anterograde tracers Substances taken up by and transported from the cell body toward the distal portion of an axon. After appropriate processing, these reveal which target structures receive connections from the injected region.

arbor The distal terminal portion of an axon, which carries the synaptic specializations.

axon a single process, usually emitted from the cell body, with membrane specializations adapted to the conduction of the nerve impulse.

bouton (French ''button'') General term that refers to the light microscopic appearance (swellings, beads, or stalked endings) of what are usually synapses. Some swellings, however, may be just accumulations of mitochondria, without synaptic vesicles. Boutons can also be called terminal specializations.

dendrites Extensions, usually multiple, of the cell body. Dendrites receive synaptic inputs and are usually much shorter than axons. For some neurons, such as local circuit neurons, dendrites and axons occupy about the same volume.

feedback A type of cortical connection that originates mainly from neurons in layer 6, terminates in layer 1, and has an elongated arbor (>1.0 mm long).

feedforward A type of cortical connection that originates mainly from neurons in layer 3, terminates in layer 4, and has one to four small arbors (<0.2 mm in diameter).

retrograde tracers Substances taken up by synaptic endings and transported back to the cell body. After appropriate processing, these reveal which areas send projections to the injected region.

synapse (Greek ''to fasten together'') Intercellular junctions that are specialized for the transmission of nerve impulses. Neurotransmitter substances are packaged within synaptic vesicles, as seen by electron microscopy.

Almost all nerve cells, and only nerve cells, have an axon.

This structure can be several microns (50-200) to several centimeters in length, and is specialized for conduction of the nerve impulse This article begins with a brief historical survey, followed by a discussion of the cell biology of axons and network properties. Because much of the experimental research on axons employs invasive techniques, this article emphasizes work carried out in animal models. The current understanding, supported by selective verifications, is that the results in most cases will extrapolate to humans, at least from closely related mammalian phyla.

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