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Dendrites (from Greek δένδρον ''déndron'', "tree") (also dendron) are the branched projections of a neuron that act to propagate the electrochemical stimulation received from other neural cells to the cell body, or soma, of the neuron from which the dendrites project. Electrical stimulation is transmitted onto dendrites by upstream neurons (usually their axons) via synapses which are located at various points throughout the dendritic tree. Dendrites play a critical role in integrating these synaptic inputs and in determining the extent to which action potentials are produced by the neuron.〔 Dendrites are one of two types of protoplasmic protrusions that extrude from the cell body of a neuron, the other type being an axon. Axons can be distinguished from dendrites by several features including shape, length, and function. Dendrites often taper off in shape and are shorter, while axons tend to maintain a constant radius and be relatively long. Typically, axons transmit electrochemical signals and dendrites receive the electrochemical signals. Although, some types of neurons in certain species lack axons and simply transmit signals via their dendrites.〔 Dendrites provide an enlarged surface area to receive signals from the terminal buttons of other axons allowing for a chemical signal to pass simultaneously to many target cells. Synapses involving dendrites can be axodendritic, involving an axon signaling to a dendrite, or dendrodendritic, involving signaling between dendrites. Dendritic branching is also called "dendritic arborization" and "dendritic ramification.〔" When an electrochemical signal stimulates a neuron it causes changes in the electrical potential across the neuron’s plasma membrane. This change in the membrane potential will passively spread across the dendrite but becomes weaker with distance without an action potential. The action potential propagates the electrical activity along the membrane of the dendrite to the cell body and then afferently down the axon to the terminal buttons where it crosses the synapse.〔 Certain classes of dendrites contain small projections referred to as dendritic spines that increase receptive properties of dendrites to isolate signal specificity. Increased neural activity and the establishment of long-term potentiation at dendritic spines change the size, shape, and conduction. This ability for dendritic growth is thought to play a role in learning and memory formation. There are approximately 200,000 spines per cell, each of which serves as a postsynaptic process for individual presynaptic axons.〔 Dendritic branching can be extensive and in some cases is sufficient enough to receive as many as 100,000 inputs to a single neuron.〔 There are three main types of neurons; multipolar, bipolar, and unipolar. Multipolar neurons, such as the one shown in the image, are composed of one axon and many dendritic trees. Pyramidal cells are multipolar cortical neurons with pyramid shaped cell bodies and large dendrites called apical dendrites that extend to the surface of the cortex. Bipolar neurons have one axon and one dendritic tree at opposing ends of the cell body. Unipolar neurons have a stalk that extends from the cell body that separates into two branches with one containing the dendrites and the other with the terminal buttons. Unipolar dendrites are used to detect sensory stimuli such as touch or temperature.〔 The morphology of dendrites such as branch density and grouping patterns are highly correlated to the function of the neuron. Malformation of dendrites is also tightly correlated to impaired nervous system function.〔 ==History== Some of the first intracellular recordings in a nervous system were made in the late 1930s by Kenneth S. Cole and Howard J. Curtis. German anatomist Otto Friedrich Karl Deiters is generally credited with the discovery of the axon by distinguishing it from the dendrites. Swiss Rüdolf Albert von Kölliker and German Robert Remak were the first to identify and characterize the axon initial segment. Alan Hodgkin and Andrew Huxley also employed the squid giant axon (1939) and by 1952 they had obtained a full quantitative description of the ionic basis of the action potential, leading the formulation of the Hodgkin–Huxley model. Hodgkin and Huxley were awarded jointly the Nobel Prize for this work in 1963. The formulas detailing axonal conductance were extended to vertebrates in the Frankenhaeuser–Huxley equations. Louis-Antoine Ranvier was the first to describe the gaps or nodes found on axons and for this contribution these axonal features are now commonly referred to as the Nodes of Ranvier. Santiago Ramón y Cajal, a Spanish anatomist, proposed that axons were the output components of neurons. He also proposed that neurons were discrete cells that communicated with each other via specialized junctions, or spaces, between cells, now known as a synapse. Ramón y Cajal improved a silver staining process known as Golgi's method, which had been developed by his rival, Camillo Golgi. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「dendrite」の詳細全文を読む スポンサード リンク
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