Dictionary
a cell that is specialized to conduct nerve impulses
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Wikipedia
Neurons (also spelled neurones or called nerve cells) are the primary cell (biology) cells of the nervous system.In vertebrates, they are found in the brain, the spinal cord and in the nerves and ganglionganglia of the peripheral nervous system.
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 classificationPseudounipolar cellsPseudounipolar- single dendrite longer than single axonBipolar - single axon and single dendrite equal lengthMultipolar - more than two dendrites*pyramidal cellGolgi I- pyramidal cell*granuleGolgi II- granule (stellate) cell
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: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_(biology)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 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).
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.
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 saltatory conductionfaster 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 node of Ranviernodes 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.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. It has been estimated that Glial cells outnumber neurons by as many as 50:1.
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.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).
Neurons of the brain - The nematode worm (''Caenorhabditis elegans'') has 302 neurons. Scientists have mapped all of the nematode's neurons.The human brain has about 100 billion () neurons and 100 trillion () connections (synapsesynapses) between them.
See also - Artificial neuron F wave Neural oscillations Mirror neuron Neuroscience
External links - primate-brain.org - High-Resolution Cytoarchitectural Primate Brain Atlases purl.net - NeuroWiki, a wiki website for Neuroscience related topics. ccdb.ucsd.edu - Cell Centered Database UC San Diego images of !neurons.Category:NeuroscienceC ategory:Neuronsbg:Неврон ca:Neuronacs:Neuronda:Neuronde :Nervenzellees:Neuronaeo:Neŭr onofr:Neuroneis:Taugafrumait:N euronehe:נוירוןmk:Нев ронnl:Zenuwcelja:ニュー ンno:Nevronpl:Neuronpt:Neur ónioru:Нейроныsimple:N euronfi:Neuronisv:Neuronuk:Н йронzh:神经元
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Websites
Neuron
Neuroscience journal published by Cell press. Abstracts since 1988 available for free. Fulltext issues available for 1996-current issues.
http://www.neuron.org/
NASA Quest
Dedicated to bringing NASA people and science to classrooms through the internet. Space Team Online, Aerospace Team Online, Solar System Online, and Deep Space Online. NASA Quest is the ultimate resource for educators and kids interested in meeting and learning about NASA people and space science.
http://quest.arc.nasa.gov/
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