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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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Neuron

Neuron

Structure and function

Structural classification

Glial cells

Functional classification

Resources

Neurons are nerve cells (neurocytes), which, together with neuroglial cells, comprise the nervous tissue of the nervous system. The neuron is the basic element of our mental faculties, including memory, pain, the senses, emotion, and rational thought.

A neuron consists of a nerve cell body (or soma), an elongated projection (axon), and short branching fibers (called dendrites). Neurons receive nerve signals (action potentials), integrate action potentials, and transmit these signals to other neurons or effector organs, such as muscles and glands. The structure and function of neurons is essentially the same in all animals, although the human nervous system is much more specialized and complicated than that of lower animals. Humans are born with a large, but finite, supply of neurons and those cells that are lost through aging, injury, or disease cannot be replaced.

The unique morphological and intercellular structure of the neuron is dedicated to the efficient and rapid transmission of neural signals. Within the neuron, the neural signal travels electrically. At the synapse, the gap between neurons, neural signals are conveyed chemically by a limited number of chemicals termed neurotransmitters. Specialized parts of the neuron facilitate the production, release, binding, and uptake of these neurotransmitters.

Structure and function

Although there are variations related to function, a typical neuron consists of dendrites (also termed dendritic processes), a cell body, an axon, and an axon terminus.

Dendrites are the (filamentous) terminal portions of neuron that bind neurotransmitter chemicals migrating across the synaptic gaps separating neurons. Depending on the type and function of a particular neuron, neurotransmitters may cause or inhibit the transmission of neural impulses. The cell body contains the cell nucleus and a concentration of cellular organelles. The cell body is the site of the normal metabolic reactions that allow the cell to remain

viable. Neurotransmitters synthesized within the cell body are transported to the axon terminus by micro-filaments and microtubules.

The nerve cell body contains a nucleus, a nucleolus, and cytoplasm containing the cell (such as mitochondria, endoplasmic reticulum, and so on). Unique to the nerve cell body are Nissl bodies, which are rough surfaced vesicles in the endoplasmic reticulum (cytoplasm located near the nucleus), and are involved with protein synthesis. Another characteristic structure of nerve cells are the neurofibrils, which are delicate threadlike structures that help to maintain the shape of the cell, and which transport substances between the cell body and the axon terminals. The plasma membrane around the cell separates the cytoplasm on the inside of the cell from the extracellular fluid on the outside. Cell membranes of neurons contain electrically gated channels, which when properly stimulated allow electrically charged particles (such as sodium and potassium ions) to pass across the barrier. This ionic exchange is the basis for the flow, or action potential, of the nerve impulse.

The axon is a cytoplasmic continuation of the cell body specialized for the electrical conduction of neural signals. The axon may be longup to a yard in length in humansor short, depending upon the neurons position and function. The cell membranes of the neural axon transmit neural signals via changes in action potentials that sweep down the membrane.

At the junction of the cell body and axon is a region termed the axon hillock. At the axon hillock, chemical signals received by the dendrites may reach a threshold level to cause a wave of electrical depolarization and hyperpolarization of the axon cell membrane. The net movements of ions across the cell membrane are responsible for these changes that move down the axon to the axon terminus as an action potential.

At the axon terminus, neurotransmitters are released into the synaptic gap. Through synaptic gaps, a typical neuron may interconnect with thousands and tens of thousands of other neurons. Axon terminals have knoblike swellings at the very end called synaptic knobs or end buttons. Each synaptic knob communicates with a dendrite or cell body of another neuron, the point of contact being a synapse. Under very high magnification, a very tiny space, the synaptic cleft or gap (about one millionth of an inch, or mm), can be detected between the synaptic knob and dendrite or cell body. Synaptic knobs contain hundreds of neurovesicles that contain a transmitter substance (or neurotransmitter). When a nerve impulse reaches the synaptic knob the neurotransmitter is ejected into the synaptic cleft and serves as a stimulus to the next adjacent neuron. The vast majority of all impulses transmitted occur at the synaptic gaps, although recent research indicates that chemical transmission can occur at other points along the axon. Many neurological diseases and psychiatric disorders result from a disturbance or alteration of synaptic activity. Drugs such as tranquilizers, anesthetics, nicotine, and caffeine target the synapse and can cause an alteration of impulse transmission.

Structural classification

Neurons exist in many shapes and sizes. Their structure, like that of other cells in the body or in nature, illustrates that structure often determines function. There are three basic structural and functional classifications of neurons.

The structural classification of a neurons depends upon the number of dendrites extending from the cell body. Multipolar neurons have several dendrites; the majority of neurons in the spinal chord and brain are multipolar. Bipolar neurons have only two processes: a single dendrite and an axon. Bipolar neurons are found in the sense organs-and in the retina of the eye and in olfactory cells. Unipolar neurons lack dendrites and have a single axon, and are also sensory neurons.

Multipolar neurons have many processes and serve principally as motor neurons. Motor neurons, efferent because they conduct impulses away from the central nervous system (the brain and spinal cord) regulate the function of muscles and glands. Afferent neural pathways that send signals to the central nervous system (CNS) are generally composed of unipolar neurons. Unipolar neurons also serve as sensory neuronstheir filamentous dendritic processes exposed and elaborated into or connected to sensory receptor

cells. Interneurons are neurons that connect neurons along a neural pathway.

Glial cells

One cannot discuss the neuron without mentioning glial cells or neuroglia. It was once thought that these cells simply held everything together (gloios means glue, in Greek), but we now know that neuroglia are highly specialized cells. For example, neuroglia are responsible for physical support, protection against infection (through phagocytosis), and the connection of nerve cells to blood vessels. The Schwann cell (or neurolemmocyte) is a common type of glial cell found in peripheral nerve axons. Schwann cells wrap jelly roll style around the axon, forming a whitish phospholipid (fatty) protective and insulating cover known as the myelin sheath. The myelin sheath of the axon of peripheral nerves has interruptions or exposed gaps known as the Nodes of Ranvier. Schwann cells also make up the

KEY TERMS

Action potential A transient change in the electrical potential across a membrane which results in the generation of a nerve impulse.

Afferent neuron A sensory neuron that carries an impulse toward the central nervous system.

Axon The threadlike projection of a neuron that carries an impulse away from the cell body of the neuron.

Consciousness A mental state involving awareness of the self and the environment.

Dendrites Branched structures of nerve cell bodies which receive impulses from axons and carry them to the nerve cell body.

Efferent neuron A motor neuron that carries an impulse from the central nervous system to muscles or glands.

Memory The ability to recall thoughts or events.

Myelin sheath A white phospholipid (fat) covering of peripheral nerve axons.

Neuroglia Specialized nerve cells also called glial cells. The Schwann cell is a glial cell.

Neurolemma Protective nerve covering made by Schwann cells.

neurolemma, a continuous sheath that covers both the myelin sheath and the axon at the Nodes of Ranvier. Action potentials traveling down the axon occur only at the Nodes of Ranvier, jumping rapidly from gap to gap (saltatory conduction), which conducts impulses significantly faster than in nonmyelinated nerves. The neurolemma is found only in peripheral nerve fibers and plays a crucial part in nerve fiber regeneration. Damaged axons will regenerate; damaged cell bodies will not. The myelinated sheaths of the axons of neuron in the brain and spinal cord (the central nervous system) are made from different glial cells (oligodendrocytes), which lack a neurolemma, so making the regeneration of their axons impossible. Multiple sclerosis is a serious demyelinating disease of the central nervous system. Not all axons are myelinated; the presence of myelin is one difference between white matter (which has myelinated axons) and gray matter (which does not).

Functional classification

Sensory neurons transduce physical stimuli, such as smell, light, or sound, into action potentials that are then transmitted to the spinal cord or brain, bringing information into the central nervous system. Sensory neurons are also referred to as afferent neurons. Motor neurons transmit nerve impulses away from the brain and spinal cord to muscles or glands and are also called efferent neurons. Interneurons transmit nerve impulses between sensory neurons and the motor neurons. Interneurons are responsible for receiving, relaying, integrating, and sending nerve impulses. Interneurons are found exclusively in the central nervous system and account for almost 99% of all the nerve cells in the body.

See also Adenosine triphosphate; Nerve impulses and conduction of impulses; Neuromuscular diseases; Reflex; Touch.

Resources

BOOKS

Bear, Mark F., et al. Neuroscience: Exploring the Brain. Philadelphia: Lippincott Williams & Wilkins, 2006.

Johnson, Steven. Mind Wide Open: Your Brain and the Neuroscience of Everyday Life. New York: Scribner, 2005.

Koch, Christof. Biophysics of Computation: Information Processing in Single Neurons. New York: Oxford University Press, USA, 2004.

Thibodeau, Gary A., and Kevin T. Patton. Anatomy & Physiology. 5th ed. St. Louis: Mosby, 2002.

K. Lee Lerner

Christine Miner Minderovic

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Neuron

Neuron

Cells of the central nervous system are divided into two categories, neurons and glial cells. The following entry deals with the characteristics of neurons in the vertebrate central nervous system. Neurons are independent morphological, trophic, and functional entities; they develop from the neural plate of the ectoderm. They differ from glial cells in their ability to generate propagated action potentials (spikes), in the release of neuroactive substances called neurotransmitters, and in their ability to communicate with other cells through specialized membrane junctions called synapses. A long debate in the first half of the twentieth century pitted those who maintained that the brain was a continuous reticulum of fibers against those who proposed that elements of the nervous system were discrete cells (for a historical account see Peters, Palay, and Webster, 1991). The first electron microscopic studies decisively resolved the issue by showing that each neuron is delineated by a continuous plasma membrane and is separated from other cells by a gap. However, like other cells of the body, neurons in some parts of the nervous system are interconnected through continuous cytoplasmic bridges organized into gap junctions that are permeable to ions and small molecules.

Neurons are polarized cells receiving information at certain locations on their plasma membrane and releasing neurotransmitters to other cells, usually from other sites (Kandel and Schwartz, 1985; Peters, Palay, and Webster, 1991; Shepherd, 1990). They emit several processes originating from the cell body or soma. One (occasionally two or three) of the processes is an axon propagating the action potential to the transmitter-releasing nerve terminals. The other processes are called dendrites and are usually shorter and branch less frequently than the axon. The shape and three-dimensional distribution of the processes are characteristic of each category of neuron and reflect their connections with other cells and the neuron's place in the neuronal network. The general arrangement is that information arrives through afferent (i.e., inward-transmitting) synapses on the dendritic processes and the cell body, and is transmitted to other cells through axonal enlargements, also called boutons, present on the axonal arborization. However, significant exceptions to this rule occur in some parts of the brain. Neurons in invertebrates usually have only one process originating from the cell body that gives rise to branches, all of which both receive and give information and are involved in different operations.

The Soma

The soma has a diameter of five to fifty microns and contains the nucleus and the usual cell organelles present in most cells, with great similarity to those present in secretory cells. This is in line with the observation that most neurons secrete proteins and peptides in addition to small transmitter molecules. For example, the rough endoplasmic reticulum (ER), the site of protein synthesis, is often highly developed and is organized into parallel lamellae forming large Nissl bodies. The Golgi apparatus is similarly highly developed and often extends into the proximal dendritic processes, which also contain ER and ribosomes. The axons are usually devoid of ribosomes and, together with the nerve terminals, lack the ability for significant protein synthesis. Thus the neuron is also a polarized biochemical machine where protein and other components synthesized in the cell body are transported through the axon to the nerve terminals.

The transport of molecules and organelles between the processes and the soma is bidirectional and is supported by the cytoskeleton, which also maintains the shape of the processes (Kandel and Schwartz, 1985). The cytoskeleton consists of microtubules (polymers of tubulin dimers, external diameter twenty-five to twenty-eight nanometers); neurofilaments (polymers of cytokeratins, diameter ten nanometers); and microfilaments (polymers of actin, diameter five to seven nanometers).

In addition to rough ER, many neurons are rich in smooth ER that is involved in intracellular Ca2 + storage and release. Cysternae of the ER are often closely aligned with the plasma membrane of both the soma and dendrites forming subsurface cysternae.

Lysosomes are found in all neurons. Secondary lysosomes accumulate throughout the life of the cell and coalesce into lipofuscin granules showing characteristic distribution for each neuronal type.

The Dendrites

The dendrites are rarely longer than one millimeter and can be as short as ten to fifty microns with a diameter of three to 0.05 microns, tapering toward their tip and decreasing in diameter at branching points. The main difference between them and the axon is that dendrites lack the morphologically distinct initial segment at their origin from the soma. With the exception of peripheral sensory neurons, all neurons have dendrites. They are generally postsynaptic to axon terminals; from scores to tens of thousands of synapses converge on the dendritic tree of a single neuron.

In some parts of the nervous system, most prominently in the retina (amacrine cells), the olfactory bulb, the thalamus, the substantia gelatinosa of the brain stem and spinal cord, and the superior colliculus, dendrites of some classes of cells can be both pre- and postsynaptic. In these cases the dendrite at the presynaptic site contains synaptic vesicles and presynaptic membrane specialization as well as nearby postsynaptic membrane specializations at synapses received by the neuron. Often these combined preand postsynaptic sites are located on protrusions, grapelike clusters or gemmules, isolating the formations from each other on the same cell, and providing a basis for independent action. Synapses between two dendrites can be reciprocal, each partner receiving as well as giving synapses to the other at closely located sites.

Dendrites have various short postsynaptic extrusions, the best-known of them being dendritic spines (see Figure 1). Spines are particularly prominent and numerous on cortical pyramidal and spiny stellate cells, on Purkinje cells, and on the spiny neurons of the neostriatum. Spines frequently contain a specialized organelle, the spine apparatus, consisting of parallel membrane saccules and continuous with the smooth ER of the dendritic shaft. The spine apparatus is thought to be involved in Ca2+ sequestration. Spines usually receive excitatory synaptic input and occasionally an additional inhibitory input. Numerous theories have been put forward for the role of spines. Of these, the formation of a biochemical compartment, semi-independent from the dendritic shaft and from other spines, seems the most attractive. The integrative properties of dendrites are determined by 1. their shape; 2. their intrinsic membrane properties, underlined by the presence and distribution of different ion channels ; and 3. the location of synaptic inputs and their relationship to other inputs (for more detail, see Shepherd, 1990).

In addition to receiving and sometimes giving synaptic junctions, dendrites can also be connected to other dendrites and nerve terminals through small cytoplasmic bridges forming gap junctions. Gap junctions are sites of electrotonic transmission because they are permeable to ions and can facilitate the synchronization of neurons.

The Axon

Most neurons have axons; the few exceptions are retinal amacrine cells and granule cells of the olfactory bulb (Shepherd, 1990). Axons usually originate from the soma, rarely from a major dendrite, and begin with the axon hillock. A specialized trilaminar inner coat of the membrane, recognizable with the electron microscope, identifies the axon initial segment, which has the highest density of voltage-sensitive sodium channels. It is thought to be the site of the generation of the propagated-action potential. A similar membrane undercoating is found in the axon at nodes of Ranvier between myelin segments. The other unique feature of the initial segment is the presence of interconnected microtubules organized into fascicles. In some regions of the brain the axon initial segment can receive numerous synapses. Such synapses are provided by the chandelier cell, a specialized inhibitory neuron unique to the cortex, which makes synapses exclusively on the axon initial segment of pyramidal and spiny stellate neurons (see Figure 2). Most axons emit several collaterals along their course, addressing particular brain areas or groups of cells in the same brain area (see Figure 3).

Myelin Sheath

The axons of neurons in the brain can be myelinated for part or for the whole of their course, or can be completely unmyelinated (Peters, Palay, and Webster, 1991). Some types of neurons, such as corticospinal cells and Purkinje cells, always have myelinated axons. The myelin is segmented, and each segment is formed by the plasma membrane of an oligodendroglial cell. Segments are interrupted by nodes of Ranvier, where axon collaterals often originate. The axons may acquire myelin for part of their course as they traverse a particularly heavily myelinated part of the brain. Axons may contain synaptic vesicles at nodes of Ranvier, and they may be presynaptic to neighboring dendrites.

Nerve Terminals

The terminal axon arborizations are characteristic of each cell type. The transmitter-releasing sites are bulbs or varicose enlargements having a diameter usually of 0.5 to three microns; they may have a position on the end of axon branches as boutons terminaux, or may be varicosities along the axon forming boutons en passant (see Figure 1D-G). Many terminal boutons sit on the ends of short stalks branching off from main axon collaterals (see Figure 1D, vertical arrow). Specialized formations of nerve terminals evolved, such as large mossy fiber terminals providing multiple localized input to the same target (see Figure 1E), and climbing fibers providing multiple synapses distributed over the postsynaptic dendritic tree of the same target dendrite or cell. Boutons form synaptic junctions. Boutons are usually only presynaptic to other cells, but terminals of a few cell types, most prominently the primary sensory afterents in the brain stem and spinal cord, may receive synapses and are post-synaptic to inhibitory terminals. One bouton may make synaptic junctions with only one postsynaptic element or may provide input to up to about ten different postsynaptic targets originating from about the same number of individual neurons. In some cases almost every bouton of the same axon establishes synapses with a different cell, providing a large degree of divergence in information transfer. Cortical cells, for example, may make synapses with thousands of other cortical neurons in a given area (see Figure 3).

The boutons contain synaptic vesicles, which are membrane-delineated discrete structures. The morphology of the vesicles is characteristic of cell types and to some degree correlates with the chemistry of their neurotransmitter content. The two most common families of vesicles are the small clear vesicles with a diameter of thirty to fifty nanometers and the large granulated vesicles with a fine electron dense core and a diameter of about eighty to two hundred nano-meters. Boutons are also rich in mitochondria.

The synaptic vesicle-containing varicosities of some neurons do not establish morphologically recognizable synaptic junctions at all of their boutons. This applies in particular to neurons that use monoamines as transmitters.

Analysis of Neuronal Circuits

Connectivity patterns of morphologically identified neurons can be traced via the transport of marker molecules through the processes (Heimer and Zaborszky, 1989). The active transport in the living cell can be exploited by introducing suitable tracers into the neuron that are carried to the dendritic and axonal processes (see Figures 1 and 3). Tracer molecules can be introduced directly into the cell or into the surrounding extracellular space from which the cell can take them up by an active process. It is also possible to label the processes of neurons that have been fixed with chemical agents (see Figure 2). The visualization of processes makes it possible to identify the connections of particular types of neurons in the same area of the brain or among different brain regions. The morphological appearance of neurons reflects their patterns of connections. In many cases synapses from a given source terminate on certain parts of the neuron because the operation that the given input provides is best carried out in that part of the cell (see Figures 1E-G, and 2). Homologous parts of numerous postsynaptic cells in a given area of the brain tend to align with inputs arriving at that part of the cell, and this leads to the development of laminated structures. For example, axons originating from the CA3 region of the hippocampus terminate mainly on the main apical dendrites of pyramidal cells in the CA1 region, and the local recurrent collaterals of pyramidal cells in the CA1 sector address the basal dendritic region of the pyramidal cells (see Figure 1A). The separation of inputs and the minimum amount of axon necessary to achieve addressing is ensured by the alignment of pyramidal cells.

The terminals of some neurons are all localized in the same brain area where the cell body is located. These cells play a role in the local processing of information and are called local circuit neurons (see Figure 2). Other cells connect different brain regions or supply the periphery with their axons; these are called projection neurons. Many projection neurons also have axon collaterals within the same brain area where the cell is located, and thus play both a local circuit and a projection role (see Figure 3). Accurate knowledge of the connectivity, especially in quantitative terms, is a prerequisite of establishing the operations taking place in real neural networks.

See also:NEUROTRANSMITTER SYSTEMS AND MEMORY

Bibliography

Heimer, L., and Zaborszky, L., eds. (1989). Neuroanatomical tracttracing methods, Vol. 2: Recent progress. New York: Plenum Press.

Kandel, E. R., and Schwartz, J. H., eds. (1985). Principles of neural science, 2nd edition, New York: Elsevier.

Kisvarday, Z. F., Martin, K. A. C., Freund, T. F., Magloczky, Z., Whitteridge, D., and Somogyi, P. (1986). Synaptic targets of HRP-filled layer III pyramidal cells in the cat striate cortex. Experimental Brain Research 64, 541-552.

Peters, A., Palay, S. L., and Webster, H. deF., eds. (1991). The fine structure of the nervous system, neurons and their supporting cells, 3rd edition. New York: Oxford University Press.

Shepherd, G. M., ed. (1990). The synaptic organization of the brain, 3rd edition. New York: Oxford University Press.

Somogyi, P., Freund, T. F., Hodgson, A. J., Somogyi, J., Beroukas, D., and Chubb, I. W. (1985). Identified axo-axonic cells are immunoreactive for GABA in the hippocampus and visual cortex of the cat. Brain Research 332, 143-149.

PeterSomogyi

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Neuron

Neuron

Neurons are nerve cells (neurocytes), which, together with neuroglial cells, comprise the nervous tissue making up the nervous system . The neuron is the integral element of our five senses and of countless other physical, regulatory, and mental faculties, including memory and consciousness. A neuron consists of a nerve cell body (or soma), an elongated projection (axon), and short branching fibers (called dendrites). Neurons receive nerve signals (action potentials), integrate action potentials, and transmit these signals to other neurons or effector organs, such as muscles and glands . The structure and function of neurons is essentially the same in all animals, although the human nervous system is much more specialized and complicated than that of lower animals. Humans are born with a large, but finite, supply of neurons and those cells that are lost through aging, injury, or disease cannot be replaced.

The unique morphological and intercellular structure of the neuron is dedicated to the efficient and rapid transmission of neural signals. Within the neuron, the neural signal travels electrically. At the synapse , the gap between neurons, neural signals are conveyed chemically by a limited number of chemicals termed neurotransmitters. Specialized parts of the neuron facilitate the production, release, binding, and uptake of these neurotransmitters.


Structure and function

Although there are variations related to function, a typical neuron consists of dendrites (also termed dendritic processes), a cell body, an axon, and an axon terminus.

Dendrites are the (filamentous) terminal portions of neuron that bind neurotransmitter chemicals migrating across the synaptic gaps separating neurons. Depending on the type and function of a particular neuron, neurotransmitters may cause or inhibit the transmission of neural impulses. The cell body contains the cell nucleus and a concentration of cellular organelles. The cell body is the site of the normal metabolic reactions that allow the cell to remain viable. Neurotransmitters synthesized within the cell body are transported to the axon terminus by microfilaments and microtubules.

The nerve cell body contains a nucleus, a nucleolus, and cytoplasm containing the cell (such as mitochondria, endoplasmic reticulum, and so on). Unique to the nerve cell body are Nissl bodies, which are rough surfaced vesicles in the endoplasmic reticulum (cytoplasm located near the nucleus), and are involved with protein synthesis. Another characteristic structure of nerve cells are the neurofibrils, which are delicate threadlike structures that help to maintain the shape of the cell, and which transport substances between the cell body and the axon terminals. The plasma membrane around the cell separates the cytoplasm on the inside of the cell from the extracellular
fluid on the outside. Cell membranes of neurons contain electrically gated channels, which when properly stimulated allow electrically charged particles (such as sodium and potassium ions) to pass across the barrier. This ionic exchange is the basis for the flow, or action potential , of the nerve impulse.

The axon is a cytoplasmic continuation of the cell body specialized for the electrical conduction of neural signals. The axon may be long—up to a yard in length in humans—or short, depending upon the neuron's position and function. The cell membranes of the neural axon transmit neural signals via changes in action potentials that sweep down the membrane.

At the junction of the cell body and axon is a region termed the axon hillock. At the axon hillock, chemical signals received by the dendrites may reach a threshold level to cause a wave of electrical depolarization and hyperpolarization of the axon cell membrane. The net movements of ions across the cell membrane are responsible for these changes that move down the axon to the axon terminus as an action potential.

At the axon terminus, neurotransmitters are released into the synaptic gap. Through synaptic gaps, a typical neuron may interconnect with thousands and tens of thousands of other neurons. Axon terminals have knob-like swellings at the very end called synaptic knobs or end buttons. Each synaptic knob communicates with a dendrite or cell body of another neuron, the point of contact being a synapse. Under very high magnification, a very tiny space, the synaptic cleft or gap (about one millionth of an inch, or mm), can be detected between the synaptic knob and dendrite or cell body. Synaptic knobs contain hundreds of neurovesicles that contain a transmitter substance (or neurotransmitter). When a nerve impulse reaches the synaptic knob the neurotransmitter is ejected into the synaptic cleft and serves as a stimulus to the next adjacent neuron. The vast majority of all impulses transmitted occur at the synaptic gaps, although recent research indicates that chemical transmission can occur at other points along the axon. Many neurological diseases and psychiatric disorders result from a disturbance or alteration of synaptic activity. Drugs such as tranquilizers , anesthetics, nicotine , and caffeine target the synapse and can cause an alteration of impulse transmission.

Structural classification

Neurons exist in many shapes and sizes. Their structure, like that of other cells in the body or in nature, illustrates that structure often determines function. There are three basic structural and functional classifications of neurons.

The structural classification of a neurons depends upon the number of dendrites extending from the cell body. Multipolar neurons have several dendrites; the majority of neurons in the spinal chord and brain are multipolar. Bipolar neurons have only two processes: a single dendrite and an axon. Bipolar neurons are found in the sense organs-and in the retina of the eye and in olfactory cells. Unipolar neurons lack dendrites and have a single axon, and are also sensory neurons.

Multipolar neurons have many processes and serve principally as motor neurons. Motor neurons, efferent because they conduct impulses away from the central nervous system (the brain and spinal cord) regulate the function of muscles and glands. Afferent neural pathways that send signals to the (CNS) are generally composed of unipolar neurons. Unipolar neurons also serve as sensory neurons—their filamentous dendritic processes exposed and elaborated into or connected to sensory receptor cells. Interneurons are neurons that connect neurons along a neural pathway.


Glial cells

One cannot discuss the neuron without mentioning glial cells or neuroglia. It was once thought that these cells simply held everything together (gloios means glue, in Greek), but we now know that neuroglia are highly specialized cells. For example, neuroglia are responsible for physical support, protection against infection (through phagocytosis), and the connection of nerve cells to blood vessels. The Schwann cell (or neurolemmocyte) is a common type of glial cell found in peripheral nerve axons. Schwann cells wrap "jelly roll style" around the axon, forming a whitish phospholipid (fatty) protective and insulating cover known as the myelin sheath. The myelin sheath of the axon of peripheral nerves has interruptions or exposed gaps known as the Nodes of Ranvier. Schwann cells also make up the neurolemma, a continuous sheath that covers both the myelin sheath and the axon at the Nodes of Ranvier. Action potentials traveling down the axon occur only at the Nodes of Ranvier, jumping rapidly from gap to gap (saltatory conduction), which conducts impulses significantly faster than in nonmyelinated nerves. The neurolemma is found only in peripheral nerve fibers and plays a crucial part in nerve fiber regeneration. Damaged axons will regenerate; damaged cell bodies will not. The myelinated sheaths of the axons of neuron in the brain and spinal cord (the central nervous system) are made from different glial cells (oligodendrocytes), which lack a neurolemma, so making the regeneration of their axons impossible. Multiple sclerosis is a serious demyelinating disease of the central nervous system. Not all axons are myelinated; the presence of myelin is one difference between white matter (which has myelinated axons) and gray matter (which does not).

Functional classification

Sensory neurons transduce physical stimuli, such as smell , light , or sound, into action potentials, which are then transmitted to the spinal cord or brain. Sensory neurons, which bring information into the central nervous system, are also referred to as afferent neurons. Motor neurons transmit nerve impulses away from the brain and spinal cord to muscles or glands and are also called efferent neurons. Interneurons transmit nerve impulses between sensory neurons and the motor neurons. Interneurons are responsible for receiving, relaying, integrating, and sending nerve impulses. Interneurons are found exclusively in the central nervous system and account for almost 99% of all the nerve cells in the body.

See also Adenosine triphosphate; Nerve impulses and conduction of impulses; Neuromuscular diseases; Reflex; Touch.

Resources

books

Cooper, Geoffrey M. The Cell—A Molecular Approach. 2nd ed. Sunderland, MA: Sinauer Associates, Inc., 2000.

Guyton, Arthur C., and John E. Hall. Textbook of Medical Physiology. 10th ed. Philadelphia: W.B. Saunders Co., 2000.

Kandel, E.R., J.H. Schwartz, and T.M. Jessell, ed. Principles ofNeural Science. 4th ed. Boston: Elsevier, 2000.

Thibodeau, Gary A., and Kevin T. Patton. Anatomy & Physiology. 5th ed. St. Louis: Mosby, 2002.


K. Lee Lerner
Christine Miner Minderovic

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Action potential

—A transient change in the electrical potential across a membrane which results in the generation of a nerve impulse.

Afferent neuron

—A sensory neuron that carries an impulse toward the central nervous system.

Axon

—The threadlike projection of a neuron that carries an impulse away from the cell body of the neuron.

Consciousness

—A mental state involving awareness of the self and the environment.

Dendrites

—Branched structures of nerve cell bodies which receive impulses from axons and carry them to the nerve cell body.

Efferent neuron

—A motor neuron that carries an impulse from the central nervous system to muscles or glands.

Memory

—The ability to recall thoughts or events.

Myelin sheath

—A white phospholipid (fat) covering of peripheral nerve axons.

Neuroglia

—Specialized nerve cells also called glial cells. The Schwann cell is a glial cell.

Neurolemma

—Protective nerve covering made by Schwann cells.

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