Dendrites, Axons, and Telodendria PDF

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Dendrites, Axons, and Telodendria...

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Dendrites, Axons, and Telodendria A variable number of slender, sensitive processes (extensions) known as dendrites extend and branch out from the cell body (see Figure 12–2a). Dendrites play key roles in intercellular communication. Typical dendrites are highly branched, and some branches are studded with fine 0.5 to 1 µm long projections called dendritic spines, which participate in synapses. In the CNS, a neuron receives information from other neurons primarily at the dendritic spines, which may represent 80–90 percent of the neuron’s total surface area. An axon is a long cytoplasmic process, in fact up to a meter in length, which is capable of propagating an electrical impulse known as an action potential. p. 304 The axoplasm (AK-so . - plazm), or cytoplasm of the axon, contains neurofibrils, neurotubules, small vesicles, lysosomes, mitochondria, and various enzymes. The axolemma (lemma, husk), the plasma membrane of the axon, surrounds the axoplasm. In the CNS, the axolemma may be exposed to the interstitial fluid or, as we’ll see, it may be covered by the cellular processes of neuroglia. The base, or initial segment, of the axon in a typical neuron joins the cell body at a thickened region known as the axon hillock (see Figure 12–2b). An axon may branch along its length, producing side branches known as collaterals. Collaterals enable a single neuron to communicate with several other cells. The main axon trunk and any collaterals end in a series of fine extensions called telodendria (tel-o . -DEN-dre . -uh; telo-, end + dendron, tree), or terminal branches. The telodendria, in turn, end at axon terminals (also called synaptic terminals), which play a role in communication with another cell. A synapse is where a neuron communicates with another cell, possibly another neuron (covered in Section 12-6). The movement of materials between the cell body and axon terminals is called axonal (axoplasmic) transport. These materials travel the length of the axon on neurotubules in the axoplasm, pulled along by protein “molecular motors,” called kinesin and dynein, which use ATP. Some materials travel slowly, at rates of a few millimeters per day. This transport mechanism is known as the “slow stream.” Some vesicles move much more rapidly, traveling in the “fast stream” at a rate as high as 1000 mm per day, but the average is around 300 mm per day. Axonal

transport occurs simultaneously in both directions. The flow of materials from the cell body to the axon terminal is anterograde (AN-ter-o . -gra . d; antero-, forward) flow, carried by kinesin. At the same time, other substances are transported from the axon terminal toward the cell body in retrograde (RET-ro . -gra . d) flow (retro, backward), carried by dynein. If debris or unusual chemicals appear in the axon terminal, retrograde flow soon delivers them to the cell body. There they may then alter the activity of the cell by turning certain genes on or off....