nervous system to effectors (e.g. muscles). (See illustration.) See also bipolar neuron; multipolar neuron; unipolar neuron.

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neuron

neu·ron / ˈn(y)oŏrän/ (chiefly Brit. also neu·rone / -rōn/ ) • n. a specialized cell transmitting nerve impulses; a nerve cell. DERIVATIVES: neu·ron·al / ˈn(y)oŏrənl; n(y)oŏˈrōnl/ adj. neu·ron·ic / n(y)oŏˈränik/ adj. ORIGIN: late 19th cent.: from Greek neuron, special use of the literal sense ‘sinew, tendon.’ See nerve.

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neuron

neuron (nerve cell) Basic structural unit of the nervous system, which enables rapid transmission of impulses between different parts of the body. It is composed of a cell body, containing a nucleus, and a number of trailing processes. The largest of these is the axon, which carries outgoing impulses; the rest are dendrites, which receive incoming impulses.

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neuron

neuron, specialized cell in animals that, as a unit of the nervous system, carries information by receiving and transmitting electrical impulses.

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neuron

neuron A node in a neural network.

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neuron

neuron See NEURONE.

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neuron

neuronAgamemnon, Memnon •ninon, xenon •noumenon • Trianon • xoanon •organon • Simenon • Maintenon •crampon, kampong, tampon •Nippon • coupon •Akron, Dacron, macron •electron • natron • Hebron • positron •Heilbronn • micron •boron, moron, oxymoron •neutron • interferon •fleuron, Huron, neuron •Oberon • mellotron • aileron •cyclotron • Percheron • Mitterrand •vigneron • croissant • Maupassant •garçon • Cartier-Bresson • exon •frisson • Oxon • chanson • Tucson •soupçon • Aubusson • Besançon •penchant • torchon • cabochon •Anton, canton, Danton •lepton •piton, Teton •krypton • feuilleton • magneton •chiton •photon, proton •croûton, futon •eschaton • peloton • contretemps •telethon •talkathon, walkathon •Avon • tableau vivant • vol-au-vent

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