Neuron: Nerve Cell
Drawing by Santiago Ramón y Cajal of neurons in the pigeon cerebellum. (A) Denotes Purkinje cells, an example of a multipolar neuron. (B) Denotes granule cells, which are also multipolar.
NeuroLex ID	sao1417703748
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A neuron ( /ˈnjʊərɒn/ NEWR-on; also known as a neurone or nerve cell) is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form neural networks. Neurons are the core components of the nervous system, which includes the brain, spinal cord, and peripheral ganglia. A number of specialized types of neurons exist: sensory neurons respond to touch, sound, light and numerous other stimuli affecting cells of the sensory organs that then send signals to the spinal cord and brain. Motor neurons receive signals from the brain and spinal cord, cause muscle contractions, and affect glands. Interneurons connect neurons to other neurons within the same region of the brain or spinal cord.

A typical neuron possesses a cell body (often called the soma), dendrites, and an axon. Dendrites are thin structures that arise from the cell body, often extending for hundreds of micrometres and branching multiple times, giving rise to a complex "dendritic tree". An axon is a special cellular extension that arises from the cell body at a site called the axon hillock and travels for a distance, as far as 1 m in humans or even more in other species. The cell body of a neuron frequently gives rise to multiple dendrites, but never to more than one axon, although the axon may branch hundreds of times before it terminates. At the majority of synapses, signals are sent from the axon of one neuron to a dendrite of another. There are, however, many exceptions to these rules: neurons that lack dendrites, neurons that have no axon, synapses that connect an axon to another axon or a dendrite to another dendrite, etc.

All neurons are electrically excitable, maintaining voltage gradients across their membranes by means of metabolically driven ion pumps, which combine with ion channels embedded in the membrane to generate intracellular-versus-extracellular concentration differences of ions such as sodium, potassium, chloride, and calcium. Changes in the cross-membrane voltage can alter the function of voltage-dependent ion channels. If the voltage changes by a large enough amount, an all-or-none electrochemical pulse called an action potential is generated, which travels rapidly along the cell's axon, and activates synaptic connections with other cells when it arrives.

With the exception of neural stem cells and a few other types of neurons, neurons do not undergo cell division. In most cases, neurons are generated by special types of stem cells. Astrocytes, a type of glial cell, have also been observed to turn into neurons by virtue of the stem cell characteristic pluripotency. In humans, neurogenesis largely ceases during adulthood—only for two brain areas, the hippocampus and olfactory bulb, is there strong evidence for generation of substantial numbers of new neurons.^[1][2]

Overview[link]

Structure of a typical neuron

Neuron
Dendrite Soma Axon Nucleus Node of Ranvier Axon terminal Schwann cell Myelin sheath

A neuron is a specialized type of cell found in the bodies of most animals (all members of the group Eumetazoa). Only sponges and a few other simpler animals have no neurons. The features that define a neuron are electrical excitability and the presence of synapses, which are complex membrane junctions that transmit signals to other cells. The body's neurons, plus the glial cells that give them structural and metabolic support, together constitute the nervous system. In vertebrates, the majority of neurons belong to the central nervous system, but some reside in peripheral ganglia, and many sensory neurons are situated in sensory organs such as the retina and cochlea.

Although neurons are very diverse and there are exceptions to nearly every rule, it is convenient to begin with a schematic description of the structure and function of a "typical" neuron. A typical neuron is divided into three parts: the soma or cell body, dendrites, and axon. The soma is usually compact; the axon and dendrites are filaments that extrude from it. Dendrites typically branch profusely, getting thinner with each branching, and extending their farthest branches a few hundred micrometres from the soma. The axon leaves the soma at a swelling called the axon hillock, and can extend for great distances, giving rise to hundreds of branches. Unlike dendrites, an axon usually maintains the same diameter as it extends. The soma may give rise to numerous dendrites, but never to more than one axon. Synaptic signals from other neurons are received by the soma and dendrites; signals to other neurons are transmitted by the axon. A typical synapse, then, is a contact between the axon of one neuron and a dendrite or soma of another. Synaptic signals may be excitatory or inhibitory. If the net excitation received by a neuron over a short period of time is large enough, the neuron generates a brief pulse called an action potential, which originates at the soma and propagates rapidly along the axon, activating synapses onto other neurons as it goes.

Many neurons fit the foregoing schema in every respect, but there are also exceptions to most parts of it. There are no neurons that lack a soma, but there are neurons that lack dendrites, and others that lack an axon. Furthermore, in addition to the typical axodendritic and axosomatic synapses, there are axoaxonic (axon-to-axon) and dendrodendritic (dendrite-to-dendrite) synapses.

The key to neural function is the synaptic signaling process, which is partly electrical and partly chemical. The electrical aspect depends on properties of the neuron's membrane. Like all animal cells, every neuron is surrounded by a plasma membrane, a bilayer of lipid molecules with many types of protein structures embedded in it. A lipid bilayer is a powerful electrical insulator, but in neurons, many of the protein structures embedded in the membrane are electrically active. These include ion channels that permit electrically charged ions to flow across the membrane, and ion pumps that actively transport ions from one side of the membrane to the other. Most ion channels are permeable only to specific types of ions. Some ion channels are voltage gated, meaning that they can be switched between open and closed states by altering the voltage difference across the membrane. Others are chemically gated, meaning that they can be switched between open and closed states by interactions with chemicals that diffuse through the extracellular fluid. The interactions between ion channels and ion pumps produce a voltage difference across the membrane, typically a bit less than 1/10 of a volt at baseline. This voltage has two functions: first, it provides a power source for an assortment of voltage-dependent protein machinery that is embedded in the membrane; second, it provides a basis for electrical signal transmission between different parts of the membrane.

Neurons communicate by chemical and electrical synapses in a process known as synaptic transmission. The fundamental process that triggers synaptic transmission is the action potential, a propagating electrical signal that is generated by exploiting the electrically excitable membrane of the neuron. This is also known as a wave of depolarization.

Anatomy and histology[link]

Diagram of a typical myelinated vertebrate motoneuron

Neurons are highly specialized for the processing and transmission of cellular signals. Given the diversity of functions performed by neurons in different parts of the nervous system, there is, as expected, a wide variety in the shape, size, and electrochemical properties of neurons. For instance, the soma of a neuron can vary from 4 to 100 micrometers in diameter.^[3]

• The soma is the central part of the neuron. It contains the nucleus of the cell, and therefore is where most protein synthesis occurs. The nucleus ranges from 3 to 18 micrometers in diameter.^[4]

• The dendrites of a neuron are cellular extensions with many branches, and metaphorically this overall shape and structure is referred to as a dendritic tree. This is where the majority of input to the neuron occurs.

• The axon is a finer, cable-like projection that can extend tens, hundreds, or even tens of thousands of times the diameter of the soma in length. The axon carries nerve signals away from the soma (and also carries some types of information back to it). Many neurons have only one axon, but this axon may—and usually will—undergo extensive branching, enabling communication with many target cells. The part of the axon where it emerges from the soma is called the axon hillock. Besides being an anatomical structure, the axon hillock is also the part of the neuron that has the greatest density of voltage-dependent sodium channels. This makes it the most easily-excited part of the neuron and the spike initiation zone for the axon: in electrophysiological terms it has the most negative action potential threshold. While the axon and axon hillock are generally involved in information outflow, this region can also receive input from other neurons.

• The axon terminal contains synapses, specialized structures where neurotransmitter chemicals are released to communicate with target neurons.

Although the canonical view of the neuron attributes dedicated functions to its various anatomical components, dendrites and axons often act in ways contrary to their so-called main function.

Axons and dendrites in the central nervous system are typically only about one micrometer thick, while some in the peripheral nervous system are much thicker. The soma is usually about 10–25 micrometers in diameter and often is not much larger than the cell nucleus it contains. The longest axon of a human motoneuron can be over a meter long, reaching from the base of the spine to the toes. Sensory neurons have axons that run from the toes to the dorsal columns, over 1.5 meters in adults. Giraffes have single axons several meters in length running along the entire length of their necks. Much of what is known about axonal function comes from studying the squid giant axon, an ideal experimental preparation because of its relatively immense size (0.5–1 millimeters thick, several centimeters long).

Fully differentiated neurons are permanently postmitotic;^[5] however, recent research shows that additional neurons throughout the brain can originate from neural stem cells found throughout the brain but in particularly high concentrations in the subventricular zone and subgranular zone through the process of neurogenesis.^[6]

Histology and internal structure[link]

Golgi-stained neurons in human hippocampal tissue

Nerve cell bodies stained with basophilic dyes show numerous microscopic clumps of Nissl substance (named after German psychiatrist and neuropathologist Franz Nissl, 1860–1919), which consists of rough endoplasmic reticulum and associated ribosomal RNA. The prominence of the Nissl substance can be explained by the fact that nerve cells are metabolically very active, and hence are involved in large amounts of protein synthesis.

The cell body of a neuron is supported by a complex meshwork of structural proteins called neurofilaments, which are assembled into larger neurofibrils. Some neurons also contain pigment granules, such as neuromelanin (a brownish-black pigment, byproduct of synthesis of catecholamines) and lipofuscin (yellowish-brown pigment that accumulates with age).

There are different internal structural characteristics between axons and dendrites. Typical axons almost never contain ribosomes, except some in the initial segment. Dendrites contain granular endoplasmic reticulum or ribosomes, with diminishing amounts with distance from the cell body.

Classes[link]

Image of pyramidal neurons in mouse cerebral cortex expressing green fluorescent protein. The red staining indicates GABAergic interneurons.^[7]

Neurons exist in a number of different shapes and sizes and can be classified by their morphology and function. The anatomist Camillo Golgi grouped neurons into two types; type I with long axons used to move signals over long distances and type II with short axons, which can often be confused with dendrites. Type I cells can be further divided by where the cell body or soma is located. The basic morphology of type I neurons, represented by spinal motor neurons, consists of a cell body called the soma and a long thin axon covered by the myelin sheath. Around the cell body is a branching dendritic tree that receives signals from other neurons. The end of the axon has branching terminals (axon terminal) that release neurotransmitters into a gap called the synaptic cleft between the terminals and the dendrites of the next neuron.

Structural classification[link]

Polarity[link]

Different kinds of neurons:
1 Unipolar neuron
2 Bipolar neuron
3 Multipolar neuron
4 Pseudounipolar neuron

Most neurons can be anatomically characterized as:

• Unipolar or pseudounipolar: dendrite and axon emerging from same process.
• Bipolar: axon and single dendrite on opposite ends of the soma.
• Multipolar: more than two dendrites:
  • Golgi I: neurons with long-projecting axonal processes; examples are pyramidal cells, Purkinje cells, and anterior horn cells.
  • Golgi II: neurons whose axonal process projects locally; the best example is the granule cell.

Other[link]

Furthermore, some unique neuronal types can be identified according to their location in the nervous system and distinct shape. Some examples are:

• Basket cells, interneurons that form a dense plexus of terminals around the soma of target cells, found in the cortex and cerebellum.
• Betz cells, large motor neurons.
• Medium spiny neurons, most neurons in the corpus striatum.
• Purkinje cells, huge neurons in the cerebellum, a type of Golgi I multipolar neuron.
• Pyramidal cells, neurons with triangular soma, a type of Golgi I.
• Renshaw cells, neurons with both ends linked to alpha motor neurons.
• Granule cells, a type of Golgi II neuron.
• Anterior horn cells, motoneurons located in the spinal cord.

Functional classification[link]

Direction[link]

• Afferent neurons convey information from tissues and organs into the central nervous system and are sometimes also called sensory neurons.
• Efferent neurons transmit signals from the central nervous system to the effector cells and are sometimes called motor neurons.
• Interneurons connect neurons within specific regions of the central nervous system.

Afferent and efferent also refer generally to neurons that, respectively, bring information to or send information from the brain region.

Action on other neurons[link]

A neuron affects other neurons by releasing a neurotransmitter that binds to chemical receptors. The effect upon the target neuron is determined not by the source neuron or by the neurotransmitter, but by the type of receptor that is activated. A neurotransmitter can be thought of as a key, and a receptor as a lock: the same type of key can here be used to open many different types of locks. Receptors can be classified broadly as excitatory (causing an increase in firing rate), inhibitory (causing a decrease in firing rate), or modulatory (causing long-lasting effects not directly related to firing rate).

The two most common neurotransmitters in the brain, glutamate and GABA, have actions that are largely consistent. Glutamate acts on several different types of receptors, and have effects that are excitatory at ionotropic receptors and a modulatory effect at metabotropic receptors. Similarly GABA acts on several different types of receptors, but all of them have effects (in adult animals, at least) that are inhibitory. Because of this consistency, it is common for neuroscientists to simplify the terminology by referring to cells that release glutamate as "excitatory neurons," and cells that release GABA as "inhibitory neurons." Since over 90% of the neurons in the brain release either glutamate or GABA, these labels encompass the great majority of neurons. There are also other types of neurons that have consistent effects on their targets, for example "excitatory" motor neurons in the spinal cord that release acetylcholine, and "inhibitory" spinal neurons that release glycine.

The distinction between excitatory and inhibitory neurotransmitters is not absolute, however. Rather, it depends on the class of chemical receptors present on the target neuron. In principle, a single neuron, releasing a single neurotransmitter, can have excitatory effects on some targets, inhibitory effects on others, and modulatory effects on others still. For example, photoreceptor cells in the retina constantly release the neurotransmitter glutamate in the absence of light. So-called OFF bipolar cells are, like most neurons, excited by the released glutamate. However, neighboring target neurons called ON bipolar cells are instead inhibited by glutamate, because they lack the typical ionotropic glutamate receptors and instead express a class of inhibitory metabotropic glutamate receptors.^[8] When light is present, the photoreceptors cease releasing glutamate, which relieves the ON bipolar cells from inhibition, activating them; this simultaneously removes the excitation from the OFF bipolar cells, silencing them.

Discharge patterns[link]

Neurons can be classified according to their electrophysiological characteristics:

• Tonic or regular spiking. Some neurons are typically constantly (or tonically) active. Example: interneurons in neurostriatum.
• Phasic or bursting. Neurons that fire in bursts are called phasic.
• Fast spiking. Some neurons are notable for their high firing rates, for example some types of cortical inhibitory interneurons, cells in globus pallidus, retinal ganglion cells.^[9][10]

Classification by neurotransmitter production[link]

This section requires expansion.

Neurons differ in the type of neurotransmitter they manufacture. Some examples are:

• Cholinergic neurons—acetylcholine. Acetylcholine is released from presynaptic neurons into the synaptic cleft. It acts as a ligand for both ligand-gated ion channels and metabotropic (GPCRs) muscarinic receptors. Nicotinic receptors, are pentameric ligand-gated ion channels composed of alpha and beta subunits that bind nicotine. Ligand binding opens the channel causing influx of Na⁺ depolarization and increases the probability of presynaptic neurotransmitter release.

• GABAergic neurons—gamma aminobutyric acid. GABA is one of two neuroinhibitors in the CNS, the other being Glycine. GABA has a homologous function to ACh, gating anion channels that allow Cl- ions to enter the post synaptic neuron. Cl- causes hyperpolarization within the neuron, decreasing the probability of an action potential firing as the voltage becomes more negative (recall that for an action potential to fire, a positive voltage threshold must be reached).

• Glutamatergic neurons—glutamate. Glutamate is one of two primary excitatory amino acids, the other being Aspartate. Glutamate receptors are one of four categories, three of which are ligand-gated ion channels and one of which is a G-protein coupled receptor (often referred to as GPCR).

1. AMPA and Kainate receptors both function as cation channels permeable to Na⁺ cation channels mediating fast excitatory synaptic transmission
2. NMDA receptors are another cation channel that is more permeable to Ca²⁺. The function of NMDA receptors is dependant on Glycine receptor binding as a co-agonist within the channel pore. NMDA receptors do not function without both ligands present.
3. Metabotropic receptors, GPCRs modulate synaptic transmission and postsynaptic excitability.

Glutamate can cause excitotoxicity when blood flow to the brain is interrupted, resulting in brain damage. When blood flow is suppressed, glutamate is released from presynaptic neurons causing NMDA and AMPA receptor activation moreso than would normally be the case outside of stress conditions, leading to elevated Ca²⁺ and Na⁺ entering the post synaptic neuron and cell damage.

• Dopaminergic neurons—dopamine. Dopamine is a neurotransmitter that acts on D1 type (D1 and D5) Gs coupled receptors, which increase cAMP and PKA, and D2 type (D2, D3, and D4) receptors, which activate Gi-coupled receptors that decrease cAMP and PKA. Dopamine is connected to mood and behavior, and modulates both pre and post synaptic neurotransmission. Loss of dopamine neurons in the substantia nigra has been linked to Parkinson's disease.

• Serotonergic neurons—serotonin. Serotonin,(5-Hydroxytryptamine, 5-HT), can act as excitatory or inhibitory. Of the four 5-HT receptor classes, 3 are GPCR and 1 is ligand gated cation channel. Serotonin is synthesized from tryptophan by tryptophan hydroxylase, and then further by aromatic acid decarboxylase. A lack of 5-HT at postsynaptic neurons has been linked to depression. Drugs that block the presynaptic serotonin transporter are used for treatment, such as Prozac and Zoloft.

Connectivity[link]

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Neurons communicate with one another via synapses, where the axon terminal or en passant boutons (terminals located along the length of the axon) of one cell impinges upon another neuron's dendrite, soma or, less commonly, axon. Neurons such as Purkinje cells in the cerebellum can have over 1000 dendritic branches, making connections with tens of thousands of other cells; other neurons, such as the magnocellular neurons of the supraoptic nucleus, have only one or two dendrites, each of which receives thousands of synapses. Synapses can be excitatory or inhibitory and either increase or decrease activity in the target neuron. Some neurons also communicate via electrical synapses, which are direct, electrically-conductive junctions between cells.

In a chemical synapse, the process of synaptic transmission is as follows: when an action potential reaches the axon terminal, it opens voltage-gated calcium channels, allowing calcium ions to enter the terminal. Calcium causes synaptic vesicles filled with neurotransmitter molecules to fuse with the membrane, releasing their contents into the synaptic cleft. The neurotransmitters diffuse across the synaptic cleft and activate receptors on the postsynaptic neuron.

The human brain has a huge number of synapses. Each of the 10¹¹ (one hundred billion) neurons has on average 7,000 synaptic connections to other neurons. It has been estimated that the brain of a three-year-old child has about 10¹⁵ synapses (1 quadrillion). This number declines with age, stabilizing by adulthood. Estimates vary for an adult, ranging from 10¹⁴ to 5 x 10¹⁴ synapses (100 to 500 trillion).^[11]

Mechanisms for propagating action potentials[link]

A signal propagating down an axon to the cell body and dendrites of the next cell

In 1937, John Zachary Young suggested that the squid giant axon could be used to study neuronal electrical properties.^[12] Being larger than but similar in nature to human neurons, squid cells were easier to study. By inserting electrodes into the giant squid axons, accurate measurements were made of the membrane potential.

The cell membrane of the axon and soma contain voltage-gated ion channels that allow the neuron to generate and propagate an electrical signal (an action potential). These signals are generated and propagated by charge-carrying ions including sodium (Na⁺), potassium (K⁺), chloride (Cl^-), and calcium (Ca²⁺).

There are several stimuli that can activate a neuron leading to electrical activity, including pressure, stretch, chemical transmitters, and changes of the electric potential across the cell membrane.^[13] Stimuli cause specific ion-channels within the cell membrane to open, leading to a flow of ions through the cell membrane, changing the membrane potential.

Thin neurons and axons require less metabolic expense to produce and carry action potentials, but thicker axons convey impulses more rapidly. To minimize metabolic expense while maintaining rapid conduction, many neurons have insulating sheaths of myelin around their axons. The sheaths are formed by glial cells: oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system. The sheath enables action potentials to travel faster than in unmyelinated axons of the same diameter, whilst using less energy. The myelin sheath in peripheral nerves normally runs along the axon in sections about 1 mm long, punctuated by unsheathed nodes of Ranvier, which contain a high density of voltage-gated ion channels. Multiple sclerosis is a neurological disorder that results from demyelination of axons in the central nervous system.

Some neurons do not generate action potentials, but instead generate a graded electrical signal, which in turn causes graded neurotransmitter release. Such nonspiking neurons tend to be sensory neurons or interneurons, because they cannot carry signals long distances.

Neural coding[link]

Neural coding is concerned with how sensory and other information is represented in the brain by neurons. The main goal of studying neural coding is to characterize the relationship between the stimulus and the individual or ensemble neuronal responses, and the relationships amongst the electrical activities of the neurons within the ensemble.^[14] It is thought that neurons can encode both digital and analog information.^[15]

All-or-none principle[link]

The conduction of nerve impulses is an example of an all-or-none response. In other words, if a neuron responds at all, then it must respond completely. Greater intensity of stimulation does not produce a stronger signal but can produce a higher frequency of firing. There are different types of receptor response to stimulus, slowly adapting or tonic receptors respond to steady stimulus and produce a steady rate of firing. These tonic receptors most often respond to increased intensity of stimulus by increasing their firing frequency, usually as a power function of stimulus plotted against impulses per second. This can be likened to an intrinsic property of light where to get greater intensity of a specific frequency (color) there have to be more photons, as the photons can't become "stronger" for a specific frequency.

There are a number of other receptor types that are called quickly-adapting or phasic receptors, where firing decreases or stops with steady stimulus; examples include: skin when touched by an object causes the neurons to fire, but if the object maintains even pressure against the skin, the neurons stop firing. The neurons of the skin and muscles that are responsive to pressure and vibration have filtering accessory structures that aid their function.

The pacinian corpuscle is one such structure. It has concentric layers like an onion, which form around the axon terminal. When pressure is applied and the corpuscle is deformed, mechanical stimulus is transferred to the axon, which fires. If the pressure is steady, there is no more stimulus; thus, typically these neurons respond with a transient depolarization during the initial deformation and again when the pressure is removed, which causes the corpuscle to change shape again. Other types of adaptation are important in extending the function of a number of other neurons.^[16]

History[link]

Drawing by Camillo Golgi of a hippocampus stained with the silver nitrate method

Drawing of a Purkinje cell in the cerebellum cortex done by Santiago Ramón y Cajal, demonstrating the ability of Golgi's staining method to reveal fine detail

The term neuron was coined by the German anatomist Heinrich Wilhelm Waldeyer. The neuron's place as the primary functional unit of the nervous system was first recognized in the early 20th century through the work of the Spanish anatomist Santiago Ramón y Cajal.^[17] Ramón y Cajal proposed that neurons were discrete cells that communicated with each other via specialized junctions, or spaces, between cells.^[17] This became known as the neuron doctrine, one of the central tenets of modern neuroscience.^[17] To observe the structure of individual neurons, Ramón y Cajal improved a silver staining process known as Golgi's method, which had been developed by his rival, Camillo Golgi.^[17] Cajal's improvement, which involved a technique he called "double impregnation", is still in use. The silver impregnation stains are an extremely useful method for neuroanatomical investigations because, for reasons unknown, it stains a very small percentage of cells in a tissue, so one is able to see the complete micro structure of individual neurons without much overlap from other cells in the densely packed brain.^[18]

The neuron doctrine[link]

The neuron doctrine is the now fundamental idea that neurons are the basic structural and functional units of the nervous system. The theory was put forward by Santiago Ramón y Cajal in the late 19th century. It held that neurons are discrete cells (not connected in a meshwork), acting as metabolically distinct units.

Later discoveries yielded a few refinements to the simplest form of the doctrine. For example, glial cells, which are not considered neurons, play an essential role in information processing.^[19] Also, electrical synapses are more common than previously thought,^[20] meaning that there are direct, cytoplasmic connections between neurons. In fact, there are examples of neurons forming even tighter coupling: the squid giant axon arises from the fusion of multiple axons.^[21]

Ramón y Cajal also postulated the Law of Dynamic Polarization, which states that a neuron receives signals at its dendrites and cell body and transmits them, as action potentials, along the axon in one direction: away from the cell body.^[22] The Law of Dynamic Polarization has important exceptions; dendrites can serve as synaptic output sites of neurons^[23] and axons can receive synaptic inputs.^{[citation needed]}

Neurons in the brain[link]

The number of neurons in the brain varies dramatically from species to species.^[24] One estimate puts the human brain at about 100 billion (Failed to parse (Missing texvc executable; please see math/README to configure.): 10^{11} ) neurons and 100 trillion (Failed to parse (Missing texvc executable; please see math/README to configure.): 10^{14} ) synapses.^[24] Another estimate is 86 billion neurons, of which 16.3 billion are in the cerebral cortex, and 69 billion in the cerebellum.^[25] By contrast, the nematode worm Caenorhabditis elegans has just 302 neurons making it an ideal experimental subject as scientists have been able to map all of the organism's neurons. The fruit fly Drosophila melanogaster, a common subject in biological experiments, has around 100,000 neurons and exhibits many complex behaviors. Many properties of neurons, from the type of neurotransmitters used to ion channel composition, are maintained across species, allowing scientists to study processes occurring in more complex organisms in much simpler experimental systems.

Neurological disorders[link]

Charcot-Marie-Tooth disease (CMT), also known as Hereditary Motor and Sensory Neuropathy (HMSN), Hereditary Sensorimotor Neuropathy (HMSN), or Peroneal Muscular Atrophy, is a heterogeneous inherited disorder of nerves (neuropathy) that is characterized by loss of muscle tissue and touch sensation, predominantly in the feet and legs but also in the hands and arms in the advanced stages of disease. Presently incurable, this disease is one of the most common inherited neurological disorders, with 37 in 100,000 affected.

Alzheimer's disease (AD), also known simply as Alzheimer's, is a neurodegenerative disease characterized by progressive cognitive deterioration together with declining activities of daily living and neuropsychiatric symptoms or behavioral changes. The most striking early symptom is loss of short-term memory (amnesia), which usually manifests as minor forgetfulness that becomes steadily more pronounced with illness progression, with relative preservation of older memories. As the disorder progresses, cognitive (intellectual) impairment extends to the domains of language (aphasia), skilled movements (apraxia), and recognition (agnosia), and functions such as decision-making and planning become impaired.

Parkinson's disease (PD), also known as Parkinson disease, is a degenerative disorder of the central nervous system that often impairs the sufferer's motor skills and speech. Parkinson's disease belongs to a group of conditions called movement disorders. It is characterized by muscle rigidity, tremor, a slowing of physical movement (bradykinesia), and in extreme cases, a loss of physical movement (akinesia). The primary symptoms are the results of decreased stimulation of the motor cortex by the basal ganglia, normally caused by the insufficient formation and action of dopamine, which is produced in the dopaminergic neurons of the brain. Secondary symptoms may include high level cognitive dysfunction and subtle language problems. PD is both chronic and progressive.

Myasthenia Gravis is a neuromuscular disease leading to fluctuating muscle weakness and fatigability during simple activities. Weakness is typically caused by circulating antibodies that block acetylcholine receptors at the post-synaptic neuromuscular junction, inhibiting the stimulative effect of the neurotransmitter acetylcholine. Myasthenia is treated with immunosuppressants, cholinesterase inhibitors and, in selected cases, thymectomy.

Demyelination[link]

Demyelination is the act of demyelinating, or the loss of the myelin sheath insulating the nerves. When myelin degrades, conduction of signals along the nerve can be impaired or lost, and the nerve eventually withers. This leads to certain neurodegenerative disorders like multiple sclerosis and chronic inflammatory demyelinating polyneuropathy.

Axonal degeneration[link]

Although most injury responses include a calcium influx signaling to promote resealing of severed parts, axonal injuries initially lead to acute axonal degeneration (AAD), which is rapid separation of the proximal and distal ends within 30 minutes of injury. Degeneration follows with swelling of the axolemma, and eventually leads to bead like formation. Granular disintegration of the axonal cytoskeleton and inner organelles occurs after axolemma degradation. Early changes include accumulation of mitochondria in the paranodal regions at the site of injury. Endoplasmic reticulum degrades and mitochondria swell up and eventually disintegrate. The disintegration is dependent on Ubiquitin and Calpain proteases (caused by influx of calcium ion), suggesting that axonal degeneration is an active process. Thus the axon undergoes complete fragmentation. The process takes about roughly 24 hrs in the PNS, and longer in the CNS. The signaling pathways leading to axolemma degeneration are currently unknown.

Nerve regeneration[link]

It has been demonstrated that neurogenesis can sometimes occur in the adult vertebrate brain, a finding that led to controversy in 1999.^[26] However, more recent studies of the age of human neurons suggest that this process occurs only for a minority of cells, and the overwhelming majority of neurons comprising the neocortex were formed before birth and persist without replacement.^[27]

It is often possible for peripheral axons to regrow if they are severed. A report in Nature suggested that researchers had found a way to transform human skin cells into working nerve cells using a process called transdifferentiation in which "cells are forced to adopt new identities."^[28]

See also[link]

Neuroscience portal

• List of neuroscience databases

Notes[link]

References[link]

• Kandel E.R., Schwartz, J.H., Jessell, T.M. 2000. Principles of Neural Science, 4th ed., McGraw-Hill, New York.
• Bullock, T.H., Bennett, M.V.L., Johnston, D., Josephson, R., Marder, E., Fields R.D. 2005. The Neuron Doctrine, Redux, Science, V.310, p. 791–793.
• Ramón y Cajal, S. 1933 Histology, 10th ed., Wood, Baltimore.
• Roberts A., Bush B.M.H. 1981. Neurones Without Impulses. Cambridge University Press, Cambridge.
• Peters, A., Palay, S.L., Webster, H, D., 1991 The Fine Structure of the Nervous System, 3rd ed., Oxford, New York

External links[link]

Wikimedia Commons has media related to: Neuron

Wikiversity has learning materials about Neuron

• IBRO (International Brain Research Organization). Fostering neuroscience research especially in less well-funded countries.
• NeuronBank an online neuromics tool for cataloging neuronal types and synaptic connectivity.
• High Resolution Neuroanatomical Images of Primate and Non-Primate Brains.
• The Department of Neuroscience at Wikiversity, which presently offers two courses: Fundamentals of Neuroscience and Comparative Neuroscience.
• NIF Search – Renshaw Cell via the Neuroscience Information Framework
• Cell Centered Database – Neuron
• Complete list of neuron types according to the Petilla convention, at NeuroLex.
• NeuroMorpho.Org an online database of digital reconstructions of neuronal morphology.
• Immunohistochemistry Image Gallery: Neuron

Edward "Ed" S. Boyden is a neuroscientist at MIT. He is a faculty member in the MIT Media Lab and an associate member of the McGovern Institute for Brain Research. He is recognized for his work on optogenetics, a technology that he co-invented with Karl Deisseroth at Stanford University.^[1] In this technology, a light-sensitive ion channel such as channelrhodopsin-2 is genetically expressed in neurons, allowing neuronal activity to be controlled by light. Since it was first described in 2005,^[2] this method has been widely adopted by neuroscientists as a research tool, and it is also thought to have many potential therapeutic applications.^[3] Boyden joined the MIT faculty in 2007, and continues to develop new optogenetic tools as well as other technologies for the manipulation of brain activity.

In 2008 Boyden was named by Discover Magazine as one of the top 20 scientists under 40.^[4] In 2006, he was named to the MIT Technology Review TR35 as one of the top 35 innovators in the world under the age of 35.^[5]

References[link]

1. ^ United States Patent Application 20070261127
2. ^ Millisecond-timescale, genetically targeted optical control of neural activity.Boyden ES, Zhang F, Bamberg E, Nagel G, Deisseroth K. Nat Neurosci. 2005 Sep;8(9):1263-8.
3. ^ http://www.wired.com/science/discoveries/news/2009/03/neuroengineering1
4. ^ http://web.mit.edu/newsoffice/2008/discover-1113.html
5. ^ "2006 Young Innovators Under 35". Technology Review. 2006. http://www.technologyreview.com/tr35/?year=2006. Retrieved August 15, 2011. 

External links[link]

• Ed Boyden's personal home page
• Boyden lab page at MIT Media Lab
• McGovern Institute for Brain Research
• Ed Boyden: A light switch for neurons at TED

Bobby McFerrin
Bobby McFerrin in March 2011
Background information
Birth name	Robert McFerrin, Jr
Born	(1950-03-11) March 11, 1950 (age 62) Manhattan, New York
Genres	Jazz, Reggae, World Music, Classical Music
Occupations	Musician Songwriter Conductor Arranger Producer
Instruments	Vocals, piano, bass guitar, clarinet
Years active	1977–present
Labels	Manhattan Records Blue Note Records Elektra Records Sony Classical
Associated acts	Chick Corea Herbie Hancock
Website	www.bobbymcferrin.com

Robert "Bobby" McFerrin, Jr. (born March 11, 1950) is a virtuoso American vocalist and conductor. He is best known for his 1988 hit song "Don't Worry, Be Happy". He is a ten-time Grammy Award winner. He is well known for his unique vocal techniques and singing styles.

Life[link]

Bobby McFerrin was born in Manhattan, New York City, the son of operatic baritone Robert McFerrin and singer Sara Copper.^[1] Robert Sr. was the first African American to be a regular with New York's Metropolitan Opera. Sara is a former soloist with regional opera companies and in Broadway shows, and is a professor emeritus of music at Fullerton College in Fullerton, California.

Bobby McFerrin married Debbie Green in 1975. They have three children.^[2]

McFerrin's song "Don't Worry, Be Happy" was a No. 1 U.S. pop hit in 1988 and won Song of the Year and Record of the Year honors. McFerrin has also worked in collaboration with instrumental performers, including pianists Chick Corea, Herbie Hancock, and Joe Zawinul, drummer Tony Williams, and cellist Yo-Yo Ma.

Vocal technique[link]

As a vocalist, McFerrin often switches rapidly between modal and falsetto registers to create polyphonic effects, performing both the main melody and the accompanying parts of songs. He makes use of percussive effects created both with his mouth and by tapping on his chest. McFerrin is also capable of multiphonic singing, as observed in his song "Drive" from the 2005 DVD Live in Montreal.^[3]

A notable document of McFerrin's approach to singing is his 1984 album The Voice, the first solo vocal jazz album recorded with no accompaniment or overdubbing.^[4]

Career[link]

In 1986, McFerrin was the voice of Santa Bear in Santa Bear's First Christmas, and in 1987 he was the voice of Santa Bear/Bully Bear in the sequel Santa Bear's High Flying Adventure. That same year, he performed the theme song for the opening credits of Season 4 of The Cosby Show, as well as the music for a Cadbury chocolate commercial.^{[citation needed]}

In 1988, McFerrin recorded the hit song "Don't Worry, Be Happy", which brought him widespread recognition across the world. However, the song's success "ended McFerrin's musical life as he had known it," and he began to pursue other musical possibilities – on stage and in recording studios.^[5] The song was used in George H. W. Bush's 1988 U.S. presidential election as Bush's 1988 official presidential campaign song, without Bobby McFerrin's permission or endorsement. In reaction, Bobby McFerrin publicly protested that particular use of his song, including stating that he was going to vote against Bush, and completely dropped the song from his own performance repertoire, to make the point even clearer.^{[citation needed]}

In 1989, he composed and performed the music for the Pixar short film Knick Knack. The rough cut to which McFerrin recorded his vocals had the words "blah blah blah" in place of the end credits (meant to indicate that he should improvise). McFerrin spontaneously decided to sing "blah blah blah" as lyrics, and the final version of the short film includes these lyrics during the end credits. Also in 1989, he formed a ten-person "Voicestra" which he featured on both his 1990 album Medicine Music and in the score to the 1989 Oscar-winning documentary Common Threads: Stories from the Quilt. The song "Common Threads" has frequently reappeared in some public service advertisements about AIDS. McFerrin also performed with the Vocal Summit.^{[citation needed]}

As early as 1992, widespread rumors circulated that falsely claimed McFerrin committed suicide. The rumors intentionally made fun of the distinctly positive nature of his popular song "Don't Worry, Be Happy" by claiming McFerrin ironically took his own life.^[6]

In 1993, McFerrin sang Henry Mancini's "Pink Panther" theme for the movie Son of the Pink Panther.

In addition to his vocal performing career, in 1994, Mr. McFerrin was appointed as creative chair of the Saint Paul Chamber Orchestra. He makes regular tours as a guest conductor for symphony orchestras throughout the United States and Canada, including the San Francisco Symphony (on his 40th birthday), the New York Philharmonic, the Chicago Symphony Orchestra, the Cleveland Orchestra, the Israel Philharmonic Orchestra the Philadelphia Orchestra, the Los Angeles Philharmonic, the London Philharmonic, the Vienna Philharmonic and many others. In McFerrin's concert appearances, he combines serious conducting of classical pieces with his own unique vocal improvisations, often with participation from the audience and the orchestra. For example, the concerts often end with McFerrin conducting the orchestra in an a cappella rendition of the "William Tell Overture," in which the orchestra members sing their musical parts in McFerrin's vocal style instead of playing their parts on their instruments.

For a few years in the late 1990s, he toured a concert version of Porgy and Bess, partly in honor of his father, who sang the role for Sidney Poitier in the 1959 film version, and partly "to preserve the score's jazziness" in the face of "largely white orchestras" who tend not "to play around the bar lines, to stretch and bend". McFerrin says that because of his father's work in the movie, "This music has been in my body for 40 years, probably longer than any other music."^[7]

McFerrin also participates in various music education programs and makes volunteer appearances as a guest music teacher and lecturer at public schools throughout the U.S. McFerrin has collaborated with his son, Taylor, on various musical ventures. Taylor has recently been singing, rapping, and playing minimal keyboard accompaniment with Vernon Reid (leader-guitarist of Living Colour) in the eclectic metal-fusion-funk group Yohimbe Brothers.^{[citation needed]}

In 2009, McFerrin and musician-scientist Daniel Levitin served as co-hosts of The Music Instinct, a two-hour award-winning documentary produced by PBS and based on Levitin's best-selling book This Is Your Brain On Music. Later that year, the two appeared together on a panel at the World Science Festival, where McFerrin demonstrated audience participation with the ubiquitous nature of human understanding of the pentatonic scale by singing and dancing, and having the audience sing while following his movements.^[8]

Discography[link]

As leader[link]

• Bobby McFerrin (1982)
• The Voice (1984)
• Spontaneous Inventions (1987)
• Elephant's Child (1987)
• Simple Pleasures (1988)
• Don't Worry, Be Happy (1988)
• How the Rhino Got His Skin/How the Camel Got His Hump (1990)
• Medicine Music (1990)
• Play, with Chick Corea, 1992
• Hush, with Yo-Yo Ma, 1992
• Many Faces of Bird (1991)
• "The Siamese Cat Song" on Simply Mad About the Mouse (1991)
• Sorrow Is Not Forever (1994)
• Paper Music (1995, Doick with the St. Paul Chamber orchestra)
• Bang! Zoom (1996)
• The Mozart Sessions, with Chick Corea, 1996
• Circlesongs (1997)
• Mouth Music (2001)
• Beyond Words (2002) – featuring Chick Corea, Cyro Baptista and Richard Bona
• Konzert für Europa -The Schönbrunn Concert (2004)
• VOCAbuLarieS (2010)

As sideman[link]

• Pharoah Sanders, Journey to the One, 1980
• Grover Washington, Jr., The Best Is Yet to Come, 1982
• Various Artists, The Young Lions, 1983
• Charles Lloyd Quartet, A Night in Copenhagen (Blue Note, 1984)
• Various Artists, A Tribute to Thelonius Monk, 1984
• Chico Freeman, Tangents, 1984
• Michael Hedges, Watching My Life Go By, 1985
• The Manhattan Transfer, Vocalese, 1985
• Joe Zawinul, Di•a•lects, 1986
• Weather Report, Sportin' Life, 1985
• Al Jarreau, Heart’s Horizon, 1988
• Quincy Jones, Back on the Block, 1989
• Laurie Anderson, Strange Angels, 1989
• Gal Costa, The Laziest Gal in Town, 1991
• Dizzy Gillespie, Bird Songs: The Final Recordings (Telarc, 1992), To Bird with Love (Telarc, 1992)
• Jack DeJohnette, Extra Special Edition (Blue Note, 1994)
• Yellowjackets, Dreamland, 1995
• George Martin, In My Life, 1998 – on "Come Together" with Robin Williams
• En Vogue, Masterpiece Theatre, 2000
• Bela Fleck and the Flecktones, Little Worlds, 2003
• Chick Corea, Rendezvous in New York, 2003
• Wynton Marsalis, The Magic Hour, 2004
• size2shoes, 2006 (free track)^[9]

Grammys[link]

• 1985, Best Jazz Vocal Performance, male, "Another Night In Tunisia" with Jon Hendricks
• 1985, Best Vocal Arrangement for two or more voices, "Another Night In Tunisia" with Cheryl Bentyne
• 1986, Best Jazz Vocal Performance, male, "Round Midnight"
• 1987, Best Jazz Vocal Performance, male, "What Is This Thing Called Love"
• 1987, Best Recording for Children, "The Elephant's Child" with Jack Nicholson
• 1988, Song of the year, Best Pop Vocal Performance, male, Record of the year, "Don't Worry, Be Happy"
• 1988, Best Jazz Vocal Performance, male, "Brothers"
• 1992, Best Jazz Vocal Performance, "Round Midnight"

References[link]

External links[link]

Wikimedia Commons has media related to: Bobby McFerrin

• Bobby McFerrin