Dendrites
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The membrane of the neuron functions as a receptive surface over its entire extent; however, specific inputs (termed afferents) from other cells are received primarily on the surface of the cell body and on the surface of the specialized processes known as dendrites. The dendritic processes may branch extensively and are often covered with projections known as
dendritic spines. Spines provide a tremendous increase in the surface area available for synaptic contacts. The dendritic processes and spines of neurons are essentially expansions of cytoplasm containing most of the organelles found in the cell body. Dendrites contain numerous orderly arrays of microtubules and fewer
neurofilaments (see below). The microtubule associated proteins (MAPs) in the dendrite have a higher molecular weight than those found in the axon. An example is MAP2. In addition, microtubules in dendrites have their positive ends toward the cell soma.
Mitochondria are often arranged longitudinally.
Rough endoplasmic reticulum and
ribosomes are present in large but not small dendrites. The shape and extent of the "dendritic tree" of an individual neuron is indicative of the quantity and variety of information received and processed by that neuron. The dendritic spines often contain
microfilaments which is the
cytoskeletal element responsible for changes in spine shape observed in some examples of synaptic plasticity.
Information is received by the dendrite through an array of receptors on dendrite surface that react to transmitters released from the axon terminals of other neurons. Dendrites may consist of a single twig-like extension from the soma or a multi-branched network capable of receiving inputs from thousands of other cells. For instance, an average spinal motor neuron with a moderate-sized dendritic tree, receives 10,000 contacts, with 2,000 of these on the soma and 8,000 on the dendrites.
