Chapter 1/Cytology and Organization of Cell Types 13 -loum 7p四 7. Dendrites labeled with antibodies to microtubule-associated protein (MAP)2. The technique of immunocytochemistry antibodies to localize specific proteins in neurons. These antibodies are detected by secondary antibodies tagged with an enzyme or gold particle. (A)The enzyme, such as peroxidase, catalyzes a reaction that results in an electron-dense precipitate. a seen in the light micrograph, in which antibodies to MAP 2 label the dendritic tree (arrowheads) of a Purkinje cell. B)Alternatively, the secondary antibody can be tagged with gold particles, seen as black dots in the electron micrograph, in which they localize MAP 2 to microtubules(arrowheads) and cross-bridges(arrows)between them. Pu, Purkinje cell; b, basket cell( Part A is reprinted from Bernhardt R, Matus A J Comp Neurol 1984; 226: 207, with permission of Wiley-Liss, a division of ohn Wiley Sons, Inc, New York 3.2. 4. CYTOSKELETAL-BASED TRANSPORT SYSTEM 8 mm/day, respectively. The fast component carries OF NEURONS membranous vesicles, and the slow compartment Neuronal processes span great distances to reach carries cytoskeletal proteins Fast transport involves movement of vesicles along be meters away from the cell body in large organ- tracks composed of microtubules. Microtubule motors isms Neuropeptides synthesized and packaged in the generate the force required to propel organelles along perikaryon must embark on long journeys to far the path. Two of these motors, kinesin and dynein,are removed nerve terminals. Although some synaptic ATPases that are activated on binding to the micro- components can be synthesized and recycled locally, of the microtubule and dynein toward the minus end returned there for degradation by lysosomes, for In axons, most microtubules have their positive end from these remote areas requires active mechanisms direction. In dendrites. about half of the microtubules because diffusion alone would take an inordinate are oriented with their plus ends toward the dendritic amount of time. Bidirectional traffic in axons deli- tip, and the other half have their minus ends toward the vers proteins and organelles to and from the nerve tip Ditferences in microtubule orientation in axons terminal by anterograde and retrograde transport, and dendrites may be one of the mechanisms under- respectively. Anterograde transport delivers neuro lying the selective transport of organelles into dendrites peptide-containing vesicles and cytoskeletal pro- or axons teins; retrograde transport returns endocytotic and other vesicles. Axonal transport has two compo 4. NEURONAL SYNAPSES nents: a fast component, traveling at rates of 200 to 400 mm/day, and a slow component, which consists Electrical signals can be conveyed directly from cell of a slow component a(SCa)and a slow component to cell at special sites called gap junctions( Fig 8) Ions b(SCb), moving at rates of 0. 2 to l mm/day and 2 to pass from one cell to another through relatively large3.2.4. CYTOSKELETAL-BASED TRANSPORT SYSTEM OF NEURONS Neuronal processes span great distances to reach their presynaptic and postsynaptic targets, which can be meters away from the cell body in large organ￾isms. Neuropeptides synthesized and packaged in the perikaryon must embark on long journeys to far￾removed nerve terminals. Although some synaptic components can be synthesized and recycled locally, others must be imported from the cell body or returned there for degradation by lysosomes, for example. Molecular and organelle traffic to and from these remote areas requires active mechanisms, because diffusion alone would take an inordinate amount of time. Bidirectional traffic in axons deli￾vers proteins and organelles to and from the nerve terminal by anterograde and retrograde transport, respectively. Anterograde transport delivers neuro￾peptide-containing vesicles and cytoskeletal pro￾teins; retrograde transport returns endocytotic and other vesicles. Axonal transport has two compo￾nents: a fast component, traveling at rates of 200 to 400 mm/day, and a slow component, which consists of a slow component a (SCa) and a slow component b (SCb), moving at rates of 0.2 to 1 mm/day and 2 to 8 mm/day, respectively. The fast component carries membranous vesicles, and the slow compartment carries cytoskeletal proteins. Fast transport involves movement of vesicles along tracks composed of microtubules. Microtubule motors generate the force required to propel organelles along the path. Two of these motors, kinesin and dynein, are ATPases that are activated on binding to the micro￾tubule. Kinesin directs movement toward the plus end of the microtubule and dynein toward the minus end. In axons, most microtubules have their positive end toward the terminal and can guide movement in either direction. In dendrites, about half of the microtubules are oriented with their plus ends toward the dendritic tip, and the other half have their minus ends toward the tip. Differences in microtubule orientation in axons and dendrites may be one of the mechanisms under￾lying the selective transport of organelles into dendrites or axons. 4. NEURONAL SYNAPSES Electrical signals can be conveyed directly from cell to cell at special sites called gap junctions(Fig. 8). Ions pass from one cell to another through relatively large Fig. 7. Dendrites labeled with antibodies to microtubule-associated protein (MAP) 2. The technique of immunocytochemistry uses antibodies to localize specific proteins in neurons. These antibodies are detected by secondary antibodies tagged with an enzyme or gold particle. (A) The enzyme, such as peroxidase, catalyzes a reaction that results in an electron-dense precipitate, a seen in the light micrograph, in which antibodies to MAP 2 label the dendritic tree (arrowheads) of a Purkinje cell. (B) Alternatively, the secondary antibody can be tagged with gold particles, seen as black dots in the electron micrograph, in which they localize MAP 2 to microtubules (arrowheads) and cross-bridges (arrows) between them. Pu, Purkinje cell; b, basket cell. (Part A is reprinted from Bernhardt R, Matus A. J Comp Neurol 1984; 226:207, with permission of Wiley-Liss, a division of John Wiley & Sons, Inc., New York.) Chapter 1 / Cytology and Organization of Cell Types 13