Neurons (also spelled neurones or called nerve cells) are the primary cells of the nervous system. In vertebrates, they are found in the brain, the spinal cord and in the nerves and ganglia of the peripheral nervous system. Jump to: navigation, search Cells in culture, stained for keratin (red) and DNA (green) The cell is the structural and functional unit of all living organisms, sometimes called the building blocks of life. ... The nervous system of an animal coordinates the activity of the muscles, monitors the organs, constructs and processes input from the senses, and initiates actions. ... Jump to: navigation, search Groups Conodonta Hyperoartia Petromyzontidae (lampreys) Pteraspidomorphi (early jawless fish) Thelodonti Anaspida Cephalaspidomorphi (early jawless fish) Galeaspida Pituriaspida Osteostraci Gnathostomata (jawed vertebrates) Placodermi Chondrichthyes (cartilaginous fish) Acanthodii Osteichthyes (bony fish) Actinopterygii (ray-finned fish) Sarcopterygii (lobe-finned fish) Actinistia (coelacanths) Dipnoi (lungfish) Tetrapoda Amphibia Amniota Sauropsida/(Reptiles... Jump to: navigation, search In the anatomy of animals, the brain, or encephalon, is the higher, supervisory center of the nervous system. ... Jump to: navigation, search The spinal cord is a part of the vertebrate nervous system that is enclosed in and protected by the vertebral column (it passes through the spinal canal). ... Jump to: navigation, search A nerve is an enclosed, cable-like bundle of nerve fibers or axons, which includes the glia that ensheath the axons in myelin. ... Jump to: navigation, search This is a dorsal root ganglion (DRG) from a chicken embryo (around stage of day 7) after incubation overnight in NGF growth medium stained with anti-neurofilament antibody. ... The peripheral nervous system or PNS, is part of the nervous system, and consists of the nerves and neurons that reside or extend outside the central nervous system--to serve the limbs and organs, for example. ...

Classes

There are three classes of neurons: afferent neurons, efferent neurons, and interneurons.

• Afferent neurons convey information from tissues and organs into the central nervous system.
• Efferent neurons transmit signals from the central nervous system to the effector cells.
• Interneurons connect neurons within the central nervous system.

Structural classification An afferent neuron is a neuron that carries information from sensory receptors at its peripheral endings to the central nervous system. ... The Central Nervous System (CNS) represents the largest part of the Nervous System. ... In vertebrates, motoneurons (also called motor neurons) are efferent neurons that originate in the spinal cord and synapse with muscle fibers to facilitate muscle contraction and with muscle spindles to modify proprioceptive sensitivity. ... The Central Nervous System (CNS) represents the largest part of the Nervous System. ... Effector cells are a type of Lymphocyte cells that are actively engaged in secreting antibody. ... Interneuron: noun An interneuron is a neuron that communicates only to other neurons. ... The Central Nervous System (CNS) represents the largest part of the Nervous System. ...

• Pseudounipolar- single dendrite longer than single axon
• Bipolar - single axon and single dendrite equal length
• Multipolar - more than two dendrites
  • Golgi I- pyramidal cell
  • Golgi II- granule (stellate) cell

Pseudounipolar cells (Pseudo- false, uni- one) are sensory neurons in the peripheral nervous system. ... A pyramidal cell is a multipolar neuron located in the hippocampus and cerebral cortex. ... An assortment of grains The word grain has a great many meanings, most being descriptive of a small piece or particle. ...

Anatomy and histology

Many highly specialized types of neurons exist, and these differ widely in appearance. Characteristically, neurons are highly asymmetric in shape. Neurons consist of: File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ...

• The dendrite, a short, branching arbour of cellular extensions. Each neuron has very many dendrites with profuse dendritic branches. These structures form the main information receiving network for the neuron.
• The soma, or cell-body, the relatively large central part of the cell between the dendrites and the axon.
• The axon, a much finer, cable-like projection which may extend tens, hundreds, or even tens of thousands of times the diameter of the soma in length. This is the structure which carries nerve signals away from the neuron. Neurons have only one axon, but this axon may - and usually will - undergo extensive branching, enabling communication with many target cells.

Axons and dendrites in the central nervous system are typically only about a micrometre thick, while some of those in the peripheral nervous system are much thicker. The soma is usually about 25 micrometres in diameter and not much larger than than the cell nucleus it contains. The axon of a human motoneuron can be over a metre long, reaching from the base of the spine to the toes, while giraffes have single axons running along the whole length of their necks, several feet in length. Much of what we currently know about axonal transport comes from studying the squid neuron, an ideal neuron for research due to it's relatively immense size (0.5 - 1 millimetres thick, several centimetres long). (The term dendrite may also refer to the metallurgical dendrite. ... The soma is the bulbous end of a neuron, containing the nucleus. ... An axon, or nerve fiber, is a long slender projection of a nerve cell, or neuron, which conducts electrical impulses away from the neurons cell body or soma. ... A micrometre (American spelling: micrometer), symbol µm, is an SI unit of length. ... In cell biology, the nucleus (from Latin nucleus or nuculeus, kernel) is found in all eukaryotic cells that contains most of the cells genetic material. ... In vertebrates, motoneurons (also called motor neurons) are efferent neurons that originate in the spinal cord and synapse with muscle fibers to facilitate muscle contraction and with muscle spindles to modify proprioceptive sensitivity. ... Jump to: navigation, search Binomial name Giraffa camelopardalis Linnaeus, 1758 The Giraffe (Giraffa camelopardalis) is an even-toed ungulate mammal, the tallest of all land living animal species. ... Jump to: navigation, search Suborders Myopsina Oegopsina Squids are the large, diverse group of marine cephalopods, popular as food in cuisines as widely separated as the Korean and the Italian. ...

Connectivity

Neurons communicate with one another and to other cells through synapses, where the axon tip of one cell impinges upon a dendrite or soma of another, or less commonly to an axon. Neurons of the cortex in mammals, such as the Purkinje cells, can have over 1000 dendrites each, enabling connections with tens of thousands of other cells. Jump to: navigation, search Illustration of the major elements in a prototypical synapse. ... Jump to: navigation, search Cortex (Latin for bark) has different meanings, depending on the context: In neuroanatomy: the cerebral cortex (often simply called cortex) is the thin wrinkled outermost layer of the brain. ... Jump to: navigation, search Orders Subclass Multituberculata (extinct) Plagiaulacida Cimolodonta Subclass Palaeoryctoides (extinct) Subclass Triconodonta (extinct) Subclass Placentalia Afrosoricida Artiodactyla Carnivora Cetacea Chiroptera Creodonta (extinct) Dermoptera Desmostylia (extinct) Embrithopoda (extinct) Hyracoidea Insectivora Lagomorpha Litopterna (extinct) Macroscelidea Notoungulata (extinct) Perissodactyla Pholidota Primates Proboscidea Rodentia Scandentia Sirenia Tubulidentata Xenarthra Subclass Marsupialia Dasyuromorphia... Drawing of pigeon Purkinje cells (A) by Santiago Ramon y Cajal Purkinje cells are a class of GABAergic neuron located in the cerebellar cortex. ...

Types of signalling

Neurons communicate with one another across synapses. This communication is usually chemically mediated by rapid secretion of neurotransmitter molecules. Pre-synaptic neurons (i.e. the neurons which release the neurotransmitter) may produce in the post-synaptic neurons (i.e. the neurons being affected by the neurotransmitter) an electrical stimulation (an electrical excitation) which will spread to the axon hillock generating an action potential which then travels as a wave of electrical excitation along the axon. Arrival of an action potential at the tip of an axon triggers the release of neurotransmitter at a synaptic gap. Neurotransmitters can either stimulate or suppress (inhibit) the electrical excitability of a target cell. An action potential will only be triggered in the target cell if neurotransmitter molecules acting on their post-synaptic receptors cause the cell to reach its threshold potential. Exocytosis is the process of a biological cell releasing substances into the extracellular fluid (its environment). ... Neurotransmitters are chemicals that are used to relay, amplify and modulate electrical signals between a neuron and another cell. ... Jump to: navigation, search A molecule is the smallest particle of a pure chemical substance that still retains its chemical composition and properties. ... A. Schematic of an electrophysiological recording of an action potential showing the various phases which occur as the wave passes a point on a cell membrane. ... In biology, depolarization is the event a cell undergoes when its membrane potential grows more positive with respect to the extracellular solution. ... Jump to: navigation, search Illustration of the major elements in a prototypical synapse. ... Transmembrane receptors are integral membrane proteins, which reside and operate typically within a cells plasma membrane, but also in the membranes of some subcellular compartments and organelles. ... The threshold potential is the membrane potential to which a membrane must be depolarized to initiate an action potential. ...

Another less common form of communication between neurons is through electrical synapses mediated by gap junctions. Jump to: navigation, search An electrical synapse is a mechanical and electrically conductive link between two abutting neurons that is formed at a narrow gap between the pre- and postsynaptic cells known as a gap junction. ... A gap junction is a junction between certain animal/plant cell-types that allows different molecules and ions to pass freely between cells. ...

Adaptations to carrying action potentials

The narrow cross-section of axons and dendrites lessens the metabolic expense of carrying action potentials, although thicker axons convey the impulses more rapidly, generally speaking.

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 the action potentials to travel faster than in unmyelinated axons of the same diameter whilst simultaneously spending less energy to "recharge" the action potential after. The myelin sheath in peripheral nerves normally runs along the axon in sections about 1 mm long, punctuated by unsheathed nodes of Ranvier. Multiple sclerosis is a neurological disorder which results from abnormal demyelination of peripheral nerves. Neurons with demyelinated axons do not conduct electrical signals properly. In neuroscience, myelin is an electrically insulating fatty layer that surrounds the axons of many neurons, especially those in the peripheral nervous system. ... Neuroglia cells of the brain shown by Golgis method. ... Oligodendrocytes are a variety of neuroglia. ... Schwann cells are a variety of neuroglia that wrap around axons in the peripheral nervous system, forming the myelin sheath. ... Saltatory conduction is a means by which action potentials are transmitted along myelinated nerve fibers. ... Nodes of Ranvier are regularly spaced gaps in the myelin sheath around an axon or nerve fiber. ...

Neurons and glia make up the two chief cell types of the central nervous system. There are far more glial cells than neurons, and recent experimental results have suggested that glial cells play a vital role in information processing among neurons. Neuroglia cells of the brain shown by Golgis method. ... Neuroglia cells of the brain shown by Golgis method. ...

Histology and internal structure

Nerve cell bodies stained with basophilic dyes will 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 ribosomes. 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 numbers of protein synthesis. Franz Nissl Franz Nissl (1860-1919) was born in Frankenthal in the Bavarian Palatinate, the son of Theodor Nissl and Maria Haas. ... Jump to: navigation, search --68. ... Figure 1: Ribosome structure indicating small subunit (A) and large subunit (B). ...

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). Intermediate filaments are one component of the cytoskeleton - important structural components of living cells. ... Lipofuscin is brown pigment granules representing lipid-containing residues of lysosomal digestion and considered one of the aging or wear and tear pigments; found in the liver, kidney, heart muscle, adrenals, nerve cells, and ganglion cells. ...

Neurons of the brain

The nematode worm (Caenorhabditis elegans) has 302 neurons. Scientists have mapped all of the nematode's neurons. Binomial name Caenorhabditis elegans Maupas, 1900 Caenorhabditis elegans (pronounced see-no-rab-DYE-tis) is a free-living nematode (a roundworm), about 1 mm in length, which lives in a temperate soil environment. ...

The human brain has about 100 billion (10¹¹) neurons and 100 trillion (10¹⁴) connections (synapses) between them. Jump to: navigation, search Illustration of the major elements in a prototypical synapse. ...

See also

• Artificial neuron
• F wave
• Neural oscillations
• Mirror neuron
• Neuroscience

The artificial neuron (also called node) is the basic unit of an artificial neural network, simulating a biological neuron. ... In neuroscience, an F wave is the second of two voltage changes observed after electrical stimulation is applied to the skin surface above the distal region of a nerve. ... The concept of neural oscillations is close to the concept of brain waves. ... A mirror neuron is a neuron which fires both when performing an action and when observing the same action performed by another (possibly conspecific) creature. ... Neuroscience is a field of study which deals with the structure, function, development, genetics, biochemistry, physiology, pharmacology and pathology of the nervous system, divided most generally into the central nervous system (the brain and spinal cord), and the peripheral nervous system, consisting of the myriad nerve pathways running throughout the...

External links

• High-Resolution Cytoarchitectural Primate Brain Atlases
• NeuroWiki, a wiki website for Neuroscience related topics.
• Cell Centered Database UC San Diego images of neurons.