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dendrites (Jan and Jan, 2003). The hamlet gene is expressed
in externalsensory neurons that have very simple mono-
polar dendrites, and when these neurons are mutant for
hamlet , their dendritic trees become highly branched so
they look very much like another class of sensory neurons
called multidendritic neurons, which normally do not
express hamlet . Conversely, multidendritic neurons
express high levels of another transcription factor called
cut , which is expressed at low levels in the external
sensory neurons. Overexpression of cut in the latter
causes them to acquire multiple dendrites, while loss
of cut function in multidendritic neurons causes them to
lose their dendritic complexity. Active axonal inputs are
important for dendrite growth and branching. The com-
plexity of the dendritic tree is, in many systems, propor-
tional to the amount of innervation. Thus, for example,
the dendritic trees of the principal sympathetic neurons
of the superior cervical ganglion (SCG) are larger and
more complex in larger mammals that have more inputs
onto these cells. Moreover, if inputs to the SCG are
reduced or silenced, this causes a concomitant decrease in
the complexity of the dendritic tree that develops (Purves
and Lichtman, 1985; Voyvodic, 1989). Tectal dendrites are
innervated by the axons of RGCs. Time-lapse recordings
of tectal dendrites show that dendritic branches are very
dynamic and can appear or disappear within minutes.
Active visually driven input on these dendrites enhances
their growth in early development through the activation
of NMDA receptors, CAM kinase II and Rho GTPases.
Activity stabilizes dendritic branches at later stages
(Cline, 2001). Many dendritic trees are dynamic through-
out the life of the animal. The plasticity of these branches
in response to experience and neuronal activity will be
discussed in Chapter 9.
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